Chlorine has two main 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 A-Level Chemistry (Wales) — 2.1 Periodic table · 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 contain 17 protons.
Both isotopes also contain 17 electrons.
Chlorine‑35 has 18 neutrons, whereas chlorine‑37 has 20 neutrons, so the neutron numbers differ.
Consequently the two isotopes have different mass numbers – 35 for chlorine‑35 and 37 for chlorine‑37 – because the mass number equals the total of protons and neutrons.

Examiner tips

  • Define isotopes first, then list shared properties (protons, electrons) before differences (neutrons, mass number).
  • Use the exact wording from the mark scheme – e.g. ‘different number of neutrons’ and ‘different mass numbers’.
  • Show the neutron counts (18 vs 20) to justify the mass numbers.
  • Keep the answer concise – 5 points, no extra explanation.

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. Chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons, so the number of neutrons is different
  5. The two isotopes have different mass numbers (35 and 37) because the mass number is the total number of protons and neutrons

Key terms in this question

isotope

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