Q.Comment on each of the following observations:

(a) Ionic mobility Li$^+$ < Na$^+$ < K$^+$ < Rb$^+$ < Cs$^+$

Although the bare (crystallographic) ionic radius increases down the group, the degree of hydration decreases down the group: Li$^+$, being smallest with the highest charge density, is hydrated most strongly and carries the largest shell of water molecules with it in solution, giving it the largest effective (hydrated) size and hence the lowest mobility. Cs$^+$, being largest with the lowest charge density, is only weakly hydrated, has the smallest hydrated radius, and therefore moves fastest under an applied field, giving it the highest mobility.

(b) Lithium is the only alkali metal to form a nitride directly

Lithium's small size gives Li$^+$ an unusually high charge density/polarising power for an alkali metal ion (diagonal relationship with Mg, which also forms a nitride directly). This lets Li combine directly with atmospheric N$_2$ at ordinary temperature to form the ionic nitride:

The heavier alkali metals lack the charge density to stabilise a nitride lattice in this way, so they do not react directly with N$_2$.

(c) $E^\ominus$ for M$^{2+}$(aq) + 2e$^-$ → M(s) is nearly constant for Ca, Sr, Ba …

For each observation, identify the underlying thermochemical/structural cause: (a) hydrated-ion size versus bare-ion size and its effect on mobility;

(b) Li's exceptional charge density (diagonal relationship) enabling direct nitride formation; …

• For (a), reasoning purely from crystallographic ionic radius and concluding Li$^+$ should be most mobile — the correct factor is the larger, hydrated (effective) radius in solution.
• For (b), forgetting to connect the answer to Li's small size/diagonal relationship with Mg, and just stating 'Li is reactive' without the specific charge-density reasoning. …