Chlorine has two stable isotopes: chlorine-35 and chlorine-37. Describe what is meant by the term isotope and state how the two isotopes of chlorine are similar and how they are different in terms of their subatomic particles.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.3 Atomic structure · Describe · 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 isotopes of chlorine have 17 protons.
Both isotopes have 17 electrons.
Chlorine‑35 has 18 neutrons, while chlorine‑37 has 20 neutrons.

Examiner tips

  • Use the definition of isotope first. Mention protons and electrons are identical. State the neutron numbers for each isotope. Keep the answer concise to fit 4 marks.

Common mistakes

  • Confusing mass number with neutron number. Forgetting to mention the electrons. Saying the isotopes have different numbers of protons instead of 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 each)
  3. Both isotopes have the same number of electrons (17 electrons each)
  4. The two isotopes have different numbers of neutrons (chlorine-35 has 18 neutrons; chlorine-37 has 20 neutrons)

Key terms in this question

isotope

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