# A nucleus of uranium-238 undergoes alpha decay to form a nucleus of thorium-234. Explain what an alpha particle is and describe the changes that occur to the nucleus of uranium-238 during this decay.

> WJEC A-Level Physics (Wales) — 3.5 Nuclear decay · Explain · 5 marks

## Mark scheme (5 marks)

1. An alpha particle consists of 2 protons and 2 neutrons
2. The alpha particle is the same as a helium nucleus (or has the same structure)
3. The mass number (nucleon number) of the nucleus decreases by 4
4. The atomic number (proton number) of the nucleus decreases by 2
5. The nucleus emits the alpha particle, which carries away the 2 protons and 2 neutrons / the nucleus loses those particles to form the new (thorium) nucleus

## Key terms

- [alpha particle](https://www.gradenine.co.uk/glossary/alpha-particle)
- [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-nucleus-of-uranium-238-undergoes-alpha-cd2c48e5) · Published by Druglandscape Ltd.