Physics · GCSE · Interactive lesson
Radioactive decay and half-life
Learn what half-life means for a large sample. Follow repeated halving and understand why one nucleus cannot be predicted.
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Explore an interactive example, then try the questions at your own pace.
GCSE · half-life and random decay
By the end of this lesson: Explain half-life as a sample trend rather than a deadline for each nucleus.
The interactive diagram and controls are in Stage 2, “Explore one example”. Read the idea here, then try it next.
- The nucleus is the tiny centre of an atom. Some nuclei are unstable and can decay, releasing radiation.
- We cannot predict exactly when one unstable nucleus will decay.
- For a large sample, one half-life leaves about half the original nuclei still undecayed.
Useful starting points
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What do the words mean?
- Nucleus / nuclei
- A nucleus is the tiny centre of one atom, containing protons and neutrons. Nuclei is the plural.
- Radioactive decay
- A random change in an unstable nucleus that releases radiation.
- Half-life
- Time for the number of undecayed nuclei, or the sample’s activity, to halve on average.
- Activity
- Number of decays each second, measured in becquerels (Bq).
- Isotope
- A type of atom. Isotopes of one element have the same number of protons but different numbers of neutrons.
Change the example
This is the lesson example. Your practice question uses its own numbers.
Something to try
Increase the number of half-lives one at a time. Does each interval remove the same number, or the same fraction, of the nuclei left?
Notice what changes
How particle motion works +
Starts paused. Press Play, then add energy to see the warmer particles move faster.
Particles keep the same size. Liquid particles move around; solid particles vibrate around fixed positions. This is a simplified model, with the change in motion exaggerated to make it easier to see.
About the playback +
About this model
Read the bars from left to right, from 0 to 2 half-lives. Each bar shows the average percentage of the original nuclei still undecayed in a large sample. Decay is random, so real counts vary around this pattern. The bars do not show when individual nuclei decay. The half-life is a made-up example.
More about this diagram
Still picture. Play particle motion in Stage 2.
About 50% remains after 4 days
Still picture of the example from Stage 2. Use the back arrow to change it.
Work through it
Starting example: read the steps below
- The example half-life is 4 days.
- 1 half-life lasts 4 days altogether.
- Start with 100% and halve it 1 time: about 50%.
- This is an average trend. We cannot say when one particular nucleus will decay.
Try on your own
Your turn. No rush.
Question 1
Can we predict the exact decay time of one unstable nucleus?
About these questions
Questions appear in a mixed order. Work at your own pace and stop whenever you like.
These questions use their own numbers, separate from the lesson example. You can stop whenever you like. Answers last for this visit to the page.
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Can you explain what half-life tells us about a large sample, and why it cannot predict one nucleus’s decay time?
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Notes for parents and tutors
These lessons cover selected KS3 and GCSE ideas for England; they are not a complete course. The level notes explain where GCSE methods or optional extensions begin. Maurice can help you choose material for your course and exam board.
The content draws on the DfE Maths curriculum, DfE Science curriculum and AQA GCSE Physics specification. Level labels are guidance, not exam-board endorsements. For AQA Radioactivity, calculating net decline as a ratio is Higher-tier work. Net means the overall change; ask Maurice whether this extension is relevant to your course. Explanations and practice were reviewed during development on 21 October 2026; Maurice should check their fit before assigning work.
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