WAEC Chemistry 2025 Paper

Concept:Isoelectronic ions have the same number of electrons, so their ionic size decreases as nuclear charge increases.Explanation:All four ions contain $10$ electrons: $\mathrm{F^-}$, $\mathrm{Na^+}$, $\mathrm{Mg^{2+}}$, and $\mathrm{Al^{3+}}$.Their nuclear charges are $\mathrm{F} = 9$, $\mathrm{Na} = 11$, $\mathrm{Mg} = 12$, and $\mathrm{Al} = 13$.A higher nuclear charge pulls the surrounding electrons more strongly toward the nucleus, reducing the ionic radius.Therefore, the decreasing order of ionic size is:$$\mathrm{F^-} > \mathrm{Na^+} > \mathrm{Mg^{2+}} > \mathrm{Al^{3+}}$$Answer:Option A