# Explain why two atoms of the same element can have different mass numbers, and explain why this does not change the chemical properties of those atoms.

> WJEC GCSE Chemistry (Wales) — 2.5 The production of important chemicals · Explain · 4 marks

## Mark scheme (4 marks)

1. Atoms of the same element with different mass numbers are called isotopes
2. The mass number differs because isotopes have different numbers of neutrons (in the nucleus)
3. Isotopes of the same element have the same number of protons / the same atomic number
4. Chemical properties depend on the number of electrons, which is the same in isotopes of the same element / electrons are responsible for chemical reactions and the number of electrons is unchanged

## Key terms

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

## Related

- [Revision notes for WJEC GCSE 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-d65a07c0) · Published by Druglandscape Ltd.