Question

What is the correct order for the ionisation enthalpies for

C,N,O,F

Answer 1

As we move from left to right in periodic table ionisation enthalpy increases due to increase in number of protons and decrease in size. So the expected order is C < N < O < F.

But the outer most electronic configuration of the elements are as below
C: 1s² 2s² 2p²
N: 1s² 2s² 2p³
O: 1s² 2s² 2p⁴
F: 1s² 2s² 2p⁵
As nitrogen have half field orbital so its stability is higher (Hund’s rule), and removal of electron form the outer most shell of nitrogen is more difficult.
Therefore actual order of ionisation enthalpy is C < N > O < F.