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 to, and different from, each other in terms of their subatomic particles.

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.
Both chlorine‑35 and chlorine‑37 have 17 protons.
Both isotopes also have 17 electrons.
The difference lies in the neutrons: chlorine‑35 contains 18 neutrons, whereas chlorine‑37 contains 20 neutrons.

Examiner tips

  • Use the definition of isotope first, then list the shared proton and electron numbers, finally state the differing neutron counts.
  • Show the neutron numbers explicitly (18 and 20) to match the mark scheme.
  • Keep the answer concise – one sentence per point is enough for full marks.

Common mistakes

  • Confusing the number of neutrons with the mass number, e.g. saying chlorine‑35 has 35 neutrons.
  • Omitting the electron count or stating that electrons differ, which is incorrect.
  • Using vague terms like “different” without specifying the exact neutron numbers.

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. 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: chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons.

Key terms in this question

isotope

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