Explain why the electron gain enthalpy of fluorine is less negative than that of chlorine.
The added electron in fluorine atom goes to the second quantum level, and due to very small size of fluorine, it experiences repulsion from other electrons much more in comparison to that in chlorine because in chlorine, the electron is added to 3rd quantum level and chlorine, on the other hand, is not that small, so it experiences a much lower repulsion.