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 — 2.1 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 contain 17 protons, so they are the same element.

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

The difference lies in the neutrons: chlorine‑35 has 18 neutrons, while chlorine‑37 has 20 neutrons.

Examiner tips

  • Use the definition of isotope first, then list the common features (protons, electrons) before the difference (neutrons).
  • Show the neutron numbers explicitly (35‑18=17, 37‑20=17) to demonstrate calculation.
  • Keep the answer concise – 4 marks, so one sentence per point is sufficient.

Common mistakes

  • Confusing the number of electrons with the number of neutrons; students sometimes say the isotopes have different electrons.
  • Failing to state the exact neutron numbers for each isotope; giving only a general statement like "different numbers 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).
  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

Related

More Bonding, structure and properties questions

▶ Try answering this question with AI marking (free) →