I have seen in my text book that for group 1 and 2 metals the hydration enthalpy decreases down the group.The reason I have found on the postWhich d-block cation has the maximum hydration enthalpy? is that the charge density decreases down the group.

I know that in aqua complexes water is joined to the central metal $\ce{[M(H2O)_n]^{z+}}$ atom mostly by coordinate bonding so we should not consider charge density of the central atom.

My assertion is that as the size of cation increases it becomes easier for the central atom to bond with water molecules,so hydration enthalpy should increase down the group as it is easier for central metal ion to accomodate water molecules.

  1. Why do we consider charge density instead of size of cation for considering hydration enthalpy?
  2. Why hydration enthalpy decreases instead of increasing down the group?

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