Why Softness of alkali metals increases from Li to Cs ?
I have to admit, that physical properties of elements depend on the chemical. Therefore, softness is a result of one (ns1) valence electron configuration in the outermost s-orbital. Increasing atomic and ionic radii in the group from Li to Cs and close packing of atoms results metallic crystal is weak. That's why a softness tends to increase in Group I of Periodic System.