Question

why the ionization enthalpy of oxygen is lower than that of Nitrogen​

Answer 1

Answer:

Oxygen also has an unexpectedly low ionisation energy, less than that of nitrogen. This is due to an electron being added to an already half full orbital in oxygen, which results in electron electron repulsion, which will lower the ionisation energy.

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Answer 2

electronic configuration:-

of nitrogen-1s22s22p3

of oxygen-1s22s22p4

hence in last orbital nitrogen has less electron so it's size is smaller than oxygen and due to smaller size,it's ionization enthalpy is more that oxygen.