Hydration and Ionic Mobility Trends

Going down Group 1, the bare (unhydrated) ionic radius increases (Li+ smallest, Cs+ largest), but the CHARGE DENSITY (charge concentrated over a smaller volume) is highest for the smallest ion. A small, high-charge-density ion like Li+ attracts and holds water molecules around itself much more strongly than a large, diffuse ion like Cs+ does, so the degree of hydration follows Li+ > Na+ > K+ > Rb+ > Cs+, decreasing down the group even though the bare ionic radius itself is increasing. The same logic applies across Group 2: Be2+, being both small and doubly charged, hydrates most strongly of all common s-block cations, giving it the largest effective hydrated radius and making the hydrated beryllium ion, [Be(H2O)n]2+, the LEAST mobile ion in solution among typical Group 1/2 cations — its true (hydrated) size in solution is far larger than its bare ionic radius would suggest.

Ionic mobility in solution depends on the EFFECTIVE (hydrated) size of the ion, not the bare ionic radius — a small ion carrying a bulky, tightly-bound shell of water molecules has to drag that whole hydration shell through solution, making it move sluggishly despite its small bare size. Because hydration decreases down Group 1 (as established above), the hydrated ion size actually DECREASES down the group too, so ionic mobility INCREASES down the group: Cs+ > Rb+ > K+ > Na+ > Li+, exactly the reverse order of the degree-of-hydration ranking. Recognise this pair of trends as two sides of the same coin — whichever ion hydrates the most heavily is, for that very reason, the least mobile in solution, not the most mobile.