(a) The alkali metal ions are highly hydrated. The smaller the size of the ion, the greater is the degree of hydration. The Li+ ion gets most hydrated and Cs+ ion least.
The result in decrease in hydrated ionic radii in the order :
Li+ > Na+ > K+ > Rb+ > Cs+
As a result, the hydrated Li+ ion being largest in ionic size has the lowest mobility in water and Cs+ ion has the highest mobility and follow the order
Li+ < Na+ < K+ < Rb+ < Cs+
©© Uses of plaster of paris :
(i) It is used for casting statues and busts.
(ii) It is used for immobilising the affected part of the organ where there is bone fracture or sprain.