Comment on each of the following observations:
$(a)$ The mobilities of the alkali metal ions in aqueous solution are $Li^{+} < Na^{+} < K^{+} < Rb^{+} < Cs^{+}$
$(b)$ Lithium is the only alkali metal to form a nitride directly.
$(c)$ The standard electrode potential $E^{\circ}$ for the reaction $M^{2+}_{(aq)} + 2e^{-} \to M_{(s)}$ (where $M = Ca, Sr$ or $Ba$) is nearly constant.

(N/A) On moving down the alkali group,the ionic sizes of the metals increase in the order: $Li^{+} < Na^{+} < K^{+} < Rb^{+} < Cs^{+}$. Smaller ions have a higher charge density and are more heavily hydrated in aqueous solution. Thus,the extent of hydration decreases as: $Li^{+} > Na^{+} > K^{+} > Rb^{+} > Cs^{+}$. Since a larger hydrated ion moves more slowly,the ionic mobility increases as the size of the hydrated ion decreases,resulting in the order: $Li^{+} < Na^{+} < K^{+} < Rb^{+} < Cs^{+}$.
$(b)$ $Li$ is the only alkali metal that reacts directly with nitrogen to form lithium nitride $(Li_{3}N)$. This is because the small size of the $Li^{+}$ ion is highly compatible with the small $N^{3-}$ ion,leading to a very high lattice energy that compensates for the energy required to form the $N^{3-}$ ion.
$(c)$ The standard electrode potential $(E^{\circ})$ for $M^{2+}/M$ depends on the sum of sublimation enthalpy,ionization enthalpy,and hydration enthalpy. For $Ca, Sr,$ and $Ba$,the combined effect of these energy terms remains nearly constant,resulting in similar electrode potentials.