$\mathrm{Rb} \& \mathrm{Cs}$ have nearly same electron gain enthalpy Their values are nearly $=-46 \mathrm{kJ}/\mathrm{mol}$

$\mathrm{Ar} \& \mathrm{Kr}$ also have same ${\mathrm{\Delta H}}_{\mathrm{e}.\mathrm{g}.}$. Value is $+96 \mathrm{kJ}/\mathrm{mol}$.

$\mathrm{Na} \mathrm{and} \mathrm{K}$ have different values $(-53 \mathrm{and} -48 \mathrm{kJ}/\mathrm{mol})$. And $\mathrm{I} \mathrm{and} \mathrm{At} \mathrm{has} \mathrm{values} \mathrm{of} -295 \mathrm{and} -270 \mathrm{kJ}/\mathrm{mol}$. 

As we move down the group value of electron gain enthalpy decreases, with few exceptions.