Hello student,

In order to understand the lower ionization energy of oxygen and higher ionization energy of fluorine let's look at their electronic configuration first.

The outer electronic configuration of Oxygen : 2s 2 2p 4

The outer electronic configuration of Fluorine : 2s 2 2p 5

In case of oxygen, the removal of electron from the 2p subshell will make it attains a half filled configuration, thus lesser energy is required to remove an elecron from oxygen. Whereas, in case of fluorine it has to gain one more electron to attain noble gas configuration and become stable and thus more energy is required to remove an electron from flourine. Hence ionisation enthalpy of oxygen is lesser than flourine.

Thank you.

Dear Student,

so in nitrogen the three 2p electrons occupy different atomic orbitals. But in the case of oxygen, two out of the four 2p electrons occupy the same atomic orbital and this results in increased electron-electron repulsion in the oxygen atom.  This is why oxygen has less ionization enthalpy than fluorine.