Neon has two naturally occurring isotopes, neon-20 and neon-22. Explain what is meant by the term isotope and explain why the relative atomic mass of neon is not a whole number.

Eduqas A-Level Chemistry — 2.3 Transition metals (A-Level only) · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.
Neon‑20 and neon‑22 are present in different proportions (abundances) in nature.
The relative atomic mass of neon is an average value that takes account of the relative abundances of its isotopes.
Because the weighted average of the isotope masses is not a whole number, the relative atomic mass of neon is not a whole number.

Examiner tips

  • Define isotopes and state that they differ in neutrons. Mention the two isotopes and their natural abundances. Explain that the atomic mass is a weighted average. Show why the average is not an integer.

Common mistakes

  • Confusing isotopes with ions or isotones. Forgetting to mention the different abundances. Saying the atomic mass is a simple sum instead of a weighted average.

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. Neon-20 and neon-22 exist in different proportions / abundances
  3. Relative atomic mass is an average value that takes account of the abundance of the isotopes
  4. Because the weighted average of the isotope masses is not a whole number, the relative atomic mass is not a whole number

Key terms in this question

isotope · relative atomic mass

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