the first ionisation energy of nitrogen is greater then that of oxygen , whereas the second ionisation energy of oxygen is greater than nitrogen....WHY..??????

The electronic configuration:

N – 1s² 2s² 2p³

O- 1s² 2s² 2p⁴

Nitrogen has 3 electrons in 3p subshell which gives it a stable half – filled sub shell configuration whereas oxygen has 4 electrons in 3p sub- shell.Therefore, removal of one electron from 3p subshell of nitrogen requires more energy than to remove one electron from 3p sub shell of oxygen atom.

The electronic configuration after removal of one electron:

N⁺ – 1s² 2s² 2p²

 O⁺ - 1s² 2s² 2p3

Now, oxygen has stable half filled sub shell configuration.Therefore, the first ionization energy of nitrogen is greater then that of oxygen, whereas the second ionisation energy of oxygen is greater than nitrogen