Question

explain in brief for oxygen has lesser first ionization enthalpy than nitrogen

Answer 1

Nitrogen has more first ionization energy than first ionization energy as electronic configuration of nitrogen is



The 2p orbital of nitrogen is half filled as it is considered as stable configuration because there is equal exchange of energies between the electrons of 2p orbital 
       




Secondly the half filed orbital has a plane of symmetry thus it is considered as stable configuration. 




When electron is being removed form a stable configuration the energy requires would be higher because  a stable configuration is shifted to an unstable configuration.   
But in case of Oxygen the configuration is one  more than half filled configuration. 



So oxygen would readily loose an electron to attain a stable configuration that is half filled configuration.

So first Ionization energy of nitrogen is more than first ionization energy of Oxygen.

Answer 2

since the electronic configuration of oxygen is 1s²2s²2px²2py¹2z¹. and the electronic configuration of nitrogen is 1s²2s²2px¹2py¹2pz¹. so we know that half filled orbitals are stable. and partial filled orbitals are not. so extra energy is required for removing the valence electrons from nitrogen than oxygen.
and hence the first ionization enthalpy of oxygen is greater than nitrogen.