Does Sulfur or Phosphorus have a larger ionization energy?

[Mario Reyes 1C]

Using the trends it would seem like Sulfur would have a larger ionization energy, but just like oxygen and nitrogen, the phosphorus is half filled. Does this mean Phosphorus has a larger ionization energy?

[tmehrazar]

I think phosphorus does have a higher ionization energy because when you look at the shells filled up, sulfur has one pair of electrons and phosphorus has all unpaired electrons. Either all unpaired or all paired makes it stable, so if phosphorus is all unpaired it take more energy to remove an electron than it would from sulfur in which one electron can be removed making it more stable. Because it takes more energy for phosphorus, it has a higher ionization energy.

[Madison Hacker 1L]

I think that sulfur has a higher ionization energy because generally ionization energy increases across the periodic table.

[Bryan Jiang 1F]

I think that sulfur has a higher ionization energy because generally ionization energy increases across the periodic table.

You are right in that ionization energy generally increases across a period, but only generally. For the case between phosphorus and sulfur, phosphorus would actually have the slightly higher ionization energy, despite it being behind sulfur on the period. This is because once you reach sulfur, there are now four valence p-electrons, meaning you must start pairing electrons in the first p-orbital (compare this to phosphorus having only three valence p-electrons, and thus no pairing). This pairing causes increased electron-electron repulsion forces within the orbital, making it slightly easier to remove that electron, and thus decreasing the ionization energy of sulfur just enough to be less than that of phosphorus.

This phenomenon is also observed between nitrogen and oxygen, where oxygen actually has the lower ionization energy for the very same reason.