# Explain what is meant by the term 'half-life' of a radioactive substance and explain why it is not possible to predict exactly when a particular nucleus will decay.

> WJEC GCSE Physics (Wales) — 2.9 Nuclear decay and nuclear energy · Explain · 4 marks

## Mark scheme (4 marks)

1. Half-life is the time taken for half the (number of) radioactive nuclei in a sample to decay
2. Half-life is constant / the same for a given isotope (regardless of external conditions)
3. Nuclear decay is a random process
4. Because decay is random, the exact moment any individual nucleus decays cannot be predicted / only probabilities can be stated

## Key terms

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

## Related

- [Revision notes for WJEC GCSE Physics (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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