# Oxygen has two naturally occurring isotopes: oxygen-16 and oxygen-18. Explain what is meant by the term 'isotope' and explain why the relative atomic mass of oxygen is not a whole number.

> WJEC GCSE Chemistry (Wales) — 2.4 Chemical analysis · 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 takes account of / is an average of the abundances of all isotopes of the element
3. Because it is an average of two isotopes with different masses (16 and 18), the relative atomic mass lies between these values
4. The isotopes are present in different abundances / not in a 1:1 ratio, so the weighted average 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)

## Related

- [Revision notes for WJEC GCSE Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/oxygen-has-two-naturally-occurring-isotopes-ed3438f4) · Published by Druglandscape Ltd.