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

AQA GCSE Chemistry (8462) — 4.1.1 A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Chlorine atoms have the same number of protons but different numbers of neutrons, giving two isotopes, chlorine‑35 and chlorine‑37.

The relative atomic mass is calculated as the weighted average of the masses of all naturally occurring isotopes, using their relative abundances.

Because chlorine‑35 and chlorine‑37 are present in unequal proportions (about 75 % and 25 %), the weighted average of their masses is not a whole number – it is approximately 35.5.

Thus the non‑whole‑number relative atomic mass reflects the mixed‑isotope composition of chlorine.

Examiner tips

  • Use the term ‘isotope’ correctly – same protons, different neutrons. Show the weighted‑average idea – mention abundances. Give the approximate average (≈35.5) to demonstrate calculation.
  • common_mistakes
  • :
  • Confusing isotope definition with atomic number. Forgetting to mention unequal abundances. Giving a whole‑number average or omitting the weighted‑average concept.

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 is an average value that takes account of the abundance of the isotopes of the element
  3. Chlorine-35 and chlorine-37 exist in different (non-equal) abundances
  4. The average of the two isotope masses weighted by their abundances gives a non-whole-number result (approximately 35.5)

Key terms in this question

isotope · relative atomic mass

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