Question

The ionic radii of alkali metal ions in water (hydrated radii) are in the order

• A. Li+> Na+> K+>Rb+< Cs+
• B. Li⁺ (aq.) > Na+ (aq.) > K+ (aq.) >Rb+ (aq.) > Cs+ (aq.)
• C. LI+< Na+> K+>Rb+> Cs+
• D. Li+ (aq.) > Na+ (aq.) < K+ (aq.) <Rb+ (aq.) < Cs+ (aq.)

Answer 1

The correct option is B

Li⁺ (aq.) > Na+ (aq.) > K+ (aq.) >Rb+ (aq.) > Cs+ (aq.)

If we just look at only the ionic radii - then clearly $L{i}^{+}$ should be the smallest and common sense might compel us to think that the small ion should exhibit the best conductivity. However, among alkali metal cations - $L{i}^{+}$ has the highest charge density and is thus hydrated to the largest extent. Thus $L{i}^{+}$ has the highest hydrated ion radius and least conductivity in water.