# Explain why the atomic radius of elements decreases across Period 3 from sodium to chlorine.

> AQA A-Level Chemistry (7405) — 3.2.1 Periodicity · Explain · 5 marks

## Mark scheme (5 marks)

1. All elements in Period 3 have the same number of electron shells / electrons are in the same outer shell (third shell)
2. The number of protons increases across the period
3. The nuclear charge / number of protons increases but the number of electron shells stays the same / shielding stays the same
4. The greater nuclear charge pulls the outer electrons closer to the nucleus
5. Therefore the atomic radius decreases (across the period)

## Key terms

- [atomic radius](https://www.gradenine.co.uk/glossary/atomic-radius)
- [Period 3](https://www.gradenine.co.uk/glossary/period-3)

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](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-atomic-radius-of-e23a3253) · Published by Druglandscape Ltd.