JEE Main · 2019 · 08 Apr 2019 · Shift 1 · s-Block Elements

All alkali-metal ions carry the same $+1$ charge, so their hydration enthalpy follows the charge density, which rises as the ion shrinks. The ionic radius grows steadily down the group, $\mathrm{L}{\mathrm{i}}^{+}<\mathrm{N}{\mathrm{a}}^{+}<{\mathrm{K}}^{+}<\mathrm{R}{\mathrm{b}}^{+}<\mathrm{C}{\mathrm{s}}^{+}$, so the smallest $\mathrm{L}{\mathrm{i}}^{+}$ binds water most strongly and the hydration enthalpy falls as $\mathrm{L}{\mathrm{i}}^{+}>\mathrm{N}{\mathrm{a}}^{+}>{\mathrm{K}}^{+}>\mathrm{R}{\mathrm{b}}^{+}>\mathrm{C}{\mathrm{s}}^{+}$.