# Chlorine has two isotopes: chlorine-35 and chlorine-37. Explain why the relative atomic mass of chlorine is not a whole number, and state what is meant by the term isotope.

> AQA GCSE Chemistry (8462) — 4.1.1 A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes · Explain · 4 marks

## Mark scheme (4 marks)

1. Isotopes are atoms of the same element with the same number of protons but a different number of neutrons
2. Relative atomic mass is an average value that takes account of the abundance of the isotopes of the element
3. Chlorine-35 and chlorine-37 exist in different (non-equal) abundances
4. The average of the two isotope masses weighted by their abundances gives a non-whole-number result (approximately 35.5)

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/chlorine-has-two-isotopes-chlorine-35-and-e261993e) · Published by Druglandscape Ltd.