Question

arrange F.Cl,Br and I in the order of increasing electronic gain enthalpy

Answer 1

Task:

arrange F.Cl,Br and I in the order of increasing electronic gain enthalpy

Answer:

In a group from top to bottom, the electron gain enthalpy become less negative due to increasing size from F to I.

But Fluorine shows less negative electron gain enthalpy than chlorine. This is due to small size and $2s^2$, $2p^5$ configuration of fluorine. There is strong electron-electron repulsion in compact 2p-orbital.