Chlorine has two naturally occurring isotopes, chlorine-35 and chlorine-37. Explain what is meant by the term isotope and describe how these two isotopes of chlorine are similar and how they are different in terms of their subatomic particles.

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.

Both chlorine‑35 and chlorine‑37 have 17 protons.

Both isotopes also have 17 electrons.

The two isotopes differ in the number of neutrons: chlorine‑35 has 18 neutrons, chlorine‑37 has 20 neutrons.

Consequently they have different mass numbers – 35 and 37 – so chlorine‑37 is heavier than chlorine‑35.

Examiner tips

  • Define isotopes first, then list shared properties (protons, electrons) before differences (neutrons, mass).
  • Use the exact wording from the mark scheme: "different number of neutrons" and "different mass numbers".
  • Keep the answer concise – 5 points, so one sentence per point.

Common mistakes

  • Confusing the number of electrons with the number of protons; students often say both have 17 electrons but then forget to mention protons.
  • Writing that the isotopes have different numbers of protons or electrons, which is incorrect.
  • Giving the mass numbers in the wrong order or omitting the fact that chlorine‑37 is heavier.

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 isotopes have the same number of protons (17 protons).
  3. Both isotopes have the same number of electrons (17 electrons).
  4. The two isotopes have different numbers of neutrons.
  5. This means the two isotopes have different mass numbers (35 and 37), so chlorine-37 has a greater mass than chlorine-35.

Key terms in this question

isotope

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