Ionization energy exception; O

[Natalie LeRaybaud 1G]

Can someone please explain why Oxygen is an exception to the ionization energy trend of the periodic table, like when it is compared to Nitrogen or Fluorine?

[Hannah Chew 2A]

Oxygen has a lower ionization energy than fluorine, as fluorine is farther to the right on the periodic table and follows the trend that IE increases as you go right. However, the anomaly is that oxygen has a lower ionization energy than nitrogen. If we look at the electron configurations, oxygen is p4 while nitrogen is p3. Nitrogen has three unpaired electrons, while oxygen only has two. Oxygen is the first element to "pair" its electrons. This pairing of electron creates more electron-electron repulsions, and thus, the ionization energy of oxygen is lower than nitrogen, which doesn't have any paired electrons in the p orbital.

[Yashaswi Dis 1K]

Basically, atoms love to either have completely filled orbitals or half-filled orbitals. Nitrogen loves its half-shells, so it will be very hard to remove an electron, thus it will take a high amount of energy to remove one of the electrons from a half-orbital, making Nitrogen have a high ionization than Oxygen. Oxygen has one paired orbital and it wants to be like Nitrogen w/ the half-filled orbitals because it's more stable, so Oxygen will readily give away that extra electron (especially also b/c of electron-electron repulsion) and so it has a lower ionization energy than Nitrogen b/c it doesn't take much energy to remove that extra electron from Oxygen.

Oxygen has a lower ionization energy than Flourine b/c the ionization trend increases across a period. The reason for that is b/c Flourine has more protons than Oxygen and so it can attract more electrons to its nucleus( high nuclear charge) and so electrons in Flourine don't want to let go, so it will take a very high ionization energy to remove an electron from Flourine.

Hope this gives a clearer idea! :)