What is What are isotopes??

Atomic Structure — GCSE Physics Revision

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The basics

Mostly Empty Space

⚛️ Mostly Empty Space

Here's a mind-bending fact: atoms are 99.9999999999999% empty space! If a nucleus were the size of a marble in the centre of a football stadium, the electrons would be tiny specks orbiting in the outer stands. All the "solid" matter around you is almost entirely nothing. You can't fall through your chair because of electromagnetic repulsion between electrons, not because atoms are solid!

What are isotopes?: Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. This means they have the same atomic number but different mass numbers.

Hotspot diagram

The nuclear model of the atom — click each part to explore its properties

Interactive atomic structure diagram. Click each part to explore protons, neutrons, electrons and electron shells.

1
Proton —
2
Neutron —
3
Electron —
4
Electron shell —

Figure 1: The nuclear model of the atom — click each part to explore its properties

Key terms

Physics glossary

What are isotopes?: Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. This means they have the same atomic number but different mass numbers.

Spotlight

Inside the Atom

An atom is made of three types of subatomic particles: protons, neutrons, and electrons. Protons and neutrons cluster together in a tiny, dense nucleus at the centre. Electrons orbit the nucleus in shells (energy levels) at relatively vast distances.

Exam tip

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🧠 Memory Aids

Particles and charges: "ProTons are PosiTive" — both have the letter T. Neutrons are Neutral (no charge). Electrons are negative (they're on the outside, "ex" means out, and they're negative).

Atomic notation: "Big number on top = Mass (more letters = top), Small number below = atomic (Z = proton count)." Think: Z is at the bottom of the alphabet, and the atomic number is at the bottom of the symbol.

Neutrons formula: N = A − Z. Think "Neutrons Are (A) Zapped (minus Z)."

Now try it yourself

Quiz · Question 1 of 16

What does the atomic number of an element tell you?

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This topic in real past papers

Every real exam question we've found on atomic structure, with a full worked answer.

Edexcel Paper 1

Estimating atom counts, reading a scattering graph, and linking results to the nuclear model

11 marks

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Radioactive Decay