# Chlorine has an atomic number of 17 and its most common isotope has a mass number of 35. State the number of protons, neutrons and electrons in this isotope of chlorine. Then explain why the relative atomic mass of chlorine, shown in the periodic table, is approximately 35.5 rather than a whole number.

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

## Mark scheme (5 marks)

1. Number of protons = 17
2. Number of neutrons = 18 (mass number minus atomic number: 35 − 17)
3. Number of electrons = 17 (equal to number of protons in a neutral atom)
4. Chlorine has more than one isotope / chlorine exists as isotopes with different numbers of neutrons (e.g. chlorine-35 and chlorine-37)
5. Relative atomic mass is an average value that takes account of the abundance of the isotopes, so it is not a whole number

## 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)
- [relative atomic mass](https://www.gradenine.co.uk/glossary/relative-atomic-mass)
- [neutron](https://www.gradenine.co.uk/glossary/neutron)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "State and Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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