Final answer:
Oxygen has a lower first ionization energy than both nitrogen and fluorine due to its electron configuration. Electrons repel each other, making it slightly easier to remove the electron from the paired set in an oxygen atom compared to removing an unpaired electron from a nitrogen atom.
Step-by-step explanation:
The reason oxygen has a lower first ionization energy than both nitrogen and fluorine is because of the electron configuration of oxygen. In an oxygen atom, there are four electrons in the 2p sublevel, with one orbital containing a pair of electrons. It is this second electron in the paired set that is removed in the ionization of an oxygen atom. Since electrons repel each other, it is slightly easier to remove the electron from the paired set in the oxygen atom than it is to remove an unpaired electron from the nitrogen atom.