Oxygen has two naturally occurring isotopes: oxygen-16 and oxygen-18. Explain what is meant by the term 'isotope' and explain why the relative atomic mass of oxygen is not a whole number.

WJEC GCSE Chemistry (Wales) — 2.4 Chemical analysis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.

The relative atomic mass of an element is an average that takes account of the abundances of all its naturally occurring isotopes.

Because oxygen has two isotopes, ^16O and ^18O, the average mass lies between 16 and 18.

The two isotopes are present in different abundances (not a 1:1 ratio), so the weighted average is not a whole number.

Examiner tips

  • Define isotopes correctly, include protons and neutrons. Explain that relative atomic mass is a weighted average of isotope masses. Show the average lies between 16 and 18. Mention unequal abundances to justify non‑whole number.

Common mistakes

  • Confusing isotopes with ions or isotones. Saying the average is exactly 17 without explaining weighting. Forgetting to mention unequal abundances or the concept of a weighted average.

Mark scheme (4 marks)

  1. Isotopes are atoms of the same element with the same number of protons but a different number of neutrons
  2. Relative atomic mass takes account of / is an average of the abundances of all isotopes of the element
  3. Because it is an average of two isotopes with different masses (16 and 18), the relative atomic mass lies between these values
  4. The isotopes are present in different abundances / not in a 1:1 ratio, so the weighted average is not a whole number

Key terms in this question

isotope · relative atomic mass

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