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Learn0625Papers 2, 4 · ExtendedNuclear physicsThe nuclear atom
Nuclear physics · Subtopic 1 of 4

The Nuclear Atom

An atom is mostly empty space: a tiny, dense, positive nucleus of protons and neutrons, with electrons around it. The proton number ZZ says which element it is; the nucleon number AA counts protons plus neutrons. Isotopes have the same ZZ but different numbers of neutrons. Rutherford's alpha-scattering experiment revealed the nucleus.

Cambridge 5054 Paper 1Cambridge 5054 Paper 2Cambridge 0625 Papers 2, 4Cambridge 0625 Papers 1, 3Edexcel 4PH1 Paper 1PEdexcel 4PH1 Paper 2P≈ 25 minRevision 5 · updated 2026-10-27
Section 1 of 14

Learning objectives

  • Describe the structure of an atom and the charges of its particles.
  • Use nuclide notation ZAX^A_Z\text{X} to find numbers of protons, neutrons and electrons.
  • Explain what isotopes are, and how atoms become ions.
  • Describe the alpha-scattering experiment and its conclusions.
Pause and think

If an atom were blown up to the size of a football stadium, how big would its nucleus be?

Section 2 of 14

Core idea

Start hereA fly in a cathedral

If an atom were the size of a cathedral, its nucleus would be a fly in the middle, yet it holds almost all the mass. Everything else is empty space with a few electrons. Rutherford discovered this by firing particles at gold foil.

The big idea

Tiny dense positive nucleus (protons + neutrons), electrons around. ZAX^A_ZX: AA nucleons, ZZ protons; neutrons =A−Z=A-Z. Isotopes: same ZZ, different AA.

1. Structure

A carbon-12 atom (not to scale)
●●●●●●6 protons (+)6 neutrons (0)6 electrons (−)
Written   612C^{12}_{\;6}\text{C}. The real nucleus is about 100 000 times smaller than the atom: mostly empty space.
ZAX:neutrons=A−Z^A_Z\text{X}:\quad\text{neutrons}=A-Z
where
AA
nucleon (mass) number.
ZZ
proton (atomic) number.
Worked exampleCarbon isotopes

 612C^{12}_{\ 6}\text{C} and  614C^{14}_{\ 6}\text{C}.

Goal
Particles in each.
Strategy
ZZ protons, A−ZA-Z neutrons.
  1. C-12: 6p, 6n, 6e\text{C-12: }6p,\ 6n,\ 6e
    12−6=612-6=6.
  2. C-14: 6p, 8n, 6e\text{C-14: }6p,\ 8n,\ 6e
    14−6=814-6=8.
AnswerSame element, different neutrons: isotopes.
✓ CheckSame ZZ means same chemistry ✓.

2. Evidence: alpha scattering

Alpha particles past a gold nucleus
bounces back
Far away: undeflected. Closer: bent. Head-on: turned back.
  • Most alphas pass straight through: atom mostly empty.
  • A few deflected strongly: small, concentrated, positive nucleus.
Common misconceptionNucleon number = neutrons AA counts protons and neutrons.
Pause and think

Why are atoms neutral?

Summary

  • AA = protons + neutrons; ZZ = protons.
  • Rutherford: small dense nucleus.
Key words
Isotope
Same protons, different neutrons.
History"Like firing a shell at tissue paper." When alpha particles bounced back off gold foil in 19091909, Ernest Rutherford said it was "as if you fired a 1515-inch shell at a piece of tissue paper and it came back and hit you." The old "plum pudding" model, with positive charge spread out, couldn't explain it. In 19111911 he proposed the nucleus.
Fun factYou are mostly nothing. If all the empty space were squeezed out of every atom in every human on Earth, all 88 billion people would fit into a volume about the size of a sugar cube. It would weigh about 500500 million tonnes.
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