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

> Pearson Edexcel International GCSE Chemistry (4CH1) — 1.2 Atoms · Explain · 4 marks

> Carbon-12 and carbon-14 are both atoms of the element carbon, yet they have different mass numbers.

## Mark scheme (4 marks)

1. Atoms of the same element always have the same number of protons (atomic number)
2. These atoms have a different number of neutrons
3. Mass number is the total number of protons and neutrons (in the nucleus)
4. Because the number of neutrons differs, the total (mass number) differs — these 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)
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