Dear Student,
Helium has highest ionisation enthalpy.
The first ionization energy decreases from top to bottom in groups, and increases from left to right across in a period. Therefore, helium has the highest first ionization enthalpy.
Chlorine has more electron gain enthalpy as compared to fluorine because of the small size of the fluorine so addition of electron is not favorable process because repulsion between the electrons takes place.
Regards