Hello Aman, 

The heat energy released when new bonds are made between the ions and water molecules is known as the hydration enthalpy of the ion.

The hydration enthalpy is the enthalpy change when 1 mole of gaseous ions dissolve in sufficient water to give an infinitely dilute solution. Hydration enthalpies are always negative.

The standard reduction potentials (E°) of the alkali metals do not follow the trend based on ionization energies. Unexpectedly, Lithium is the strongest reductant, and sodium is the weakest. Because Li+ is much smaller than the other alkali metal cations, it's hydration energy is the highest.