The negative value of electron gain enthalpy is less for fluorine than for chlorine . Why?

Explanation

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 CI atom. Consequently, the energy released during formation of `F^(-)`(g) form F(g) is little less than during the formation of `CI^(-)`(g) from CI (g)
`F(g) + e^(-) rarr F^(-)(g), Delta_(eg) = -333 kJ mol^(-1) and CI(g) + e^(-) rarr CI^(-)(g), Delta_(eg) = -349 kJ mol^(-1)`
Thus, negative electron gain enthalpy of F is lower than that of Cl.