Why does hydration enthalpy decrease with decrease in size.

Enthalpy of hydration, H_hyd, of an ion is the amount of energy released when a mole of the ion dissolves in a large amount of water forming an infinite dilute solution in the process,

 

M^z+(g) + mH₂O ® M^z+(aq)

 

where M^z+(aq) represents ions surrounded by water molecules and dispersed in the solution. 

 

As the atomic numbers increases, so do the ionic size, leading to a decrease in absolute values of enthalpy of hydration. For exmple; 

 

Enthalpy of Hydration (H_hyd kJ/mol) of Some Typical Ions

Ion	Hhyd	Ion	Hhyd	Ion	Hhyd
H+	-1130	Al3+	-4665	Fe3+	-4430

Li+	-520	Be2+	-2494	F-	-505
Na+	-406	Mg2+	-1921	Cl-	-363