why fluorine and oxygen have different behaviour towards electron gain enthalapy and electronegativity? [closed]

Question

I have seen following definitions in my textbook:

Electron gain enthalapy may be defined as enthalpy change taking place when an isolated gaseous atom of the element accepts an electron to form a monovalent gaseous anion.

Electronegativity may be defined as the tendency of an atom in molecule to attract towards itself the shared pair of an electron.

I know that fluorine and Oxygen have low ${\Delta H_{eg}}$ than Chlorine and Sulpher respectively due to the fact when an electron is added to these atoms, the electron goes to the relatively compact second energy level which results in significant repulsion from other electrons present in the shell.

But I have seen that Fluorine and Oxygen have very high electronegativity.

(i)I can't understand how can an atom attract shared pair of electron very well when it is unwilling to add elctron in its valence shell?

Answer 1

The reaction between atomic hydrogen and atomic halogen can be considered as a two-step reaction: single electron transfer furnished by the formation of bond between halogen anion and hydrogen cation. Even if the real mechanism is different, it is of no importance as long as we consider thermodynamics.

The second step, from the point of view of pure electrostatics, should release more energy in the case of fluorine than in the case of chlorine because the shorter bond is formed.