# Chlorine has two naturally occurring isotopes: chlorine-35 and chlorine-37. Explain what is meant by the term isotope and describe how these two isotopes of chlorine are similar to, and different from, each other in terms of their subatomic particles.

> WJEC A-Level Chemistry (Wales) — 2.3 Transition metals (A-Level only) · Explain · 5 marks

## 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. Both isotopes have the same number of protons (17 protons).
3. Both isotopes have the same number of electrons (17 electrons).
4. Chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons, so the two isotopes have different numbers of neutrons.
5. The two isotopes therefore have different mass numbers / different masses.

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/chlorine-has-two-naturally-occurring-isotopes-0a884a5d) · Published by Druglandscape Ltd.