Question

Why f has less negative electron gain enthalpy than cl?

Answer 1

Fluorine is highly electronegative. It has a great affinity for the electrons but is not capable of being stable by taking an electron. Wheareas Chlorine has its vaccant d orbitals to give enough vaccancy for the electrons & to be stable. Thus, electron gain enthalpy that denotes stability after gaining electron is higher for the Chlorine than for the Fluorine.
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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.