Comment on each of the following observations:
(a) The mobilities of the alkali metal ions in aqueous solution are `Li^(+)ltNa^(+) lt K^(+) lt Rb^(+) lt Cs^(+)`
(b) Lithium is the only alkali metal to form a nitride directly
(c) `E^(Theta)` for `M^(2+)` (aq) `+2e^(-)rarrM(s)` (where M=Ca, Sr or Ba) is nearly constant

Explanation

The value of `E^@` for a system depends upon three factors (i) enthalpy of vaporisation, (ii) ionisation enthalpy and (iii) hydration enthalpy. The combined effect of these three factors is approximately the same for Ca, Sr and Ba. Therefore, they possess nearly the same value of standard electrode potential.