Worked example 1
a Draw diagrams to show how substances change from solids to liquids.
The particles move faster when they are heated and break away from the solid structure to move more freely.
solid
liquid
b Describe what kind of process is happening during this change and explain whether this is a physical change or a chemical change.
When a solid turns into a liquid it is called ‘melting’. This is a physical change because the particles don’t join together. 2
Describe the structure of an atom that has atomic number 3 and mass number 7.
This atom has 3 protons and 3 electrons around them and 1 neutron.
3
a Describe what is meant by ‘isotopes’ and draw an isotope of hydrogen that has 2 neutrons.
Isotopes are atoms that have the same number of protons but different numbers of neutrons. This means that their atomic number is the same but their mass numbers are different.
This answer shows both diagrams and has an added explanation.
The process is correct and an explanation is given of why it is not a chemical reaction. The student should add that no new substance is made and that the substance will go back to the same solid if cooled. The numbers of protons and electrons are correct. The number of protons is equal to the number of electrons and is the atomic number of an element. The number of neutrons is incorrect and should be 7. The mass number is not equal to the sum of the numbers of protons, electrons and neutrons – it is equal to the sum of the numbers of just the protons and neutrons. The description is correct and the diagram is correct.
b Describe the charges and masses of the subatomic particles you have drawn and explain how they relate to the atomic number and mass number.
The red dot is a proton that has a positive charge and a mass of 1. The blue dot is an electron that has no mass and a negative charge. The green dots are neutrons with no charge and a mass of 1. The atomic number is 1 and the mass number is 4.
The three descriptions of the charge and mass of the proton, neutrons and electron are correct. The atomic number is correct. However, even though the description of the mass of the electron is correctly given as 0, this has been included in the mass number (4). There are 2 neutrons and 1 proton so the mass number is 3.
Worked example
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8/10/16 1:50 PM