Chlorine has two isotopes: chlorine-35 and chlorine-37. Explain what is meant by the term isotope and explain why the relative atomic mass of chlorine is not a whole number.

Eduqas GCSE Chemistry — 2.5 The production of important chemicals · 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 is an average value that takes account of the abundance of the isotopes.

Because chlorine‑35 and chlorine‑37 exist in different proportions, the average is not a whole number.

The average value (≈35.5) falls between the two mass numbers 35 and 37, so it cannot be a whole number.

Examiner tips

  • Use the definition of isotope first, then explain the averaging process.
  • Show the two isotopes and their different abundances to justify the non‑whole number.

Common mistakes

  • Confusing isotopes with ions or isotones.
  • Giving only the definition of isotope without linking to the average atomic mass.

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. The relative atomic mass is an average value that takes account of the abundance of the isotopes
  3. Chlorine-35 and chlorine-37 exist in different proportions / abundances, so the average is not a whole number
  4. The average value (approximately 35.5) falls between the two mass numbers 35 and 37, so it cannot be a whole number

Key terms in this question

isotope · relative atomic mass

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