# Chlorine has two main 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) — 2.1 Periodic table · 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 number of neutrons is different
5. The two isotopes have different mass numbers (35 and 37) because the mass number is the total number of protons and neutrons

## Key terms

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

## Related

- [Revision notes for WJEC A-Level 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/chlorine-has-two-main-isotopes-chlorine-35-6043353e) · Published by Druglandscape Ltd.