Ionic mobility of which of the following alkali metal ions is lowest when aqueous solution of their salts are put under an electric field?
As we go down the group, the size of the ionic radius increases. This implies that the charge per surface area (or charge density = charge per unit volume) is the highest for Li+. Hence, in the solvated state, the hydration sphere is the largest for Li+. Hence, its ionic mobility will also be the lowest.