Chlorine has two naturally occurring isotopes: chlorine-35 and chlorine-37. Describe what is meant by the term isotope and explain what is the same and what is different about the nuclei of these two isotopes of chlorine.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.2 Atoms · 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 in their nuclei.
Chlorine‑35 contains 18 neutrons, whereas chlorine‑37 contains 20 neutrons.
Consequently the mass numbers differ: 35 for chlorine‑35 and 37 for chlorine‑37.

Examiner tips

  • Use the definition of isotope first, then list the common proton number, the differing neutron numbers and the resulting mass numbers.
  • Show the neutron counts explicitly (18 vs 20) to demonstrate the difference.
  • Mention the mass numbers to cover all points of the mark scheme.

Common mistakes

  • Confusing the number of protons with the number of neutrons; students sometimes state the wrong neutron count.
  • Omitting the mass numbers or only giving one of them; the scheme requires both 35 and 37 to be mentioned.

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) in the nucleus
  3. Chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons / the number of neutrons is different
  4. The mass numbers (nucleon numbers) of the two isotopes are different / chlorine-35 has mass number 35 and chlorine-37 has mass number 37

Key terms in this question

isotope

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