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.

Eduqas GCSE Chemistry — 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 contain 17 protons.
Both isotopes also contain 17 electrons.
Chlorine‑35 has 18 neutrons, whereas chlorine‑37 has 20 neutrons.

Examiner tips

  • Use the definition of isotope first; this shows you understand the concept. State the common proton and electron numbers before giving the neutron difference. Keep the answer concise – 4 marks can be earned with 4 clear points.

Common mistakes

  • Confusing isotopes with ions – students may say the electron number differs. Giving the wrong neutron count (e.g. 17 neutrons for Cl‑35). Using vague terms like “different” without specifying neutrons.

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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