AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Students should be able to explain the concept of half-life and how it is…
Full specification wording
Half-lives and the random nature of radioactive decay: Students should be able to explain the concept of half-life and how it is related to the random nature of radioactive decay.
Radioactive decay is random: you cannot predict which nucleus decays next or exactly when.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Radioactive decay is random: you cannot predict which nucleus decays next or exactly when. Yet a large sample shows a steady pattern because many nuclei are present, so the proportion decaying per unit time is stable. Half-life is the time for half the nuclei, or half the count rate, to remain. For example, a source of 600 counts per minute falls to about 300, then 150, then 75 over equal half-lives. Individual decays are chance events, but the overall halving is predictable. This lets scientists date rocks and trace medical isotopes.
What earns the marks
- Explain that radioactive decay is random, so individual nuclei decay at unpredictable times.
- Explain that a large number of nuclei gives a statistically predictable overall pattern.
- Link the predictable pattern to half-life as the time for half the nuclei or half the count rate to remain.
- Use a numerical example, such as 600 → 300 → 150 → 75 counts per minute, to show repeated halving.
- Distinguish between the random behaviour of one nucleus and the reliable average behaviour of a large sample.
- Apply the concept to a context such as carbon dating or medical tracers, choosing an isotope with a suitable half-life.
Common mistakes and how to put them right
Saying that half-life makes each nucleus decay at a fixed time.
Put it right by stating that individual decay times remain random.
Confusing half-life with the time for the count rate to reach zero.
Put it right by showing the halving sequence continues without reaching zero.
Believing that a small sample will halve exactly on schedule.
Put it right by explaining that statistical predictability needs a large number of nuclei.
Exam tips
- Use the phrase random decay and then explain how many nuclei produce a predictable average.
- Support your explanation with a short halving sequence and clear units.
- When asked to explain, link each statement to the next: random decay, large numbers, stable proportion, half-life.
Try the lesson
The start of the Half-lives and the random nature of radioactive decay: Students should be able to explain the concept of half-life and how it is related to the random nature of radioactive decay. lesson, made from checked teaching notes for this specification point.
Have a go first
A radioactive source has a half-life of 8 days. What is meant by the half-life of this source?
Listen
Radioactive decay is random: you cannot predict which nucleus decays next or exactly when. Yet a large sample shows a steady pattern because many nuclei are present, so the proportion decaying per unit time is stable. Half-life is the time for half the nuclei, or half the count rate, to remain. For example, a source of 600 counts per minute falls to about 300, then 150, then 75 over equal half-lives. Individual decays are chance events, but the overall halving is predictable. This lets scientists date rocks and trace medical isotopes. A strong answer earns marks for these points. Explain that radioactive decay is random, so individual nuclei decay at unpredictable times. Explain that a large number of nuclei gives a statistically predictable overall pattern. Link the predictable pattern to half-life as the time for half the nuclei or half the count rate to remain. Use a numerical example, such as 600 → 300 → 150 → 75 counts per minute, to show repeated halving. A common slip: Saying that half-life makes each nucleus decay at a fixed time. Instead, stating that individual decay times remain random. Exam tip: Use the phrase random decay and then explain how many nuclei produce a predictable average.
See it
Key words
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Randomness of individual decay events
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Statistical predictability of large samples
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Half-life as a measurable constant
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Practical applications of half-life
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See every definition
- Randomness of individual decay events
- You cannot predict which nucleus decays next or exactly when, so individual decays are chance events.
- Statistical predictability of large samples
- A large sample shows a steady pattern because many nuclei are present, so the proportion decaying per unit time is stable.
- Half-life as a measurable constant
- The time for half the nuclei, or half the count rate, to remain.
- Practical applications of half-life
- This lets scientists date rocks and trace medical isotopes.
Label the diagram
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