Key Stage 4 science · Physics – Atomic structure

GCSE atoms: the nuclear model, isotopes and ionisation

The atom students picture today was not invented in one go: it is the result of several models, each replaced when new evidence did not fit. At GCSE, students trace how the nuclear model developed, compare the sizes and masses of nuclei, atoms and small molecules, work out the numbers of protons, neutrons and electrons from the atomic number and mass number, understand isotopes, and explain ionisation in terms of electrons moving between energy levels or leaving an atom.

The nuclear model and isotopes, explained

Students learn the history of the atom as an example of how science works. In the early 1800s atoms were pictured as tiny solid spheres. After J. J. Thomson discovered the electron in 1897, the atom was pictured as a ball of positive charge with electrons embedded in it, the plum pudding model. Around 1909 Geiger and Marsden, working in Rutherford’s laboratory, fired alpha particles at thin gold foil and found that most went straight through, a few were deflected, and a very few bounced almost straight back. That could not happen with a diffuse positive ball. In 1911 Rutherford explained it with the nuclear model: the positive charge and nearly all the mass are in a tiny dense nucleus, with the electrons far outside it. Bohr then proposed that electrons orbit at fixed distances, or energy levels (1913), and James Chadwick’s discovery of the neutron in 1932 completed the picture.

The scale matters. An atom has a radius of roughly 1 × 10⁻¹⁰ metres, while the nucleus is about 10 000 times smaller, around 1 × 10⁻¹⁴ metres. Almost all of an atom is empty space, yet almost all its mass is in the nucleus, because a proton and a neutron each have a relative mass of 1 and an electron only about one two-thousandth of that. In kilograms the numbers are tiny: a hydrogen atom has a mass of about 1.7 × 10⁻²⁷ kg, and even the heaviest atoms are only a few hundred times that. Small molecules, such as water or carbon dioxide, are a few atoms joined together and are about a nanometre across or less, which is 1 × 10⁻⁹ metres. Students are expected to be comfortable comparing sizes in standard form.

A nucleus is made of protons (relative charge +1) and neutrons (charge 0). The atomic number is the number of protons, and it decides which element it is. The mass number is the total number of protons and neutrons. An atom is neutral, so the number of electrons equals the number of protons. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, such as carbon-12 and carbon-14. They have the same chemistry but different masses, and some isotopes have an unstable nucleus that decays and emits radiation, shown in equations that keep the mass numbers and the atomic numbers balanced.

Electrons occupy energy levels. If an electron absorbs electromagnetic radiation of the right frequency it can move to a higher energy level, further from the nucleus. When it falls back down, it emits electromagnetic radiation. If an atom absorbs enough energy, an outer electron can leave the atom altogether. The atom has then been ionised and becomes a positively charged ion. Ionisation matters because radiation that ionises can damage living cells, which students go on to study in radioactivity.

Words your child will hear

Nucleus: The tiny, dense, positively charged centre of an atom, made of protons and neutrons.
Atomic number: The number of protons in an atom. It identifies the element.
Mass number: The total number of protons and neutrons in a nucleus.
Isotope: An atom of the same element with the same number of protons but a different number of neutrons.
Ion: An atom that has lost or gained electrons, so it carries an overall charge.
Energy level: A fixed orbit, at a set distance from the nucleus, that an electron can occupy.
Ionisation: Turning an atom into an ion. Ionising radiation does this by knocking out electrons, leaving a positive ion.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about the nuclear model and its development in the light of changing evidence
Students should be taught about masses and sizes of nuclei, atoms and small molecules
Students should be taught about differences in numbers of protons and neutrons related to masses and identities of nuclei, isotope characteristics and equations to represent changes
Students should be taught about ionisation; absorption or emission of radiation related to changes in electron orbits

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

A worked example

Carbon has atomic number 6. (a) State the numbers of protons, neutrons and electrons in a neutral atom of carbon-12 and of carbon-14. (b) Explain why they are isotopes. (c) What happens to a carbon-12 atom if it loses one electron?

Protons: the atomic number is 6, so both atoms have 6 protons. A neutral atom has the same number of electrons, so each has 6 electrons.
```
protons   = 6
electrons = 6
```
Neutrons: take the atomic number from the mass number.
```
carbon-12: 12 − 6 = 6
carbon-14: 14 − 6 = 8
```
Both atoms have 6 protons, so they are the same element, but they differ in neutrons (6 and 8). Atoms with the same number of protons and different numbers of neutrons are isotopes.
If a carbon-12 atom loses one electron, the nucleus is unchanged. There are still 6 protons (+6) but now only 5 electrons (−5), so the atom becomes a positive ion.
```
charge = +6 − 5
       = +1
```

Carbon-12 has 6 protons, 6 neutrons and 6 electrons; carbon-14 has 6 protons, 8 neutrons and 6 electrons. They are isotopes because only the neutron number differs. Losing an electron makes a carbon ion with a charge of +1.

Common mistakes and how to help

Isotopes have different numbers of protons

“Different version of an atom” sounds like something has changed in the nucleus, and mass number and proton number are easily mixed up.

What helps: The number of protons defines the element, so changing it would make a different element. Isotopes keep the proton number and vary only in neutrons. A table of protons, neutrons and electrons for each atom makes the pattern clear.

The mass number is the number of protons

Both are whole numbers written next to the symbol, and the bigger one is easy to treat as the one that counts.

What helps: The smaller number is the atomic number (protons). The larger is the mass number (protons plus neutrons). Practise subtracting to find the neutrons every time.

The plum pudding model was just wrong, so older models do not matter

Students see a list of models and assume each was a mistake, rather than the best explanation of the evidence at the time.

What helps: A model is replaced when new evidence does not fit it. The plum pudding model explained the electron; the alpha scattering results it could not. Ask: “what observation forced the change?”

Atoms are solid, or the electrons are the big part of the atom

Pictures of atoms show a nucleus and electrons drawn close, and everything around us feels solid.

What helps: If the nucleus were the size of a pea, the outer electrons would be tens of metres away, with empty space between. It is the electric forces from the electrons that make matter feel solid.

How to help at home

Build a scale picture. If the nucleus were a pea, roughly where would the nearest electron be? Work out together that the atom is about 10 000 times wider, so a pea a centimetre across means the outer electrons are roughly 50 metres away. Walk it out in a park if you have the space.
Model Rutherford’s experiment. Put a small hidden object, such as a tin lid or a block, under a large piece of card propped up on books so you can’t see it. Roll marbles under the card from different sides and watch where they come out. Most go straight through and a few bounce off, which lets your teenager work out the hidden shape, just as the scattering of alpha particles revealed the nucleus.
Practise nucleon counting. Choose elements from a periodic table, such as chlorine-35 and chlorine-37 (atomic number 17), and ask for the protons, neutrons and electrons of each. Do five or six; the pattern becomes automatic.
Ask the “why did the model change?” question. Pick any model from the history and ask your teenager what evidence supported it and what evidence made it change. A short, clear answer to this is what examiners reward in the “changing evidence” questions.