Explain why the electron gain enthalpy of fluorine is less negative than that of chlorine?
Electron gain enthalpy of F is less negative than that of Cl because when an electron is added to F, the added electron goes to the smaller n=2 quantum level and suffers repulsion from other electrons present in this level.
In case of Cl, the added electron goes to the larger n=3 quantum level and suffers much less repulsion from other electrons.
