(a) On moving down the group, the atomic and ionic sizes of alkali metals increases. As the size increases, the extent of hydration decreases.
The extent of hydration and ionic mobility are inversely proportional.
Hence, the increasing order of the ionic mobilities of alkali metals is
$$\displaystyle Li^+\, <\, Na^+\, <\, K^+\, <\, Rb^+\, <\, Cs^+ $$.
(b) Small sized Li ion is most compatible with nitride ion. Thus, in formation of lithium nitride, large amount of lattice energy is released. This energy overcomes the high amount of energy required for formation of nitride ion.
(c) Following factors are responsible for the electrode potential value.
(a) Ionization enthalpy
(ii) Hydration enthalpy
(iii) Enthlpy of vaporisation.
For $$Ba, Sr$$ and $$Ca$$, the net effect of above factors is same.
Hence, their electrode potentials are nearly constant.