On moving down the group a new shell is progressively added. Although, the nuclear charge also increases down the group but the effect of addition of new shells is more predominant due to increasing screening effect of inner filled shell on the valence s-electrons. Hence the atomic size increases in a group.Alkali metals change into positively charged ions by losing their valence electron. The size of cation is smaller than parent atom of alkali metals. However, within the group the ionic radii increase with increases in atomic number.The alkali metal ions get extensively hydrated in aqueous solutions. Smaller the ion more is the extent or degree of hydration. Thus, the ionic radii in aqueous solution follow the orderLi+ > Na+ > K+ > Rb+ > Cs+              The charge density on Li+ is higher in comparison to other alkali metals due to which it is extensively hydrated.Ionization Energy (Ionization enthalpy)
This is because a new valence shell is added to the atoms of the elements at each succeeding elements