Oxygen has two naturally occurring isotopes, oxygen-16 and oxygen-18. Explain what is meant by the term isotope and describe how oxygen-16 and oxygen-18 are both similar to and different from each other in terms of their subatomic particles.

Eduqas A-Level Chemistry — 2.3 Transition metals (A-Level only) · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.

Both oxygen‑16 and oxygen‑18 have 8 protons.

Both have 8 electrons.

Oxygen‑16 has 8 neutrons, whereas oxygen‑18 has 10 neutrons.

Because the neutron numbers differ, the two isotopes have different mass numbers and therefore different masses.

Examiner tips

  • Use the definition of isotope first. Show the common proton/electron count. State the neutron numbers. Explain the consequence for mass.
  • Use the exact wording "different number of neutrons" and "different mass" to match the scheme.

Common mistakes

  • Confusing isotopes with ions (changing electron count). Forgetting to mention the same proton number. Saying the isotopes have different chemical properties instead of mass.

Mark scheme (5 marks)

  1. Isotopes are atoms of the same element with the same number of protons but a different number of neutrons.
  2. Both oxygen-16 and oxygen-18 have the same number of protons (8 protons).
  3. Both oxygen-16 and oxygen-18 have the same number of electrons (8 electrons).
  4. Oxygen-16 has 8 neutrons and oxygen-18 has 10 neutrons, so they have different numbers of neutrons.
  5. The different numbers of neutrons mean oxygen-16 and oxygen-18 have different mass numbers / different masses.

Key terms in this question

isotope

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