Question

Why nitrogen has higher first ionization potential than oxygen atom?

Answer 1

Ionisation energy could simply be defined as the energy required to remove the valence electron of an element.

While nitrogen has one electron occupying each of the three p orbitals in the 2ndsublevel, oxygen has two orbitals occupied by only one electron but one orbital containing a pair of electrons. The greater electron-electron repulsion experienced by these 2p electrons allows for less energy to be needed to remove one of these. Therefore, IE1 for oxygen is less than nitrogen.