Question

Why is the electon gain enthalpy of F less than that of Cl?

Answer 1

Even though Fluorine has the highest electronegativity among all the elements and it should have the highest electron gain enthalpy among all the halogens but 

this is an exception and chlorine has higher electron gain enthalpy than Fluorine. 

The reason for this is that the size of Fluorine atom is very small and hence there is very high inter-electronic repulsion among the electrons of fluorine. 

This makes incoming of another electron not very favourable. 

Even though fluorine has large negative electron gain enthalpy but for chlorine its even more negative.

Answer 2

Answer:

The Electron Gain Enthalpy of Fluorine is Less Negative than that of Chlorine

This is due to its small size. As a result of its small size, the electron-electron repulsion in the relatively compact to 2p subshell are comparatively large. Hence the incoming electron is not accepted with the same ease as is the case with chlorine.