Explain the electron gain enthalpy values of elements of the $17^{\text{th}}$ group. Give reasons and explain.

(N/A) The electron gain enthalpy values of $17^{\text{th}}$ group elements (halogens) are highly negative because they have a general electronic configuration of $ns^{2} np^{5}$. They require only one electron to achieve the stable noble gas configuration $(ns^{2} np^{6})$,making the process highly exothermic.
Although fluorine $(F)$ has the smallest size,its electron gain enthalpy is less negative than that of chlorine $(Cl)$. This is due to the strong inter-electronic repulsions in the relatively small $2p$ subshell of fluorine,which makes the addition of an electron less favorable compared to chlorine.
As we move down the group from $Cl$ to $At$,the atomic size increases,which decreases the effective nuclear attraction for the incoming electron,resulting in a decrease in the magnitude of negative electron gain enthalpy.

Element	$F$	$Cl$	$Br$	$I$	$At$
$\Delta_{eg} H \ (kJ \ mol^{-1})$	$-328$	$-349$	$-325$	$-295$	$-279$