# Silicon has two naturally occurring isotopes: silicon-28 and silicon-29. Explain why the relative atomic mass of silicon is not a whole number.

> WJEC GCSE Chemistry (Wales) — 2.4 Chemical analysis · Explain · 4 marks

> Silicon is used in the manufacture of computer chips. It has an atomic number of 14.

## 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. The two isotopes have different masses / mass numbers (silicon-28 has a lower mass than silicon-29)
3. Relative atomic mass is an average value that takes account of the abundance of the isotopes
4. Because the two isotopes exist in proportions that are not equal (not 50:50), the average mass falls between whole numbers, giving a non-whole number value

## 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/silicon-has-two-naturally-occurring-isotopes-6275a68e) · Published by Druglandscape Ltd.