Key Stage 4 science · Chemistry – Atomic structure and the Periodic Table

GCSE atomic structure, isotopes and moles: a parent’s guide

GCSE chemistry begins with the atom. Students learn the model of a nucleus surrounded by electrons, how to work out the particles in any atom or isotope, how relative atomic mass is calculated, and how chemists count enormous numbers of particles by weighing a sample. These ideas underpin nearly everything else in the course, from the Periodic Table to chemical equations.

Atomic structure, isotopes and moles, explained

In the model taught at GCSE, an atom has a tiny central nucleus containing protons and neutrons, with electrons around it in energy levels, also called shells. Protons have a relative charge of +1, electrons have −1 and neutrons have no charge, so an atom has no overall charge because the numbers of protons and electrons are equal. Protons and neutrons each have a relative mass of 1, while the mass of an electron is so small that it is counted as almost nothing. Most of the atom is empty space: the nucleus is far smaller than the whole atom, but nearly all the mass is in it. Many courses show how Rutherford’s team fired positively charged alpha particles at thin gold foil and found that most passed straight through but a few bounced back, which showed there must be a tiny, dense, positive nucleus.

Two numbers describe an atom. The atomic number is the number of protons, which decides which element it is. The mass number is the total number of protons and neutrons. For carbon-12, the atomic number is 6 and the mass number is 12, so it has 6 neutrons. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, so they have different mass numbers. Carbon-12 and carbon-14 are both carbon, but carbon-14 has two more neutrons. Because isotopes have the same number of electrons, they react in the same chemical ways.

Real samples contain a mixture of isotopes, so the relative atomic mass of an element is an average, weighted by how common each isotope is. That is why chlorine’s relative atomic mass is about 35.5, even though no chlorine atom has a mass number of 35.5. Students learn to calculate it from the percentage abundance of each isotope, and they are expected to say what the number means as well as how to find it.

The second objective is about counting particles in a given mass. Atoms are far too small to count one at a time, so chemists use the mole, a fixed number of particles, 6.02 × 10²³, which is called the Avogadro constant. One mole of any substance has a mass in grams equal to its relative formula mass. For instance, one mole of carbon-12 atoms weighs 12 g, and one mole of water molecules weighs 18 g. Students use the relationship that the number of moles equals mass divided by relative formula mass, and then multiply by the Avogadro constant to find the number of particles. How far this goes depends on the course and the tier: many students go on to use moles for reacting masses and concentrations, while others stay with simpler calculations.

Words your child will hear

Atomic number: The number of protons in an atom. It tells you which element it is.
Mass number: The total number of protons and neutrons in the nucleus of an atom.
Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons.
Relative atomic mass: The average mass of the atoms of an element, weighted by how common each isotope is, on a scale where a carbon-12 atom has a mass of exactly 12.
Electron shell: An energy level around the nucleus that can hold a limited number of electrons.
Mole: The amount of substance that contains 6.02 × 10²³ particles, the Avogadro constant.
Relative formula mass: The sum of the relative atomic masses of all the atoms in a formula, such as 18 for H₂O.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about a simple model of the atom consisting of the nucleus and electrons, relative atomic mass, electronic charge and isotopes
Students should be taught about the number of particles in a given mass of a substance

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

(a) Chlorine has two main isotopes: 75% of its atoms are chlorine-35 and 25% are chlorine-37. Calculate the relative atomic mass of chlorine. (b) How many water molecules are in 36 g of water, H₂O? (Relative atomic masses: H = 1, O = 16. Avogadro constant = 6.02 × 10²³ per mole.)

Part (a). Multiply each mass number by its percentage abundance, then add the results.
```
35 × 75 = 2,625
37 × 25 =   925
total  = 3,550
```
Divide the total by 100, because the percentages are out of 100. The answer sits between 35 and 37, and closer to 35 because that isotope is more common, which is a useful check.
```
3,550 ÷ 100 = 35.5
```
Part (b). Work out the relative formula mass of water: two hydrogen atoms and one oxygen atom.
```
(2 × 1) + 16 = 18
```
Find the number of moles: mass divided by relative formula mass.
```
moles = 36 ÷ 18 = 2 mol
```
Multiply the number of moles by the Avogadro constant to find the number of molecules.
```
2 × 6.02 × 10²³ = 12.04 × 10²³
= 1.204 × 10²⁴ molecules
```

(a) The relative atomic mass of chlorine is 35.5. (b) 36 g of water contains 2 moles, which is 1.204 × 10²⁴ (about 1.2 × 10²⁴) molecules.

Common mistakes and how to help

Thinking that isotopes are different elements

Isotopes have different masses and names such as carbon-12 and carbon-14, so they sound like different substances. But the number of protons is what defines an element, and isotopes have the same number.

What helps: Ask what has changed and what has stayed the same. The neutrons differ, the protons and electrons do not, so the chemical behaviour is the same.

Confusing mass number with relative atomic mass

Both are numbers close to the mass of an atom, and the Periodic Table usually shows the relative atomic mass in the place where students expect a mass number. The mass number is a whole number for one particular isotope. The relative atomic mass is an average over a mixture and often is not a whole number.

What helps: Check that your child can explain why chlorine is 35.5 even though no chlorine atom has 35.5 particles in its nucleus. The weighted average idea is the key.

Picturing the atom as a solid ball, or the nucleus as a large part of it

Textbook diagrams have to draw the nucleus big enough to see. In fact the radius of the nucleus is less than one ten-thousandth of the radius of the atom, so the atom is mostly empty space, even though nearly all its mass is in the nucleus.

What helps: Talk about what a model is for. The shell diagram is good for showing energy levels and predicting bonding, but it is not to scale. Ask what the model gets right and what it hides.

Treating a mole as a mass, or thinking one mole of different substances weighs the same

The mole is introduced alongside grams and relative masses, so students blur the units. A mole is a number of particles. One mole of water weighs 18 g, but one mole of carbon weighs 12 g, because the particles have different masses.

What helps: Use the dozen as an analogy. A dozen eggs and a dozen melons are both 12 items but weigh very different amounts. Ask what a mole of marbles would mean.

How to help at home

Build an atom with household items. Use different coloured beads, buttons or sweets for protons, neutrons and electrons, and make a few atoms from a given element. Then change the neutron count to make an isotope. It shows what stays the same and what changes.
Practise weighted averages. Find the average mark of a pretend test where 75% of students scored 35 and 25% scored 37, then compare it with the chlorine calculation. The method is identical, which helps students see that it is ordinary maths in a chemistry context.
Explain the size of a mole. 6.02 × 10²³ is hard to imagine. Ask your child to write it out in full, then to estimate how many grains of rice would fill a room. Even that huge number is tiny compared with a mole, yet a mole of water is only 18 g, a little more than a tablespoonful. The aim is to feel how small atoms are, not to get a precise answer.
Check standard form together. Chemistry uses standard form constantly. Practise converting between 12.04 × 10²³ and 1.204 × 10²⁴, and between large and small numbers, without a calculator. Slips here often cost marks that chemistry knowledge would have won.

Builds on

Key Stage 3: The Periodic Table