# Phosphorus has atomic number 15. An atom of the most common isotope of phosphorus has a mass number of 31. State the number of protons, electrons and neutrons in this atom, and explain how the number of electrons is related to the atomic number.

> Pearson Edexcel International GCSE Chemistry (4CH1) — 1.2 Atoms · State · 4 marks

## Mark scheme (4 marks)

1. Number of protons = 15
2. Number of electrons = 15
3. Number of neutrons = 16 (mass number minus atomic number, i.e. 31 − 15)
4. The number of electrons equals the atomic number / the number of protons, because atoms are electrically neutral / have no overall charge

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Chemistry (4CH1)](https://www.gradenine.co.uk/learn)
- [How to answer "State" 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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Source: [GradeNine](https://www.gradenine.co.uk/q/phosphorus-has-atomic-number-15-an-b8a2352a) · Published by Druglandscape Ltd.