# A student says: 'When a nucleus emits a beta particle, the nucleus loses a particle so it must become lighter and its atomic number must decrease.' Explain why the student is incorrect, describing what actually happens to the nucleus during beta decay.

> WJEC A-Level Physics (Wales) — 1.7 Particles and nuclear structure · Explain · 5 marks

## Mark scheme (5 marks)

1. A beta particle is a fast-moving electron emitted from the nucleus
2. During beta decay a neutron in the nucleus changes into a proton (and a beta particle is emitted)
3. The atomic/proton number increases by 1 (not decreases)
4. The mass/nucleon number stays the same (nucleus does not become lighter)
5. The student is wrong that the nucleus becomes lighter because the number of nucleons is conserved — a neutron is replaced by a proton, so the total count of particles in the nucleus remains the same

## Key terms

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

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](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-student-says-when-a-nucleus-cd7f5816) · Published by Druglandscape Ltd.