# A neutral atom of oxygen has a proton number of 8 and a nucleon number of 16. State the number of protons, neutrons, and electrons in this atom. Then explain how a negative ion of oxygen is formed from this neutral atom.

> Cambridge International IGCSE Physics (0625) — 5.1 The nuclear model of the atom · State and Explain · 4 marks

## Mark scheme (4 marks)

1. 8 protons
2. 8 neutrons (nucleon number minus proton number = 16 − 8)
3. 8 electrons (equal to number of protons in a neutral atom)
4. A negative ion is formed when the atom gains electrons, giving it an overall negative charge

## Key terms

- [proton number](https://www.gradenine.co.uk/glossary/proton-number)
- [nucleon number](https://www.gradenine.co.uk/glossary/nucleon-number)
- [neutron](https://www.gradenine.co.uk/glossary/neutron)
- [electron](https://www.gradenine.co.uk/glossary/electron)
- [ion](https://www.gradenine.co.uk/glossary/ion)

## Related

- [Revision notes for Cambridge International IGCSE Physics (0625)](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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Source: [GradeNine](https://www.gradenine.co.uk/q/a-neutral-atom-of-oxygen-has-7d373870) · Published by Druglandscape Ltd.