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

(N/A) The electronic configuration of ${}_{7}N$ is $1s^{2}, 2s^{2}, 2p^{3}$. Nitrogen has a stable half-filled $p$-orbital configuration,which makes the addition of an extra electron unfavorable,resulting in a positive electron gain enthalpy.
The electronic configuration of ${}_{8}O$ is $1s^{2}, 2s^{2}, 2p^{4}$. Adding an electron to oxygen leads to a more stable half-filled $p$-orbital $(2p^{3})$,making the process exothermic (negative electron gain enthalpy).
Regarding ionisation enthalpy,oxygen $(1s^{2}, 2s^{2}, 2p^{4})$ has a lower value than nitrogen $(1s^{2}, 2s^{2}, 2p^{3})$ because removing an electron from oxygen results in a stable half-filled $2p^{3}$ configuration. Conversely,nitrogen has a stable half-filled configuration,making the removal of an electron more difficult.