A-Level Chemistry May/June 2024 Q1(b): State and explain the difference in the ionic radius of Al$^{3+}$ compared to Mg$^{2+}$.

1 answer

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  The Al³⁺ ion is smaller than the Mg²⁺ ion.

  Both Al³⁺ and Mg²⁺ are isoelectronic, meaning they have the same number of electrons (10) and the same electron configuration (1s²2s²2p⁶). Therefore, the shielding effect from inner electron shells is the same for both ions.

  However, an aluminium nucleus contains 13 protons, whereas a magnesium nucleus contains 12 protons. This means Al³⁺ has a greater nuclear charge. The greater nuclear charge results in a stronger nuclear attraction for the same number of electrons, pulling the electron shells closer to the nucleus and resulting in a smaller ionic radius.

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  How the marks are awarded

  • M1 — Correctly stating that the Al³⁺ ion is smaller than the Mg²⁺ ion.
  • M2 — Explaining that the result is a greater or stronger nuclear attraction for the remaining electrons in Al³⁺.
  • M3 — Stating both that the shielding effect is the same (as they are isoelectronic) AND that Al³⁺ has a greater nuclear charge due to having more protons.

  Common mistakes

  • Stating that Al³⁺ is smaller because it has a greater ionic charge (+3 vs +2), without mentioning nuclear charge or protons.
  • Incorrectly stating that Al³⁺ has less shielding because it has lost more electrons, failing to recognise the ions are isoelectronic.
  • Only mentioning the greater number of protons in Al³⁺ without explaining that shielding is the same, which is an incomplete explanation.
  • Getting the size comparison wrong and stating that Mg²⁺ is smaller.

  Examiner tip: When comparing the radii of isoelectronic species, always analyse the number of protons (nuclear charge) versus the number of electrons and shells (shielding) to determine the net electrostatic attraction.

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