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.