# Explain why the relative atomic mass of chlorine is 35.5, rather than a whole number.

> Eduqas GCSE Chemistry — 2.5 The production of important chemicals · Explain · 4 marks

> Chlorine has two naturally occurring isotopes: chlorine-35 and chlorine-37. Chlorine-35 is more abundant than chlorine-37.

## 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 takes account of the abundance of each isotope
3. Chlorine-35 is more abundant than chlorine-37, so the average is pulled closer to 35
4. The result is a non-whole number (35.5) because it is an average of two isotopes with different masses

## Key terms

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

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