Grade 8 Chemistry

IL FOUNDATION SERIES

CHEMISTRY

A Reliable Companion for JEE | NEET | Olympiads

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ISBN 978-81-985385-0-5 Second Edition

Contents 1.

Coal and Petroleum

01

2. Combustion and Flame

23

3. Structure of the Atom

44

4. Atoms and Molecules

71

5. Periodic Classification of Elements

89

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COAL AND PETROLEUM

1.1 INTRODUCTION We make use of a variety of materials to meet our daily needs. While some of them are created by humans, others can be found in nature. The resources that human beings obtain from nature (E.g., air, water, soil, etc.) for the fulfilment of their basic needs are called natural resources. Based on their availability, natural resources can be broadly classified into two kinds: inexhaustible and exhaustible natural resources.

Natural Resources Inexhaustible natural resources

Exhaustible natural resources

Fig. 1.1 Natural resources

1.1.1 Inexhaustible natural resources The resources that are present in unlimited quantity and cannot be exhausted by humans are known as inexhaustible natural resources. They get replenished at a faster rate than that at which they are consumed.

Examples: • Tidal energy, wave energy, ocean thermal energy • Air • Geothermal energy: Heat energy inside the earth • Solar energy: Solar cooker, solar water heater

Sunlight

Air

Fig. 1.2 Inexhaustible resources

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COAL AND PETROLEUM

1.1.2 Exhaustible natural resources The resources that are present in limited quantity in nature and can be exhausted by human activities are known as exhaustible natural resources. These are consumed at a rate faster than that at which they are replenished.

Examples: • Forests, wildlife, minerals • Fossil fuels like coal, petroleum, natural gas, etc.

Forest

Coal

Natural gas

Minerals Fig. 1.3 Exhaustible resources

1.1.3 Fuel Burning substances to release energy is one of the most significant chemical changes in our daily lives. Substances that we use to produce energy by burning them are called fuels. When these fuels burn, they react with oxygen and release energy. Some examples of fuels include natural gas, biogas, LPG, kerosene, diesel, firewood, coal, and petrol.

Calorific value of fuel

The calorific value of a fuel is the amount of heat liberated by the complete combustion (burning) of a unit mass or volume of a fuel in the presence of oxygen. For liquid or gaseous fuels, the volume

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of the fuel is considered, while for solid fuels, the mass of the fuel is considered to find the calorific value. The calorific value is measured in kilojoules per kilogram (kJ/kg).

Fuel

Calorific value (kJ/kg)

Cow dung cake

6000-8000

Wood

17000-22000

Coal

25000-33000

Petrol

45000

Kerosene

45000

Diesel

45000

Methane

50000

CNG

50000

LPG

55000

Biogas

35000-40000

Hydrogen

150000 Table 1.1 Calorific value of some fuels

Classification of fuels

Based on the states of matter Solid fuels

Liquid fuels

Gaseous fuels

They exist in a solid state at room temperature.

They are liquid at room temperature and are mainly obtained from petroleum through fractional distillation.

They occur in a gaseous state at room temperature.

They leave residue after burning.

They leave relatively very little residue or smoke after burning.

They do not leave any residue after burning.

Also, they do not produce smoke during combustion.

Examples: Coal, wood, Cow dung cake, etc.

Examples: Petrol, diesel, kerosene, etc.

Examples: LPG, CNG, water gas, biogas, etc.

Table 1.2 Difference between solid, liquid, and gaseous fuels

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COAL AND PETROLEUM

Based on the source Based on the sources, fuels are of two types: A) Primary or natural fuel: The fuel that is obtained directly from nature.

Examples: Coal, wood, natural gas, etc.

B) Secondary or derived fuel: The fuel that is derived from natural fuels.

Examples: Coke, kerosene, water gas, etc.

Characteristics of an ideal fuel

• It should have a high calorific value.

• It should not cause any pollution or produce any harmful gases during combustion.

• It should be low-cost and easily available.

• It should be easy to handle, store, and transport.

• It should have a moderate ignition temperature.

• It should have a moderate rate of combustion.

1.1.4 Fossil fuels The remains of dead plants and animals buried under the rocks millions of years ago are called fossils. The fuel formed over millions of years by decomposing the fossil remains of ancient plants and animals is called fossil fuel. Coal, petroleum, and natural gas are important fossil fuels.

Fossil fuels are used to generate electricity. They are generally used in power stations to heat water to produce steam to drive large turbines, which in turn produce electricity.

1.2 COAL Coal is a black or brown sedimentary rock that is combustible. It is a complex mixture of carbon, hydrogen, and oxygen compounds. Compounds like nitrogen, sulphur, and phosphorus are also present in it. It is found in coal mines deep under the surface of the Earth. Coal is mostly used by burning it to produce heat energy.

Fig. 1.4 Coal

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IL Foundation Series Class 8

1.2.1 Story of coal About 300 million years ago, there were thick forests in low-lying areas on Earth. These forests were buried under the surface of the Earth due to natural events such as floods, earthquakes, and volcanic eruptions. Thus, the plants had no contact with oxygen. Over time, successive layers of soil were deposited on the buried plants. Due to this, there was an increase in the temperature. Over millions of years, these plants were subjected to tremendous high pressure and heat, finally transforming them into coal.

Carbonisation: The chemical process involved in the transformation of matter into coal is called carbonisation. The plants buried under the soil undergo carbonisation and change into coal.

Coal burns and produces mainly carbon dioxide when heated in the presence of the air.

1.2.2 Types of coal The variety of coal depends on the carbon content present in it. The higher the temperature and pressure of the Earth, and the longer the coal has been buried under the Earth, the more carbon content in it.

The different varieties of coal are: peat, lignite, bituminous coal, anthracite coal.

Peat

Lignite

Bituminous coal

Anthracite coal

Peat is the youngest type of coal. It is light brown and has the lowest carbon content, resulting in less heat and more smoke when burned. Its calorific value is 10 to 15 kJ/g.

Lignite is often called soft coal. It is brown and has more carbon than peat.

Its calorific value is 15 to 20 kJ/g.

This type is common in households.

It is compact, black, contains more carbon, and produces more heat than peat and lignite. Its calorific value is 30 to 35 kJ/g.

The hardest coal with the highest carbon content. It burns with difficulty due to very low volatile matter.

It is not used for the household; it's mainly utilised for industry purposes.

Its calorific value is 28 to 30 kJ/g.

Table 1.3 Types of coal

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COAL AND PETROLEUM

Many giant plants died in the swamps.

Over millions of years, the plants were buried underwater and dirt.

Swamp 300 million years ago

Water 100 million years ago

dirt dead plants

Heat and pressure turned the dead plants into coal.

rocks and dirt coal

Fig. 1.5 Formation of coal

1.2.3 Destructive distillation of coal Heating coal without air is called the destructive distillation of coal. Coal contains several elements, such as carbon, hydrogen, oxygen, nitrogen, and sulphur. When coal is heated in the absence of air, several products are obtained. The main products obtained by the destructive distillation of coal are: 1. Coke 2. Coal tar 3. Coal gas 1. Coke Coke contains 98% carbon. It is porous, tough, black, and the purest form of coal. Like charcoal, it is a good fuel and burns without smoke. It is largely used as a reducing agent in the extraction of metals from their ores. It is also used in making fuel gases like water gas and producer gas.

2. Coal tar It is a mixture of different compounds of carbon. Coal tar is a thick, black-coloured liquid with an unpleasant smell. The fractional distillation of coal tar gives many chemical substances that are used in the preparation of dyes, paints, explosives, drugs, synthetic fibres, and pesticides. Some of these chemical substances are benzene, toluene, phenol, and aniline. Naphthalene balls used to repel moths and other insects are also obtained from coal tar. Bitumen is used in place of coal tar for metaling roads.

3. Coal gas Coal gas is mainly a mixture of methane and carbon monoxide. The gases present in coal gas are combustible, and hence, it is an excellent fuel. It has a high calorific value.

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IL Foundation Series Class 8

Coal

Solid

Liquid

Gas

Coke

Coal tar

Coal gas

Fig. 1.6 Products obtained from coal

1.3 PETROLEUM Petroleum is a dark brownish to green-coloured viscous liquid fuel. It has a strong smell due to the presence of sulfur-containing compounds. It is commonly called crude oil. A nation's economy depends greatly on petroleum; that's why petroleum is called black gold.

1.3.1 Story of petroleum Like coal, the dead bodies of plants and animals were buried at the bottom of the sea millions of years ago. They were covered with layers of sand and clay over a period of time, which led to the decomposition of organic matter. Due to high pressure, increased temperature, and absence of oxygen, the dead plants and animals are transformed into petroleum. Petroleum is obtained by digging into the Earth’s crust.

300 million years

100 million years

Today

Ocean

Sand and sediment

Sand sediment and rock

Decomposing organic remains

Oil and gas accumulations

Fig. 1.7 Formation of petroleum

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COAL AND PETROLEUM

1.3.2 Refining of petroleum • The crude oil obtained from an oil well is a dark and viscous liquid. It is a mixture of hydrocarbons. This liquid is separated into its components by the process of fractional distillation.

• The process of separation of mixtures with different boiling points to obtain the components with similar boiling points is called fractional distillation.

• Each separated part is called a fraction. For example, petrol is one of these fractions, and sometimes we use it as it is. But if we need pure substances, we separate each fraction even more.

• The process of separating crude oil into its components by fractional distillation is called refining.

• To refine crude oil, a special column called a fractionating column is used.

• Crude oil is heated to about 400 degrees Celsius. The vapours that are formed are fed into the fractionating column.

• The lightest part, like petroleum gas, goes to the top. Below that, fractions like petrol, naphtha, kerosene, diesel oil, lubricating oil, and fuel oil are obtained.

• The residue at the bottom is further distilled to make things like paraffin wax and asphalt.

PETROLEUM GAS < 25OC < 25-60OC

GASOLINE

< 60-180OC

NAPHTHA

< 180-220OC

PARAFFIN

< 220-250OC

DIESEL

< 250-300OC

FUEL OIL

< 300-350OC

LUBRICATING OIL

< 350OC

CRUDE OIL

FURNACE

BITUMEN Fig. 1.8 Refining of petroleum

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IL Foundation Series Class 8

1.4 PETROCHEMICAL PRODUCTS Diesel and petrol are two well-known petroleum products. Apart from these two, many other petroleum products are used for various purposes. Some of them are mentioned: Constituents of Petroleum

Uses

Petroleum Gas in Liquid form (LPG)

Fuel used at home and in some industries

Petrol

Motor fuel, aviation fuel, solvent for dry cleaning

Paraffin wax

Candle, vaseline, ointments, etc.

Diesel

Fuel for heavy motor vehicles, electric generators

Kerosene

Fuel for stoves, lamps, and jet aircraft

Lubricating oil

Lubrication

Bitumen

Paints, road surfacing Table 1.4 Petrochemical products and their uses

1.5 NATURAL GAS Millions of years ago, natural gas and petroleum were formed when tiny sea plants and animals died and were buried under sand and mud. In anaerobic conditions, these organisms transformed into gas.

1.5.1 Composition It primarily contains methane (around 85%), ethane (approximately 10%), and propane (about 3%). When natural gas is compressed at high pressure, it becomes CNG (compressed natural gas).

CNG is utilised for power generation and is gaining popularity as a fuel for vehicles due to its lower environmental impact. One notable advantage of CNG is its direct use in homes and factories through pipelines, as seen in Vadodara (Gujarat) and certain parts of Delhi.

1.5.2 Occurrence Natural gas is commonly found trapped between impermeable rocks, sometimes in association with petroleum and sometimes independently. In India, natural gas reserves have been discovered in Tripura, Rajasthan, Maharashtra, and the Krishna Godavari Delta.

1.5.3 Uses of natural gas • As a fuel since it has a high calorific value of 55 kJ/g.

• As a source of hydrogen and carbon.

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COAL AND PETROLEUM

1.6 LIMITED NATURAL RESOURCES Coal and petroleum are classified as fossil fuels, formed over millions of years from the remains of dead organisms. However, despite the extended duration of their formation, the known reserves of these fuels are expected to last only a few hundred years. Additionally, the combustion of these fuels significantly contributes to air pollution and is associated with global warming. Hence, these resources should be used judiciously, reserving their use for essential purposes. This approach will not only contribute to a healthier environment but also mitigate the risk of global warming and extend the availability of these fuels.

In India, the Petroleum Conservation Research Association (PCRA) guides fuel conservation.

Their recommendations include: • Maintaining a constant and moderate speed whenever possible.

• Turning off the engine at traffic lights or when waiting.

• Ensuring correct tire pressure.

• Regularly maintaining the vehicle.

QUICK REVIEW • Based on their availability, natural resources are classified into inexhaustible and exhaustible natural resources.

• A fossil fuel is formed over millions of years by decomposing the fossil remains of ancient plants and animals.

• Coal, petroleum, and natural gas are types of fossil fuels.

• They were made from the remains of plants and animals that lived a long time ago.

• Fossil fuels are an exhaustible source of energy.

• Coal is a black or brown sedimentary rock that is combustible and is a complex mixture of carbon, hydrogen, and oxygen compounds.

• Coal gives us things like coke, coal tar, and coal gas.

• Petroleum is a dark brownish to green-coloured viscous liquid fuel commonly called crude oil.

• Petroleum gives us different things like petrol, diesel, kerosene, etc.

• These fuels should be used carefully because there is only a limited amount of coal and petroleum.

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IL Foundation Series Class 8

WORKSHEET - 1 MULTIPLE CHOICE QUESTIONS WITH SINGLE CORRECT ANSWER I. Exhaustible and inexhaustible natural resources

1. Which of the following are examples of exhaustible natural resources? a. Water and sunlight

b. Wildlife and minerals

c. Coal and sunlight

d. All of these

2. Minerals are: a. Inexhaustible natural resources b. Renewable natural resources c. Exhaustible natural resources d. None of these 3. Which non-renewable source of energy is associated with the greenhouse effect and climate change? a. Geothermal energy

b. Nuclear energy

c. Hydroelectric power

d. Coal

4. Which of the following is not a fossil fuel? a. Coal

b. Solar Energy

c. Petroleum

d. Natural gas

5. The resources formed from the dead remains of living organisms are called _________. a. Renewable resources

b. Inexhaustible resources

c. Fossil fuels

d. Man-made fuels

6. Good fuel ______. a. Is readily available

b. Is cheap

c. Burns easily in the air

d. All of these

7. Which of the following is an inexhaustible natural resource? a. Metals

b. Groundwater

c. Sunlight

d. Coal

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COAL AND PETROLEUM

8. Which of the following statements is true? a. Groundwater is considered an inexhaustible natural resource. b. Topsoil is considered an inexhaustible natural resource. c. Solar energy is considered an inexhaustible natural resource. d. Forest is considered an inexhaustible natural resource.

II. Coal

1. Consider the following statements: i) Coal and natural gas are examples of exhaustible substances. ii) Coal, petroleum, and natural gas all are considered fossil fuels. iii) Fossil fuels are present in unlimited amounts. iv) During the manufacturing of steel, coke is used.

The correct statements are : a. i and iii

b. ii and iii

c. i, ii, and iv

d. i, ii, iii, and iv

2. Several materials can be obtained from coal tar; some of them are mentioned below: i) Naphthalene balls

ii) Perfumes

iii) Drugs

iv) Cookware v) Fuel

The correct materials are: a. i and iii

b. i, ii and iii

c. ii, iii and iv

d. iii, iv and v

3. The process of converting coal into coke is called: a. Fractional distillation

b. Destructive distillation

c. Refining

d. None of these

4. Which type of coal is typically used in thermal power plants due to its balance of energy and relatively low sulfur content? a. Humus

b. Bituminous

c. Anthracite

d. Peat

5. The process of coal formation from plant material involves various stages. Which stage comes before the formation of bituminous coal?

12

a. Peat

b. Lignite

c. Humus

d. Anthracite

IL Foundation Series Class 8

6. In order of quality of coal, the four main varieties of coal can be graded from superior to inferior as: a. Peat, bituminous, lignite, anthracite b. Bituminous, peat, lignite, anthracite c. Anthracite, bituminous, lignite, peat d. Lignite, anthracite, bituminous, peat 7. One word is given for a few statements: i) Resources that will either never run out or are replaced within a reasonable period Renewable resources. ii) A gaseous substance obtained during the processing of coal to coke-coal gas. iii) Conversion of vegetation into coal over millions of years due to high temperature Carbonisation.

The correct statements are: a. i and ii

b. ii and iii

c. Only iii

d. i, ii, and iii

8. Which one of the following is used for making printer ink and shoe polish? a. Graphite

b. Coke

c. Lamp black

d. Activated Charcoal

9. Assertion (A): Coal gas is used for lighting.

Reason (R): Charcoal is used as a decolourising agent in the sugar industry. a.

Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).

b. Both Assertion (A) and Reason (R) are true, but Reason(R) is not the correct explanation of Assertion (A). c. Assertion (A) is true, but Reasoning (R) is false. d. Both Assertion (A) and Reasoning (R) are false.

10. Assertion (A): Coke is used in the manufacturing of steel.

Reason (R): Coal tar is used to prepare explosives. a.

Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).

b. Both Assertion (A) and Reason (R) are true, but Reason(R) is not the correct explanation of Assertion (A).

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COAL AND PETROLEUM

c. Assertion (A) is true, but Reasoning (R) is false. d. Both Assertion (A) and Reasoning (R) are false.

11. Which of the following natural resources is an ideal replacement for coal to generate electricity? a. Wood

b. Water

c. Coke

d. Petroleum

12. Match the varieties of coal given in Column I with their carbon content given in Column II and choose the correct answer using the codes given below.

Column I

Column II

A) Lignite

i) 95% carbon

B) Anthracite

ii) 60% carbon

C) Peat

iii) 85% carbon

D) Bituminous

iv) Brown coal

a. A – iv, B – iii, C – ii, D – i c. A – ii, B – i, C – iii, D – iv

b. A – iv, B – i, C – ii, D – iii d. A – ii, B – i, C – iv, D – iii

13. Coal tar is a _______liquid. a. Blue

b. White

c. Grey

d. Black

14. Which of the following is used in making roads? a. Coal gas

b. Parrafin wax

c. Lubricating oils

d. Coal tar

15. Carbonisation of dead plants yields: a. Coal

b. Petrol

c. Diesel

d. Natural gas

III. Petroleum

1. Which of the following is not an application of fractional distillation of petroleum? a. Petrol

b. Paraffin wax

c. Lubricating oil

d. Coke

2. Which of the following statements is true regarding the formation of coal and petroleum? a. Coal forms from aquatic plants, and petroleum from terrestrial plants. b. Coal and petroleum both come from aquatic plant remains. c. Coal forms from terrestrial plants, and petroleum from aquatic plants and animals.

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IL Foundation Series Class 8

d. Both coal and petroleum come from terrestrial plants.

3. Which petroleum product is a raw material for the production of plastics, synthetic fibres, and other petrochemicals? a. Diesel

b. Jet fuel

c. Lubricating oil

d. Naphtha

4. Various constituents from petroleum are obtained by: a. Fractional distillation

b. Simple distillation

c. Destructive distillation

d. Vapourisation

5. Which of the following gases is produced during the burning of coal? a. Oxygen

b. Nitrogen

c. Argon

d. Carbon dioxide

6. Match the items given in Column I with the items given in Column II and choose the correct answer using the codes given below : Column I

Column II

A) Petroleum gas in liquid form

i) Liquid gold

B) Natural gas

ii) LPG

C) Petroleum

iii) CNG

D) Paraffin wax

iv) Vaseline

a. A – ii, B – iii, C – iv, D – i c. A – ii, B – iii, C – i, D – iv

b. A – iv, B – iii, C – i, D – ii d. A – iii, B – ii, C – iv, D – i

7. Petroleum is formed in _____ pressure and _____ temperature conditions. a. High, low

b. Low, high

c. High, high

d. Low, low

8. Which of the following are the main components in the formation of petroleum? a. Dead plants and animals

b. Destructed buildings

c. Water

d. Stones of the mountain

9. How long does it take to form petrol? a. Hundreds of years

b. A few weeks

c. A few days

d. Millions of years

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COAL AND PETROLEUM

10. Petroleum is formed due to _____________. a. Decomposition of organic matter

b. Decomposition of inorganic matter

c. Displacement of organic matter

d. Displacement of inorganic matter

11. Which one of the following conditions is not responsible for the transformation of dead organisms into petroleum and natural gas? a. High-pressure

b. Absence of air

c. High-temperature

d. High amounts of oxygen

12. Which of the following is not a product of petroleum refining? a. Formic acid

b. Gasoline

c. Kerosene oil

d. Diesel oil

IV. Natural gas and limited natural resources

1. What is the process called when natural gas is converted into a liquid for easier transportation? a. Vaporisation

b. Liquefaction

c. Sublimation

d. Condensation

2. Which of the following statements accurately compares and analyses the characteristics of fossil fuels, specifically natural gas? a. Natural gas is less efficient than coal and oil due to its lower energy content. b. Natural gas releases fewer pollutants than coal and oil when burnt. c. Natural gas is not environmentally sustainable. d. Natural gas is mainly carbon and hydrogen, lacking the abundant methane found in oil.

3. Study the characteristics given below: It is a dark-coloured viscous liquid. It has a strong smell. Identify the compound among the following based on the above characteristics. a. Kerosene

b. Petrol

c. Petroleum

d. Alcohol

4. Assertion (A): CNG is also used for power generation.

Reason (R): The boiling point of diesel ranges between 250-350°C. a.

Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).

b. Both Assertion (A) and Reason (R) are true, but Reason(R) is not the correct explanation of Assertion (A). c. Assertion (A) is true, but Reasoning (R) is false. d. Both Assertion (A) and Reasoning (R) are false.

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5. What is CNG used for? a. Power generation

b. Petroleum generators

c. Solvent

d. None of these

6. Natural gas is mainly composed of: a. Methane

b. Ethyl alcohol

c. Methyl alcohol

d. None of these

7. PCRA stands for: a. Public Conservations Research Association c. Petroleum Conservation Research

b. Public Council of Research Association d. Partial Counting of Remaining

Association

Amendment

8. Which of the following statements accurately compares and analyzes the properties of petroleum and its impact on the environment? a. Petroleum emits fewer greenhouse

b. Petroleum's vast reserves make it

gases than coal, making it eco-friendly. c. Petroleum refineries consume

renewable for future generations. d. Petroleum spills have a minimal

significant energy and water.

ecological impact, evaporating quickly.

9. Which of the following gases is called a cleaner fuel? a. Coal gas

b. CNG

c. Petrol

d. Diesel

10. Natural gas is found : a. In a layer over petroleum oil.

b. In a layer below the petroleum oil.

c. Mixed with petroleum oil.

d. None of the above.

WORKSHEET - 2 MULTIPLE CHOICE QUESTIONS WITH SINGLE CORRECT ANSWER 1. Match the following: Column I

Column II

A) Carbonisation

1) Coke

B) Destructive distillation

2) Coal

C) Cracking

3) Petroleum

D) Refining

4) Hydrocarbons

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COAL AND PETROLEUM

a. A-1, B-2, C-3, D-4

b. A-2, B-1, C-4, D-3

c. A-3, B-2, C-4, D-1

d. A-4, B-3, C-2, D-1

2. Assertion: Coal is considered an inexhaustible natural resource as it can be recreated by the deposition of fossils of plants and animals over millions of years.

Reason: Inexhaustible natural resources can be finished after being used over the years. a.

Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).

b. Both Assertion (A) and Reason (R) are true, but Reason(R) is not the correct explanation of Assertion (A). c. Assertion (A) is true, but Reasoning (R) is false. d. Both Assertion (A) and Reasoning (R) are false.

3. Petroleum is a mixture of __________. a. Carbohydrates

b. Esters

c. Hydrocarbons

d. Alcohols

4. ________ is a thick black liquid with an unpleasant odour. a. Gasoline

b. Coal tar

c. Diesel

d. CNG

5. Which one of the following has the highest calorific value? a. Kerosene

b. Petrol

c. LPG

d. Biogas

6. Which of the following is not a use of petrol? a. Fuel for automobiles

b. Aviation fuel

c. Manufacture of vaseline

d. Solvent of dry cleaning

7. Which gas is produced when coal is heated in the air? a. CO2

b. H2

c. N2

d. NO2

8. Which of the following states of India does not have a petroleum oil well? a. Assam

b. Maharastra

c. Punjab

d. Gujarat

9. Which of the following fractions of petroleum has the lowest boiling point? a. Kerosene

b. Diesel oil

c. Gasoline

10. Which among the following is a source of kerosene?

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a. Coal tar

b. Wood

c. Crude petroleum

d. Coal

d. Heavy oil

IL Foundation Series Class 8

11. The gas that is the cause of explosions in coal mines is: a. Methane

b. Carbon monoxide

c. Hydrogen

d. Carbon dioxide

12. Crude petroleum is also known as : a. Black gold

b. Yellow gold

c. Brown gold

d. White gold

13. Mark the incorrect statement: a. Burning of coal with a sufficient amount of oxygen produces carbon dioxide. b. When coal burns an insufficient amount of oxygen, carbon monoxide is formed. c. Charcoal is a better fuel than kerosene to be used as cooking fuel at home. d. LPG is considered to be a good fuel for domestic use.

14. Which fuel is expected to last longest? a. Wood

b. Coal

c. Petroleum

d. Kerosene

15. Which of the following statements is true? a. Our energy demand has decreased over the years c. Coal as a fuel will last forever

b. We must use fossil fuels sparingly d. Fossil fuels do not create any pollution

16. Which among the following is a greenhouse gas? a. Carbon dioxide

b. Oxygen

c. Hydrogen

d. Nitrogen

17. Petrol is used as a fuel in automobiles such as motorcycles, scooters, and cars, while heavy vehicles like trucks and tractors run on which fuel? a. Diesel

b. Fuel oil

c. Lubricating oil

d. Petroleum gas

18. Coal is used in generating electricity in a/an: a. Nuclear power station

b. Thermal power station

c. Hydropower station

d. Geothermal power station.

19. Which of the following is not a petroleum product? a. Petrol

b. Paraffin wax

c. Beeswax

d. Kerosene

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COAL AND PETROLEUM

20. Different varieties of coal differ in their: a. Moisture

b. Volatile nature

c. Carbon content

d. Number of hydrogen atoms.

21. Which form of coal contains the highest carbon content? a. Anthracite

b. Peat

c. Lignite

d. All have the same carbon content

22. Which of the following will be obtained first from the fractional distillation of crude oil? a. Bitumen

b. Lubricating oil

c. Kerosene

d. Fuel gas

23. Which fraction of petroleum is used in making skin ointments and Vaseline? a. Lubricating oil

b. Paraffin wax

c. Bitumen

d. Fuel oil

24. Read the following statements and mark the correct ones from the given options. i) Coal, petroleum, and natural gas are called fossil fuels. ii) Coal and natural gas are two exhaustible substances. iii) Coke is used in the manufacturing of steel. iv) Fossil fuels are present in limited quantities. a. i and ii

b. i and iv

c. i, ii, and iii

d. i, ii, iii, and iv

25. Coal is mainly carbon, but it has some other elements like: a. Oxygen, hydrogen, nitrogen, and sulphur c. Sulphur, phosphorus, iodine, and oxygen

b. Chlorine, nitrogen, sulphur, and helium d. Bromine, nitrogen, phosphorus, and hydrogen

26. Butane gas is used for filling cylinders to be used as LPG because: a. It is easily available. c. It is stored in a gaseous state only in cylinders.

20

b. It is easily compressed into a liquid and stored in cylinders. d. It is the cheapest gas available.

IL Foundation Series Class 8

27. Y is formed when X is heated in the absence of air. Y is a tough, porous, and black substance.

Both X and Y are carbon-rich materials. What could X and Y be? a. X= Coal, Y= Coke

b. X= Petroleum, Y= Petrol

c. X= Coal, Y= Coal tar

d. X= Petroleum, Y= Diesel

28. Which of the following are the constituents of petroleum? a. Diesel, paraffin

b. Coal tar, bitumen

c. CNG, coal tar

d. Kerosene, coal gas

29. The gaseous fuels: i) Burns without producing smoke ii) Have a low calorific value iii) They are easy to ignite iv) They have high calorific value The correct statements are: a. i and ii

b. ii and iii

c. iii and iv

d. iv and ii

30. Coal is a fossil fuel, and it cannot be prepared in a laboratory or industry because the formation of coal: i) Is a very slow process. ii) Need very low pressure and low temperature. iii) Need very high pressure and high temperature. iv) Causes air pollution.

The correct statements are a. i and ii

b. i and iii

c. ii and iv

d. iv and iii

31. Consider the following statements: i) Natural gas can be supplied to homes and factories through pipes. ii) Natural gas is obtained by fractional distillation of crude oil. iii) Natural gas is a cleaner fuel because it produces only water while burning. iv) Natural gas is an exhaustible source of energy like fossil fuels.

The correct statements are: a. i and iii

b. ii and iii

c. i and iv

d. i, iii, and iv

32. _________________ is the renewable source of energy. a. Coal

b. Diesel

c. Oil

d. Biodiesel

21

COAL AND PETROLEUM

33. What percentage of carbon is present in bituminous? a. 20 - 25%

b. 25 - 35%

c. 65 - 85%

d. 98%

34. The petroleum product which is not used as fuel is: a. Petrol

b. Kerosene

c. Diesel

d. Petroleum jelly

35. The main constituent of LPG is: a. Methane

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b. Butane

c. Ethane

d. Ethylene

2

COMBUSTION AND FLAME

We use different fuels for various purposes at home, in industries, and for cars. At home, we use materials such as cow dung, wood, coal, charcoal, and gases like compressed natural gas (CNG).

Industries use fuels such as coal, petrol, and diesel. Cars run on petrol and diesel mostly.

Now, think about a burning candle.

Fig 2.1 Burning candle

It has a flame, right? Well, when we burn things like coal, there's no flame. Many things can burn without a flame. Let's look at why and the types of flames that can happen during burning.

2.1 WHAT IS COMBUSTION? We can say that some materials burn, and some do not burn. Combustion is the chemical process where a substance undergoes a reaction with the oxygen present in the atmosphere, producing heat as a result.

Combustible substances are those that can ignite and burn when exposed to a flame, and some of them are suitable for use as fuels like- petrol, paper, wood, etc. On the other hand, noncombustible substances are those that do not undergo combustion and do not catch fire when subjected to a flame, like stone, glass, etc.

Various materials ignite at distinct temperatures. The point at which a substance initiates combustion is referred to as its ignition temperature. When a material undergoes combustion, it

23

COMBUSTION AND FLAME

generates heat, sustaining the continuous burning process. The ignition temperature varies among different substances.

2.1.1 Inflammable substances Things that are very susceptible to burning when placed near an open flame due to their very low ignition temperature are referred to as inflammable substances. These materials include, but are not limited to, gasoline, alcohol, and liquefied petroleum gas (LPG).

2.2 CONTROL OF FIRE Water helps put out fires because it makes things that can catch fire cooler, stopping them from getting hot enough to start burning.

Fig 2.2 Firefighter controlling fire

When you pour water on something that's on fire, it also makes steam, which wraps around the burning stuff and blocks the air it needs to keep burning. So, the fire goes out because it can't get the air it needs.

To make a fire, you need three things: fuel (something that can burn), air (which provides oxygen), and heat (to make the fuel hot enough to catch fire). To stop a fire, you need to take away something it needs. A fire extinguisher helps by stopping the air it uses, making the fuel colder, or both.

Removing combustible material from a fire is a challenging task; therefore, it is advisable to reduce the temperature and cut off the air supply.

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IL Foundation Series Class 8

2.3 TYPES OF COMBUSTION During the summer, dry grass in forests can easily catch fire on its own, and if not controlled, the fire can spread rapidly, affecting trees and the entire forest. Controlling such fires is a challenging task.

2.3.1 Rapid combustion When a gas burns really fast, it gives off heat and light. This kind of burning is called rapid combustion. It's like when you see a quick and bright flame from a gas, and it happens because the burning happens superfast.

Fig 2.3 Burning of gas

2.3.2 Spontaneous combustion Now, let's talk about how matchsticks catch fire. The head of a matchstick has a special mixture made of antimony trisulphide and potassium chlorate, along with some glue and starch. In the past, white phosphorus was used, but it turned out to be dangerous for both workers and users.

Fig 2.4 Matchsticks

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COMBUSTION AND FLAME

These days, safety matchsticks have a safer formula. The matchstick head contains antimony trisulphide and potassium chlorate, and the striking surface has powdered glass and a safer type of phosphorus called red phosphorus. When you strike the match against a rough surface, a bit of red phosphorus turns into white phosphorus. This white phosphorus reacts with potassium chlorate, creating enough heat to ignite antimony trisulphide and start the combustion, making the match catch fire. Spontaneous combustion is the process in which a material catches fire on its own due to internal heat accumulation, without an external ignition source.

2.3.3 Explosion During festivals, we often enjoy fireworks. When you light a firecracker, it undergoes a quick change that produces heat, light, and sound. A lot of gas is released in this process, and we call this kind of fast change an explosion.

Fig. 2.5 Crackers

2.4 FLAME You might notice that when a candle is burning, there's a flame, but when charcoal is burning, there isn't. Different things burn in different ways – some make flames, and others don't.

Fig. 2.6 Burning of a candle and charcoal

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IL Foundation Series Class 8

When we use kerosene oil or molten wax in a lamp, they go up through the wick and turn into gas, creating flames. However, charcoal is different because it can't turn into a gas, so it doesn't produce a flame. Fuels that are already in a gas form catch fire easily. For example, cooking gas lights up right away. Spirit and petrol can also quickly catch fire because they turn into gas at room temperature.

2.5 STRUCTURE OF A FLAME Light a wax candle and pay close attention to the flame. Look carefully for the various colours in the flame. Can you count how many colours there are? Starting from the bottom of the flame, try to identify different sections or zones in the flame. Now, what colour do you see on the outermost part of the flame?

Take a look at the bottom of the candle flame. When the wax turns into vapour and mixes with air, it burns all the way and makes a blue flame. This blue part is called the blue zone.

2.5.1 Zones of a flame 1) Outermost or Non-luminous zone: This is the outermost zone of flame and is the hottest part of the flame. It is blue in colour where complete combustion takes place.

2) Middle or Luminous zone: It is the middle zone of flame and is moderately hot. It is yellow in colour, where incomplete combustion takes place.

3) Innermost or Dark zone: It is the flame's innermost zone and the least hot zone. It is black due to unburnt carbon, where no combustion takes place.

HottestHOTTEST part PART MODERATELY Moderately hot HOT LEAST Least hot HOT

OUTER Outermost zone MOST ZONE

Middle zone

MIDDLE ZONE

Dark zone Blue zone

DARK ZONE BLUE ZONE

Fig. 2.7 Different zones of a candle flame

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COMBUSTION AND FLAME

2.6 WHAT IS A FUEL? Fuels like gasoline, coal, wood, charcoal, LPG, and kerosene are the main sources of heat energy for use in homes and businesses. These are referred to as fuels. A good fuel is inexpensive, readily available, burns cleanly in the air, produces a lot of heat, and leaves no harmful residue behind.

Fig. 2.8 Fuels

Although there isn't a perfect fuel, we should select one that meets our needs well. Different fuels have different prices; some are less costly than others.

2.7 FUEL EFFICIENCY 2.7.1. Introduction to fuel efficiency Choosing the right fuel depends on what you're using it for. The best fuel for cooking might not be the best for powering a vehicle or machinery. In short, fuel efficiency matters. There are a few things to think about when picking a fuel, like what you're using it for, availability, affordability, ease of use, safety, and ignition characteristics. It's also important that the fuel burns at a good speed and doesn't cause too much pollution.

2.7.2 Calorific value Let's say you have 1 kilogram of coal and 1 kilogram of cow dung, and you burn them. But here's the interesting part: different things produce different amounts of heat when they burn. Heat is measured in kilojoules, which is a way to talk about energy. The amount of heat energy produced on the complete combustion of 1 kg of a fuel is called its calorific value.

So, if you burn 1 kilogram of coal and 1 kilogram of cow dung, which one gives off more heat? Well, the calorific value helps us figure that out. It's like saying, "How much energy do we get from

28

IL Foundation Series Class 8

burning 1 kilogram of this stuff?" In simple terms, some things make more heat when they burn, and we can measure that with calorific value in kilojoules.

Example: When 4 kg of fuel completely burns, the heat produced was measured to be 1,60,000 kJ. What is the calorific value of the fuel?

Solution: We know that calorific value means the amount of heat energy produced on the complete conversion of 1 kg of fuel.

4 kg → 1, 60,000 kJ 1 kg → ?

4x = 1, 60,000 x = 1, 60,000/4 = 40,000 kJ Characteristics of a good fuel

• It is readily available.

• It is cheap.

• It burns easily in the air at a moderate rate.

• It produces a large amount of heat.

• It does not leave any residue.

• It should have a low ignition temperature.

• It has a high calorific value.

2.8 BURNING OF FUELS LEADS TO HARMFUL PRODUCTS Using too much fuel is bad for the environment in several ways. Some of them are• When we burn things like wood, coal, and petroleum, they release tiny particles of unburnt carbon. These particles can be harmful and make it difficult to breathe, leading to illnesses like asthma.

• If we don't burn these fuels completely, they create a poisonous gas called carbon monoxide.

This gas is really dangerous, especially if we burn coal in a closed room, as it can even be deadly for people sleeping in that room.

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COMBUSTION AND FLAME

Fig. 2.9 Global warming

• When fuels burn, they also release a gas called carbon dioxide. Having too much of this gas in the air is thought to be a big problem because it can make the Earth warmer, causing issues like global warming.

So, using too much fuel isn't just bad for our environment; it can also affect our health and the planet's temperature.

Fig. 2.10 Acid rain

Burning coal and diesel makes a gas called sulfur dioxide, which is bad for breathing and can damage things. Also, when we use gasoline in engines, it creates gases with nitrogen that can be harmful. When these sulfur and nitrogen gases mix with rain, they turn into acids. This kind of rain is called acid rain, and it's not good for plants, buildings, and soil.

30

IL Foundation Series Class 8

QUICK REVIEW • Combustible substances are things that can catch fire.

• Burning a material in the presence of air (oxygen) is called combustion.

• Oxygen in the air is needed for things to burn.

• When things burn, they give off heat and light.

• Ignition temperature is the lowest temperature when things catch fire.

• Things that catch fire easily have a low ignition temperature.

• Fire controlling methods: Water is commonly used to control fire but can’t be used to control fire that involves electrical equipment or oils.

• There are different types of burning, like rapid combustion, spontaneous combustion, and explosions.

• Flames have three zones: Non-luminous zone, luminous zone and dark zone.

• The best fuel is cheap, easy to find, burns well, and doesn't cause pollution. It should have lots of energy and not produce pollutants.

• Fuels are different in how good they are and how much they cost.

• Fuel efficiency is how good the fuel is, measured in units called kilojoules per kilogram.

• Polluted air with small bits of unburnt stuff can make it hard to breathe.

• When fuel doesn't burn completely, it makes a bad gas called carbon monoxide.

• Too much carbon dioxide in the air can make the Earth too warm.

• Things from burning coal, diesel, and petrol can make acid rain, which damages plants, buildings, and soil.

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COMBUSTION AND FLAME

WORKSHEET - 1 MULTIPLE CHOICE QUESTIONS WITH SINGLE CORRECT ANSWER I.

Combustion and control of fire

1. Select all the necessary conditions for combustion: I. Presence of air

II. Presence of fuel

IV. Presence of carbon dioxide

III. Attainment of ignition temperature

V. Presence of candle

a. I, II, and III only

b. I, III, and IV only

c. II, IV, and V only

d. I and III only

2. A substance which reacts with oxygen, giving heat, is called a combustible substance. Which one of the following is a combustible substance? a. Iron nail

b. Glass

c. Stone piece

d. Wood

3. The incomplete combustion of LPG fuel leads to which of the following oxides: a. Sulphur and silicon

b. Carbon

c. Nitrogen and phosphorus

d. Nitrogen and silicon

4. The head of the matchstick does not contain: a. Antimony trisulphide

b. Potassium chlorate

c. Red phosphorus

d. Black phosphorus

5. How is fire extinguished at airports and petrol pumps? a. By using water

b. By using sand

c. By using a foam fire extinguisher

d. By using blankets

6. Burning coal in a closed room will produce ____________ a. Nitrogen oxide

b. Carbon dioxide

c. Carbon monoxide

d. Oxygen

7. Once, in a village, there was a fire in a hut. When a lot of dry powder of a substance 'A' was released over the fire, the fire was extinguished. What among the following is substance A?

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IL Foundation Series Class 8

a. Sodium chloride

b. Sodium hydrogen carbonate

c. Sodium sulfate

d. Potassium chloride

8. Rajesh takes three substances: X, Y, and Z. On heating, these substances burn at different temperatures. He finds out that X catches fire immediately upon heating. On further heating of Y and Z, he observes that Y catches fire while Z does not. The correct arrangement of X, Y, and Z in order of their ignition temperature is: a. X > Y > Z

b. X = Y = Z

c. X = Y > Z

d. X < Y < Z

9. Consider the following substances I) Hydrogen

II) Water

III) Petrol

IV) LPG

The inflammable substance(s) /fuel(s) is/are a. Only I

b. I and IV

c. I and II

d. I, III and IV

10. Incomplete combustion of methane forms: a. CO2 + H2O

b. CO + H2O

c. CH4 + O2

d. C + O2

II. Types of combustion

1. The example of spontaneous combustion is/are: a. Burning of a matchstick

b. Burning of phosphorus

c. Burning of fireworks

d. Burning of oxygen

2. Which of the following options stands out as odd based on the type of combustion? a. Burning of petrol

b. Burning of kerosene

c. Burning of LPG

d. Forest fire

3. Burning of a spirit lamp is an example of _________ combustion. a. Explosive

b. Rapid

c. Slow

d. Spontaneous

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COMBUSTION AND FLAME

4. Which of the following is not an example of rapid combustion? a. Burning of LPG

b. Burning of CNG

c. Combustion of coal dust in mines

d. Burning of a spirit lamp

5. When a substance burns instantly and produces heat and light, the combustion is called _________. a. Explosive combustion

b. Spontaneous combustion

c. Rapid combustion

d. Slow combustion

6. Which statement best explains spontaneous combustion? a. It is the ability of materials to burn with the help of some source of heat like flame or spark. b. It is the ability of certain materials to start burning without any flame, spark, heat, or ignition source. c. It is the ability of certain materials to remain unburnt. d. It is the ability of certain materials to burn without oxygen.

7. ’Firework’ is an example of ____________ . a. Rapid combustion

b. Explosion

c. Spontaneous combustion

d. Slow combustion

8. The combustion reaction which occurs on its own, without any external supply of heat, is called ________. a. Explosion

b. Spontaneous combustion

c. Fire

d. Rapid combustion

9. Choose the correct option: The burning of LPG is an example of _____. a. Explosive combustion

b. Spontaneous combustion

c. Slow combustion

d. Rapid combustion

10. Which of the following is not an example of spontaneous combustion?

34

a. Forest fire

b. Bursting of crackers

c. Burning of phosphorous in air

d. Burning of sodium in the air

IL Foundation Series Class 8

III. Flame and structure of a flame

1. What will happen if you hold a piece of iron wire with a pair of tongs inside a candle flame or a Bunsen burner flame? a. Iron wire becomes red hot and burns with a flame only. b. Iron wire becomes red hot and starts glowing. c. Iron wire does not burn with a flame and glow d. Iron wire becomes red hot and burns with a flame and glow 2. Which of the following does not produce flames on burning? a. LPG

b. Candle wax

c. Charcoal

d. Kerosene oil

3. When the wick of a candle is touched by a lit match, the candle begins and continues to burn.

What is the role of the match in the reaction involving the candle wax? a. It acts as a catalyst.

b. It supplies the heat energy.

c. It lowers the heat energy.

d. It decreases the rate of the combustion.

4. Which zone of a flame does a goldsmith use for melting gold and silver? a. Outer zone

b. Middle zone

c. Inner zone

d. Lower zone

5. The middle part of the flame is yellow because _____. a. Here, partial combustion takes place b. Here, complete combustion takes place c. This part is the hottest zone of the flame d. This part is the least hot zone of the flame 6. The outer zone of the flame is the hottest part of the candle flame because of: a. Complete combustion

b. Partial combustion

c. Presence of unburnt wax

d. None of the above

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COMBUSTION AND FLAME

7. The innermost zone of unburnt wax particles gives ________ colour. a. Blue

b. Yellow

c. Black

d. Green

8. The middle zone of a flame has a _____________ colour. a. Blue

b. Yellow

c. Black

d. Green

9. The _____ colour of the flame indicates complete combustion of LPG. a. Yellow

b. Red

c. Orange

d. Blue

10. A _________ is formed on the glass plate when brought near a candle. a. Circular reddish ring

b. Circular blackish ring

c. Circular blue ring

d. Circular green ring

IV. Fuel, fuel efficiency, and burning of fuel

1. Which types of coal will have the highest calorific value? a. Lignite

b. Anthracite

c. Nituminous

d. Sub-bituminous

2. Despite having a very high calorific value, hydrogen gas is still not used commonly as fuel.

Why? a. It is not available easily. b. It is available only in deep oceans. c. It has a very explosive nature. d. It cannot be formed easily in a laboratory.

3. In an experiment, 50 kg of fuel was burned entirely. The heat produced measured to be 150,000 kJ. The calorific value of the fuel is 'X' × 1000 kJ/kg. Then 'X' is _________.

36

a. 5

b. 3

c. 2

d. 4

IL Foundation Series Class 8

4. 70 kg of fuel was completely burnt for an experiment. The amount of heat energy was found to be 1,40,000 kJ. Calculate the calorific value of the fuel. a. 3,000 kJ/kg

b. 2,000 kJ/kg

c. 3,500 kJ/kg

d. 4,000 kJ/kg

5. Which of the following represents the correct increasing order of calorific value of the given fuels? a. Hydrogen < LPG < cow dung cake < coal b. Cow dung cake < LPG < biogas < hydrogen c. Cow dung cake < biogas < LPG < hydrogen d. Hydrogen < cow dung cake < LPG < biogas 6. Which of the following fuels is used for running automobiles? a. Cow dung

b. Petrol

c. LPG

d. Wood

7. Which among the following is a cleaner fuel? a. CNG

b. Petrol

c. Diesel

d. Wood

8. Which of the following is closest to being an ideal fuel? a. Kerosene

b. Petrol

c. CNG

d. Diesel

9. Select the correct alternative.

Statement 1: CNG is a better fuel than petrol.

Statement 2: The calorific value of CNG is greater than of petrol. a. Statement 1 is correct, and statement 2 is incorrect. b. Statement 1 is incorrect, and statement 2 is correct. c. Both statements are correct, and statement 2 is the correct reason for statement 1. d. Both statements are correct, and statement 2 is not the reason for statement 1.

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COMBUSTION AND FLAME

10. Global warming can be controlled by _________________. a. Reducing deforestation and cutting down the use of fossil fuels b. Reducing afforestation and cutting down the use of fossil fuels c. Reducing deforestation and increasing the use of fossil fuels d. Reducing afforestation and increasing the use of fossil fuels

WORKSHEET - 2 MULTIPLE CHOICE QUESTIONS WITH SINGLE CORRECT ANSWER 1. A luminous flame appears _________________. a. Green

b. Blue

c. Yellow

d. Red

2. In which zone of a candle does complete combustion take place? a. Inner

b. Outer

c. Middle

d. All three zones

3. What are the substances that can catch fire called? a. Acids

b. Bases

c. Combustible

d. Burners

4. The calorific value of a fuel is expressed in___. a. Kilojoule per kilogram

b. Joule per kg

c. Joule per milligram

d. Kilojoule per milligram

5. Which of the following is not a necessary condition for combustion? a. Presence of combustible substance b. Presence of supporter of combustion c. Attainment of ignition temperature of the fuel d. Presence of carbon dioxide

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IL Foundation Series Class 8

6. Which of the following is not the attribute of a good fuel? a. Low calorific value

b. Moderate rate of combustion

c. Cheap and easily available

d. Safe to handle

7. The calorific value of a fuel may be defined as _________________. a. The amount of heat produced when 1 kg of a fuel is completely burnt b. The amount of heat produced when 1 gm of a fuel is completely burnt c. The amount of heat produced when 10 gm of a fuel is completely burnt d. The amount of heat absorbed in kilojoules when 1 gm of a fuel is completely burnt 8. Which of the following fuels has the highest calorific value? a. Hydrogen gas

b. LPG

c. Petrol

d. Natural gas

9. Global warming can be controlled by _________. a. Increase in oxygen level

b. Decrease in greenhouse gases

c. Increase in greenhouse gases

d. Increase in ethane

10. Which of the following is not a fossil fuel? a. Coal

b. Petroleum

c. Natural gas

d. Water gas

11. In which zone of a candle does combustion not take place? a. Inner

b. Outer

c. Middle

d. None of these

12. A temperature at which the substance burns is called _________. a. Melting

b. Boiling temperature

c. Kindling temperature

d. Evaporation

13. Which of the following is not an effect of global warming? a. Melting of polar ice caps

b. A rise in sea level

c. Heating and expansion of ocean water

d. Good fungal growth in soil 39

COMBUSTION AND FLAME

14. The amount of heat energy produced on complete combustion of 1 kg of a fuel is called _______. a. Calorific value

b. Significant value

c. Heal value

d. Internal energy

15. Which of these is able to control fires? a. NH3

b. H2

c. CO2

d. F2

16. Which gas helps in the process of combustion? a. Chlorine

b. Nitrogen gas

c. Oxygen gas

d. Producer gas

17. What will the burning coal in a closed room produce? a. Nitrogen oxide

b. Carbon dioxide

c. Carbon monoxide

d. Oxygen

18. Acid rain mainly contains _______. a. Oxygen and nitrogen gas

b. Fluorine and chlorine gas

c. Magnesium oxide

d. Nitrogen oxide and sulphur dioxide

19. Which zone represents the partial combustion in candle flame? a. Outer zone

b. Middle zone

c. Inner zone

d. Lower zone

20. Which is a non-renewable source of energy? a. Natural gas

b. Wind energy

c. Tidal energy

d. Mechanical energy

21. Which is a non-combustible substance? a. Wood

b. Paper

c. Iron nails

22. Complete burning of a substance in the presence of O2 is:

40

a. Combustion

b. Evaporation

c. Condensation

d. Filtration

d. Straw

IL Foundation Series Class 8

23. Combustion produces _________. a. Light only

b. Heat only

c. Light & heat

d. Water

24. Coal is formed by the process of _________. a. Distillation

b. Vaporisation

c. Carbonisation

d. Evaporation

25. The minimum temperature at which a combustible substance catches fire is called _________. a. Ignition temperature

b. Combustion temperature

c. Burning temperature

d. Optimum temperature

26. A ____________ produces sulphur dioxide. a. Nuclear power plant

b. Hydroelectric power plant

c. Coal power plant

d. Wind power plant

27. ______ is the region of combustion of gaseous fuel. a. Flame

b. Fuel

c. Oxidation

d. Vaporisation

28. ______ is the luminous zone of a flame. a. Outermost zone

b. Middle zone

c. Dark zone

d. None of these

29. In a foam type extinguisher, ______ is present in the bottle. a. NaHCO3

b. Al2(SO4)3

c. Al(OH)3

d. Na2SO4

30. Water cannot be used as a fire extinguisher to put out: a. Burning cloth

b. Burning paper

c. Electrical fire

d. Burning charcoal

31. In the case of good fuels, the ignition temperature is X, and the calorific value is Y. X and Y respectively, are _________. a. Low and high

b. Low and low

c. High and low

d. High and high 41

COMBUSTION AND FLAME

32. Global warming is a serious environmental problem that leads to rising sea levels, resulting in floods in coastal areas.

The cause of global warming is: a. Sulphur dioxide released by combustion of fuels and resulting in increased temperature b. Carbon monoxide added to the environment due to the combustion of fuel and results in decreased temperature. c. Carbon dioxide added to the environment due to combustion of fuel and resulting in increased temperature. d. None of the above 33. Consider the following substances: I) Hydrogen

II) Petrol

III) Water

IV) Coke

The inflammable substance(s)/fuel(s) is/are: a. Only I

b. I and IV

c. I and II

d. I, III, and IV

34. Some of the features of fuel are given below.

I) Readily available II) Low cost III) Catch fire readily in the air IV) No residue after burning V) Limited heat while burning Which of the above features should be present in a fuel so that it can be considered as an ideal fuel? a. I, II, and III

b. I, II, III, and V

c. I, II, III, and IV

d. All of the above

35. Consider the consequences of the greenhouse effect: I) Existence of life on the Earth II) Melting of ice on the Earth's pole III) Floods on low-lying areas of the Earth IV) Warmth of the Earth The correct statements are:

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IL Foundation Series Class 8

a. I and IV

b. II and III

c. I, II, and III

d. all of the above

36. Oxides of which element(s) is/are present in acid rain?

I) Carbon

II) Nitrogen

III) Sulphur

The correct answer is: a. I and II

b. I, II, and III

c. I and III

d. II and III

37. Consider the following factors: I) Fuel

II) Oxygen

III) Heat

Which of these has to be removed to put off a fire? a. I and II

b. II and III

c. I or II, or III

d. I, II, and III

38. Some processes are given below: I) Burning of candle II) Burning of wood III) Burning of paper From these, the example(s) of slow combustion is/are: a. Only II

b. Only III

c. II and III

d. I, II, and III

39. Fire extinguishers extinguish the fire by _________.

I) Cutting off the air supply II) Bringing down the temperature of the fuel III) Supplying air to the fuel The correct statements are: a. I and II

b. II and III

c. I and III

d. I, II, and III

40. The main contributors of acid rain are _________. a. Sulphur oxides and carbon oxides

b. Nitrogen oxides and sulphur oxides

c. Carbon dioxide and carbon monoxide

d. Nitrogen oxides and carbon oxides

43

STRUCTURE OF THE ATOM

3

3.1 THE STRUCTURE OF AN ATOM 3.1.1 Introduction to the structure of an atom The electrical nature of matter was indicated by frictional studies; for example, when substances like glass or ebonite are rubbed with silk or fur, they generate electricity. However, the greatest breakthrough in the atomic theory of matter came to light with the discovery of Faraday's laws of electrolysis, which proved the electrical nature of matter. Faraday suggested that there is some relationship between matter and electricity.

The electrical nature of matter was further supported by Thomson's experiment on the electrical conduction of gases through discharge tubes.

Year

Landmarks

1896

J.J. Thomson's discovery of the electron

1909

Rutherford's nuclear atom

1913

Mosley's determination of atomic number

1913

Bohr's atom

1921

Bohr - Bury Scheme of electronic arrangement

1932

Chadwick's discovery of the neutron

Table 3.1 The main landmarks in the evolution of atomic structure Discharge tube experiment

The discovery of electrons was a result of the study of the electric discharge (tube) in the discharge tube.

Discharge tube

-

Air at very low pressure

Green glow

- - - - - -

+

Cathode rays Cathode

Anode

To vacuum pump

-+

High voltage generator

Fig. 3.1 Production of cathode rays

44

IL Foundation Series Class 8

Properties of cathode rays

1. They travel in straight lines away from the cathode and cast shadows of opaque objects placed in their path.

2. Cathode rays cause mechanical motion of a small pin-wheel placed in their path. Thus, they possess kinetic energy and must be material particles.

3. They produce fluorescence (a glow) when they strike the glass wall of the discharge tube.

4. They heat up a metal foil to incandescence, which they impinge upon.

5. Cathode rays produce X-rays when they strike a metallic target.

6. The speed of cathode rays is less than the speed of light.

7. Cathode rays are deflected in an electric field towards the positive end, which indicates the presence of a negative charge in cathode rays. In a magnetic field, cathode rays are deflected in a direction perpendicular to both the field and their motion.

Properties of anode rays

1. They travel in a straight line in a direction opposite to the cathode.

2. Anode rays are deflected in an electric field towards the negative end, which indicates the presence of a positive charge in anode rays. In a magnetic field, anode rays are deflected towards the Southpole.

3. The charge-to-mass ratio (e/m) of positive particles varies with the nature of the gas placed in the discharge tube.

4. They possess mass many times that of an electron.

5. They cause fluorescence on zinc sulphide.

3.1.2 Thomson’s model of an atom An atom may be considered as a sphere of positive charge in which the electrons are distributed uniformly. This model of an atom is known as the plum pudding or watermelon model. This model could explain the electrical neutrality of atoms but failed to explain the observations of Rutherford's α-ray scattering experiment.

Thomson's model of an atom

1. Thomson proposed the first atomic model in 1904.

2. A ccording to him, an atom contains an equal number of negative and positive charges, so an atom as a whole is electrically neutral.

3. T homson believed that electrons are embedded in the positively charged atomic mass like the seeds embedded in the fibrous mass of a watermelon fruit.

4. The whole positive charge and mass of the atom are uniformly distributed inside the atom.

45

STRUCTURE OF THE ATOM

Atom model

Watermelon Positive charge

- + -+-+ - ++ + - - +-

Electron

Fig. 3.2 Thomson’s atomic model Limitations of J.J. Thomson's model of an atom

1. J.J. Thomson attributed the mass of an atom to electrons and protons, which are evenly spread throughout the atom. This does not meet with the observations of Rutherford, who concluded that the mass is concentrated in a very small space called the nucleus.

2. However, this model also failed to explain how the positively charged particles are shielded from the negatively charged particles and electric charges, when squeezed in a small volume, are expected to repel each other.

3.1.3 Rutherford’s model of an atom α-particles

movable screen gold foil Source of alpha rays

radioactive substance (polonium) deflected α-particles lead plate

+

ZnS screen

The scattering pattern on a gold foil

Rutherford alpha ray scattering experiment

Fig. 3.3 Rutherford's atomic model (planetary model) Alpha-particle scattering experiment

• Ernest Rutherford designed an experiment to check the validity of Thomson's atomic model.

• A stream of high energy α-particles (nuclei of helium atoms) from a radioactive source was directed at a thin foil (thickness nearly 100 nm ) of gold metal. The thin gold foil had a circular fluorescent zinc sulphide screen around it. Whenever an α-particle struck the screen, a tiny flash of light was produced at that point.

• α-rays consist of 24 He2+ .

46

IL Foundation Series Class 8

Observations made from this α-ray scattering experiment

• Most of the α-particles passed through the gold foil undeflected.

• A small fraction of the alpha particles were deflected by small angles.

• A very few alpha particles (1 in 20,000) bounced back, i.e., were deflected by an angle of nearly 180°.

Conclusion from alpha-particle scattering

From this experiment, he concluded that: • Most of the space inside the atom is empty (as most of the alpha particles passed through the gold foil undeflected).

• Very few α-particles were deflected from their path, indicating that the positive charge of the atom occupies very little space at the centre.

On the basis of his experiment, Rutherford put forward the nuclear model of an atom, which has the following features (postulates): • There is a positively charged centre in the atom called the nucleus. Almost all the mass of an atom is present in the nucleus.

• The electrons revolve around the nucleus just like the planets revolve around the sun.

• The size of the nucleus is very small compared to the size of the atom. The size of the nucleus is in the order of 10-13 cm, and the size of an atom is in the order of 10-8 cm.

Drawbacks of Rutherford's model

• He could not explain the stability of the atom. Any particle moving in a circular path would undergo acceleration. According to the electromagnetic theory, during acceleration, a charged particle radiates energy continuously. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom would be highly unstable, and hence, matter would not exist. But we know that matter exists and atoms are stable.

• He could not explain the line spectrum of an atom. If the electron loses energy continuously, then the atomic spectra should be continuous. Experimentally, atomic spectra are made up of discrete spectral lines.

Ernest Rutherford was born in New Zealand but went to Cambridge University in England to pursue his PhD in 1895. In 1899, he moved to McGill University in Canada.

In 1907, Rutherford moved from Canada to Manchester University in England, where he performed the experiments that gave us the modern view of the atom. Rutherford was awarded the 1908 Nobel Prize in Chemistry for his investigations on the chemistry of radioactive substances. In a series of experiments analysing the radiation emitted E. Rutherford by elements such as uranium, he discovered that such radiation consisted of three components: alpha, beta, and gamma rays.

47

STRUCTURE OF THE ATOM

3.1.4 Bohr's model of an atom Rutherford's atomic model could not explain electromagnetic radiation and, therefore, was not able to account for the existence of spectral lines even for the simplest atom, i.e. the hydrogen spectrum. To seek a theoretical explanation of the existence of spectral lines and their regularities and also to solve the conflict between the conclusions of conventional mechanics and the laws of electrodynamics, Bohr, in 1913, put forward a theory to improve Rutherford’s model of (atom) structure of an atom.

His theory was based on the principles of Max Planck about the absorption and emission of radiation by an electron. In order to overcome the drawbacks of Rutherford's model of an atom, Niels Bohr, a brilliant Danish Physicist, pointed out that the old laws of physics just did not work in the submicroscopic world of the atom. He closely studied the behaviour of electrons, radiations and atomic spectra.

In 1913, Bohr proposed a new model of the atom based on the modern quantum theory of energy.

With his theoretical model, he was able to explain why an orbiting electron did not collapse into the nucleus and how the atomic spectra were caused by the radiations emitted when electrons moved from one orbit to the other.

Bohr's postulates

1. Electrons revolve around the nucleus in specified circular paths called orbits or shells. These orbits are numbered as 1, 2, 3, 4,… etc., or represented as K, L, M, N,...etc., respectively and are represented by the symbol 'n'.

2. Each orbit is associated with a definite amount of energy. Hence, these orbits are also called energy levels.

3. As long as the electron revolves in a particular orbit, the electron neither gains nor loses energy.

Therefore, these orbits are called stationary orbits or main energy states, and the electrons are said to be in stationary energy states.

4. The energy associated with the energy levels increases with the increase in the 'n' value.

5. When an electron absorbs energy, it jumps from a lower energy level to a higher energy level, and by emitting energy, it jumps from a higher energy level to a lower energy level. This absorption and emission take place in the form of electromagnetic radiation.

Let E1 be the energy of the lower energy orbit and E2 be the energy of the higher energy orbit.

Then E2- E1 = ΔE = hϑ Where h = Planck's constant and ϑ = Frequency of radiation h = 6.6256 × 10-34 joule.sec (or) h = 6.6256 × 10-27 erg.sec 6. ΔE is the energy difference between the two orbits.

7. The angular momentum of an electron revolving in a particular orbit is quantised and is an integral multiple of h/2π. This is known as the quantisation of angular momentum.

8. The angular momentum is given by the formula mvr = n h/2π, which is called 'Bohr's quantum condition'.

48

IL Foundation Series Class 8

Where 'n' is an integer( n = 1, 2, 3,…..), this is also called 'principal quantum number.' m = mass of the electron, v = velocity of the electron r = distance of the electron from the nucleus, h = Planck's constant Niels Bohr, a Danish physicist, received his PhD from the University of Copenhagen in 1911. He then spent a year with J.J. Thomson and Ernest Rutherford in England. In 1913, he returned to Copenhagen, where he remained for the rest of his life. In 1920, he was named Director of the Institute of Theoretical Physics. After the First World War, Bohr worked energetically for peaceful uses of atomic energy. He received the first Atoms for Peace award in 1957. Bohr was awarded the Nobel Prize in Physics in 1922.

Niels Bohr

3.1.5 Fundamental particles of an atom The three fundamental particles present in an atom are electron, proton, and neutron.

Property

Electron

Proton

Neutron

1) Charge

-ve

+ve

no charge

2) Notation

e-

p+

n0

3) Mass

9.11 × 10-28 g

1.672 × 10-24 g

1.675 × 10-24 g

(or)

(or)

(or)

0.000548 amu

1.007277 amu

1.008665 amu

- 1.602 × 10-19 coulombs

1.602 × 10-19 coulombs

-

(or) - 4.8 × 10-10 esu

(or) 4.8×10-10 esu

-

-1

+1

-

4) Charge

5) Relative charge

Table 3.2 Fundamental particles in an atom

In a neutral atom, the number of positive charges is equal to the number of negative charges (or the number of protons is equal to the number of electrons).

49

STRUCTURE OF THE ATOM

-

Oxygen Atom electron <10-16 cm proton (neutron) ~10-13 cm

atom ~10-8 cm

Electrons

Second Electron Shell

-

First Electron Shell

-

-

+

+

quark ~10-16 cm

+

+

-

nucleus ~10-12 cm

+

-

Nucleus

-

-

Fig. 3.4 Fundamental particles of an atom

3.2 ELECTRON DISTRIBUTION IN DIFFERENT ORBITS (SHELLS) The electrons in an atom revolve around the nucleus in definite paths called orbits or shells. These orbits are numbered 1, 2, 3, 4,… and are represented as K, L, M, N,… respectively and are denoted by 'n'. Each orbit consists of a subshell.

The first orbit ( n = 1, K- shell) has only one subshell and is named 1s subshell. It can accommodate a maximum of two electrons only.

The second orbit ( n = 2, L- Shell) has two types of subshells, which are named 2s and 2p.

The third orbit (n = 3, M- Shell) has three types of subshells, which are named 3s, 3p and 3d.

The fourth orbit ( n = 4, N- Shell) has four types of subshells, which are named 4s, 4p, 4d and 4f.

The capacity of s, p, d, and f subshell to hold electrons present in any subshell is 2, 6, 10, and 14, respectively.

54321

K LMNO

Fig. 3.5 Electronic distribution in different orbits

Note: The number of subshells in the nth shell is given by n.

The number of orbitals in the nth shell is given by n2.

The number of electrons in the nth shell is given by 2n2.

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IL Foundation Series Class 8

3.3 VALENCY Definition - Valency is the combining capacity of an element.

(OR) Modern definition of valency: According to the new concept, valency can be defined as 'the number of electrons that are lost or gained or shared with one atom of an element, to acquire the stable configuration of the nearest noble gas element.'

(OR) The number of hydrogen atoms (or) chlorine atoms (or) double the number of oxygen atoms with which one atom of an element combines is called its valency.

Element

Na

Mg

Al

11

12

13

2, 8, 1

2, 8, 2

2, 8, 3

1

2

3

Atomic number Electronic configuration Valency

Capacity of an atom to give, accept, or share electrons to achieve the octet state Table 3.3 Valency

Need 1eHydrogen Valency 1

Oxygen (2,6)

Valency

2

Fig. 3.6 Valency

Example: Element

With hydrogen

With chlorine

With oxygen

Valency

Na

NaH

NaCl

Na2O

1

Mg

MgH2

MgCl2

MgO

2

Al

AlH3

AlCl3

Al2O3

3

Table 3.4 Compound formation

3.3.1 Valency of metals 'Number of valence electrons present in a metal atom is its valency.'

Generally, metals have 1, 2 or 3 valence electrons. So, their valencies are 1, 2 or 3, respectively.

51

STRUCTURE OF THE ATOM

Example: 11

Sodium

Na

Atomic mass: 22.989 Electron configuration: 2, 8, 1

Fig. 3.7 Sodium element

3.3.2 Valency of non-metals Valency of non-metals = (8 - number of valence electrons ) Generally, non-metals possess 4, 5, 6 or 7 valence electrons.

Thus, their valencies are 8-4, 8-5, 8-6, and 8-7, i.e., 4, 3, 2 and 1, respectively.

Example: Element

C

N

O

F

Atomic number

6

7

8

9

Electronic configuration

2, 4

2, 5

2, 6

2, 7

Valency

8-4=4

8-5=3

8-6=2

8-7=1

Compounds example

CH4

NH3

H2O

HF

Table 3.5 Non-metal compounds

Name of the element

Symbol

Lithium

Li

Beryllium

Atomic number

Distribution of electrons K

L

M

N

3

2

1

-

-

1

Be

4

2

2

-

-

2

Boron

B

5

2

3

-

-

3

Carbon

C

6

2

4

-

-

4(8-4)

Nitrogen

N

7

2

5

-

-

3(8-5)

Oxygen

O

8

2

6

-

-

2(8-6)

Fluorine

F

9

2

7

-

-

1(8-7)

Neon

Ne

10

2

8

-

-

0(8-8)

Table 3.6 Distribution of electrons

52

Valency

IL Foundation Series Class 8

3.4

ELECTRONIC CONFIGURATION OF ELEMENTS

The systematic arrangement of electrons in various atomic orbitals in the increasing order of their energies is known as electronic configuration.

The increasing order of various atomic orbitals is as follows 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s <4d <5p To write the electronic configuration easily, we use Moeller's diagram, which is shown below. l=0

l=1

l=2

l=3

n=1

1s

n=2

2s

2p

n=3

3s

3p

3d

n=4

4s

4p

4d

4f

n=5

5s

5p

5d

5f

n=6

6s

6p

6d

n=7

7s

7p

n=8

8s

Fig. 3.8 Moeller's diagram

3.4.1 Electronic configuration of elements Atomic number

Element

Electronic configuration

1

H

1s1

2

He

1s2

3

Li

1s22s1

4

Be

1s22s2

53

STRUCTURE OF THE ATOM

5

B

1s22s22p1

6

C

1s22s22p2

7

N

1s22s22p3

8

O

1s22s22p4

9

F

1s22s22p5

10

Ne

1s22s22p6

11

Na

1s22s22p63s1

12

Mg

1s22s22p63s2

13

Al

1s22s22p63s23p1

14

Si

1s22s22p63s23p2

15

P

1s22s22p63s23p3

16

S

1s22s22p63s23p4

17

Cl

1s22s22p63s23p5

18

Ar

1s22s22p63s23p6

19

K

1s22s22p63s23p64s1

20

Ca

1s22s22p63s23p64s2

Table 3.7 Electronic configuration of first 20 elements

3.5 ATOMIC NUMBER AND MASS NUMBER 3.5.1 Atomic number It is defined as the number of protons (positive charges) present inside the nucleus or the number of electrons (negative charges) present outside the nucleus in a neutral atom. The atomic number is represented by the symbol 'Z'.

Mass number Electrical charge

24 12

2+

Mg

Atomic number

Fig. 3.9 Magnesium element

54

IL Foundation Series Class 8

3.5.2 Mass number It is defined as the sum of the number of protons and the number of neutrons present in the nucleus of an atom. It is represented by 'A'.

Protons and neutrons together are called nucleons.

Atomic Mass on the Periodic Table

Atomic Number

11

Symbol

Na

Atomic Mass

22.99

Fig. 3.10 Atomic mass of sodium element

An element can be represented as ZXA, where X is an element, A is the mass number, and Z is the atomic number.

A = Number of protons + Number of neutrons ⇒A = Z + n ⇒ n = A - Z 3.5.3 The atomic numbers and atomic masses of the first 30 elements Sr. No

Element

Symbol

1 2 3 4 5 6 7 8 9 10 11 12 13

Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium

H He Li Be B C N O F Ne Na Mg Al

Atomic number Atomic mass (Rounded values) 1 2 3 4 5 6 7 8 9 10 11 12 13

1 4 7 9 11 12 14 16 19 20 23 24 27 55

STRUCTURE OF THE ATOM

Sr. No

Element

Symbol

Atomic number Atomic mass (Rounded values)

14 15 16 17 18

Silicon Phosphorus Sulphur Chlorine Argon

Si P S Cl Ar

14 15 16 17 18

28 31 32 35.5 40

19 20 21 22 23 24 25 26 27 28 29 30

Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn

19 20 21 22 23 24 25 26 27 28 29 30

39 40 45 48 51 52 55 56 59 59 63.5 65

Table 3.8 Atomic numbers and atomic masses of the first 30 elements

3.6 ISOTOPES, ISOBARS, ISOTONES 3.6.1 Isotopes • The atoms of the same element which have the same atomic number but different mass number are called isotopes.

Examples: 1) H11, H12, H13 35 2) Cl17 , Cl37 , 17 235 3) U92 , U238 92

• Isotopes differ in number of neutrons. They have the same chemical properties but differ in physical and radioactive properties.

56

IL Foundation Series Class 8

Hydrogen (H)

Deuterium (D)

Tritium (T)

Electron

1

1

1

Proton

1

1

1

Neutron

0

1

2

Table 3.9 Isotopes of Hydrogen

Proton

Proton N-

P+

N-

P+

Neutron

P+

N-

Electron

Electron

Neutron Proton Electron

Protium 1 H 1

Deuterium 2 H 1

Tritium 3 H 1

Fig. 3.11 Isotopes of hydrogen

3.6.2 Isobars • The atoms of different elements which have the same mass number but different atomic numbers are called isobars.

• Isobars differ in number of electrons, protons and neutrons. They differ in chemical, physical and radioactive properties.

Examples: 14

14

1) C 6 & N 7 40

40

2) Ar18 , Ca20 131

I

Xe

131

Isobars

54 Protons 77 Neutrons

53 Protons 78 Neutrons 131

same

131

Fig. 3.12 Isobars

57

STRUCTURE OF THE ATOM

3.6.3 Isotones • Isotones are the atoms of different elements which have the same number of neutrons.

Examples: 1) Si1430, P1531, S1632 2) C614 & O816, 23 24 3) Na11 & Mg12

• They differ in physical and chemical properties.

Isotones X A=131 N=53 Z=78

Same

Y A=127 N=53 Z=74

Proton

Proton

Neutron

Neutron

Fig. 3.13 Isotones

QUICK REVIEW • Properties of cathode rays i. They heat up a metal foil to incandescence, which they impinge upon. ii. Cathode rays produce X-rays when they strike a metallic target. iii. The speed of cathode rays is less than the speed of light.

• Properties of anode rays i. They travel in a straight line in a direction opposite to the cathode. ii. They possess mass many times that of an electron. iii. They cause fluorescence on zinc sulphide.

58

IL Foundation Series Class 8

• Specific charge (e/m ratio): The charge to mass ratio is known as the specific charge. The specific charge for cathode rays is the same for different gases, but the specific charge for anode rays is different for different gases present in the discharge tube.

• Structure of α-ray: 42He2+

• According to Rutherford, most of the α-particles passed through the gold foil undeflected.

• The size of the nucleus is very small compared to the size of the atom. The size of a nucleus is in the order of 10-13 cm, and the size of an atom is in the order of 10-8 cm.

• Bohr's postulates: Electrons revolve around the nucleus in specified circular paths called orbits or shells. These orbits are numbered as 1, 2, 3, 4… etc., or represented as K, L, M, N..., etc., respectively and are represented by the symbol ' n'.

E2-E1= ΔE = hϑ mvr = n h/2π) • Atomic number: The number of protons present in the nucleus, denoted by ’Z’.

• Mass number: The sum of the protons and neutrons in its nucleus, denoted by ’A’.

• Isotopes: The atoms of the same element which have the same atomic number but different mass numbers.

• Isobars: The atoms of different elements which have the same mass number but different atomic numbers.

• Isotones: The atoms of different elements which have the same number of neutrons.

WORKSHEET - 1 MULTIPLE CHOICE QUESTIONS WITH SINGLE CORRECT ANSWER I.

Structure of an atom

1. The specific charge for cathode rays is a. Constant b. Variable c. Dependent upon the material of the cathode d. Dependent upon the nature of the gas in the discharge tube

59

STRUCTURE OF THE ATOM

2. Specific charge is a. Charge to mass ratio

b. Mass to charge ratio

c. Velocity to mass ratio

d. Mass to velocity ratio

3. Which of the following statements about cathode ray discharge tubes is incorrect? a. In cathode rays, a negative charge is present b. These rays are deflected by the magnetic field c. It necessitates a high-voltage supply d. In cathode rays, protons are present 4. The speed of cathode rays is (C = 3×108 m/s) a. Less than C

b. Greater than C

c. Greater than C

d. Equal to C

5. Choose a value for the charge to mass ratio of electrons from the options a. 1.758820 × 1011 C/Kg.

b. 1.738820 × 1011 C/Kg

c. 1.718820 × 1011 C/Kg

d. 1.708820 × 1011 C/Kg

6. Statement (A): J.J. Thomson said that the atom consists of uniformly distributed positive and negative charged particles.

Statement (B): Cathode rays produce X-rays when they strike a metallic target.

Statement (C): Anode rays travel in a straight line in a direction opposite to the cathode. a. All the statements A, B, and C are correct b. All the statements A, B, and C are incorrect c. A and B are correct, and C is incorrect d. A and B are incorrect, and C is correct 7. Rutherford's atomic model is known as a. Planetary model

b. Apple pie model

c. Plum pudding model

d. Watermelon model

8. The size of the nucleus is in the order of

60

a. 10-13 cm

b. 10-8 m

c.

d. Both a and c

1Å

IL Foundation Series Class 8

9. In the Rutherford atomic model, alpha particles are bombarded at a. Gold

b. Silver

c. Titanium

d. Aluminium

10. Rutherford's model of the atom accounts for the a. Scattering of alpha particles by metal foils b. Stability of the electron orbits c. Stability of the atom d. Line spectra of light elements 11. The average distance of an electron from the nucleus in an atom is of the order of a. 1 cm

b. 10-8 cm

c. 10-13 cm

d. 10-6 cm

12. Gold foil is selected for α rays scattering experiment due to a. A very thin gold plate with a thickness equal to 1000 atoms can be made b. The internal structure, sub-atomic particles, and nucleus can be observed easily c. High malleability capacity of gold d. All

13. Rutherford's experiment on the scattering of α particles showed for the first time that the atom has a. Electrons

b. Protons

c. Neutrons

d. Nucleus

14. Rutherford's scattering experiment is related to the size of the a. Nucleus

b. Atom

c. Electron

d. Neutron

15. According to Bohr's atomic model, the electrons revolve in a. Circular orbits

b. Elliptical orbits

c. Stationary orbits

d. Both a and c

16. In Bohr's atomic model, orbits are represented by a. K, L, M, N

b. 1, 2, 3, 4

c. A, B, C, D

d. Both a and b

17. Assertion (A): In stationary orbits, the energy of electrons is constant.

Reason (R): Electrons do not revolve in atoms. a. Both A and R are correct, and R is the correct explanation for A

61

STRUCTURE OF THE ATOM

b. Both A and R are correct, but R is not the correct explanation for A c. A is correct, and R is incorrect d. A is incorrect, and R is correct 18. Statement (A): The angular momentum of electrons in orbits is quantised.

Statement (B): While revolving in stationary orbits, electrons do not lose energy.

Statement (C): mvr = nh/2π. a. All the above statements are correct b. All the above statements are incorrect c. A and B are correct, and C is incorrect d. A and B are incorrect, and C is correct 19. The fundamental particle with no charge is a. Electron

b. Proton

c. Neutron

d. Positron

II. Electronic configuration of elements and valency

1. The maximum number of electrons that can be accommodated in a 4f subshell is a. 10

b. 12

c. 14

d. 16

2. The number of electrons present in the penultimate shell of nitrogen is a. 2

b. 3

c. 5

d. 8

c. 3s1

d. 3s2

c. 4

d. 6

c. 3

d. 1

c. 3

d. 4

3. The valence electronic configuration of calcium is a. 4s1

b. 4s2

4. The valency of carbon in CH4 is a. 1

b. 2

5. The valency of aluminium in Al2O3 is a. 6

b. 2

6. The combining capacity of oxygen in water is a. 1

b. 2

7. An element has X valence electrons in its outermost orbit; its valency may be

62

a. Equal to X

b. Equal to 8 - X

c. Both a & b

d. None

IL Foundation Series Class 8

8. Identify the possible number of valence electrons for metals a. 4

b. 5

c. 6

d. 1

III. Atomic number and mass number

1. In a neutral atom, there are 12 electrons, and its mass number is 24 then the number of neutrons in it are a. 24

b. 12

c. 15

d. 16

c. 39

d. 1

c. 28

d. All

2. The atomic mass of potassium is a. 19

b. 31

3. The number of protons present in nitrogen is a. 14

b. 7

4. Among the following, the element with an odd number as its atomic number is a. C

b. Mg

c. Al

d. O

c. Z-A

d. A×Z2

c. 18

d. 15

5. In an atom, the number of neutrons is equal to a. A+Z

b. A-Z

6. The atomic number of argon is a. 2

b. 10

7. Among the following, the element with an even number of neutrons is a. Na

b. Al

c. Ne

8. Atomic number is equal to the

d. All (AFMC)

a. number of neutrons in the nucleus

b. number of protons in the nucleus

c. sum of protons and neutrons

d. atomic mass of the element

9. A & Z can be a. Negative

b. Fractional

c. Zero

d. Whole number

10. The number of protons, electrons and neutrons in 80 35 Br are, respectively a. 35, 35, 80

b. 35, 35, 45

c. 80, 80, 35

d. 45, 45, 35

63

STRUCTURE OF THE ATOM

IV. Isotopes, isobars, and isotones

1. The number of nucleons in the isotope of an atom zXm is a. m

b. z

c. m+z

d. m-z

2. Which one of the following elements contains 20 neutrons? a. Sodium

b. Magnesium

c. Potassium

d. Aluminium

3. Two nuclides, X and Y, are isotonic to each other with mass numbers 70 and 72, respectively. If the atomic number of X is 34, then that of Y would be a. 32

b. 34

c. 36

d. 38

4. Natural chlorine has 2 isotopes, 1735Cl and 1737Cl, in a ratio of 1:3. Calculate the atomic mass of natural chlorine. a. 35 a.m.u

b. 37 a.m.u

c. 35.5 a.m.u

d. 34 a.m.u

c.

d.

5. Which one of the following is an isobar of 6C14? a.

C13

6

b.

6

C12

7

N14

N15

7

6. Two nuclides, A and B, are isoneutronic. Their mass numbers are 76 and 77, respectively. If the atomic number of A is 32, then the atomic number of B will be a. 33

b. 34

c. 32

d. 30

WORKSHEET - 2 MULTIPLE CHOICE QUESTIONS WITH SINGLE CORRECT ANSWER 1. The ratio of the specific charge (e/m) of a proton to that of an α particle is a. 1:2

b. 1:1/4

c. 1:4

d. 1:1/2

2. If the specific charge of a proton (e/m) is 9.6×107 C/kg, then that for an α-particle will be a. 2.4×107 C/kg

b. 4.8×107 C/kg

c. 19.2×107 C/kg

d. 38.4×107 C/kg

3. ____ are the lightest and tiniest particles produced by hydrogen (a positive ion).

64

a. Electron

b. Proton

c. Neutron

d. Particle

IL Foundation Series Class 8

4. Match the following Column - I

Column - II

A) Cathode rays possess kinetic energy

1. When they strike a metallic target

B) Cathode rays produce X-rays

2. In the presence of an electric field

C) Cathode rays deflect towards positive end

3. Cathode rays cause mechanical motion

a. A-3, B-1, C-2

b. A-1, B-2, C-3

c. A-2, B-3, C-1

d. A-1, B-3, C-2

5. Assertion(A): In the α-particle scattering experiment, one out of 20,000 α-particles bounced back.

Reason(R): At the centre of the atom, positively charged particles are present. a. Both A and R are correct, and R is the correct explanation of A b. Both A and R are correct, and R is not the correct explanation of A c. A is correct, R is incorrect d. A is incorrect, R is correct 6. Statement A: Alpha particles are much heavier than electrons Statement B: Alpha particles are positively charged.

Statement C: Alpha particles move with very high velocity. a. All the statements A, B, and C are correct b. All the statements A, B, and C are incorrect c. A and B are correct, and C is incorrect d. A and B are incorrect, and C is correct 7. Match the following.

Set A

Set B

A) Size of the nucleus

1) Doubly charged helium

B) Size of the atom

2) Fluorescent screen

C) α-particle

3) 10-8 cm

D) Zinc sulphide screen

4) 10-13 cm

a. A-4, B-3, C-1, D-2

b. A-4, B-2, C-3, D-1

c. A-4, B-3, C-2, D-1

d. A-4, B-1, C-2, D-3

65

STRUCTURE OF THE ATOM

8. According to Bohr's atomic model a. An electron in an atom of hydrogen can have only certain values of angular momentum b. Electrons have particle as well as wave characteristics. c. Atomic spectrum of an element should contain only five lines. d. All the statements are incorrect 9. According to Bohr's atomic model mvr = a. nh/2π

b. nh/π

c. h/4π

d. h/2nπ

c. Both (a) & (b)

d. None

10. Bohr's theory cannot explain the following factor a. Stark effect

b. Zeeman effect

11. The value of Planck's constant is a. 6.6×10-32 gm2 sec

b. 6.6×10-34 kgm2 sec-1

c. 6.6×10-32 kgm sec-1

d. 6.6×10-34 gm2 sec

12. Assertion (A): Bohr's orbits are called stationary orbits.

Reason (R): Electrons remain stationary in these orbits for some time. a. Both A and R are correct, and R is the correct explanation of A b. Both A and R are correct, and R is not the correct explanation of A c. A is correct, R is incorrect d. A is incorrect, R is correct 13. During the transition of electrons from orbits of higher energy to orbits of lower energy, energy is a. Absorbed

b. Radiated

c. Either (a) or (b)

d. None of these

14. In Bohr's model of an atom, which of the following is an integral multiple of h/2π. a. Kinetic energy

b. Radius of an atom

c. Potential energy

d. Angular momentum

15. Bohr's model can explain a. The spectrum of hydrogen atoms only b. The spectrum of an atom or ion containing one electron only c. The spectrum of hydrogen molecule d. The solar spectrum 66

IL Foundation Series Class 8

16. The difference in angular momentum of a revolving electron in the 3rd to the 6th orbit is a. 6 h/π

b. 4.5 h/π

c. 3 h/π

d. 1.5 h/π

17. The number of subshells present in M shell is a. 1

b. 2

c. 3

d. 4

18. The number of valence electrons present in Sodium is a. 1

b. 2

c. 3

d. 5

19. The maximum number of electrons that can be accommodated in the K shell is a. 8e-

b. 2e-

c. 18e-

d. 32e-

c. (n-2)

d. All

20. The valence shell is represented as a. n

b. (n-1)

21. The increasing order of energy of the orbitals 3s, 3p, 3d, 4s is a. 3s <3p <3d <4s

b. 3s <3p <4s <3d

c. 3d <4s <3p <3s

d. 3s = 3p = 3d < 4s

22. 1s2 2s2 2p6 3s2 3p5 is the electronic configuration of a. Al

b. Cl

c. P

d. Ca

23. The energy of a 3p orbital is ______ than a 3d orbital a. Greater

b. Lesser

c. Neither greater nor lesser

d. All

24. Match the following.

Column - I

Column - II

A) Ultimate shell

1) n - 1

B) Penultimate shell

2) n - 2

C) Antipenultimate shell

3) n

a. A - 3, B - 1, C - 2

b. A - 2, B - 1, C- 3

c. A - 2, B - 3, C- 1

d. A - 3, B - 2, C - 1

67

STRUCTURE OF THE ATOM

25. Match the following.

Column - I

Column - II

A) K shell

1) s, p, d, f

B) L shell

2) s, p, d

C) M shell

3) s, p

D) N shell

4) s

a. A - 4, B - 3, C - 2, D - 1

b. A - 3, B - 4, C - 1, D - 2

c. A - 4, B - 2, C - 1, D - 3

d. A - 4, B - 3, C - 1, D - 2

26. The valency of neon is a. 8

b. 1

c. 0

d. 7

b. 3

c. 1

d. 4

27. The valency of lithium is a. 2

28. Which of the following numbers of electrons is not possible in the valence shell of metals? a. 4

b. 3

c. 2

d. 1

c. 3

d. 4

29. The valency of copper in cupric chloride is a. 2

b. 1

30. Statement (A): Valency of metals is equal to their valence electrons.

Statement (B): Valency of non-metals is equal to 8 - valence electrons.

Statement (C): Valency of carbon is 4. a. All the above statements are correct

b. All the above statements are incorrect

c. A and B are correct, and C is incorrect

d. A and B are incorrect, and C is correct.

31. Match the following.

Elements

68

Valency

A) Beryllium

1) 0

B) Boron

2) 4

C) Carbon

3) 3

D) Neon

4) 2

IL Foundation Series Class 8

a. A - 4, B - 3, C - 2, D - 1

b. A - 4, B - 2, C - 3, D - 1

c. A - 4, B - 2, C - 1, D - 3

d. A - 4, B - 1, C - 2, D - 3

32. The nucleus of an atom contains a. Electrons and protons

b. Electrons and beta particles

c. Protons and neutrons

d. Protons and alpha particles

33. The isotopes of neutral atoms of an element differ in a. Atomic number

b. Mass number

c. Number of electrons

d. Chemical properties

34. The nucleus of tritium consists of a. 1 proton + 1 neutron

b. 1 proton + 3 neutrons

c. 1 proton + zero neutrons

d. 1 proton + 2 neutrons

35. Sodium ion is isoelectronic with atom a. Mg2+

b. Al2+

c. Ne

d. N3-

36. An atom differs from its ion in a. Nuclear charge

b. Mass number

c. Number of electrons

d. Number of neutrons

37. The ratio between the neutrons present in carbon atoms and silicon atoms with mass numbers 12 and 28 is a. 7:3

b. 3:7

c. 1:2

d. 2:1

38. An oxide of nitrogen has a molecular weight of 30. The total number of electrons in one molecule of the compound is a. 15

b. 30

c. 45

d. 60

39. Maximum sum of the number of neutrons and protons in an isotope of hydrogen (IIT) a. 6

b. 5

c. 4

d. 3

40. In two elements Z Az and Z BM . M1 ≠ M2, and Z1 ≠ Z2 but M1- Z1 = M2- Z2 These elements are 3

2

2

2

a. Isotonic

b. Isobaric

c. Isotopic

d. Isoelectronic

69

STRUCTURE OF THE ATOM

41. If two neutrons are added to an element X, then it will get converted to its

70

a. Isotope

b. Isotone

c. Isobar

d. None of the above

4

ATOMS AND MOLECULES

4.1 INTRODUCTION TO ATOMS AND MOLECULES An Indian philosopher, Maharishi Kanad, postulated that if we go on dividing matter (padarth), we shall get smaller and the smallest particles.

He called these smallest particles beyond which further division will not be possible 'paramanu'.

Around the same era, Greek philosopher Democritus called these indivisible particles as atoms (atom means indivisible).

Fig. 4.1 Maharishi Kanad, Democritus

The foundation of all this was based on philosophical ideas. Until the eighteenth century, there weren’t many chances to test these ideas through experiments. At that time, experimental work was limited, making it hard to prove these early thoughts.

By the end of the 18th century, scientists recognised the difference between elements and compounds. Elements are substances having molecules of the same type of atoms, i.e., elements composed of identical atoms.

A compound is composed of two or more elements chemically combined with each other in a definite proportion by weight.

4.2 LAWS OF CHEMICAL COMBINATION The combination of two or more elements in a chemical reaction is governed by some combination laws. Due to these laws, changes often occur in the matter. These laws are known as the laws of chemical combination. Following are some of the laws from the different laws of combination1.

Law of conservation of mass

2.

Law of constant proportions 71

ATOMS AND MOLECULES

4.2.1 Law of conservation of mass When a chemical reaction occurs, does the mass of the elements involved in the reaction change?

No! French chemist Antoine Lavoisier gave a law regarding the change of mass in a chemical reaction.

His given law states that mass can neither be created nor destroyed in a chemical reaction. That means during chemical changes, the sum of masses of all the reactants remains equal to the mass of products. That isThe sum of masses of all the reactants = The sum of masses of all the products.

A simple experiment to demonstrate the LAW OF CONSERVATION OF MASS string sodium hydroxide solution

small test tube

copper sulfate solution

small test tube

sodium sulphate solution copper hydroxide precipitate

MIX Throughly

67.25g

67.25g

one pan electric balance

one pan electric balance

Fig. 4.2 Law of conservation of mass

4.2.2 Law of constant proportions The law of constant proportions was established by Joseph L. Proust. This law is also known as the law of definite proportions. It states that in a chemical substance, the elements are always present in definite proportions by mass.

For example i. Water obtained from any source contains hydrogen and oxygen combined in the ratio of 1:8 by weight. ii. Sodium chloride obtained from rock salt or seawater always contains sodium and chlorine in the ratio of 23: 35.5 by weight. iii. Carbon dioxide is prepared by two methods, but the ratio of carbon and oxygen is always 3:8.

4.3 WHAT IS AN ATOM? The word 'atom' is derived from the Greek word 'a-tomio' (which means indivisible).

According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture, is composed of small particles called atoms. An atom is the smallest particle of the element that can retain all its chemical properties. More than millions of atoms, when stacked, would make a layer barely as thick as this sheet of paper. Atomic radius is measured in nanometres.

1 1 nm or 1 m 109 nm 109 1 nm 10 9 m

72

IL Foundation Series Class 8

4.3.1 Modern-day symbols of atoms of different elements In daily life, abbreviations are generally written in place of lengthy names to save time and labour.

For example, RBI represents the Reserve Bank of India, and ATM indicates Automatic Teller Machine. Similarly, elements are represented by symbols.

Definition: Shorthand notation of an element is called a symbol. For example, oxygen is represented by the letter 'O'.

Symbols of elements with a single letter: For some elements, the first letter of their English names represents their symbols. For example: nitrogen (N).

7

N

Nitrogen 14.007

Fig. 4.3 Symbol for Nitrogen

Name of the element

Symbol

Hydrogen

H

Oxygen

O

Nitrogen

N

Carbon

C

Fluorine

F

Table 4.1 Some elements and their symbols

Similarly, boron, sulphur, phosphorus, iodine, and uranium are represented by the letters B, S, P, I, and U, respectively.

Symbol of some elements with two letters: Two-letter symbols are used to represent most of the elements. The list of such elements is given below. The first letter is always written in capitals, and the second letter is always small.

For example, the symbol of cobalt is Co but not CO because CO represents a molecule of carbon monoxide.

73

ATOMS AND MOLECULES

Fig. 4.4 Symbol for aluminium

Element

Symbol

Element

Symbol

Aluminium

Al

Helium

He

Argon

Ar

Lithium

Li

Arsenic

As

Magnesium

Mg

Astatine

At

Manganese

Mn

Barium

Ba

Molybdenum

Mo

Beryllium

Be

Neon

Ne

Bismuth

Bi

Nickel

Ni

Bromine

Br

Palladium

Pd

Table 4.2 Some elements and their symbols

There are some elements whose symbol is derived from their Latin names. Some of them are given below.

74

Element

Latin name

Symbol

Antimony

Stibium

Sb

Copper

Cuprum

Cu

Gold

Aurum

Au

Iron

Ferrum

Fe

Lead

Plumbum

Pb

Mercury

Hydragyrum

Hg

Potassium

Kalium

K

Silver

Argentum

Ag

Sodium

Natrium

Na

Tin

Stannum

Sn

Tungsten

Wolfram (German Name)

W

Table 4.3 Elements and their Latin name and symbols

74

W

Tungsten 183.84

Fig. 4.5 Symbol for tungsten

IL Foundation Series Class 8

Also, there are some elements that are named after scientists. Such asElement

Scientist name

Symbol

Curium

Madam Curie

Cm

Einsteinium

Albert Einstein

Es

Fermium

Enrico Fermi

Fm

Nobelium

Alfred Nobel

No

Mendelevium

Mendeleev

Md

Bohrium

Neils Bohr

Bh

Rutherfordium

Rutherford

Rf

Table 4.4 Elements with their Latin name and symbols

Fig. 4.6 Symbol for einsteinium

Moreover, some elements are named after the countries and the laboratories.

Element

Country and Laboratory

Symbol

Berkelium

City of Berkely

Bk

Californium

University of California

Cf

Polonium

Poland

Po

Americium

America

Am

Ruthenium

Russia

Ru

Germanium

Germany

Ge

Table 4.5 Elements with their name based on country and laboratory

Fig. 4.7 Symbol for polonium

Elements

Name of the Planet

Symbol

Uranium

Uranus

U

Neptunium

Neptune

Np

Plutonium

Pluto

Pu

Table 4.6 Elements with their name based on planets

4.3.2 Atomic mass Every tiny bit of stuff carries a certain amount of mass, no matter if it’s a little or a lot. All things are composed of atoms. The mass linked to an atomic particle is referred to as its atomic mass.

1th One atomic mass unit is equal to exactly one-twelfth (12 ) mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12.

Atomic masses of elements are expressed in 'u’ (or) a.m.u, 'u’ means unified mass.

75

ATOMS AND MOLECULES

Unified atomic mass unit 1.660538782(83)x10-27 kg

C612

/12

1 amu = 931.46 MeV/C2 Fig. 4.8 Unified atomic mass unit

Mass of one atom of an element Atomic mass of an element = 1 th 12 part of mass of an atom of C - 12 isotope Here, we have to remember one thing strictly. We cannot measure atomic mass accurately and directly, so we have to use a reference. Even though we have a number of references, nowadays, we consider carbon-12 as the reference as it gives an accurate atomic weight.

Element

Atomic mass (u)

Hydrogen

1u

Carbon

12u

Nitrogen

14u

Table 4.7 Atomic mass of a few elements

4.4 WHAT IS A MOLECULE? A molecule is the smallest particle of an element or a compound capable of independent existence under ordinary conditions. It shows all the properties of the substance.

4.4.1 Molecules of elements The number of atoms present in one molecule of an element is called the atomicity of that element.

• Monoatomic elements - Helium (He), argon (Ar), ...etc.

• Diatomic element - Oxygen, hydrogen, nitrogen, ... etc.

• Triatomic element - Ozone • Tetratomic element - Phosphorus • Polyatomic elements - Sulphur, ... etc.

76

IL Foundation Series Class 8

O2

O3 O

O

Oxygen

O

O

O

Ozone

Sulphur

Fig. 4.9 Molecules of oxygen, ozone, and sulphur

4.4.2 Molecules of compounds Atoms of different elements join together in definite proportions to form molecules of compounds.

For example- Water - H2O, Ammonia - NH3, Carbon dioxide - CO2

Fig. 4.10 Water molecule

4.4.3 What is an ion?

Definition: Charged species formed by losing (or) gaining electrons are known as ions. Ions may contain a single atom or a group of atoms.

Types of ions: Ions are two types based on the nature of the charge carried by them. a. Electropositive ion (or) Cation: An ion having a positive charge on it is known as an electropositive ion or a cation. For example: K+, Ag+, Pb+2, etc. b. Electronegative ion (or) Anion: The ion having a negative charge on it is known as an electronegative ion or an anion. For example: Cl-, O-2, SO4-2, etc.

4.5 WRITING CHEMICAL FORMULAE Definition: The chemical formula of a compound is a symbolic representation of its composition.

Importance: A Chemical formula gives the exact number of atoms of the same or different elements present in a chemical substance.

To write chemical formulae, the symbols of elements and the combining capacities of elements are required.

Note: Here, the combining capacity of an element indicates the number of bonds formed by that element 77

ATOMS AND MOLECULES

For exampleHydrogen Chlorine Its valency is 1. Its valency is 1.

Its combining capacity is 1. Its combining capacity is 1.

Number of bonds formed by it is 1. Number of bonds formed by it is 1.

H-Cl: The combining capacity of hydrogen and chlorine is one. So, only one bond is formed between H and Cl. Rules for writing chemical formulae: • The valencies or charges on the ion must be balanced.

• In writing formulae of a compound, the metal comes first, followed by the non-metal.

4.5.1 Formulae of simple compounds Criss-Cross method to write the formulae for compounds 1. First, we write the constituent elements in the compound.

2. Then, write their valencies.

3. After that, we must cross over the valencies of the combining atoms.

Examples: 1. Formula of hydrogen chloride: Cl Symbol H Hydrogen chloride (HCl) Valency 1

1

2. Formula of hydrogen Sulphide: Symbol H

S Hydrogen sulphide (H2S)

Valency 1

2

3. Formula of carbon tetrachloride:

Symbol C

Cl Carbon tetrachloride (CCl4)

Valency 4

1

4. Formula of magnesium chloride: Symbol Mg

Cl Magnesium chloride (MgCl2)

Valency 2 78

1

IL Foundation Series Class 8

4.6 MOLECULAR MASS AND MOLE CONCEPT 4.6.1 Molecular mass The Molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.

So, we can simply use the molecular formula to calculate the molecular mass. For exampleThe molecular mass of methane(CH4) will be the sum of the atomic mass of four atoms of hydrogen and the atomic mass of one atom of carbon.

Molecular mass = Atomic mass of four hydrogen atoms + Atomic mass of one carbon atom = 4 # 1 + 1 # 12 = 16 u 4.6.2 Formula unit mass The formula unit mass of a substance is the sum of the atomic masses of all atoms in a formula unit of a compound.

Note: The word formula unit mass is used for those substances whose constituent particles are ions.

For Example Formula unit mass of sodium chloride ^ NaClh = 1 # 23 + 1 # 35.5 = 58.5 u Formula unit mass of CaCl2 = Atomic mass of Ca + 2 # atomic mass of Cl 4.6.3 Mole concept

= 40 + 2 # 35.5 = 40 + 71 = 111 u

The actual meaning of a mole is a heap, but here, it represents the number of atoms or molecules or particles present in a fixed amount of the substance. This fixed amount of the substance is known as atomic mass for atoms and molecular mass for molecules.

(Or) One mole is defined as the amount of the substance that contains as many particles as atoms exactly present in 12 g of the C-12 Isotope.

(Or) The amount of the substance which contains the same number of elementary particles (atoms, molecules, ions or electrons) as the number of atoms present in 12g of Carbon (C-12).

For example Let us calculate the number of atoms present in 12g of C-12 isotope.

One C-12 atom weighs 1.9926 × 10-23 g Number of atoms in 12g per mole of C-12 isotope =

12g/mole 19926 # 10 -23 g/atom

= 6.023 # 1023 atoms/mole

79

ATOMS AND MOLECULES

This number of entities in one mole of a substance is known as Avogadro’s number. It is denoted by 'N’ or 'NA’.

Avogadro’s number = 6.023 × 1023 number of particles (atoms or molecules or ions or electrons).

This number is named in honour of the Italian scientist Avogadro. It is used as a reference for most of the calculations and equations found in chemistry.

Quite often, we use the unit dozen to represent 12 articles, irrespective of their nature. For example, one dozen books mean 12 books, whereas one dozen apples mean 12 apples. Similarly, chemists use the unit mole for counting atoms, molecules, ions, etc. So, a mole is a collection of 6.023 × 1023 particles. A mole represents 6.023 × 1023 particles.

1 mole of carbon atoms 6.023 ◊ 1023 atoms of C 6.023 ◊ 1023 atoms of H 6.023 ◊ 1023 number of that particle 6.023 ◊ 1023 numbers of molecules

12g of carbon

1 mole of hydrogen atoms

1g of H atoms

1 mole of any particle (atoms, molecules, icon) 1 mole of molecules Molecular mass in grams

Fig. 4.11 Different representations of a mole

QUICK REVIEW

80

•

All chemical reactions between elements take place according to certain laws. These are called the laws of chemical combinations.

•

The law of conservation of mass, law of definite proportions, law of multiple proportions, and law of combining volumes are important.

•

According to the law of conservation of mass, in a chemical reaction, the total mass of the products is equal to the total mass of the reactants, or the mass is neither created nor destroyed in a chemical reaction.

•

According to the law of definite proportions, a given chemical substance (compound) always contains the same elements combined in a fixed proportion by weight.

•

The shorthand notation of an element is called a symbol. For example, oxygen is represented by the letter 'O’.

•

The atomic weight or atomic mass of an element is a relative mass and is expressed in atomic mass units or atomic weight units.

IL Foundation Series Class 8

•

The SI unit of atomic weights is 'u’.

•

The latest standard for determining atomic masses is 6C12, which is assigned the mass of an th

1 atom. One a.m.u. is 12 part of the mass C612 atom.

•

One a.m.u is also known as one Dalton or one Aston.

•

Molecular weight (or) molecular mass is also a relative mass expressed in a.m.u.

•

One mole of carbon is 12g of graphite, and one mole of oxygen is 32g of oxygen.

• •

1 One atomic mass unit is equal to exactly one-twelfth ( 12 ) mass of one atom of carbon-12.

Mass of one atom of an element The atomic mass of an element = 1 th 12 part of mass of an atom of C 12 isotope

•

Ions: Charged species formed by losing (or) gaining of electrons are known as 'Ions'.

•

Cation: An ion having a positive charge on it is known as an electropositive ion or a cation.

•

Anion: The ion having a negative charge on it is known as an electronegative ion or an anion.

•

Simple ion: It is an ion which contains one or more atoms of the same element.

•

Compound ion: Ions consist of two or more atoms of different elements to form a single unit.

•

An ionic compound is a combination of two or more simple compound ions.

•

Chemical formula: The chemical formula of a compound is a symbolic representation of its composition.

•

Importance of chemical formula: A Chemical formula gives the exact number of atoms of the same or different elements present in a chemical substance.

•

Rules for writing chemical formulae : The valencies or charges on the ion must be balanced.

•

In writing formulae of a compound, the metal comes first, followed by the non-metal.

•

The Molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.

•

The formula unit mass of a substance is the sum of the atomic masses of all atoms in a formula unit of a compound.

•

Mole: A mole represents 6.023 × 1023 particles.

•

The actual meaning of a mole is a heap, but here, it represents the number of atoms or molecules or particles present in a fixed amount of the substance.

•

Mole: The amount of the substance that contains as many particles as atoms exactly present in 12 g of the C-12 Isotope.

•

Avogadro’s number (NA): The number of entities in one mole of substance is known as Avogadro’s number.

th

81

ATOMS AND MOLECULES

•

The chemical formula represents one mole of the substance.

•

The formula mass in grams represents the mass of one mole of that substance.

WORKSHEET - 1 MULTIPLE CHOICE QUESTIONS WITH SINGLE CORRECT ANSWER I.

Introduction to atoms, molecules and laws of chemical combination

1. The law which deals with the relation between the mass of the reactants and the products in physical changes or chemical reactions is

2.

a. Law of constant composition

b. Law of conservation of mass

c. Law of multiple proportions

d. None of the above

The law that deals with the composition of the various elements present in a compound is a. Law of multiple proportions

b. Law of conservation of mass.

c. Law of constant composition

d. None of these

3. According to the law of constant composition, a pure chemical compound always consists of the same elements combined together in a fixed proportion by a. Weight 4.

b. Volume

c. Density

d. Pressure

The scientist who proposed the law of definite proportions a. Lavoisier

b. Proust

c. Dalton

d. Avogadro

5.

"Total mass of the reactants is equal to the total mass of the products". This statement is based on a. Law of conservation of mass b. Law of conservation of energy c. Law of multiple proportions d. Law of definite proportions

6.

"Elements always combine in a fixed ratio of their atomic weights". This statement is based on a. Law of conservation of mass b. Law of conservation of energy c. Law of multiple proportions d. Law of definite proportions

7. Statement (A): Stoichiometric equation obeys the law of conservation of mass.

Statement (B): The weight of reactants is equal to the weight of products in a balanced chemical reaction.

Statement (C): The law of conservation of mass is given by Lavoisier. a. All the above statements are correct b. All the above statements are incorrect c. A and B are correct, but C is incorrect d. A and B are incorrect, but C is correct

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IL Foundation Series Class 8

8. If 32 g of oxygen is going to react with 32 g of sulphur, then find the weight of SO2 formed according to the law of conservation of mass a. 32 g 9.

b. 64 g

c. 128

d. 16 g

Among the following, the element which has fractional atomic mass a. Carbon

b. Magnesium

c. Chlorine

d. Calcium

c. 32

d. 15

10. The atomic mass of sulphur is a. 16

b. 31

11. Which of the following elements has the same molecular mass as its atomic mass? a. Nitrogen

b. Neon

c. Oxygen

d. Chlorine

12. The atomicity of ozone, sulphur, phosphorus and argon are respectively a. 8, 3, 4 and 1

b. 1, 3, 4 and 8

c. 4, 1, 8 and 3

d. 3, 8, 4 and 1

13. The English name of an element is potassium; its Latin name will be a. Plumbum

b. Cuprum

c. Kalium

d. Natrium

14. Out of ozone, phosphorus, sulphur and krypton, the elements having the lowest and highest atomicities, respectively, are a. Sulphur and krypton

b. Krypton and ozone

c. Phosphorus and sulphur

d. Krypton and sulphur

II.

What is an atom?

1.

Which of the following has the maximum number of atoms? a. 18 g Of CO2

2.

c. 18 g of O2

d. 18 g of CH 4

The symbol of a metal element, which is used in making thermometers is a. Ag

3.

b. 18 g of H2 O

b. Hg

c. Mg

d. Sg

Arrange the following in the order of increasing mass.

Atomic mass of O = 16u, Cu = 63u, N = 14u I.

4.

One atom of oxygen

II. One atom of nitrogen

III. 1 × 10-10 mole of oxygen gas

IV. 1 × 10-10 mole of Copper

a. II < I < III < IV

c. III < II < IV < I

b. I < II < III < IV

d. IV < II III < I

The formula of calcium sulphate is a. Ca2 SO 4

b. Ca ^SO 4h2

c. CaSO 4

d. CaSO3

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ATOMS AND MOLECULES

5.

One a.m.u equals to a. 1.66 # 1027 kg

6.

7.

b. 1.66 # 10 -27 g

c. 1.66 # 1027 g

Match the following: Column - I A. 1 a.m.u in grams B. this is a relative quantity C. 1 mole of carbon contains these many atoms D. 12 g of C - 12

d. 1.66 # 10 -27 kg

Column - II 1. Mole 2. Avogadro number 3. Relative atomic mass 4. 1.66 # 10 -24

a. A- 4, B- 3, C - 2, D - 1

b. A- 3, B - 4, C - 2, D - 1

c. A- 4, B- 3, C - 1, D - 2

d. A- 1, B - 2, C - 4, D - 3

Match the following: Column - I Column - II A. Avogadro number 1. N B. 18 g of water 2. 1/12 th of the mass of C-12 atom C. 1 a.m.u 3. U D. Unified mass 4. 1 mole of water a. A- 2, B- 4, C - 3, D - 1

b. A- 3, B - 4, C - 2, D - 1

c. A- 1, B- 4, C - 2, D - 3

d. A- 1, B - 4, C - 3, D - 2

III. What is a molecule?

1.

The number of molecules in 3 moles of CO2 are a. 18 # 1023

2.

d. 22 # 10 -23 g

b. 14.2 # 1020

c. 1020

d. 39.6 # 1020

b. 3.0115 # 1023

c. 12.046 # 1023

d. 24 # 1023

b. 2N

c. 3N

d. 4N

The weight of one carbon dioxide molecule is a. 44 g

84

c. 44 # 10 -23 g

The total number of atoms present in 1 mole of CO2 is a. N

6.

b. 11 # 10 -23 g

The total number of atoms in one gram atom is a. 6.023 # 1023

5.

d. 3 # 1023

The number of ions in 2.2 × 10-3 moles of Ca(OH)2 are a. 11 # 1020

4.

c. 12 # 1023

3 molecules of CO2 weigh a. 33 # 10 -23 g

3.

b. 6 # 1023

b. 44 a.m.u

c. 44 kg

d. 44 mg

IL Foundation Series Class 8

7.

Identify the gas whose 2-gram molecules weigh 32 g a. He

8.

b. O2

c. CH 4

d. SO2

Identify the gas whose 2-gram molecules weigh 64 g a. He

b. O2

c. CH 4

d. SO2

IV. Writing chemical formulae; molecular mass and mole concept

1.

The chemical formula represents __________ of the substance. a. 1 mole

2.

3.

a. Atomic mass number

b. Atomicity

c. No of moles

d. Charge of atom

d. 6.023 # 10 -24

b. 32 a.m.u

c. 16 g

d. 32 g

b. C-12

c. C-13

d. None of these

b. 1.9926 # 10 -23 kg

c. 1.9926 # 1023 kg

d. 1.9926 # 10 -23 g

b. 1.66 # 10 -27 g

c. 1.66 # 1027 g

d. 1.66 # 10 -27 kg

The weight of one C-12 atom is

One a.m.u equals to a. 1.66 # 1027 kg

8.

c. 6.023 # 10 -23

The element taken as a reference in measuring atomic mass is

a. 1.9926 # 1023 g 7.

b. 6.023 # 1023

Gram molecular weight of oxygen is

a. C - 14 6.

d. All of the above

The number of particles present in a mole

a. 16 a.m.u 5.

c. 1 g

In a chemical symbol or formula, the coefficient put in front represents that

a. 6.023 # 1024 4.

b. 10 a.m.u

Assertion (A): Avogadro number is denoted by NA. Reason (R) : Gram molecular weight of oxygen is 32 a.m.u. a.

Both A and R are true, and R is the correct explanation of A

b.

Both A and R are true, and R is not the correct explanation of A

c.

A is correct, and R is incorrect

d.

A is incorrect, and R is correct

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ATOMS AND MOLECULES

WORKSHEET - 2 MULTIPLE CHOICE QUESTIONS WITH SINGLE CORRECT ANSWER 1.

The number of atoms in 1 mole of iron is a. 6.023 # 1023

2.

5.

b. 3.0115 # 1023

c. Two moles of Nitrogen

d. Both (a) and (b)

d. 24 # 1023

d. 24 # 1023

Give the chemical formula of calcium sulphate. b. CaSO4

c. CaCl2

d. CaCO2

c. 2.0 # 1024

d. 1.07 # 1022

c. 9.33 # 10 -23 g

d. 40 # 1023 g

The number of atoms in 1 gram of iron are b. 1.07 # 1020

1 atom of iron weighs b. 93 # 10 -23 g

The number of moles in 2 # 1024 atoms of iron are a. 3.3

9.

c. 12.046 # 1023

b. Three moles of oxygen

a. 8 # 10 -23 g 8.

c. 18 # 1023

a. One mole of potassium

a. 2 # 1022 7.

b. 12 # 1023

One mole of KNO3 contains

a. CaSO 6.

d. 24 # 1023

The total number of atoms in one gram atom are a. 6.023 # 1023

4.

c. 12.046 # 1023

The number of atoms of carbon in 3 moles of CO2 is a. 6 # 1023

3.

b. 3.0715 # 1023

b. 4.5

c. 5.2

d. 2.1

The number of moles in 4 # 1031 atoms of iron are a. 6.6 # 107

b. 3.3 # 107

c. 1014

d. 8 # 1014

c. 17 # 1016 g

d. 14 # 1016 g

10. 6.4 × 1031 atoms of iron weighs a. 64 # 108 g

b. 59.73 # 108 g

11. The number of atoms in 140 grams of iron are a. 45 # 1023

86

b. 15 # 1023

c. 30 # 1023

d. 60 # 1023

IL Foundation Series Class 8

12. Fe2(SO4)3 is the chemical formula of: a. Ferrous sulphate b. Ferric sulphate

c. Iron sulphate

d. Iron sulphide

c. Calcium nitride

d. Carbon nitride

c. 0.1628 g

d. 162.8 g

13. The name of the compound Ca(NO3)2 is a. Calcium nitrate

b. Carbon nitrate

14. 2.2 # 10 -3 moles of Ca ^OHh2 weigh a. 1.628 g

b. 0.01628 g

15. The number of atoms in 7.4 moles of iron are a. 44.4 # 1023

b. 22.2 # 1023

c. 44 # 1023

d. 9.4 # 1023

16. The number of ions in one mole of Ca(OH)2 are a. 1.5 N

b. 3 N

c. N

d. 2 N

c. 396 g

d. 400 g

17. 4 moles of Ca ^OH h2 weigh a. 296 g

b. 300 g

18. Which of the following will represent the correct chemical formula for aluminium sulphate and calcium carbonate, respectively? a. Al2(SO4)3 and CaCO3

b. CaPO4 and AlCl3

c. Ca(HCO3)2 and AlSO4

d. Al(CO)3 and CaCl2

19. 3.1 moles of CO2 weigh a. 136.4 g

b. 144.2 g

c. 186.2 g

d. 190.5 g

20. The number of moles in 6 grams of KClO3 are a. 1.2

b. 4.8

c. 48

d. 0.048

c. 245 g

d. 402 g

c. 0.5

d. 0.4

21. Two moles of KClO3 weigh a. 142.5 g

b. 122.5 g

22. The number of moles in 20.2 g of KNO3 a. 0.1

b. 0.2

23. The weight of potassium in 0.25 moles of KNO3 is a. 7.25 g

b. 6.25 g

c. 9.75 g

d. 8.25 g

c. 4

d. 5

24. The atomicity of NO2 is a. 2

b. 3

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ATOMS AND MOLECULES

25. The charge present on one mole of electrons is a. 96,500 coulomb

b. 1 Faraday

c. Both a and b

d. None of these

26. The names of the compounds Ba(NO3)2, K2CO3, and SiO2 are: a. Barium nitrate, potassium carbonate, and silicon dioxide b. Barium nitrite, potassium carbon, and silicon oxide c. Beryllium nitrate, potassium carbonate, and silicon-oxygen d. Barium nitrate, Carbon potassium, and silicon dioxide 27. The formula of a chloride of a metal M is MCl3. The formula of phosphate of metal M will be: a. MPO4

b. M2PO4

c. M3PO4

d. 2(PO4)3

28. Which of the following contains the maximum number of molecules? a. 1g CH4

b. 1g CO2

c. 1g H2

d. 1g N2

c. 56 g

d. 40 g

c. 101

d. 404

29. 1 mole of CO2 weighs a. 22 g

b. 44 g

30. The mass of 1 mole of KNO3 in grams is a. 202

b. 303

31. What is not true about an atom? a. Atoms cannot exist independently. b. Molecular and ionic structures are formed by atoms. c. Atoms are always neutral in nature. d. Atoms aggregate in large numbers to form the matter that we can see, feel or touch.

32. The number of moles in 1.6 # 10 4 grams of Ca ^OHh2 is a. 2.162

b. 21.62

c. 0.2162

d. 216.2

33. The number of moles of KNO3 in 12 × 1023 in ion pairs of KNO3 is a. 1

b. 2

c. 3

d. 4

34. Statement (A): C - 12 is the reference for measuring atomic mass as it gives an accurate atomic mass.

Statement (B): Atomic mass can be expressed as one unified mass.

Statement (C): One gram atom of oxygen mass is 16 a.m.u.

88

a. All the above statements are correct

b. All the above statements are incorrect

c. A and B are correct, but C is incorrect

d. A and B are incorrect, but C is correct

5

PERIODIC CLASSIFICATION OF ELEMENTS

5.1 EARLY ATTEMPTS AT THE CLASSIFICATION OF ELEMENTS 5.1.1 Introduction to early attempts It is difficult to study individually the chemistry of more than one hundred elements known today and their innumerable compounds. The experimental data regarding elements and their compounds can only be systematised if proper classification is done. The basic object of classification is to arrange the facts regarding elements and their compounds in such a way that we may have greater control over their characteristics with less possible effort. The best classification would be the one that puts together the elements that resemble each other in most respects and separate each other.

At present, 118 elements are known. Of these elements, 92 are available in the elemental form, and the remaining 26 elements are human-made. The classification may help to study the elements better and to correlate the properties of elements with some fundamental properties characteristic of all the elements.

5.1.2 Döbereiner’s triads In 1829, John Wolfgang Dobereiner, a German scientist was the first to consider the idea of trends among the properties of elements. He noted the physical and chemical properties of several sets of three elements, arranged in the increased order of their atomic weights called "triads".

The law of triads states: When elements are arranged in order of their increasing atomic mass, the mass of the middle element is approximately the arithmetical mean of the remaining two elements of the triad.

Sr. No. 1

2

3

Elements of triad

Symbol

Atomic Mass

Lithium

Li

7.0

Sodium

Na

23.0

Potassium

K

39.0

Chlorine

Cl

35.5

Bromine

Br

80.0

Iodine

I

127.0

Calcium

Ca

40

Strontium

Sr

87.5

Barium

Ba

137

Arithmetic Mean 7+39 2

= 23.0

35.5 + 127 2

= 81.25

40 + 137 2

= 88.5

Table 5.1 Examples of Döbereiner triads

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PERIODIC CLASSIFICATION OF ELEMENTS

Limitations of Döbereiner's system

Few elements were known at the time of Döbereiner. The law of triads seemed to work only for a few elements. Quite a large number of similar elements could not be grouped into triads.

Examples: Fe, Mn, Ni, Co, Zn, and Cu are similar elements, but cannot be placed in the triads.

Similarly, it was possible that entirely dissimilar elements could be grouped into triads. Like, Carbon (12), Nitrogen (14), and Oxygen (16) can form a triad, but their properties are entirely different from each other.

5.1.3 Newlands’ law of octaves The English chemist John Alexander Newlands (1865), a lover of music, founded the law of octaves.

He arranged many of the known elements in the increasing order of their atomic masses and noticed that the properties of every eighth element were a kind of repetition of the first element, just like the eighth note of octave in music, Indian as well as Western. On the basis of this observation Newlands gave a law. It states: When the known elements were arranged in the increasing order of their atomic mass, every eighth element had properties similar to the first one.

Octaves of Music Newlands' Arrangement

Indian Western

sa do

re re

ga me

ma fa

pa so

da la

ne ti

Li (7.0)

Be (9.0)

B (11.0)

C (12.0)

N (14.0)

O (16.0)

F (19.0)

Na (23.0)

Mg (24.0)

Al (27.0)

Si (28.0)

P (31.0)

S (32.0)

Cl (35.5)

K (39.0)

Ca (40.0)

Cr (52.7)

Ti (48.0)

Mn (55.0)

Fe (56.0)

Br (80.0)

Table 5.2 Newlands' arrangement of elements

From Newlands' classification, a very important conclusion was made that there is some systematic relationship between the order of atomic mass and repetition of properties, which gives rise to a new term 'periodicity'.

5.2 MENDELEEV’S PERIODIC TABLE 5.2.1 Introduction to Mendeleev’s periodic table In 1869, Dmitri Ivanovich Mendeleev (Father of periodic classification), an eminent Russian chemist, was the first one to successfully arrange elements.

After a thorough study of the elements and their properties, he proposed that when elements were arranged in the increasing order of their atomic masses, the elements with similar properties

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IL Foundation Series Class 8

appeared at regular intervals. He fully recognised the significance of periodicity and used a broader range of physical and chemical properties to classify the elements.

Mendeleev was responsible for publishing the periodic law for the first time. It states as follows: The properties of the elements are a periodic function of their atomic weights.

The main points of periodic law, as stated by Mendeleev in his original paper published, are as follows : 1. If the elements are arranged according to their atomic masses, they exhibit an evident periodicity of properties.

2. The elements with similar properties have either almost the same atomic weights, e.g., Fe(56), Co(59), Ni(59) or increase regularly, e.g., K(39), Rb(85), Cs(133); Ca(40), Sr(99), Ba(137).

3. The magnitude of atomic weights determines the character of the element.

4. The arrangement of elements in the order of their atomic weights corresponds to their so-called 'valencies' as well as their properties.

5. The elements with low atomic weights were found to be widely distributed in nature and possess sharply defined properties.

6. Many yet unknown elements may be discovered, and their characteristic properties can be foretold from their atomic weights.

Mendeleev arranged the elements between hydrogen and uranium, providing 90 spaces between them in the order of increasing atomic masses.

Elements with similar properties were arranged in vertical columns called 'groups'. The horizontal rows were called 'periods'.

II RO

III R2O3

IV RO2/ RH4

V R2O5/ RH3

VI RO3/ RH2

VII R2O7/ RH

Li 7

Be 9.4

B 11

C 12

N 14

O 16

F 19

3

Na 23

Mg 24

Al 27.3

Si 28

P 31

S 32

Cl 35.5

4

K 39

Ca 40

?

44

Ti 48

V 51

Cr 52

Mn 55

5

Cu 63

Zn 65

?

68

?

72

As 75

Se 78

Br 80

Series

I R2O

1

H 1

2

VIII RO4

Fe 56

Co 59

Ni 59

Cu 63

91

PERIODIC CLASSIFICATION OF ELEMENTS

6

Rb 85

Sr 87

Yt 88

Zr 90

Nb 94

Mo 96

?

100

7

Ag 108

Cd 112

In 113

Sn 118

Sb 122

Te 127

I 127

8

Cs 133

Ba 137

Di 138

Ce 140

?

?

?

9

?

?

?

?

?

?

?

10

?

?

Er 178

La 180

Ta 182

W 184

?

11

Au 199

Hg 200

Ti 204

Pb 207

Bi 208

?

?

12

?

?

?

Th 231

?

U 240

Ru 104

Rh 104

Pd 106

Ag 108

Pt 198

Au 199

?

Os 195

Ir 197

Table 5.3 Mendeleev’s original periodic table of elements (1871)

Mendeleev arranged all the known 63 elements in the increasing order of their atomic masses. This arrangement showed that the elements having similar chemical properties came directly under one another in the same group.

In this periodic table, Mendeleev left a few gaps (shown by ?) as unknown elements could be placed there. These missing elements are called "Eka" elements.

Some of these elements are viz - Scandium (Sc) - Eka Boron, Gallium (Ga) - Eka Aluminium and Germanium (Ge) - Eka Silicon.

Noble gases were not known at that time. When they were discovered towards the end of the nineteenth century, they were easily accommodated as a separate column in the periodic table.

Description of Mendeleev’s periodic table

Mendeleev’s modern periodic table consists of nine vertical columns called 'groups' or 'families' and seven horizontal rows called 'periods'.

The groups are marked from 0 to VIII. The groups I to VII are divided into subgroups A and B. Groups I to VII are termed normal groups. In the VIII group, in one place, three similar elements are placed together, which are known as transition triads - Fe, Co, Ni; Ru, Rh, Pd; Os, Ir, Pt; Hs (U' no), Mt (Une), Uun. The first three periods of Mendeleev’s periodic table are called short periods, and the other periods are known as long periods. The elements present in the short periods are referred to as typical elements.

The long periods of Mendeleev’s periodic table consist of two rows of elements, and each row of elements is called a series.

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5.2.2 Achievements of Mendeleev’s periodic table I.

Classification of elements

Mendeleev’s periodic system is more elaborate and is far superior to all earlier classifications. He fully recognised the importance of periodicity and utilised a wide range of physical and chemical characteristics, as well as the formulae and the properties of the compounds formed by the elements in the classification of elements.

II. Prediction of new elements

A number of gaps were left in Mendeleev’s periodic table for unknown elements. He even predicted the properties of these unknown elements, helping scientists discover the unknown elements more readily and accurately. Thus, the periodic system hastened the discovery of many new elements.

The following table shows a comparison of properties predicted by Mendeleev for the elements, and those found experimentally after their discovery.

Property

Name of the element Eka Boron

Scandium

Atomic weight

44

43.80

Specific gravity

3.5

3.864

Formula of oxide

(EkaB)2O3

Sc2O3

Formula of sulphate

(EkaB)2 (SO4 )3

Sc2 (SO4 )3

Eka Aluminium

Gallium

Atomic weight

68

69.90

Specific gravity

5.90

5.94

Formula of oxide

(EkaAl)2O3

Ga2O3

Formula of chloride

(EkaAl)2Cl3

GaCl3

Solubility in acids and

Dissolves slowly in both

Dissolves slowly in both

Eka Silicon

Germanium

Atomic weight

72

72.6

Specific gravity

5.50

5.47

Valency

4

4

Formula of oxide

(EkaSi)O2

GeO2

Formula of chloride

(EkaSi)Cl4

GeCl42

By the reduction of its oxide

By the reduction of GeO2 with C or of K2 GeF6 with Na

Isolation

(or) its fluoro complex with Na

Table 5.4 Comparison of properties of elements

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PERIODIC CLASSIFICATION OF ELEMENTS

III. Correction of atomic mass

The atomic mass of an element is related to the equivalent mass of the element by the formula, Atomic mass = Equivalent mass × Valency.

The Valency of the element can be known from its position in the periodic table.

For example, the atomic mass of Beryllium was corrected from 13.5 to 9 (4.55 ◊ 2 = 9.10). Similarly, the atomic masses of In, Au, Pt etc., were also corrected.

5.2.3 Limitations of Mendeleev’s classification In spite of its great usefulness in the study of various elements, Mendeleev’s periodic table suffers from the following limitations.

I. Position of hydrogen

The position of hydrogen in the periodic table is uncertain. It is sometimes placed in the first group and sometimes in the seventh group, as Hydrogen shows resemblance with alkali metals and also with halogens.

II. Grouping of elements

Certain elements which show similar properties have been separated in the periodic table. For e.g. Ba and Pb resemble each other in many respects but have been placed in the second and fourth groups, respectively. Similarly, Ag and Tl, having similar properties, have been placed in the first and third groups, respectively. Certain chemically dissimilar elements have been grouped together. For example, elements Cu, Ag, and Au were grouped together with IA group elements such as Li, Na, and K, which have quite dissimilar properties.

III. Anomalous pairs of elements

For certain pairs of elements, the chemical properties observed were not in apparent agreement with the positions allotted according to atomic weights.

In Mendeleev’s periodic table, four pairs of elements are in the reverse order of their atomic masses. These elements are called anomalous pairs of elements. These are:

94

Element

Atomic Number

Atomic weight

Argon (Ar)

18

40

Potassium(K)

19

39

Cobalt(Co)

27

58.9

Nickel (Ni)

28

58.6

Tellurium (Te)

52

127.60

Iodine (I)

53

126.90

Thorium (Th)

90

232

Protactinium (Pa)

91

231

Table 5.5 Anomalous pairs of elements

IL Foundation Series Class 8

IV. Position of isotopes

Isotopes are atoms of the same elements having different atomic mass but same atomic number.

Example: Hydrogen has three Isotopes - 1H1, 1H2, and 1H3.

According to Mendeleev’s periodic law, these elements should be placed at three separate places in the periodic table. However, isotopes have not been given separate places in the periodic table.

5.3 THE MODERN PERIODIC TABLE The empirical evolution of the periodic table reached its peak in 1913 when Henry Moseley showed that atomic number is a more fundamental property of an element than its atomic weight. The position of an element in the periodic table depends on its atomic number and the reason for the anomalies in the original periodic table becomes clear at once.

5.3.1 Introduction to the modern periodic table Moseley modified Mendeleev’s periodic law and stated: The physical and chemical properties of the elements are periodic functions of their atomic numbers.

Periodic Table of the Elements (Long Form) (Representing Electron Configuration)

Main group Elements p-subshell is gradually filled up

Main group Elements s-subshell is gradually filled up Group Period

2 3 4 5 6 7

H

1

1s1

H

1

Li

Be

2s1

2s2

Transition Elements d-subshell is gradually filled up

Mg

3s1

3s2 20

IIIB 22

Ca

Sc

4s

4s

3d 4s

37

IVB

21

K 1

2

38

5

VIIIB

12

Na 19

2

IIIA

4

11

0 (zero)

IIA

1s1 3

Atomic number Symbol Valence-shell Configuration

1

IA

1

VB 23

Ti 2

39

24

3d 4s

2

40

3

2

41

IB 26

27

Cr

Mn

Fe

3d 4s

3d 4s

3d 4s

4

2

42

5

2

43

6

2

44

6

28

IIB

29

30

VA 7

VIA 8

9

C

N

O

F

Ne

2s22p2

2s22p3

2s22p4

2s22p5

2s22p6

14

15

16

17

Si

P

S

Cl

Ar

3s23p1

3s23p2

3s23p3

3s23p4

3s23p5

3s23p6

31

32

33

34

35

Cu

Zn

Ga

Ge

As

Se

3d 4s

3d 4s

3d 4s

3d 4s

4s 4p

4s 4p

4s 4p

4s 4p

45

8

2

46

10

1

47

10

2

48

18

Al

Ni

2

1s1 10

B

Co 7

He

VIIA

2s22p1 13

VIIB 25

V

3d 4s

2

VIB

IVA

2

1

49

2

2

50

2

3

51

2

36

Br 4s 4p

4

52

2

Kr 4s24p6

5

53

54

Rb

Sr

Y

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Ag

Cd

In

Sn

Sb

Te

I

Xe

5s1

5s2

4d15s2

4d25s2

4d45s1

4d55s1

4d55s6

4d75s1

4d85s1

4d10

4d105s1

4d105s2

5s25p1

5s25p2

5s25p3

5s25p4

5s25p5

5s25p6

55

56

57

Cs

Ba

La

6s

6s

5d 6s

1

87

2

88

1

*

72

2

89

73

Ta

5d 6s

5d 6s

2

**

74

Hf 2

104

3

75

76

W 5d 6s

2

105

4

2

106

77

78

Re

Os

Ir

5d 6s

5d 6s

5d 6s

5

2

107

6

2

108

7

109

Ra

Ac

Rf

Db

Sg

Bh

Hs

Mt

7s1

7s2

6d17s2

6d27s2

6d37s2

6d47s2

6d57s2

6d67s2

6d77s2

80

Au

5d 6s

1

10

1

111

Ds

6d97s1

81

82

Hg

Tl

5d 6s

6s 6p

10

2

112

2

1

113

83

84

Pb

Bi

6s 6p

6s 6p

2

2

114

2

3

115

85

86

Po

At

6s 6p

6s 6p

2

4

116

2

Rn 6s26p6

5

117

118

Rg

Cn

Uut

Fl

Uup

Lv

Uus

Uuo

6d107s1

6d107s2

7s27p1

7s27p2

7s27p3

7s27p4

7s27p5

7s27p6

Inner-Transition Elements f-subshell is gradually filled up

58

Ce

59

60

Pr

4f26s2

Th 6d27s2

61

Nd

4f36s2 91

90

**Actinides

5d 6s 9

110

Fr

*Lanthanides

79

Pt

2

4f46s2 92

62

Pm 4f56s2 93

63

Sm 4f66s2 94

64

Eu 4f76s2 95

65

Gd 4f75d16s2 96

66

Tb 4f96s2 97

67

Dy 4f106s2 98

69

68

Ho

Er

4f116s2 99

4f126s2 100

70

Tm

71

Yb

4f136s2

4f146s2 102

101

Lu 4f145d16s2 103

Pa

U

Np

Pu

Am

Cm

Bk

Cf

Es

Fm

Md

No

Lr

5f26d17s2

5f36d17s2

5f46d17s2

5f67s2

5f77s2

5f76d17s2

5f97s2

5f107s2

5f117s2

5f127s2

5f137s2

5f147s2

5f146d17s2

Fig. 5.1 Periodic table of elements 95

PERIODIC CLASSIFICATION OF ELEMENTS

The atomic number is equal to the nuclear charge or the number of electrons in the neutral atom.

Further, it was recognised that the periodic law is essentially the consequence of the periodic variation in electronic configurations. The configurations determine the properties of elements and their compounds, and are the basis for the modern periodic law.

The modern periodic law is stated as the physical and chemical properties of the elements are periodic functions of their atomic numbers or their electronic configurations.

The original form of the periodic table has since been modified due to the structural elucidation of atoms and the discovery of noble gas elements. Numerous forms of the periodic table have been devised from time to time. Moseley constituted the periodic table by unfolding Mendeleev’s table, where the elements are arranged according to the atomic numbers. This is closely similar to the Bohr - Thomson table.

The most convenient version of the periodic table was constructed by Bohr based on the modern periodic law, where the elements were arranged in the order of their electronic configurations.

5.3.2 Position of elements in the modern periodic table It is very difficult to study and remember the properties of all the elements. So to study the properties of elements in a better way and to know the relation between one element and another, a classification of elements is necessary.

Groups

The vertical columns of the periodic table are called groups or families. There are eighteen groups.

Among these groups, eight are important. These groups are: 1, 2, 13, 14, 15, 16, 17, and 18. A part of group 3 elements are separately shown at the bottom of the periodic table in the form of Lanthanides and Actinides. Fourteen elements coming after lanthanum are called lanthanides. Lanthanides are from cerium (Z = 58) to lutetium(Z = 71). Fourteen elements coming after actinium are called actinides. Actinides are from thorium (Z = 90) to lawrencium(Z = 103). Lanthanides are commonly called rare earths. Most of the actinides are mainly synthetic elements.

Hydrogen is the only element which can be placed in two different groups of the periodic table: Group 1 and Group 17.

Groups number

Common name

IA Group elements Alkali Metals

96

IIA Group elements

Alkaline Earth Metals

IIIA Group elements

Boron family

IVA Group elements

Carbon family

VA Group elements

Nitrogen family (pnictogens)

IL Foundation Series Class 8

VIA Group elements

Oxygen family (Chalcogens)

VIIA Group elements

Halogens

VIIIA Group elements

Noble gases / Inert gases / Aerogens / Zero group elements Table 5.6 Groups in a periodic table

Periods

There are 7 horizontal rows in the periodic table. These are called periods.

• First Period: It contains only two elements: Hydrogen and Helium. So, it is called the 'shortest period’.

• Second Period: It contains 8 elements: Li, Be, B, C, N, O, F, Ne. • Third Period: It contains 8 elements: Na, Mg, Al, Si, P, S, Cl, Ar. The second and the third periods are called 'Short periods’.

• Fourth Period: It contains 18 elements from K to Kr (Z=19 to 36).

• Fifth Period: It contains 18 elements from Rb to Xe (Z=37 to 54).

The fourth and fifth periods are called ’Long periods’.

• Sixth Period: It contains 32 elements from Cs to Rn and is called the 'Longest period' ( Z=55 to 86 ).

• Seventh period: It contains 32 elements from Fr to Og and is called the 'Longest period' ( Z=87 to 118 ).

On the basis of the differentiating electron, the periodic table is divided into four main blocks - s, p, d, and f. i) Elements of groups 1 and 2 are s - block elements. ii) Elements of groups 13, 14, 15, 16, 17 and 18 are p - block elements. iii) Elements of groups 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 are d-block elements. iv) The f-block elements comprise two horizontal rows placed at the bottom of the periodic table.

97

PERIODIC CLASSIFICATION OF ELEMENTS

s-block 1

2

H

He

1s1

3

p-block

1s1

5

4

Li

B

Be

2s1

11

13

12

Mg

3s1

3s2

21

20

K

4s1 37

56

Ba

6s1

6s2

88

Fr 7s

1

Ra 7s

2

41

* * * *

*

La Lu

72

Ac Lr

**

104

106

6d 7s

2

3

76

107

6d 7s 4

108

6d 7s 5

5d76s2 109

Hs 6

110

7

6d 7s 9

2

82

83

Pb

6s26p1

6s26p2

35

52

6s26p3

36

4s24p6 54

I

5s25p4

Xe

5s25p5 85

6s26p4

Kr

4s24p5 53

Po

3s23p6

Br

Te

84

Bi

Ar

3s23p5

4s24p4

5s25p3

18

Cl

Se

Sb

5s25p2

Tl

5d106s2

51

2s22p6

5s25p6 86

At

6s26p5

Rn

6s26p6

111

Ds

6d 7s

2

81

Hg

34

4s24p3

Sn

5s25p1

80

5d106s1

50

17

3s23p4

As

4s24p2

In

4d105s2

Au

5d96s1

Mt

6d 7s

2

79

49

Cd

33

Ne

2s22p5

S

3s23p3

Ge

4s24p1

48

4d105s1

Pt

Ir

5d66s2

Bh

2

4d10

32

16

10

F

2s22p4

P

3s23p2

Ga

3d104s2

Ag

78

Os

5d56s2

Sg

2

77

47

Pd

31

Zn

3d104s1

46

4d85s1

30

Cu

3d84s2

Rh

4d75s1

Re

5d46s2

Db

6d 7s 2

45

29

Ni

3d74s2

Ru

4d55s6 75

5d36s2 105

Rf

1 2 2

44

W

28

Co

3d64s2

Tc

4d55s1 74

5d26s2

5f6d 6d 7s7s 14 1

43

Ta

27

15

9

O

2s22p3

Si

3s23p1

Fe

3d54s2

Mo

4d45s1 73

14 1 1 2 2 6s6s 4f5d 5d

89 103

42

Hf

26

Mn

3d44s2

Nb

4d25s2

57 71

25

Cr

3d34s2

Zr

4d15s2

5s2

Cs 87

40

Y

Sr

24

V

3d24s2

39

38

5s1

23

Ti

3d14s2

4s2

Rb 55

22

Sc

Ca

14

8

N

2s22p2

Al

d-block

7

C

2s22p1

2s2

Na 19

6

Rg

6d107s1

1

f-block 57

*

La

58

Ce

5d16s2 89

Ac

**

6d17s2

59

Pr

4f26s2 90

60

6d27s2

Nd

4f36s2 91

Th

61

5f26d17s2

Pm

4f46s2 92

Pa

62

5f36d17s2

Sm

4f56s2

93

U

63

4f66s2

94

Np

5f46d17s2

64

Eu

4f76s2 95

Pu

5f67s2

65

Gd

4f75d16s2 96

Am 5f77s2

66

Tb

4f96s2 97

Cm

5f76d17s2

67

Dy

4f106s2 98

Bk

5f97s2

5f107s2

Er

4f116s2 99

Cf

69

68

Ho

4f126s2 100

Es

5f117s2

70

Tm

5f127s2

4f146s2 102

101

Fm

Yb

4f136s2

Md 5f137s2

No

5f147s2

Fig. 5.2 Different blocks in a periodic table

5.3.3 Trends in the modern periodic table Atoms of the elements present in a group of the periodic table have similar outer shell configurations. Repetition of similar valence shell configuration is the cause of periodicity in properties.

In the periodic table, the properties of elements change gradually with a change in their electronic configurations. This trend repeats itself at regular intervals. This repetition of a character is called 'periodicity' or periodic trends. Some of the periodic trends are listed below• Valency • Atomic size • Metallic and Non-metallic properties Valency

Valency is the combining capacity of an element. (OR) According to the new concept, valency may be defined as the number of electrons that are lost or gained or shared with one atom of that element to acquire the stable configuration of the nearest noble gas element.

The valency of metals is given by the number of valence electrons present in an atom. The valency of non-metals is given by subtracting the number of valence electrons present in an atom from 8.

98

IL Foundation Series Class 8

The combining capacity of elements is often compared to that of hydrogen, chlorine or oxygen.

Valency is defined as the number of hydrogen atoms or the number of chlorine atoms or twice the number of oxygen atoms with which one atom of the element combines.

The maximum valency of an element is its group number. The minimum valency is zero. The highest valency is exhibited by Os or Xe. The value is 8.

In the second period, the highest valency of 4 is exhibited by carbon, and in the third period, the highest valency of 7 is exhibited by chlorine.

Atomic size

Atomic radius is the distance between the centre of the atomic nucleus and the electron cloud of the outermost energy level. Atomic radius is also commonly referred to as atomic size. However, the atomic size is truly regarded as the diameter of the atom.

Variation of atomic radii in period

The atomic radii decrease from left to right along a period in the periodic table. In a period, the atomic number increases, and distinguishing electrons enter the same outer shell; hence, the nuclear charge increases. This increases the attraction between the nucleus and the extra nuclear electrons as the number of orbitals remains the same. Due to this, all electrons in orbitals are pulled closer to the nucleus. This goes on from atom to atom in a period. The atomic radius of an inert gas is shown to be the largest in a period, because of its Vanderwaal’s radius, which is generally larger than the covalent radii.

Example: 2nd period

Li > Be > B > C > N > O > F < Ne

Atomic radii (in A) 1.23, 0.89, 0.82, 0.77, 0.75, 0.73, 0.72, 1.60 Variation of atomic radii in group

In a group from top to bottom, the atomic number increases, the valence shell increases, and hence, the atomic radii increase. For example, Li (1.23), Na (1.57), K (2.03), Rb (2.16), Cs (2.35). The atomic size increases due to the presence of extra energy shells in the elements as we go down the group.

Metallic and non-metallic properties

Metallic character: It is the tendency to lose electron(s) and form a positive ion. It decreases along a period and increases as we move down a group. The metals with low melting and boiling points are Ga, Na, K, Rb, etc. The metals with high melting and boiling points are W, Fe, Mo, etc.

Non-metallic character: It is the tendency to gain one or more electron(s) to form a negative ion.

It increases along a period and decreases as we move down a group due to an increase in the atomic size.

99

PERIODIC CLASSIFICATION OF ELEMENTS

QUICK REVIEW • The first periodic table was constructed by Mendeleev. The periodic law is that the physical and chemical properties of elements are periodic functions of their atomic weights.

• The elements with low atomic weights were found to be widely distributed in nature. They are referred to as typical elements which are present in three short periods of Mendeleev’s table.

• Group VIII of Mendeleev's table contains three triads.

• Newlands’ Law of Octaves states that when the known elements were arranged in the increasing order of their atomic mass, every eighth element has properties similar to the first one.

• The long form of the periodic table is called Bohr's table. Here, the elements are arranged in the increasing order of their atomic numbers.

• There are 18 vertical columns in the long form called groups, and 7 horizontal rows called periods.

• The first period is the shortest period, it has only 2 elements. The second and third periods are short periods with eight elements each. The fourth and fifth periods are normal periods with eighteen elements each. The sixth period is the longest period with 32 elements. The seventh period is incomplete.

• The period number indicates the valence shell and the group number in Roman letters denotes the number of electrons in the outer most shell. However, elements of the zero group g enerally have eight electrons in the outermost shell.

• The atomic radius is defined as the distance between the centre of the nucleus and the outermost shell where electrons are present.

• The atomic radius increases as we move down a group because of the presence of new shells.

• In any period, the atomic radius generally decreases from left to right, with the smallest radius typically found in noble gases.

• The elements in a group generally show the same valency. Across a period, the valency increases unit by unit. The transition metals show a variable valency.

• The maximum valency of an element is its group number.

• The minimum valency is zero. The highest valency is exhibited by Os or Xe. The value is 8. In the second period, the highest valency of 4 is exhibited by carbon, and in the third period, the highest valency of 7 is exhibited by chlorine.

• The tendency of an element to lose an electron is called metallic nature. It increases down the group as the size increases and decreases across a period as the size decreases.

100

IL Foundation Series Class 8

• The metallic nature increases and the non-metallic nature decreases down the group. The metallic nature decreases across a period.

WORKSHEET - 1 MULTIPLE CHOICE QUESTIONS WITH SINGLE CORRECT ANSWER I. Early attempts at the classification of elements

1. The maximum number of known elements are a. 26

b. 92

c. 111

d. 118

c. Ru, Rh, Pd

d. Os, Ir, Pt

2. Which of the following is Dobereiner's triad : a. Li, Na, K

b. Fe, Co, Ni

3. Identify the set of elements showing similar properties according to Newlands : a. F and Cl

b. Al and Cr

c. Mg and Ca

d. All

II. Mendeleev’s periodic table

1. According to Mendeleev's periodic law the physical and chemical properties of the elements are periodic functions of their : a. Atomic number

b. Atomic mass

c. Both a and b

d. None

2. Anomalous pair among the following is : a. Boron - Silicon

b. Beryllium - Indium

c. Aluminium - Gallium

d. Cobalt - Nickel

3. The triad not present in Group VIII of Mendeleev's table is : a. Li, Na, K

b. Fe, Co, Ni

c. Ru, Rh, Pd

d. Os, Ir, Pt

4. Which of the following pairs of atomic numbers represents elements belonging to the same group? a. 11, 20

b. 12, 30

c. 13, 31

d. 14, 33

5. As per the modern periodic law, the physical and chemical properties of elements are periodic functions of their: a. Atomic mass number

b. Electronic configuration

c. Atomic weight

d. Atomic size

6. Among the following, the element with metallic properties is: a. P

b. N

c. K

d. S 101

PERIODIC CLASSIFICATION OF ELEMENTS

7. As the electropositivity increases, the metallic nature: a. Increases

b. Decreases

c. Remains constant

d. No change

8. Statement A: The metallic property decreases along a period from left to right.

Statement B: The non-metallic property increases over a period.

Statement C: Metals have the tendency to form cations by gaining electrons. a. All the statements are correct.

b. All the statements are incorrect.

c. A and B are correct, and C is incorrect.

d. A and B are incorrect, and C is correct.

9. Which of the following pair is against the Mendeleev's periodic law: a. Cr, Mn

b. Cu, Zn

c. Te, I

d. Na, Mg

10. Which of the following set of elements are known as transition triads? a. Fe, Co, Ni

b. Ru, Rh, Pd

c. Os, Ir, Pt

d. All

11. Which of the following is not a defect in Mendeleev's periodic table? a. Position of hydrogen

b. Grouping of elements

c. Anomalous pairs

d. Common valency of elements in a group

12. Mendeleev's periodic table is based on: a. Increasing order of atomic numbers

b. Decreasing order of atomic masses

c. Decreasing order of atomic numbers

d. Increasing order of atomic masses

III. The modern periodic table

1. In a periodic table, the vertical columns and horizontal rows are respectively called: a. Groups, periods

b. Periods, groups

c. Groups, groups

d. Periods, periods

2. The element Sc is known as: a. Eka - Aluminium

b. Eka - Boron

c. Eka - Silicon

d. Eka - Mercury

3. The correct order of atomic radii is: a. N < Be < B

102

b. F- < O2- < N3-

c. Na < Li<K

d. Fe3+ < Fe2+ < Fe4+

IL Foundation Series Class 8

4. According to the Periodic Law of elements, the variation in properties of elements is related to their: a. Nuclear neutron - proton number ratios

b. Atomic masses

c. Nuclear masses

d. Atomic number

5. The metallic character of elements _______ with an increase in atomic size: a. Increases

b. Decreases

c. Remains the same

d. Increases first then decreases

6. The increasing order of the atomic radii of Si, S, Na, Mg, Al is: a. S < Si < Al < Mg < Na

b. Na < Al < Mg < S < Si

c. Na < Mg < Si < Al < S

d. Na < Mg <Al < Si < S

7. The increasing order of atomic radii of the following Group 13 elements is: a. Al < Ga < In < Tl

b. Ga < Al < In < Tl

c. Al < In < Ga < Tl

d. Al < Ga < Tl < In

8. Rare earths are generally: a. Actinides

b. f-Block elements

c. Inner transition elements

d. Lanthanides

9. The lanthanum element with z = 57 belongs to: a. s-block

b. p-block

c. d-block

d. f-block

10. In the periodic table, the transition elements begin with: a. Scandium

b. Zinc

c. Copper

d. Mercury

11. The triad not present in Group VIII of Mendeleev's table is: a. Li, Na, K

b. Fe, Co, Ni

c. Ru, Rh, Pd

d. Os, Ir, Pt

12. In the periodic table, inversion of atomic weights took place in this pair: a. Argon - Potassium

b. Boron - Scandium

c. Hydrogen - Helium

d. Beryllium - Boron

13. The period that contains only gaseous elements is: a. 1

b. 2

c. 3

d. 4

103

PERIODIC CLASSIFICATION OF ELEMENTS

14. The starting and last elements in the largest period in the modern periodic table are: a. Rb and Xe

b. Cs and I

c. Cs and Rn

d. Fr and Kr

15. Which of the following has both members from the same period of the periodic table? a. Na, F

b. Mg, Ca

c. Na, Cl

d. Be, Al

16. The elements with atomic number 10, 18, 36, 54, and 86 are all: a. Light metals

b. Inert gases

c. Halogens

d. Rare earths

17. Which of the following pairs has elements containing the same number of electrons in the outermost orbit? a. N, O

b. Na, Cl

c. Ca, Cl

d. Cl, Br

18. Among the s-block metals and transition metals, which are more metallic? a. s-block metals

b. Transition metals

c. Both are equally metallic

d. Cannot be predicted

19. The pair of atomic numbers which represent the p-block elements is: a. 6, 12

b. 7, 53

c. 19, 35

d. 38, 51

20. Which of the following is an element present in the d-block, but not a transition element? a. Cd

b. Cu

c. Ca

d. Cr

WORKSHEET - 2 MULTIPLE CHOICE QUESTIONS WITH SINGLE CORRECT ANSWER 1. The correct arrangement of increasing atomic radii of Na, K, Mg, Rb: a. Mg < K < Na < Rb

b. Mg < Na < K < Rb

c. Mg < Na < Rb < K

d. Na < K < Rb < Mg

2. Which of the following has the smallest atomic radius? a. Al 3.

b. Si

c. Cl

d. Na

c. Sb

d. Sc

Among the following, the symbol of Antimony is: a. Au

b. Ag

4. Arrange fundamental particles of an atom in increasing order of its weight:

104

IL Foundation Series Class 8

a. Neutron > Electron > Proton

b. Electron < Proton < Neutron

c. Proton < Neutron < Electron

d. Electron < Neutron < Proton

5. Among the following, the element with the atomic number 21 is: a. Ca

b. Sc

c. K

d. Ti [EAMCET 1993]

6. In the Lanthanides, the differentiating electron enters the: a. d-sub shell

b. f-sub shell

c. p- sub shell

7. The Non-transition metal among the following is: a. Ag

b. Zn

d. s-sub shell

[EAMCET 1993]

c. Os

d. Pt [EAMCET 1996]

8. The 100th element is named in honour of: a. Einstein

b. Bohr

c. Fermi

d. Curie

9. Among the following pairs of atomic numbers, the elements belonging to the same group are: a. 11, 20

b. 12, 30

c. 13, 31

10. The number of periods present in the long form of the periodic table is: a. 6

b. 7

d. 14, 33 [EAMCET1999]

c. 8

d. 18

11. According to the Periodic Law of elements, the variation in properties of elements is related to their: a. Nuclear masses

b. Atomic numbers

c. Nuclear neutron - proton number ratios

d. Atomic masses

[AIEEE - 2003]

12. The element Z=114 has been discovered recently. It will belong to which of the following family group and electronic configuration?

(NEET 2017)

a. Halogen family [Rn]5f14 6d10 7s2 7p5

b. Carbon family [Rn]5f14 6d10 7s2 7p2

c. Oxygen family [Rn]5f14 6d10 7s2 7p4

d. Nitrogen family [Rn]5f14 6d10 7s2 7p6

13. Element X forms a chloride with the formula XCl4 which is a solid with a low melting point. X would most likely be in the same group of the periodic table as: a. Sodium (Na)

b. Magnesium (Mg)

c. Aluminium (Al)

d. Silicon (si)

14. Which of the following pairs of atomic numbers represent s-block elements? a. 7 and 15

b. 6 and 12

c. 9 and 17

d. 3 and 12

15. The most metallic element among Li, K, Mg, C, Al, and S is: a. Al

b. K

c. Mg

d. S

105

PERIODIC CLASSIFICATION OF ELEMENTS

16. An element X has 12 protons and 12 electrons and another element Y has 12 protons and 10 electrons. Which of the following is the correct option regarding their atomic radii? a. Element X will have larger atomic

b. Element Y will have larger atomic

radii.

radii.

c. The atomic radii of both elements will

d. The atomic radii of X and Y cannot be

be almost similar.

compared.

17. What is the chemical formula of the carbonate compound of element X, where element X has a valency of hydrogen? a. XCO2

b. X2CO2

c. X2CO3

d. XCO3

18. Elements have been arranged in the following sequence according to the Newlands’ Law of Octaves, as shown below. Which of the elements given below will show similar properties?

F, Na, Mg, Al, Si, P, S, Cl, K a. Na and Cl

b. F and K

c. Mg and Cl

d. Both Na and K, F and Cl

19. An element X belongs to the 3rd period and group 2. The nature of an element is: a. Metalloids

b. Noble gas

c. Metal

d. Non-metal

20. Which of the following is the mass of the third element if an element A has atomic mass 14 and middle element B has atomic mass 31 in Döbereiner’s triads? a. 90

b. 22.5

c. 48

d. 17

21. What will be the formula and the nature of bonding of its chloride of an element X placed in group 14? a. XCl2, ionic

b. XCl4, metallic

c. XCl4, covalent

d. XCl2, coordinate

22. The electronic configuration of an element X is 2, 8, 8, 2. What are its period number, group number and valency, respectively? a. 4, 2, and 2

b. 2, 2, and 4

c. 2, 4, and 2

d. 4, 2, and 4

23. Which among the following groups of elements can be called Döbereiner's Triads?

Group A : Li, Na, K

106

Group B : Be, Mg, Ca

a. Group A only

b. Group B only

c. Both groups A and B

d. Neither group A nor group B

IL Foundation Series Class 8

24. Which one of the following elements has the maximum number of valence electrons? a. Al

b. Si

c. Na

d. P

25. Which of the following is the outermost shell for elements of period 2? a. N shell

b. L shell

c. M shell

d. K shell

26. An element which is an essential constituent of all organic compounds belongs to: a. Group 16

b. Group 14

c. Group 1

d. Group 15

27. The elements A, B, C, D and E have atomic number 9, 11, 17, 12, and 13, respectively. Which pair of elements belong to the same group? a. A and B

b. B and D

c. A and C

d. D and E

28. Which of the following statements about the Modern Periodic Table is correct: a. It has 7 horizontal rows known as groups.

b. It has 18 horizontal rows known as periods.

c. It has 18 vertical columns known as groups.

d. It has 7 vertical columns known as periods.

29. Which of the following statement(s) about the Modern Periodic Table are incorrect? (i)The elements in the Modern Periodic Table are arranged on the basis of their decreasing atomic number ii) The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic masses iii) Isotopes are placed in adjoining group(s) in the Periodic Table iv) The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic number a. iv only

b. i, ii and iii

c. i only

d. i, ii and iv

30. The electronic configuration of the atom of an element X is 2,8,4. In the modern periodic table, the element X is placed in a. 2nd group

b. 4th group

c. 14th group

d. 8th group

31. Five elements A, B, C, D and E have atomic numbers of 2, 3, 7, 10 and 18 respectively. The elements which belong to the same period of the periodic table are a. A, B, C

b. B, C, D

c. A, D, E

d. B, D, E

c. IVB group

d. IIIB group

32. Lanthanides and actinides are placed in a. IA group

b. IIIA group

107

PERIODIC CLASSIFICATION OF ELEMENTS

33. VIIIA group elements are also called as a. Aerogens

b. Inert gases

c. Rare elements

d. All

34. VIA group elements are also called as a. Pnicogens

b. Chalcogens

c. Halogens

d. All

35. Most of the non-metals are present in the long form of the periodic table in a. p-block

b. f-block

c. d-block

d. s-block

36. In lanthanides, the differentiating electron enters into a. d – subshell

b. f – subshell

c. p – subshell

d. s – subshell

37. Non-metal having metallic properties is a. Al

b. Ag

c. Hg

d. Graphite

38. Which of the following statements is correct?

Statement 1: 4Be, 9F belong to the same period and 9F is bigger in size.

Statement 2: 4Be, 9F belong to the same period and 4Be is bigger in size.

Statement 3: 19K, 20Ca belong to the same period and 19K is bigger in size.

Statement 4: 19K, 20Ca belong to the same period and 20Ca is bigger in size. a. Statement 2 is correct.

b. Statement 3 is correct.

c. Both statements, 2 and 3 are correct.

d. Both statements, 2 and 3 are incorrect.

39. Assertion (A): Rare earths are generally lanthanides Reason(B): Lanthanides are transuranic elements. a. Both A and R are correct and R is the

b. Both A and R are correct and R is not

correct explanation of A

the correct explanation of A

c. A is correct and R is incorrect

d. A is incorrect and R is correct

40. Assertion (A): Chemistry of Actinides is more complicated than Lanthanides.

Reason (R): Actinides elements are radioactive.

108

a. A and R are true, R explains A

b. A and R are true, R does not explain A

c. A is true, but R is false

d. A is false, but R is true

ANSWER KEY 1: COAL AND PETROLEUM

III. Flame and structure of a flame

WORKSHEET 1

1. b

2. c

3. b

4. a

5. a

I. Exhaustible and inexhaustible natural resources

6. a

7. c

8. b

9. d

10. b

1. b 6. d

2. c 7. c

3. d 8. c

4. b

2. b 7. d 12. b

3. b 8. c 13. d

4. b 9. b 14. d

5. c

II. Coal 1. c 6. c 11. b

5. b 10. b 15. a

III. Petroleum 1. d 6. c 11. d

2. c 7. c 12. a

3. d 8. a

4. a 9. d

5. d 10. a

IV. Natural gas and limited natural resources 1. b 6. a

2. b 7. c

3. c 8. c

4. b 9. b

5. a 10. a

3. c 8. c 13. c 18. b 23. b 28. a 33. c

4. b 9. c 14. b 19. c 24. d 29. c 34. d

5. c 10. c 15. b 20. c 25. a 30. b 35. b

WORKSHEET 2 1. b 6. c 11. a 16. a 21. a 26. b 31. d

2. d 7. a 12. a 17. a 22. d 27. a 32. d

IV. Fuel, fuel efficiency, and burning of fuel 1. b

2. c

3. b

4. b

5. c

6. b

7. a

8. c

9. c

10. a

WORKSHEET 2 1. c

2. b

3. c

4. a

5. d

6. a

7. a

8. a

9. b

10. d

11. a

12. c

13. d

14. a

15. c

16. c

17. c

18. d

19. b

20. a

21. c

22. a

23. c

24. c

25. a

26. c

27. a

28. b

29. a

30. c

31. a

32. c

33. c

34. c

35. d

36. d

37. c

38. a

39. a

40. b

3: STRUCTURE OF THE ATOM WORKSHEET 1 I. Structure of an atom 1. a

2. a

3. d

4. a

5. a

6. a

7. a

8. a

9. a

10. a

11. b 12. d 13. d 14. a

15. d

16. d 17. c

18. a

19. c

2: COMBUSTION AND FLAME

II. Electronic configuration of elements and valency

WORKSHEET 1

1. c

2. a

I. Combustion and control of fire

6. b

7. c

3. b

4. c

5. c

8. d

9. d

5. c 10. b

III. Atomic number and mass number 1. b

2. c

3. b

4. c

5. b

7. d

8. b

9. d

10. b

1. a

2. d

3. b

4. d

6. c

7. b

8. d

II. Types of combustion 1. b

2. d

3. b

4. c

5. c

6. c

6. b

7. b

8. b

9. d

10. b

IV. Isotopes, isobars, and isotones

109

ANSWER KEY 6. d

7. c

8. a

9. a

10. b

6. a

11. b

12. b

13. a

14. c

15. a

WORKSHEET 2

16. b

17. a

18. a

19. a

20. d

1. a

2. c

3. c

4. c

5. c

1. d

2. b

3. b

4. a

5. a

21. c

22. b

23. c

24. b

25. c

6. a

7. a

8. a

9. a

10. c

26. a

27. a

28. c

29. b

30. c

11. b

12. c

13. b

14. d

15. b

31. a

32. d

33. b

34. c

16. d

17. c

18. a

19. b

20. a

21. b

22. b

23. b

24. a

25. a

5: PERIODIC CLASSIFICATION OF ELEMENTS

26. c

27. c

28. a

29. a

30. a

WORKSHEET 1

31. a

32. c

33. b

34. d

35. c

36. c

37. b

38. a

39. d

40. a

I. Early attempts at the classification of elements 3. d 2. a 1. d

41. a

II. Mendeleev’s periodic table

4: ATOMS AND MOLECULES WORKSHEET 1 I. Introduction to atoms, molecules and laws of chemical combination 1. b 2. c 3. a 4. b 5. a 6. d

7. a

8. b

9. c

11. b

12. d

13. c

14. d

10. c

II. What is an atom?

1. d

2. b

6. a

7. c

3. a

4. c

5. d

2. d

3. d

6. b

7. c

8. b

4. a

5. c

IV. Writing chemical formulae; molecular

mass and mole concept 1. a

2. c

3. b

6. d

7. d

8. c

4. d

5. b

WORKSHEET 2 1. a 110

2. c

3. a

2. d

3. a

4. c

5. b

6. c

7. a

8. c

9. c

10. d

11. d

12. d

III. The modern periodic table 1. a

2. b

3. b

4. d

5. a

6. a

7. b

8. d

9. c

10. a

11. a

12. a

13. a

14. c

15. c

16. b

17. d

18. a

19. b

20. a

WORKSHEET 2

III. What is a molecule?

1. a

1. b

4. d

5. b

1. b

2. c

3. c

4. b

5. b

6. b

7. b

8. c

9. c

10. b

11. b

12. b

13. d

14. d

15. b

16. a

17. c

18. d

19. c

20. c

21. c

22. a

23. c

24. d

25. b

26. b

27. c

28. c

29. b

30. c

31. b

32. d

33. d

34. b

35. a

36. b

37. d

38. c

39. c

40. b