Answer :
The first ionisation energy of oxygen is less than that of nitrogen due to the stability of half-filled electronic configurations and the electron-electron repulsion in oxygen's 2p orbital, which eases the removal of one electron. The correct option is C.
The correct reason for the observation that the first ionisation energy of oxygen is less than that of nitrogen is due to the stability of half-filled electronic configurations. Nitrogen has a half-filled 2p sublevel, with three unpaired electrons, which contributes to its stability and higher ionisation energy.
Oxygen, on the other hand, has four electrons in the 2p sublevel, meaning one 2p orbital contains a paired set of electrons. It is slightly easier to remove an electron from this paired set in oxygen due to electron-electron repulsion, resulting in a lower first ionisation energy compared to nitrogen.
This anomaly occurs due to oxygen's desire to reach a half-filled stable configuration upon the removal of one electron. Similar patterns can be observed with other elements in the periodic table where a dip in ionisation energy is noticeable when comparing elements with fully-filled or half-filled sublevels to those with a less stable electron configuration.
