# Explain why it is impossible to predict the exact moment at which a specific nucleus in a radioactive sample will decay, yet the half-life of a large sample of that nuclide can be known with great precision.

> IB DP Physics Standard Level (2023 syllabus) — E.3 Radioactive decay · Explain · 4 marks

## Mark scheme (4 marks)

1. Radioactive decay is a random process, so the decay of any individual nucleus cannot be predicted.
2. Each nucleus has the same constant probability of decaying per unit time (the decay constant), independent of all external factors.
3. When a very large number of nuclei is present, statistical averaging means the overall behaviour of the sample becomes predictable / fluctuations become negligible relative to the total.
4. The half-life is therefore a well-defined, measurable quantity for a large sample because it reflects the statistically predictable fraction of nuclei that will decay in a given time interval.

## Key terms

- [half-life](https://www.gradenine.co.uk/glossary/half-life)

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-it-is-impossible-to-e74ac4c2) · Published by Druglandscape Ltd.