Explain why nitrogen has positive electron gain enthalpy whereas oxygen has negative; although first ionisation enthalpy of oxygen is lower than that of nitrogen. Justify your answer.
The outermost electronic configuration of nitrogen is 1s22s22p3.
Nitrogen has stable half filled configuration and therefore it has no tendency to accept electron therefore, energy has to be supplied in order to add an electron to it. Thus, nitrogen has positive electron gain enthalpy.
On the other hand, the outermost electronic configuration of oxygen is 1s22s22p4. It does not have half filled or fully filled stable configuration so it shows tendency to accept electron and thus, it has negative electron gain enthalpy.
Although first ionisation enthalpy of oxygen is lower than that of nitrogen because O has 4 electrons in 2p subshell and it can lose one electron to acquire half filled stable electronic configuration and therefore, it has low ionization enthalpy. On the other hand N already have stable half filled electronic configuration so it would not readily lose its electron and thus, N has higher ionisation enthalpy than O.