# Explain why atoms of the same element can have different mass numbers.

> Pearson Edexcel International GCSE Chemistry (4CH1) — 1.3 Atomic structure · Explain · 4 marks

> Carbon has an atomic number of 6. One atom of carbon has a mass number of 12 and another has a mass number of 14.

## Mark scheme (4 marks)

1. Atoms of the same element always have the same number of protons (atomic number stays the same)
2. The atoms have different numbers of neutrons
3. Mass number is the total number of protons and neutrons (in the nucleus)
4. These atoms are called isotopes

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Chemistry (4CH1)](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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