Question

Although the atomic size of oxygen is small as compared to nitrogen but 1st Ionisation enthalpy of nitrogen is higher as compared to oxygen. Why?

Answer 1

the electronic configuration of nitrogen is symmetrical so less e- repulsion but in oxygen e- repulsion is more as its not symmetrical so e- can be easily removed as compared to nitrogen.

Answer 2

Answer: Because of the stable electronic configuration of nitrogen atom.

Explanation:

Ionization enthalpy is defined as the energy required to remove the outermost electron from an isolated gaseous atom.

$X(g)\rightarrow X^+(g)+e^-$

Oxygen is the 8th element of the periodic table and nitrogen is the 7th element of the periodic table.

Electronic configuration of oxygen: $(Z=8):1s^22s^22p^4$

Electronic configuration of nitrogen: $(Z=7):1s^22s^22p^3$

As, nitrogen has half filled electronic configuration which is very stable than the partially filled configuration. Thus, to remove an electron from a half filled configuration is more difficult and will require more energy.

Hence, nitrogen has higher ionization energy than the oxygen atom.