Electronic configuration of Nitrogen is 1s2 2s22px12py12pz1
Electronic configuration of Oxygen is 1s2 2s22px22py12pz1
As nitrogen has half-filled stable p orbital therefore it has a positive electron gain enthalpy.
The ionisation enthalpy of oxygen is lower than that of Nitrogen as because when we remove one electron from oxygen then it easily donates it to attain half-filled stability but in case of nitrogen it is difficult to remove one electron as it already has half-filled stability and it would become unstable after that.
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