Which has the highest first ionization energy : S or Si? And why?

Question

Which has the highest first ionization energy : S or Si? And why?

Answer 1

sulfur:

http://en.wikipedia.org/wiki/Sulfur

1st Ionization energy: 999.6 kJ·mol−1

Atomic radius 100 pm

16 protons

silicon:

http://en.wikipedia.org/wiki/Silicon

1st Ionization energy: 786.5 kJ·mol−1

Atomic radius 117.6 pm

14 protons

from the # of protons & the atomic radius data above one might expect incorrectly that sulfur would have the lower ionizatrion energy.

however as you see above, it is the silicon that has the lower ionization energy. the reasoning is given @

http://209.85.141.104/search?q=cache:iEr...

where it is explained in a comparison of oxygen vs. nitrogen that the electron repulsion of having  two electrons in the same P-orbital makes it easier to ionize =the electron from sulfur that in the case of silicon where each P-orbital electron is  unpaired