# Explain why two atoms of the same element can have different mass numbers, and describe how this affects the relative atomic mass of that element.

> WJEC A-Level Chemistry (Wales) — 2.3 Transition metals (A-Level only) · Explain · 5 marks

## Mark scheme (5 marks)

1. Atoms of the same element always have the same number of protons (atomic number)
2. But atoms of the same element can have a different number of neutrons
3. These atoms are called isotopes
4. Mass number = protons + neutrons, so different numbers of neutrons give different mass numbers
5. Relative atomic mass is an average value that takes account of the abundance of each isotope

## Key terms

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

## Related

- [Revision notes for WJEC A-Level Chemistry (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/explain-why-two-atoms-of-the-8835ca8c) · Published by Druglandscape Ltd.