Physics With Kate
Empowering Girls in Physics
Spec-aligned revision resources, group courses, and 1:1 tutoring for GCSE and A Level Physics — built by an experienced teacher and examiner.
AQA GCSE Physics · Topic 4.4 Atomic structure
Edexcel International GCSE Physics · Section 7 Radioactivity and particles
The Structure of an Atom, Isotopes and Ions
In this lesson you'll learn to: describe what an atom is made of and how big it is, work out
protons, neutrons and electrons from the mass number and atomic number, explain
why an atom is neutral but an ion is not, say what makes two atoms isotopes, and
describe how electrons move between energy levels by absorbing or emitting
electromagnetic radiation.
In this lesson you'll learn to: describe what an atom is made of, write and read symbols like
¹⁴₆C, work out protons, neutrons and electrons from the mass (nucleon) number and
atomic (proton) number, explain why an atom is neutral but an ion is not, and say
what makes two atoms isotopes.
▶
Prefer to watch? Scroll to the bottom for the video on this topic
▶
A video walkthrough is coming soon — it will appear at the bottom of this page
▶
Start here — have a play
Don't read anything yet. On tab 1, build carbon-12: pick up a particle, then click the nucleus or a shell to put it there. Watch the symbol build itself as you go, then switch to carbon-14 and see what changes. On tab 2, fire a photon at the electron. On tab 3, find out how much every atom diagram you have ever seen is lying to you.
Build
Pick up
What you are building
Shells are drawn evenly spaced so they fit on screen. Real energy levels get closer together further out.
▲ Protons and neutrons only go in the nucleus, electrons only on the shells — the animation won't let you put them anywhere else.
Built one? Now learn to describe it.
The exam asks you to describe the structure of an atom in words, work out particle numbers from the two numbers in the symbol, and explain why two isotopes are still the same element. The notes below do all three.
✎
Revision notes
What an atom is made of
Every atom has a tiny central nucleus containing protons and neutrons, with electrons moving around it in shells (energy levels).
⭐ Key fact — charges and masses
| Particle | Relative charge | Relative mass | Where it is |
|---|---|---|---|
| Proton | +1 | 1 | In the nucleus |
| Neutron | 0 | 1 | In the nucleus |
| Electron | −1 | very small (about 1/2000) | In shells around the nucleus |
The nucleus is positive, because it holds the protons and the neutrons are neutral.
Why an atom has no overall charge
An atom has the same number of protons as electrons. Each proton is +1 and each electron is −1, so the charges cancel and the atom is neutral overall.
How big is an atom?
- The radius of an atom is about 1 × 10⁻¹⁰ m.
- The radius of the nucleus is less than 1/10 000 of the radius of the atom — about 1 × 10⁻¹⁴ m.
- Almost all of the mass of the atom is in the nucleus.
💡 Exam tip
If a question asks you to compare the nucleus with the atom, quote the numbers: atom ≈ 1 × 10⁻¹⁰ m, nucleus ≈ 1 × 10⁻¹⁴ m, so the nucleus is about 10 000 times smaller. Tab 3 of the animation shows what that really looks like.
The two numbers in the symbol
Every element is written with two numbers. The bottom one is small and the top one is big — that is how you remember which is which.
| Name | Where | What it tells you |
|---|---|---|
| Atomic number | bottom | The number of protons. This is what defines the element — i.e. 6 protons, the element is always carbon. |
| Mass number | top | The number of protons + neutrons. |
So: neutrons = mass number − atomic number, and in a neutral atom electrons = protons = atomic number.
EdexcelEdexcel also uses the other names: atomic number = proton number, and mass number = nucleon number (a nucleon is any particle in the nucleus: a proton or a neutron). You must recognise both names in a question.
Isotopes
⭐ Definition
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.
Carbon always has 6 protons — that is what makes it carbon. Its isotopes differ only in neutrons:
| Isotope | Protons | Neutrons | Electrons |
|---|---|---|---|
| Carbon-12 | 6 | 6 | 6 |
| Carbon-13 | 6 | 7 | 6 |
| Carbon-14 | 6 | 8 | 6 |
Because they all have 6 protons, they are all carbon, and they all have 6 electrons, so they are all neutral.
Ions
An ion is formed when an atom loses or gains an electron. The number of protons never changes — if it did, it would be a different element.
In GCSE physics we only consider positive ions, where an atom has lost an electron.
- Loses an electron → more protons than electrons → a positive ion.
AQA onlyElectrons and energy levels
Edexcel students: this part is not on your specification, so you can skip to the exam questions below. It does help to explain why radiation is called “ionising”.
Electrons sit at different distances from the nucleus. Each distance is an energy level, and the ones further out are higher energy levels. Electrons can change level, and this is where the exam wants exact wording:
⭐ The two sentences to learn
An electron that absorbs electromagnetic radiation moves to a higher energy level, further from the nucleus.
An electron that falls to a lower energy level, closer to the nucleus, emits electromagnetic radiation.
If an electron absorbs enough energy it can leave the atom altogether. The atom has lost a negative charge, so it becomes a positive ion — that is what we mean by ionising radiation. Try the high-energy photon on tab 2.
✍
How it comes up in the exam
Nearly every question on this spec point is one of two jobs: read the numbers off a symbol, or describe a structure in words. Here is one of each.
In either case, say it explicitly: the number of protons defines the element. Examiners want that sentence written down, not implied.
Worked example · lithium-7
Lithium-7 is written with mass number 7 and atomic number 3. Give the number of protons, neutrons and electrons.
- Protons = 3 — the atomic number, the bottom number.
- Neutrons = 4 — mass number − atomic number = 7 − 3.
- Electrons = 3 — the same as the protons, because the atom is neutral.
Check it: build lithium-7 on tab 1 of the animation and the symbol fills itself in as you go.
Exam-style question · 5 marks
Carbon-12 and carbon-14 are both isotopes of carbon. Describe the structure of a carbon-14 atom, and explain why carbon-12 and carbon-14 are both still carbon.
Mark scheme:
- The nucleus contains protons and neutrons; the electrons orbit around it in shells / energy levels (1)
- Carbon-14 has 6 protons — the atomic number (1)
- and 8 neutrons (14 − 6) (1)
- and 6 electrons, equal to the number of protons, so the atom is neutral overall (1)
- Both isotopes have the same number of protons, and the number of protons is what defines the element — so both are carbon. They differ only in the number of neutrons (1)
Where marks go missing: forgetting to say the atom is neutral because the number of electrons equals the number of protons — and say why that works: electrons are negatively charged and protons are positively charged, so the charges cancel.
📘 Now do it in your workbook
Label the three carbon isotopes, fill in the lithium-7 table, draw your own atom with radiation being absorbed or emitted, then write the 5-mark carbon-14 answer and mark it yourself.
Label the three carbon isotopes and write their symbols, draw any atom you like and fill in its numbers, then write the 5-mark carbon-14 answer and mark it yourself.
AQA GCSE Physics Workbook · ATOMIC STRUCTURE · page 3 Edexcel iGCSE Physics Workbook · RADIOACTIVITY AND PARTICLES · page 4
⬇ Free: page 3 (PDF)
📄 Full workbook PDF · £2.99
Paperback on Amazon
⬇ Free: page 4 (PDF)
📄 Full workbook PDF · £2.99
Paperback on Amazon
The full workbook covers the whole of Topic 4 Atomic Structure, with exam-style questions and worked answers.
The full workbook covers the whole of Section 7 Radioactivity and Particles, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
▶
Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
🎬
Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Page 3 of the workbook has the three carbon isotopes to label, the lithium-7 table to fill in, and the 5-mark carbon-14 question with its mark scheme.
Page 4 of the workbook has the three carbon isotopes to label, an atom of your own to draw, and the 5-mark carbon-14 question. It's free below.
Proud member · Accredited · Trusted partners
© 2026 Physics With Kate · All rights reserved