IGCSE Coordinated Sciences - Chemistry Student Book

Cambridge IGCSE ™

Co-ordinated Sciences Chemistry STUDENT’S BOOK

Chris Sunley and Sam Goodman

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Atoms, elements and compounds

Δ Some crystals are found in the natural world.

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Materials, atomic structure and the Periodic Table INTRODUCTION Δ Fig. 2.1 Non metals: from left: silicon, chlorine, sulfur.

This topic is about the structure, or the makeup, of all substances. Some substances exist in nature as elements, others as compounds that are formed when elements combine chemically, and others as mixtures. The topic starts by considering the structure of the atoms that make up elements. It shows how the arrangement of elements in the Periodic Table is determined by the structure of their atoms. The link between electronic structures and metals and non-metals is introduced. The following topics look in more detail at how atoms combine together to form ions and molecules, and the structure of metals.

LEARNING OBJECTIVES ✓ Describe the differences between elements, compounds and mixtures. ✓ Describe the structure of the atom as a central nucleus containing neutrons and protons surrounded by electrons in shells. ✓ State the relative charges and relative masses of a proton, a neutron and an electron. ✓ Define proton number / atomic number as the number of protons in the nucleus of an atom. ✓ Define mass number / nucleon number as the total number of protons and neutrons in the nucleus of an atom. ✓ Determine the electronic configuration of elements and their ions with proton number 1 to 20, e.g.

2,8,3. ✓ State that: Group VIII noble gases have a full outer shell; the number of outer shell electrons is equal to the group number in Groups I to VII; the number of occupied electron shells is equal to the period number. ✓ Define isotopes as different atoms of the same element that have the same number of protons but different numbers of neutrons. − ✓ Interpret and use symbols for atoms, e.g. 126 C, and ions, e.g. 35 17 Cl ✓ SUPPLEMENT State that isotopes of the same element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration.

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ATOMS, ELEMENTS AND COMPOUNDS

KNOWLEDGE CHECK ✓ There are three states of matter and the kinetic particle theory explains what happens when one state is converted into another. ✓ Diffusion experiments provide evidence for the existence of particles. ✓ Compounds are formed when elements combine together chemically.

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ELEMENTS, COMPOUNDS AND MIXTURES All matter can be classified into the three categories of elements, mixtures and compounds.

An element is the smallest part of a substance that can exist on its own. When two or more elements combine together a compound is formed. In a compound the atoms or elements are chemically combined together.

A mixture contains more than one substance (elements or compounds). In a mixture, the individual substances can be separated by simple means. This is because the substances in a mixture have not combined chemically.

Sub-atomic particles The smallest amount of an element that still behaves like that element is an atom.

Each element has its own unique type of atom. Atoms are made up of smaller, sub-atomic particles. The three main sub-atomic particles are protons, neutrons and electrons.

These particles are very small and have very little mass. However, it is possible to compare their masses to the mass of a proton. Their charges may also be compared in a similar way. These numbers are called the relative mass and the relative charge.

The proton and neutron have the same mass, and the proton and electron have equal but opposite charges. As an atom has equal numbers of protons and electrons atoms are neutral.

Sub-atomic particle Proton Neutron

Relative mass 1 1

Electron

about

1 2000

Relative charge +1 0 –1

Δ Table 2.1 Relative masses and charges of sub-atomic particles.

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Protons and neutrons are found in the centre of the atom in a cluster called the nucleus. The electrons form a series of shells around the nucleus. The shells are different distances from the nucleus.

ATOMIC STRUCTURE AND THE PERIODIC TABLE

ATOMIC STRUCTURE AND THE PERIODIC TABLE In 1808 the British chemist John Dalton published a book outlining his theory of atoms. These were the main points of his theory: • All matter is made of small, indivisible spheres called atoms.

• All the atoms of a given element are identical and have the same mass.

• The atoms of different elements have different masses.

• Chemical compounds are formed when different elements join together.

Since 1808, atomic theory has developed considerably and yet many of Dalton’s ideas are still correct. Modern theory is built on an understanding of the particles that make up atoms – the so-called sub-atomic particles.

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Nucleus this is very small, it contains positively charged particles called protons and particles with no charge at all called neutrons

Electrons are negatively charged particles that form a series of 'shells' around the nucleus

Δ Fig. 2.2 Structure of an atom.

Proton number and mass number In order to describe the numbers of protons, neutrons and electrons in an atom, scientists use two numbers.

These are called the proton number (or atomic number) and the mass number (or nucleon number).

The proton number, as you might expect, describes the number of protons in the nucleus of an atom. The mass number describes the number of particles in the nucleus of the atom – that is, the total number of protons and neutrons.

Proton numbers are used to arrange the elements in the Periodic Table. The atomic structures of the first ten elements in the Periodic Table are shown in Table 2.2.

Hydrogen is the only atom that has no neutrons.

Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon

Proton number 1 2 3 4 5 6 7 8 9 10

Mass number 1 4 7 9 11 12 14 16 19 20

Number of protons 1 2 3 4 5 6 7 8 9 10

7 3

symbol for the element

Li

PROTON NUMBER (the number of protons, which equals the number of electrons)

Δ Fig. 2.3 The chemical symbol for lithium showing the mass number and proton number.

Number of neutrons 0 2 4 5 6 6 7 8 10 10

Number of electrons 1 2 3 4 5 6 7 8 9 10

Δ Table 2.2 Atomic structures of the first ten elements.

A Periodic Table of elements is shown on page 318 of this book and you will recognise the names of some of the more common elements that you know about, for example, carbon (C), oxygen (O), aluminium (Al) and iron (Fe).

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ATOMS, ELEMENTS AND COMPOUNDS

Element

MASS NUMBER (the number of protons + neutrons)

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QUESTIONS 1. Which sub-atomic particle has the smallest relative mass?

2. Why do atoms have the same number of protons as electrons?

3. An aluminium atom can be represented as 27 13 Al. a) What is aluminium’s mass number? b) Calculate how many neutrons this atom of aluminium has.

ISOTOPES Atoms of the same element with the same number of protons and electrons but different numbers of neutrons are called isotopes. For example, there are two isotopes of chlorine: Symbol

Number of protons

Number of neutrons

35 17

Cl

17

18

37 17

Cl

17

20

Δ Table 2.3 Isotopes of chlorine.

SUPPLEMENT

The chemical properties of an element depend on the number of electrons in the outer electron shell. As isotopes of the same element have the same electron configuration they have the same chemical properties.

● SCIENCE IN CONTEXT

ISOTOPES

Isotopes that do not decay and do not emit radiation are classed as non-radioactive isotopes.

Δ Fig. 2.4 A gamma camera is an example of radioactive isotopes used in medicine.

Radioactive isotopes emit radioactivity of one of the following types:

ISOTOPES

Some isotopes have different physical properties. Some isotopes are radioactive.

They emit radioactivity from the nucleus and decay – that is, if the radioactivity is alpha particles or beta particles, they change into other atoms with different numbers of protons and/or neutrons.

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Alpha particles (α) are helium nuclei; they contain two protons and two neutrons.

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Beta particles (β) are fast-moving electrons.

Gamma rays (γ) are high-energy electromagnetic rays.

In medicine, radioactive isotopes are used to sterilise equipment and in radiotherapy to treat cancer tumours.

Industry uses radioactive isotopes as ‘tracers’ to detect leaks in pipes.

Challenge Question: Suggest how you could use a radioactive tracer to detect a leak in a sewage pipe.

● SCIENCE IN CONTEXT

SUB-ATOMIC PARTICLES

Δ Fig. 2.5 The Large Hadron Collider at CERN in Switzerland.

To study the smallest known particles, a particle accelerator has been built underground at CERN near Geneva, Switzerland. This giant instrument, called the Large Hadron Collider (LHC), has a circumference of 27 km. It attempts to recreate the conditions that existed just after the ‘Big Bang’ by colliding beams of particles at very high speed – only about 5 m / s slower than the speed of light. It promises to revolutionise scientific understanding of the nature of atoms. Who knows – school science in 10 or 20 years’ time may be very different from your lessons today!

Challenge Question: Describe what you can find out about the ‘Big Bang’ theory.

QUESTIONS 1. What are isotopes?

2. SUPPLEMENT Explain why different isotopes of the same element have the same chemical properties.

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ATOMS, ELEMENTS AND COMPOUNDS

Protons, neutrons and electrons are the particles from which atoms are made.

However, in the past 20 years or so scientists have discovered a number of other sub-atomic particles: quarks, leptons, muons, neutrinos, bosons and gluons. The properties of some of these particles have become well known, but there is still much to learn about the others. Finding out about these, and possibly other sub-atomic particles, is one of the challenges of the 21st century.

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ELECTRONIC CONFIGURATION Arrangements of electrons in the atom An atom’s electrons are arranged in shells around the nucleus. These do not all contain the same number of electrons – the shell nearest to the nucleus can take only two electrons, whereas the next one out from the nucleus can take eight.

Electron shell 1 2 3

Maximum number of electrons 2 8 8

Δ Table 2.4 Maximum number of electrons in a shell.

Oxygen has a proton number of 8, so it has 8 electrons. Of these, two are in the first shell and six are in the second shell. This arrangement is written 2, 6. A phosphorus atom with a proton number of 15 has 15 electrons, arranged 2, 8, 5. It is the electrons in the outer electron shell that are involved in chemical bonding, which you will learn more about in the next two topics.

Electronic configuration: The first 20 elements of the Periodic Table There are over 110 different elements. They are arranged in the Periodic Table in order of increasing proton number. They are then arranged in periods and groups according to their chemical and physical properties.

The chemical properties of elements depend on the arrangement of electrons in their atoms. The electronic configuration of the first 20 elements is shown in Table 2.5.

Symbol

Hydrogen Helium Lithium

H He Li

Proton number 1 2 3

Electron number 1 2 3

Electronic configuration 1 2 2, 1

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Element

ELECTRONIC CONFIGURATION

Atom diagrams The atomic structure of an atom can be shown simply in a 6p 16p diagram. The arrangement of 6n 16n electrons in shells around the nucleus of an atom can be shown simply in a diagram, using a dot for each electron.

12 32 The arrangement of electrons in 6 C 16 S an atom is called its electronic configuration. This can also be Δ Fig. 2.6 Atom diagrams for carbon and sulfur showing the numbers of protons and neutrons and the electron arrangements. written as a sequence of numbers showing the number of electrons in each shell. For example, the electronic configuration of carbon is 2, 4 and the electronic configuration of sulfur is 2, 8, 6.

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