# Chlorine has two naturally occurring isotopes: chlorine-35 and chlorine-37. Describe what is meant by the term isotope and explain what is the same and what is different about the nuclei of these two isotopes of chlorine.

> Pearson Edexcel International GCSE Chemistry (4CH1) — 1.2 Atoms · Describe · 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. Both isotopes have the same number of protons (17 protons) in the nucleus
3. Chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons / the number of neutrons is different
4. The mass numbers (nucleon numbers) of the two isotopes are different / chlorine-35 has mass number 35 and chlorine-37 has mass number 37

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Chemistry (4CH1)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/chlorine-has-two-naturally-occurring-isotopes-c5e7b179) · Published by Druglandscape Ltd.