Explain why the negative electron gain enthalpy is less for fluorine than for chlorine.

Explanation

The 2p -subshell of F-atom is smaller than the 3p - subshell of Cl. Therefore, the electron density of the 2p - subshell of F-atom is much higher than the 3p -subshell of `Cl^(-)` atom which is less compact than 2p -subshell. Consequently, the incoming electron faces greater force of repulsion when F forms `F^(-)` ion than when Cl forms `Cl^(-)` ion. As a result, the energy released during formation of `F^(-)(g)` from F(g) is less than that of the formation of `Cl^(-)(g)` from Cl(g).
`F(g)+etoF^(-)(g),Delta_(eg)H=-333kJ.mol^(-1)`
`Cl(g)+etoCl^(-)(g),Delta_(eg)H=-349kJ.mol^(-1)`
Thus,-ve electron gain enthalpy of F is lower than that of Cl.