Question

Oxygen is more electronegative than nitrogen but nitrogen has higher ionization energy than oxygen. Why\[?\]

Answer 1

Hint: Ionization energy is the minimum amount of energy required to remove the most loosely bound electron. The amount of energy required to separate one electron from its atom depends on how tightly held the electron is. This depends on the number of protons and on the orbitals that the electron occupies.

Complete answer: Let’s first start with actual ionization energy. Ionization energy is the minimum amount of energy required to remove the most loosely bound electron of an isolated neutral gaseous atom or molecule. Oxygen has $8$ protons in the nucleus whereas nitrogen only has $7$. A bonding pair will experience more attraction from the oxygen's nucleus than from nitrogen, and so the electronegativity of oxygen is greater.
The electronic configuration of oxygen $1{s^2},2{s^2},2{p^4}$.
And, the electronic configuration of nitrogen is $1{s^2},2{s^2},2{p^3}$.
Here, the nitrogen atom attains the half-filled stability. The half-filled stability is that orbitals in which the subshell is exactly half-filled or completely filled are more stable because of the symmetrical distribution of electrons. When the orbitals are half-filled or completely filled then the number of exchanges is maximum. Therefore, its stability is maximum.
Therefore, the ionization energy of nitrogen is higher than oxygen.

Note:
Hund’s rule states that every orbital in a sublevel is singly occupied before any orbital is doubly occupied. All of the electrons in singly occupied orbitals have the same spin. When assigning electrons to orbitals, an electron first seeks to fill all the orbitals with similar energy before pairing with another electron in a half-filled orbital. Atoms at ground states tend to have as many unpaired electrons as possible.