Oxygen is the second most electronegative element in comparison to fluorine. In group - 16 family (O,S,Se,Te),O-atom is smallest in size. So, electron density on O-atom is very high in group −16. During addition of a free electron to gaseous O-atom, O(g)+e−⟶O−(g) We have to supply a significant amount of energy (endothermic) to overcome the electrostatic repulsion between the approaching electron and O-atom of very high electron density. So, the net value of electron affinity (EA) or (negative) electron gain enthalpy [∆egH or |∆egH|] of oxygen decreases to a higher extent in comparison to other elements of group - 16 who have larger size and lower electronegativity. So, the correct order of EA or |∆egH| of group −16 elements will be S>Se>Te>O.