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.

WJEC GCSE Chemistry (Wales) — 2.1 Bonding, structure and properties · 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, so they are the same element.

They also have 17 electrons, so their chemical behaviour is identical.

The difference lies in their neutrons: chlorine‑35 has 18 neutrons (35‑17) while chlorine‑37 has 20 neutrons (37‑17).

Examiner tips

  • Use the definition of isotope first, then list the common proton/electron numbers, finally state the neutron difference.
  • Show the calculation of neutrons (mass number‑protons).

Common mistakes

  • Confusing mass number with atomic number; writing 35 and 37 as proton numbers.
  • Forgetting to mention the electrons or giving the wrong neutron counts.

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; chlorine-37 has 20 neutrons).

Key terms in this question

isotope

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