The decreasing order of hydration enthalpies of alkaline earth metal ions is

Be2+ > Mg2+ > Ca2+ > Sr2+ > Ba2+  : Size of isolated M+2  ionsBe2+ > Mg2+ > Ca2+ > Sr2+ > Ba2+ : Degree of hydrationBe2+ > Mg2+ > Ca2+ > Sr2+ > Ba2+ : Hydration Enthalpies As the size of M+2 ions [isolated] increases effective nuclear charge on the ion decreases nuclear attractive forces on the water molecules decreases, number of water molecules surrounded around the M+2 ion decreases, degree of hydration decreases, hydration enthalpy decreases