Why Fluorine atom has unexpectedly less negative `Delta_("eg")` H than chlorine.

Explanation

`Delta` Hg of Fluorine is - 328 KJ/mol
`Delta` Heg of chlorine is - 349 KJ/mol
This is due to the small size of fluorine atom. 2p orbital of fluorine are smaller than the 3p orbitals of chlorine As a result of relatively small 2p - subshell in fluorine atom , the electron - electron repulsions are comparatively large and hence the incoming election is not easily accepted , when compared with chlorine . In other words , a little endothermic is involved in forcing the electron in small 2p - subshell
Consequently , `Delta` Heg Hey of fluorine i . e less negative than that of chlorine .