Question

Why is first ionisation energy of nitrogen more than oxygen?

Answer 1

heya----

Oxygen has low ionisation energy (than that of nitrogen). It 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. 
Another way of looking at it is :
Nitrogen also has the added stability of a half filled shell of electrons in the 2p shell.
And we know half filled orbitals are more stable due to more exchange energy.
As Nitrogen is more stable, therefore it is much difficult to remove the valence electron.

tysm..............@kundan

Answer 2

nitrogen: [He] 2s2 2p3

oxygen: [He] 2s2 2p4


In reality, the first ionisation energy of oxygen is greater than first ionisation energy of oxygen because nitrogen, in a stable half filled orbital state, is comparatively more stable than oxygen. Oxygen, on the other hand, would tend to lose an electron easily to achieve it's more stable half filled orbital state.

Also, as a rule, half filled and fully filled orbital states are more stable as compared to other configurations because they attribute to maximum exchange energies.