Electron Affinity
The energy change when one mole of gaseous atoms each gains an electron to form gaseous negative ions.
Formula / equation
X(g) + e⁻ → X⁻(g)
Unit
kJ/mol
In depth
First electron affinities are usually exothermic because the added electron is attracted to the nucleus, but second affinities are endothermic since the electron must overcome an existing negative charge. Chlorine exceeds fluorine because fluorine's very small 2p subshell suffers high electron–electron repulsion.
Examples in the real world
Chlorine releases 349 kJ/mol, fluorine 328 kJ/mol, while adding a second electron to oxygen costs about +744 kJ/mol.