# Describe what is meant by the term 'isotope' and explain why two isotopes of the same element have the same atomic number but different mass numbers.

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

> Silicon has three naturally occurring isotopes: silicon-28, silicon-29 and silicon-30. The relative atomic mass of silicon is 28.1.

## 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 atomic number is determined by the number of protons, which is the same in all isotopes of an element.
3. The mass number is the total number of protons and neutrons in the nucleus.
4. Because the number of neutrons differs between isotopes, the mass number (protons + neutrons) is different for each isotope.

## Key terms

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

## Related

- [Revision notes for AQA GCSE Chemistry (8462)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/describe-what-is-meant-by-the-b0c118d1) · Published by Druglandscape Ltd.