Hydration energy of the given ions follows the order:

Hydration energy of the given ions IS, ${\mathrm{Li}}^{+}>{\mathrm{Na}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Rb}}^{+}>{\mathrm{Cs}}^{+}$

It is challenging for the metal to be surrounded by water molecules due to the alkali metal's rise in size. 

Therefore, when size increases, less energy is released; for example, as one moves from top to bottom in a group, ${\mathrm{Rb}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Na}}^{+}>{\mathrm{Li}}^{+}$, size increases. hydrating energy in ascending order
${\mathrm{Li}}^{+}>{\mathrm{Na}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Rb}}^{+}$