# Explain how the discovery of the neutron changed scientists' understanding of atomic structure, and explain why neutrons are important for the existence of stable nuclei.

> AQA A-Level Physics (7408) — 3.2.1 Particles · Explain · 5 marks

## Mark scheme (5 marks)

1. Before the discovery of the neutron, scientists could not account for the extra mass in the nucleus (the mass number was greater than the proton number alone could explain).
2. The neutron is a particle found in the nucleus with no charge (electrically neutral).
3. The neutron has a relative mass of 1 (approximately equal to that of a proton), which accounts for the missing mass in the nucleus.
4. Without neutrons, the repulsion between positively charged protons in the nucleus would cause the nucleus to break apart.
5. Neutrons help hold the nucleus together / contribute to the strong nuclear force holding the nucleus stable.

## Key terms

- [neutron](https://www.gradenine.co.uk/glossary/neutron)

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-discovery-of-the-ba4c829a) · Published by Druglandscape Ltd.