A-Level Chemistry May/June 2025 Q2(e): The atomic radius of iron is 1.26 × 10–10m. Suggest the change to the radius, if any, a…

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  The radius of the ion will be smaller than the atomic radius.

  Explanation: When an iron atom (Fe) reacts to form an iron(III) ion (Fe³⁺), it loses its three outermost electrons. This results in the complete loss of the outer electron shell. Furthermore, the remaining electrons are now being attracted by the same number of protons in the nucleus (26 protons). This leads to a greater effective nuclear charge and a stronger electrostatic force of attraction on each of the remaining electrons, pulling them closer to the nucleus.

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  How the marks are awarded

  • M1 — Correctly stating that the radius will be smaller or will decrease.
  • M2 — Providing a correct explanation, such as the loss of the outer electron shell, or the increased force of attraction from the nucleus on the remaining electrons.

  Common mistakes

  • Stating that the radius increases, confusing the formation of a cation with an anion.
  • Giving a vague explanation like 'it loses electrons' without explaining the consequence on the electron shells or the force of attraction.
  • Incorrectly stating that the number of protons or the nuclear charge increases.
  • Explaining that the ion is smaller because it has fewer electrons, without linking this to the unchanged nuclear pull.

  Examiner tip: Always explain changes in atomic or ionic radius by considering the number of protons in the nucleus relative to the number and arrangement of electrons in their shells.

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