Chemistry - Complete s-block Question with Solution | TestHub

The alkali metal ions exist as hydrated ions,  ${\left[\mathrm{M}{\left({\mathrm{H}}_2\mathrm{O}\right)}_{\mathrm{x}}\right]}^{+}$, in the aqueous solution. The degree of hydration, however, decreases with the increasing ionic size as we move from ${\mathrm{Li}}^{+}$ to ${\mathrm{Cs}}^{+}$. In other words, ${\mathrm{Li}}^{+}$ ion is the most hydrated. e.g. ${\left[\mathrm{Li}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{+}$.

Since the mobility of ion is inversely proportional to the size of their hydrated ions, therefore, amongst the alkali metal ions, lithium has the lowest ionic mobility.

${\text{Rb}}_{}^{+}\left(aq\right)>{\text{K}}_{}^{+}\left(aq\right)>{\text{Na}}_{}^{+}\left(aq\right)>{\text{Li}}_{}^{+}\left(aq\right)$