Question

Why does chlorine has the highest electron gain enthalpy instead of fluorine?

Answer 1

Chlorine has a more negative electron gain enthalpy than fluorine because the incoming electron enters the larger 3p orbital of chlorine, where there is less electron–electron repulsion. In fluorine, the electron enters the very small 2p orbital, where repulsion is greater.

Therefore, more energy is released when chlorine gains an electron.