Of F or S, which element has the higher first ionization energy?
Florine has the highest first ionization energy of all the elements.
The reason behind this is because as we move across across a period, more protons are being added to the nucleus, and thus there is a stronger force of attraction between the protons and the electrons. As we move down the table, the first ionization energy will decrease as the atomic radius is larger and the electrons are farther away from the nucleus, thus giving a lesser force of attraction.
Since sulfur is both under and to the left of Florine, it will have a lesser first ionization energy.
Hope that helped!