Nitrogen has positive electron gain enthalpy whereas oxygen has negative. However, oxygen has lower ionisation enthalpy than nitrogen. Explain.

Electronic configuration of Nitrogen is 1s² 2s²2p_x¹2p_y¹2p_z¹

Electronic configuration of Oxygen is 1s² 2s²2p_x²2p_y¹2p_z¹

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.