Overview
What you’ll learn
- Describe the nuclear atom and explain how alpha-scattering evidence supports it (Supplement).
- Use proton number Z, nucleon number A and nuclide notation, and explain isotopes.
- Recall the nature, ionising power and penetration of alpha, beta and gamma radiation.
- Balance decay equations for alpha, beta and gamma emission (Supplement).
- Define half-life and calculate remaining activity from tables or decay curves.
- Describe safe handling, storage and use of radioactive sources, and matching isotopes to applications.
Syllabus checklist (0625)
Statements are paraphrased and grouped by tier. Core is studied by everyone; Supplement is examined on the Extended papers only.
Core — every candidate
- 5.1.1Describe the structure of an atom in terms of a positively charged nucleus and negatively charged electrons in orbit around the nucleus.
- 5.1.1Know how atoms may form positive ions by losing electrons or form negative ions by gaining electrons.
- 5.1.2Describe the composition of the nucleus in terms of protons and neutrons.
- 5.1.2State the relative charges of protons, neutrons and electrons as +1, 0 and −1 respectively.
- 5.1.2Define the terms proton number (atomic number) Z and nucleon number (mass number) A, and be able to calculate the number of neutrons in a nucleus.
- 5.1.2Use the nuclide notation (A top-left, Z bottom-left beside the symbol).
- 5.1.2Explain what is meant by an isotope and state that an element may have more than one isotope.
- 5.2.1Know what is meant by background radiation.
- 5.2.1Know the sources that make a significant contribution to background radiation, including radon gas (in the air), rocks and buildings, food and drink, and cosmic rays.
- 5.2.1Know that ionising nuclear radiation can be measured using a detector connected to a counter.
- 5.2.1Use count rate measured in counts per second or counts per minute.
- 5.2.2Describe the emission of radiation from a nucleus as spontaneous and random in direction.
- 5.2.2Identify alpha, beta and gamma emissions from the nucleus by recalling their nature, their relative ionising effects and their relative penetrating abilities (β⁺ are not included; β-particles will be taken to refer to β⁻).
- 5.2.3Know that radioactive decay is a change in an unstable nucleus that can result in the emission of alpha-particles or beta-particles and/or gamma radiation, and that these changes are spontaneous and random.
- 5.2.3State that during alpha-decay or beta-decay, the nucleus changes to that of a different element.
- 5.2.4Define the half-life of a particular isotope as the time taken for half the nuclei of that isotope in any sample to decay; recall and use this definition in simple calculations, which might involve information in tables or decay curves (calculations will not include background radiation).
- 5.2.5State the effects of ionising nuclear radiations on living things, including cell death, mutations and cancer.
- 5.2.5Describe how radioactive materials are moved, used and stored in a safe way.
Supplement — Extended candidates only
- 5.1.1Describe how the scattering of alpha particles by a sheet of thin metal supports the nuclear model of the atom, by providing evidence for a very small nucleus surrounded by mostly empty space, a nucleus containing most of the mass of the atom, and a nucleus that is positively charged.
- 5.1.2Describe the processes of nuclear fission and nuclear fusion as the splitting or joining of nuclei, to include the nuclide equation and qualitative description of mass and energy changes without values.
- 5.1.2Know the relationship between the proton number and the relative charge on a nucleus.
- 5.1.2Know the relationship between the nucleon number and the relative mass of a nucleus.
- 5.2.1Use measurements of background radiation to determine a corrected count rate.
- 5.2.2Describe the deflection of alpha, beta and gamma radiation in electric fields and in magnetic fields.
- 5.2.2Explain their relative ionising effects with reference to their kinetic energy and their electric charge.
- 5.2.3Know that isotopes of an element may be radioactive due to an excess of neutrons in the nucleus and/or the nucleus being too heavy.
- 5.2.3Describe the effect of alpha-decay, beta-decay and gamma-emissions on the nucleus, including an increase in stability and a reduction in the number of excess neutrons; during beta-emission a neutron changes into a proton plus an electron.
- 5.2.3Use decay equations, using nuclide notation, to show the emission of alpha-particles, beta-particles and gamma radiation.
- 5.2.4Calculate half-life from data or decay curves from which background radiation has not been subtracted.
- 5.2.4Explain how the type of radiation emitted and the half-life of an isotope determine which isotope is used for applications, including household smoke alarms, irradiating food to kill bacteria, sterilisation of equipment using gamma rays, measuring and controlling thicknesses of materials, and diagnosis and treatment of cancer using gamma rays.
- 5.2.5Explain safety precautions for all ionising radiation in terms of reducing exposure time, increasing distance between source and living tissue, and using shielding to absorb radiation.
In 30 seconds
- 1The atom: a tiny, positively charged nucleus (protons +1, neutrons 0) with electrons (−1) in orbit; N = A − Z neutrons.
- 2Alpha scattering (Supplement): most alphas passed straight through (mostly empty space); a few bounced (tiny, massive, positive nucleus).
- 3Alpha: stopped by paper, strongly ionising. Beta: stopped by a few mm of aluminium, moderately ionising. Gamma: reduced by lead/concrete, weakly ionising.
- 4Decay is spontaneous and random; α and β decay change the element (Supplement: β turns a neutron into a proton + electron).
- 5Half-life: the time for half the nuclei of a sample to decay — halve the activity again and again.
- 6Safety = time, distance, shielding, plus careful storage and handling.
Notes · 1 of 5
The nuclear atom
A tiny heavy positive core, electrons outside, and the experiment that proved it.
An atom has a positively charged nucleus containing protons (relative charge +1) and neutrons (charge 0), with negatively charged electrons (−1) in orbit around it. Atoms become ions by losing electrons (positive ions) or gaining them (negative ions). The proton number (atomic number) Z counts the protons; the nucleon number (mass number) A counts protons plus neutrons; the number of neutrons is A − Z. Nuclide notation writes the element symbol with A top-left and Z bottom-left.
Supplement: the nucleus’s charge is exactly +Z (one unit per proton) and its mass is roughly A times a nucleon’s mass. Isotopes are atoms of the same element (same Z) with different numbers of neutrons (different A) — carbon-12 and carbon-14 are both carbon.
Supplement: nuclear fission is the splitting of a large unstable nucleus into two smaller nuclei (plus neutrons), releasing energy — the process in nuclear power reactors. Nuclear fusion is the joining of two light nuclei to form a heavier one, also releasing energy — the process in the Sun. In both, the total mass of the products is slightly less than before, and that missing mass appears as energy.
Notes · 2 of 5
Background radiation and detection
We live inside a faint radioactive drizzle — and detectors count it.
Background radiation is the low-level ionising radiation around us all the time. Its main sources: radon gas seeping from rocks into the air, rocks and buildings, food and drink, and cosmic rays arriving from space. A detector connected to a counter (a Geiger–Müller tube setup) records this as a count rate in counts per second or counts per minute.
Supplement: because background is always present, measurements of a source include it. Measure the background count rate first, then subtract it from every reading: the difference is the corrected count rate due to the source alone. Decay-curve readings taken with the background still included must be corrected the same way before a half-life is read off.
Notes · 3 of 5
Alpha, beta and gamma radiation
Three emissions, three characters — one table to memorise.
| Emission | Nature | Ionising power | Penetration |
|---|---|---|---|
| Alpha (α) | A helium nucleus: 2 protons + 2 neutrons (charge +2) | Very strong | Stopped by a few cm of air or a sheet of paper |
| Beta (β⁻) | A fast-moving electron from the nucleus (charge −1) | Moderate | Stopped by a few millimetres of aluminium |
| Gamma (γ) | A high-energy electromagnetic wave (no charge, no mass) | Weak | Reduced (not fully stopped) by several cm of lead or metres of concrete |
Emission from an unstable nucleus is spontaneous (nothing you do can trigger or delay it) and random in direction (each emission fires off in an unpredictable direction). Supplement: in electric or magnetic fields, charged emissions deflect — alpha one way, beta the other (beta curves more, being far lighter); gamma passes straight on undeflected. The ionising pattern follows charge and energy: alpha, being massive and doubly charged, rips electrons off atoms at a huge rate and loses energy fast (hence tiny range); beta is lighter and singly charged; gamma, with no charge, rarely interacts at all.
Notes · 4 of 5
Decay equations
The books must balance: top numbers and bottom numbers separately.
Radioactive decay is the change of an unstable nucleus into a more stable one, emitting α, β and/or γ. It is spontaneous and random — and during α or β decay the nucleus becomes a different element. Supplement: an isotope is likely to be unstable if it has too many neutrons or is simply too heavy. In any decay equation, the nucleon numbers add up on both sides, and so do the proton numbers.
Notes · 5 of 5
Half-life, uses and safety
The radioactive countdown, and how to live safely with sources.
The half-life of an isotope is the time taken for half the nuclei of that isotope in any sample to decay. Because decay is random, the count rate halves every half-life — 800, 400, 200, 100 counts/s after successive half-lives — whether you read it from a table or from a decay curve. Supplement: if a decay curve still includes the background, subtract the background count first, then read the half-life off the corrected curve.
Supplement — choosing isotopes: the radiation type and half-life decide the job. A household smoke alarm uses alpha: strongly ionising in a tiny gap, stopped instantly by the plastic casing, with a long half-life so it works for years. Food irradiation and gamma sterilisation of equipment use gamma: penetrating enough to kill bacteria throughout, and the source lasts. Thickness control in a factory uses beta (or gamma for metal foil): paper absorbs beta proportionally to its thickness, and a long-ish half-life keeps the calibration steady. Cancer diagnosis and treatment use gamma sources chosen for penetrating power and a half-life long enough to use but short enough that the source does not stay dangerously active for ages.
Safety: ionising radiation damages living cells — cell death, mutations and cancer. Sources are handled with tongs at arm’s length, pointed away from people, used for the shortest possible time, and stored in lead-lined boxes in locked stores, with warning labels; protective clothing and film badges monitor exposure. The three levers are always the same: less time, more distance, more shielding (Supplement).
Revise
Key terms
The words examiners expect you to use precisely.
- Proton number (Z)
- The number of protons in a nucleus — the atomic number, which fixes the element.
- Nucleon number (A)
- The total number of protons and neutrons in a nucleus — the mass number.
- Isotope
- Atoms of the same element with the same proton number but different numbers of neutrons.
- Nuclide notation
- Writing an isotope as A over Z beside the symbol, e.g. ¹⁴₆C.
- Nuclear fission
- The splitting of a large nucleus into smaller nuclei, releasing energy.Supplement
- Nuclear fusion
- The joining of two light nuclei into a heavier nucleus, releasing energy.Supplement
Show 7 more key termsShow fewer key terms
- Background radiation
- The ever-present low-level radiation from radon, rocks and buildings, food and drink, and cosmic rays.
- Count rate
- The number of detector counts per second or per minute from a radioactive source.
- Alpha particle
- A helium nucleus (2p + 2n, charge +2); strongly ionising, stopped by paper.
- Beta particle
- A fast electron emitted when a neutron changes into a proton and an electron; moderately ionising, stopped by aluminium.
- Gamma radiation
- High-energy electromagnetic radiation from the nucleus; weakly ionising, reduced by lead or concrete.
- Half-life
- The time taken for half the nuclei of a sample of an isotope to decay.
- Ionising radiation
- Radiation energetic enough to remove electrons from atoms, damaging living cells.
Revise
Common mistakes
Where students most often lose marks in this topic — and what to do instead.
Mistake: Counting neutrons as A − Z but writing Z − A (giving a negative number).
The nucleon number is always the larger one.
Instead: Neutrons = A − Z. Carbon-14 (Z = 6) has 14 − 6 = 8 neutrons.
Mistake: Saying alpha particles are electrons.
Alpha particles are helium nuclei — two protons plus two neutrons.
Instead: Alpha = ⁴₂He; beta is the electron.
Show 8 more mistakesShow fewer mistakes
Mistake: Claiming gamma decay changes the element.
Gamma carries no charge or nucleons away.
Instead: Only α and β decay change the proton number; gamma emission leaves A and Z unchanged.
Mistake: Forgetting to balance proton numbers in a decay equation.
Both the A totals and the Z totals must match on the two sides.
Instead: Check the top row and the bottom row separately after writing any decay equation.
Mistake: Saying the half-life shortens as the sample gets smaller.
Half-life is a property of the isotope, not the sample size.
Instead: Every half-life halves whatever activity remains — the time per halving never changes.
Mistake: Reading a decay curve that still includes background as if it were corrected.
The curve never reaches zero — it flattens at the background count.
Instead: Subtract the background count rate first, then find the half-life (Supplement).
Mistake: Choosing gamma radiation for a smoke alarm.
Gamma penetrates everything — it would not be stopped by the casing and would expose users.
Instead: Smoke alarms use alpha: huge ionisation in the detection gap, stopped by a sheet of paper, let alone the case.
Mistake: Believing radioactive decay can be sped up by heating the sample.
Decay is spontaneous and unaffected by temperature, pressure or chemistry.
Instead: Nothing you do to the sample changes its decay rate — that is why half-life is so reliable.
Mistake: Saying beta particles are deflected more than alpha in a field because beta is stronger.
Beta is lighter, so the same force bends it more easily.
Instead: Beta curves more than alpha in the same field, and in the opposite direction; gamma does not curve at all (Supplement).
Mistake: Handling a source with bare fingers "because it is only small".
Even a tiny source at close range delivers a large dose to the skin.
Instead: Handle with tongs at arm’s length, point away from people, store in lead-lined boxes (Supplement).
Check yourself
Quick quiz
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Describe the nucleus
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Practise
Topical questions
12 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this topic, in three tiers, each with a model answer, mark scheme and examiner tip.
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Practice paper
A 30-mark paper set out like a Cambridge paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Topic 12 Practice Paper
30 marks · 40 min · 5 pages
10 multiple-choice questions and structured questions with answer lines. Topic 12: Nuclear physics (nuclear atom, background, α/β/γ, decay equations, half-life, safety).
Mark scheme
PROTopic 12 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Which paper covers this topic?
Paper 1 / 2
Multiple choice (45 min, 40 marks)
Core and Extended versions
Paper 3 / 4
Theory, short and structured (1 h 15, 80 marks)
Core and Extended versions
Paper 5 / 6
Practical test or Alternative to Practical (40 marks)
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Questions
Frequently asked questions
What is the difference between alpha, beta and gamma radiation?
Alpha is a helium nucleus (2 protons + 2 neutrons) — strongly ionising, stopped by paper. Beta is a fast electron — moderately ionising, stopped by a few mm of aluminium. Gamma is electromagnetic radiation — weakly ionising, reduced by thick lead or concrete.
How do I write a decay equation?
Write the parent in nuclide notation, add the emitted particle (⁴₂He for alpha, ⁰₋₁e for beta, γ for gamma), and make both the nucleon numbers and the proton numbers balance across the two sides.
Does half-life change if I use a bigger sample?
No. Half-life belongs to the isotope: whatever the sample size, half the nuclei decay in one half-life. A bigger sample starts higher but halves just as fast.
Why does a smoke alarm use an alpha source?
Alpha’s strong ionisation makes a small, reliable sensing current in the detection gap, and its tiny range means the plastic casing stops every particle — safe in the home (Supplement).
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No. Cambridge IGCSE and Cambridge are trademarks of Cambridge University Press & Assessment. QuizQuip is independent — not affiliated with, endorsed by, or connected to Cambridge. All notes and questions are original QuizQuip material written against the published 0625 syllabus.
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