Chlorine has two main isotopes: chlorine-35 and chlorine-37. Explain what is meant by the term isotope and describe how the atoms of these two isotopes of chlorine are similar and how they are different.

WJEC GCSE Chemistry (Wales) — 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.

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

Both isotopes also have 17 electrons, so they have the same electronic structure and therefore the same chemical properties.

The difference lies in the number of neutrons: chlorine‑35 has 18 neutrons (35−17) while chlorine‑37 has 20 neutrons (37−17), giving them different mass numbers.

Examiner tips

  • Define isotopes first, then state the common proton/electron count, then explain the neutron difference and its effect on mass.
  • Use the exact wording from the mark scheme: same protons, same electrons, different neutrons, different mass numbers.

Common mistakes

  • Confusing neutrons with protons or electrons; writing that the isotopes have different numbers of protons.
  • Failing to mention that the chemical properties are the same because the electronic structure is identical.

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) / same electronic structure / same chemical properties
  4. The two isotopes have different numbers of neutrons / chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons / different mass numbers

Key terms in this question

isotope

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