# Oxygen has two naturally occurring isotopes, oxygen-16 and oxygen-18. Explain what is meant by the term isotope and describe how oxygen-16 and oxygen-18 are both similar to and different from each other in terms of their subatomic particles.

> Eduqas A-Level Chemistry — 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 oxygen-16 and oxygen-18 have the same number of protons (8 protons).
3. Both oxygen-16 and oxygen-18 have the same number of electrons (8 electrons).
4. Oxygen-16 has 8 neutrons and oxygen-18 has 10 neutrons, so they have different numbers of neutrons.
5. The different numbers of neutrons mean oxygen-16 and oxygen-18 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/oxygen-has-two-naturally-occurring-isotopes-1591c8f5) · Published by Druglandscape Ltd.