Question

Among alkali metals in aqueous solution , li+ ion has lowest mobility . why??/

Answer 1

 In general smaller the size of the ion higher will be hydration tendency. Among alkali metals Li+ ion maximum hyration capacity. Since lithium posses higher hydration because of which its ionic radii increases thus causes reduction in its mobility

Answer 2

Since Li+ is the smallest,among the other elements of group 1,( also it has a high charge /mass ratio, which makes the water molecules more attracted towards the Li+ ion) it is more easily and densely hydrated by the water molecules, as compared to the much bigger ions of the respective group, i.e. it has the largest hydration sphere. Therefore , the dense hydration makes is difficult for Li+ ion to move freely . Thus, its mobility is hindered.