Explain why the electrons gain enthalpy of fluorine is less negative than that of chlorine?

Answer

Due to small size the electron-electron repulsions in the relatively compact (small) 2p-subshell of F are comparatively large and hence the incoming electron is not accepted with the same ease as is the case with the relatively bigger 3p subshell of Cl atom.Consequently, the energy released during formation of `F^(-)(g)` from F(g) is little less than that of the formation of `CI^(-)(g)` from `Cl_(g)`
`F(g)+e^(-)toF^(-)(g) ; Delta_(eg)=-333 kJ mol^(-1)` and `Cl (g) + e^(-)toCl^(-)(g) ; Delta_(eg)=-349 kJ mol^(-1)`
Thus, negative electron gain enthalpy of F is lower than that of cl.