Question

why chlorine have more negative electron gain enthalpy than fluorine

Answer 1

since fluorine has small size and has more screening effect due to which and hence tendency to gain an electron is less than that of chlorine

Answer 2

Answer:

Due to its small size, the electron–electron repulsions in the relatively compact 2p-sub-shell are comparatively large, and hence, the incoming electron is not accepted with the same ease as is the case with larger Cl atom. Consequently, electron gain enthalpy of Cl is more negative than that of F.