# A geologist wants to determine the age of a rock sample by analysing the proportion of decayed and undecayed radioactive isotopes it contains. The isotope found in the rock has a half-life of 700 million years. Explain how radioactive decay and the concept of half-life make this method of radioactive dating possible, and why the random nature of radioactive decay does not prevent scientists from using this technique reliably.

> OCR A-Level Physics A (H556) — 6.1 Capacitors · Explain · 5 marks

> Radioactive dating is used by scientists to estimate the age of ancient rocks and fossils. It relies on measuring the ratio of unstable nuclei remaining in a sample compared to the amount originally present.

## Mark scheme (5 marks)

1. Radioactive decay is the process by which unstable nuclei emit radiation to become more stable
2. Half-life is the time taken for the number of unstable nuclei in a sample to halve
3. By comparing the proportion of decayed to undecayed isotopes, the number of half-lives that have passed can be determined, allowing the age to be calculated
4. Radioactive decay is random, meaning it is impossible to predict when any one nucleus will decay
5. Despite individual decay being random, the half-life of a large sample is constant / reliable because large numbers of nuclei are involved, so statistical averages are consistent

## Key terms

- [radioactive decay](https://www.gradenine.co.uk/glossary/radioactive-decay)
- [half-life](https://www.gradenine.co.uk/glossary/half-life)
- [random nature of radioactive decay](https://www.gradenine.co.uk/glossary/random-nature-of-radioactive-decay)
- [isotopes](https://www.gradenine.co.uk/glossary/isotopes)
- [radioactive dating](https://www.gradenine.co.uk/glossary/radioactive-dating)

## Related

- [Revision notes for OCR A-Level Physics A (H556)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-geologist-wants-to-determine-the-b9800e88) · Published by Druglandscape Ltd.