Comment on each of the following observation:
a. The mobilities of the alkali metal ions in aqueous solution are `Li^(o+)ltNa^(o+)ltK^(o+)ltRb^(o+)ltCs^(o+)`.
b. Lithium is the only alkali metal to form a nitride directly.
c. `E^(ө)` for `M_(aq)^(2+)+2e^(-)to M_((s))` (where M=Ca,Sr or Ba) is nearly constant.

Explanation

Higher the charge density (charge/radius ratio) of the ion, more highly it is hydrated and hence greater is the mass of the hydrated ion and lesser is its ionic mobility.
Decreasing order of extent of hydration is
`Li^(o+)gtNa^(o+)gtK^(o+)gtRb^(o+)gtCs^(o+)`
therefore, ionic mobility increases in the same order:
`Li^(o+)ltNa^(o+)ltK^(o+)ltRb^(o+)ltCs^(o+)`
b. Because of the diagonal relationship of `Li` and `Mg`. lithium like magnesium forms a nitride while other alkali metals do not.
`6 Li_((s))+N_(2(g))overset(Delta)rarr2Li_(3)N_((s))`
c. `Li^(ө)` of any `M^(2+)//M` electrode depends upon three factors: (i) enthalpy of sublimation, (ii) ionisation enthalpy and (iii) enthalpy of hydration. Since the combined effect of these factors is approximately the same for `Ca,Sr` and `Ba`, therefore, their electrode potentials are nearly constant.