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

> Eduqas A-Level Chemistry — 2.1 Periodic table · Explain · 5 marks

> The relative atomic mass of chlorine is 35.5.

## Mark scheme (5 marks)

1. Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.
2. Chlorine-35 and chlorine-37 have different mass numbers (35 and 37 respectively).
3. Relative atomic mass takes account of the abundance of each isotope.
4. Chlorine-35 is more abundant than chlorine-37, so the average is pulled closer to 35.
5. Because relative atomic mass is a weighted average of the masses of all isotopes, the value (35.5) is not a whole number.

## 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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- [Revision notes for Eduqas A-Level Chemistry](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/chlorine-has-two-main-isotopes-chlorine-35-e7845306) · Published by Druglandscape Ltd.