WAEC Chemistry 2021 Paper

Concept:For isoelectronic ions, the ionic radius increases as the nuclear charge decreases.Explanation:The species $\mathrm{S}^{2-}$, $\mathrm{Cl}^{-}$, $\mathrm{K}^{+}$ and $\mathrm{Ca}^{2+}$ are isoelectronic because each contains $18$ electrons.Their nuclear charges are: sulfur $= 16$, chlorine $= 17$, potassium $= 19$, and calcium $= 20$.Since the number of electrons is the same, the ion with the smallest positive nuclear charge attracts its electrons least strongly.A weaker attraction allows the electron cloud to spread out more, giving a larger radius.Sulfur has the lowest nuclear charge among these isoelectronic ions.Therefore, the sulfide ion, $\mathrm{S}^{2-}$, has the largest ionic radius.Answer:A. $\mathrm{S}^{2-}$ (sulfide ion)