# Magnesium has three isotopes: magnesium-24, magnesium-25 and magnesium-26. The relative atomic mass of magnesium is 24.3. Explain why the relative atomic mass of magnesium is not a whole number and describe what is meant by the term isotope.

> AQA GCSE Chemistry (8462) — 4.1.1 A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes · Explain · 4 marks

## Mark scheme (4 marks)

1. Isotopes are atoms of the same element with the same number of protons but a different number of neutrons
2. Relative atomic mass is an average value that takes account of the abundance of the isotopes
3. The three isotopes have different abundances / they are not present in equal amounts
4. Because the weighted average of the isotope masses is not a whole number, the relative atomic mass is not a whole number

## Key terms

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

## Related

- [Revision notes for AQA GCSE Chemistry (8462)](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/magnesium-has-three-isotopes-magnesium-24-magnesium-25-77f7e66b) · Published by Druglandscape Ltd.