# Explain why beta decay causes a change in the atomic number of a nucleus but does not cause a change in the mass number.

> AQA GCSE Physics (8463) — 4.4.2 Atoms and nuclear radiation · Explain · 4 marks

## Mark scheme (4 marks)

1. In beta decay, a neutron converts into a proton (and an electron is emitted)
2. The number of protons increases by one, so the atomic number increases
3. The total number of protons and neutrons (mass number) stays the same because one neutron is replaced by one proton
4. The emitted beta particle (electron) comes from the nucleus and is not counted in the mass number

## Key terms

- [atomic number](https://www.gradenine.co.uk/glossary/atomic-number)
- [mass number](https://www.gradenine.co.uk/glossary/mass-number)

## Related

- [Revision notes for AQA GCSE Physics (8463)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-beta-decay-causes-a-5dd89f0e) · Published by Druglandscape Ltd.