Neon exists as three naturally occurring isotopes: neon-20, neon-21, and neon-22. State what is meant by the term isotopes, and describe how the atoms of neon-20 and neon-22 are similar and how they differ.

Cambridge International IGCSE Chemistry (0620) — 2.3 Isotopes · State and 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 different numbers of neutrons.
Neon‑20 and neon‑22 both have 10 protons and therefore 10 electrons, giving them the same electronic structure. They differ in their neutron number: neon‑20 has 10 neutrons, while neon‑22 has 12 neutrons.

Examiner tips

  • Use the definition of isotopes first, then list the common features (protons, electrons) before the difference (neutrons).
  • Show the neutron counts explicitly (10 vs 12) to demonstrate understanding.

Common mistakes

  • Confusing isotopes with isotones (same neutrons, different protons).
  • Omitting the electron count or electronic structure.

Mark scheme (4 marks)

  1. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons
  2. Neon-20 and neon-22 have the same number of protons (10 protons)
  3. Neon-20 and neon-22 have the same number of electrons (10 electrons) / same electronic structure
  4. Neon-20 and neon-22 have different numbers of neutrons (neon-20 has 10 neutrons; neon-22 has 12 neutrons)

Key terms in this question

isotopes

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