This mini-lesson walks you through the whole of Edexcel Topic 6 — Radioactivity: the structure of the atom, how our model of the atom was built, the types of radiation, nuclear equations, activity and half-life, the difference between irradiation and contamination, and finally fission and fusion.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
An atom is a tiny positive nucleus of protons and neutrons, surrounded by negative electrons orbiting at fixed distances. Almost all the mass sits in the nucleus, yet the nucleus is tiny compared with the whole atom.
Proton: charge +1, relative mass 1
Neutron: charge 0, relative mass 1
Electron: charge −1, relative mass ≈ 1/1835 (about 0.0005)
Sense of scale: the nuclear radius (~10⁻¹⁴ m) is about 10 000 times smaller than the atomic radius (~10⁻¹⁰ m). Atoms and small molecules are of the order of 10⁻¹⁰ m across.
Every nucleus is described by two numbers, written around its symbol:
Isotopes are atoms of the same element (same Z, same protons) with different numbers of neutrons — so they have a different mass number A. Example: carbon-12, carbon-13 and carbon-14 all have 6 protons.
Edexcel wants you to know how and why the atomic model was rebuilt as new evidence arrived:
Why it changed: each new model was proposed only because an experiment produced results the old model could not explain — this is how science works.
Geiger and Marsden (directed by Rutherford) fired alpha particles at a very thin gold foil and recorded where they went:
Most went straight through → the atom is mostly empty space.
Some were deflected → the centre is positively charged (it repels the positive alpha).
A very few bounced back > 90° → the mass and positive charge are concentrated in a tiny, dense nucleus.
This destroyed the plum-pudding model: if charge were spread out evenly, the alpha particles would never have bounced straight back.
An unstable nucleus becomes more stable by emitting ionising radiation. Edexcel lists five emissions:
The big trade-off: the radiation that ionises the most (alpha) penetrates the least, because it dumps all its energy quickly. Gamma ionises least but travels furthest.
Alpha: stopped by paper / a few cm of air. Range in air: a few cm.
Beta: stopped by a few mm of aluminium. Range in air: tens of cm.
Gamma: only reduced by thick lead or concrete — never fully stopped. Range: very large.
Tap the radiation that matches each clue.
In any nuclear equation, the mass numbers must balance and the atomic numbers must balance on both sides.
Beta-plus (β⁺): a proton becomes a neutron and a positron (⁰₊₁e) is emitted, so A stays the same and Z → Z − 1. Gamma emission carries away energy only — A and Z are unchanged.
Radioactive decay is a random process — you cannot predict which nucleus will decay next, or exactly when. But with huge numbers of nuclei the behaviour is very predictable.
Activity is the number of nuclear decays per second in a source. It is measured in becquerels (Bq), where 1 Bq = 1 decay per second.
Analogy for randomness: radioactive decay is like popcorn in a pan — you know roughly how many kernels will pop each second, but never which one pops next. Same as rolling thousands of dice and removing every six.
The half-life is the time taken for the number of undecayed nuclei (or the activity) to fall to half its starting value. It is constant for a given isotope.
A sample starts with an activity of 800 Bq. Its half-life is 6 hours.
After 6 h → 400 Bq; after 12 h → 200 Bq; after 18 h → 100 Bq (that's 3 half-lives).
Higher Tier: you may be asked for the net decline as a ratio — e.g. after 3 half-lives, 800 → 100 Bq, so the net decline is (800 − 100)/800 = 7/8.
Background radiation is the low-level radiation that is around us all the time. It comes from both natural and man-made sources. Most of it is natural — the largest single source in the UK is radon gas from rocks.
Why it matters: when measuring a source's activity you must subtract the background count rate first to get the true reading.
Tap a source, then tap the box it belongs in.
This is the most-tested misconception in Topic 6 — keep them separate:
Irradiation: an object is exposed to radiation from an outside source. The object does NOT become radioactive. It stops the moment the source is removed or shielded.
Contamination: radioactive atoms get onto or into an object (e.g. dust on skin, swallowed material). The object is now itself radioactive and keeps emitting until those atoms decay.
Protection: reduce exposure by shielding (lead/concrete), keeping your distance, and limiting the time near a source. Workers wear film badges and use tongs and lead aprons.
The radiation type is chosen to match the job, using penetration and half-life:
Dangers: ionising radiation can kill cells or damage DNA, causing mutations that may lead to cancer. Inside the body, alpha is the most dangerous (highly ionising, all energy absorbed locally); outside the body, alpha is least dangerous because skin or paper stops it.
(Uses and dangers are examined in full on the separate-science Physics paper, 1PH0.)
Fission is the splitting of a large, unstable nucleus (e.g. uranium-235) into two smaller "daughter" nuclei, after it absorbs a neutron. It releases energy, gamma rays and two or three more neutrons.
Control rods (boron) absorb excess neutrons to control the rate.
Moderator (water or graphite) slows neutrons so they can be absorbed and cause more fission.
(Fission, fusion and nuclear power are on the separate-science Physics paper, 1PH0.)
Fusion is the joining of two light nuclei (e.g. hydrogen isotopes) to form a heavier nucleus, releasing huge amounts of energy. Fusion is the energy source of stars, including the Sun.
Why fusion is so hard on Earth: both nuclei are positive, so they repel each other. Only at the extreme temperatures and pressures found in stars can they get close enough to fuse — which is why a practical fusion power station is so difficult to build.
Fission: a big nucleus splits into two smaller ones (used in today's nuclear power stations).
Fusion: two small nuclei join into a bigger one (powers the stars).
Both release energy, but in opposite directions on the chart of elements.
Memory hook: "FisSION = SPLIT in two (scission); fuSION = FUSE together." Fission starts with a neutron being absorbed; fusion needs star-like heat and pressure.
Atom: tiny positive nucleus (p + n), electrons in shells; nucleus ~10⁻¹⁴ m, atom ~10⁻¹⁰ m.
Numbers: Z = protons, A = protons + neutrons, neutrons = A − Z; isotopes share Z, differ in A.
Model: Dalton → Thomson (plum-pudding) → Rutherford (nuclear, alpha-scattering) → Bohr (shells) → Chadwick (neutron).
Radiation: α (most ionising, least penetrating) · β⁻ · β⁺ · γ (least ionising, most penetrating) · neutron.
Equations: α → A−4, Z−2 · β⁻ → A same, Z+1 · β⁺ → A same, Z−1 · γ → no change.
Activity: decays per second, in becquerels (Bq); GM tube detects; decay is random.
Half-life: time for the amount/activity to halve.
Background: mostly natural (radon, cosmic, food); some man-made (medical, fallout).
Safety: irradiation ≠ contamination; shield, distance, time.
Fission & fusion: splitting vs joining nuclei (Physics paper).
You've covered the whole of Edexcel Topic 6 — Radioactivity. Press Finish to see your score.
You've worked through Radioactivity for Edexcel GCSE Physics. 🎉
Your stars: 0 / 0
Next: test yourself in the Evaluate stage Confidence Quiz, then lock it in with Verify.