The hydration enthalpy decreases with the increase in atomic number down the group due to their increasing size. Smaller atoms can hold more molecules of water but with the increase in atomic number more valence shells addds to the atom thereby ,making it large and reducing the hydration enthalpy.

In alkali metals (Group 1) the hydration enthalpy is as follows:

Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺

In alkaline earth metals (Group 2) the hydration enthalpy is as follows:

Be⁺² > Mg⁺² > Ca⁺² > Sr⁺² > Ba⁺²