CGPET 2013 Solved Paper

As we move from left to right in a period, the ionisation enthalpy increases with increasing atomic number due to increased nuclear charge and smaller atomic radii. But the first ionisation enthalpy of oxygen is lower than that of nitrogen although the nuclear charge of oxygen is higher than that of nitrogen. This is due to following reasons.
(i) The electronic configuration of N (1s22s22px12py12pz1) in which the 2 p-orbitals are exactly half-filled is more stable than the electronic configuration of O(1 s22 s22px22py12py1) in which the 2p-orbitals are neither half-filled nor completely filled. Therefore, it is difficult to remove an electron from N than from O.
(ii) The removal of an electron from O gives a stable electronic configuration with exactly half-filled 2p subshell, i.e., O+(1 s22 s22px12py12pz1) while this is not so in case of N, i.e., N+(1 s22 s22px12py12pz0). Therefore, the first ionisation enthalpy of O is lower than that of N.