# Explain why two atoms that have the same atomic number but different mass numbers are classed as isotopes of the same element.

> AQA GCSE Physics (8463) — 4.4.1 Atoms and isotopes · Explain · 4 marks

> Lithium-6 and lithium-7 are both naturally occurring forms of lithium.

## Mark scheme (4 marks)

1. The atomic number is the number of protons in an atom / both atoms have the same number of protons
2. Atoms of the same element must have the same atomic number / the number of protons determines which element an atom belongs to
3. The mass number is the total number of protons and neutrons / a different mass number means a different number of neutrons
4. Isotopes have the same number of protons but different numbers of neutrons (linking both features together to justify the classification)

## Key terms

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

## Related

- [Revision notes for AQA GCSE Physics (8463)](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/explain-why-two-atoms-that-have-8ca975e7) · Published by Druglandscape Ltd.