# Explain why the activity of a radioactive sample decreases over time, even though the decay constant of the nuclide remains constant.

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

> A freshly prepared sample of a radioactive nuclide has a measured activity of 800 Bq. After several half-lives, the activity is found to be significantly lower.

## Mark scheme (4 marks)

1. Activity is defined as the number of decays (disintegrations) per unit time.
2. Each decay removes one radioactive nucleus from the sample, so the number of undecayed nuclei decreases over time.
3. Activity is proportional to the number of undecayed nuclei present (A = λN).
4. Because N decreases while λ remains constant, the product λN decreases, so activity falls exponentially over time.

## Key terms

- [activity](https://www.gradenine.co.uk/glossary/activity)
- [decay constant](https://www.gradenine.co.uk/glossary/decay-constant)

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-activity-of-a-7c867d01) · Published by Druglandscape Ltd.