Which atom releases the most energy when it gains one electron in the gas phase?

Core Concept

Chlorine's added electron enters roomy 3p orbitals, where strong nuclear attraction meets little electron-electron repulsion, giving the most exothermic electron affinity of any element (about -349 kJ/mol). Fluorine's newcomer crowds into the compact 2p subshell, where repulsion cancels part of the pull, so fluorine releases less (about -328 kJ/mol). Highest electronegativity and most exothermic electron affinity are therefore different titles.