Question

Oxygen has ionisation enthalpy than nitrogen fluorine. Suitable reason

Answer 1

The energy required to remove one electron from an atom called first ionization energy✔
Usually, first ionization energy increases as we go across the period. As per trend oxygen should have higher first ionization energy than nitrogen. But this is not the reality✔
Oxygen has smaller first ionization energy compared to nitrogen. We can explain by writing electronic configuration and valence orbital diagrams for both N and O below:⬆️⬆️

Nitrogen has three electrons in the p-orbital and each electron fill all the three p-orbital with aligned spins and stay farther apart. As the electrons in nitrogen p-orbital are half-filled and are stable and require more energy to remove one electron✔
But in oxygen, there are four electrons in the p-orbital, as each electron fill the 3 p-orbital first and the fourth electron pairs up with already present electron in first p-orbital with opposite spins. This causes repulsion between electrons more than the ones in separate orbitals. This electron-electron repulsion causes this electron to get removed easily hence making oxygen to have lower than expected first ionization energy✔