Strictly, electron gain enthalpy ($Delta_{eg}$ H) is the IUPAC-preferred term: $Delta_{eg}$ H = enthalpy change when X(g) + $e^-$ → $X^-$(g). It is negative when energy is released (exothermic, favourable). Electron affinity (EA) is the older term, sometimes defined with opposite sign convention. In JEE, both terms are used interchangeably. The key values: Cl has the most negative $Delta_{eg}$ H (-349 kJ/mol), not F (-328). Second EA is always positive (adding electron to an already negative ion requires energy): O(g) + $e^-$ → $O^-$(g), $Delta_{eg}$ H1 = -141 kJ/mol. $O^-$(g) + $e^-$ → $O^{2-}$(g), $Delta_{eg}$ H2 = +780 kJ/mol. Overall formation of $O^{2-}$ is endothermic (+639 kJ/mol) — it exists in ionic crystals because lattice energy compensates.