Phosphorus has three electrons in its 3p sublevel and sulfur has four. Phosphorus should have the lower ionization energy but it does not, sulfur does. Why? A. Phosphorus has 2 of its electrons paired in the 3p sublevel, which makes them harder to remove, or "ionize." B. Sulfur has 2 of its electrons paired in the 3p sublevel. The mutual repulsion of their negative charges makes them easier to remove, or "ionize." C. Because sulfur has more electrons, they are further from the nucleus and thus easier to remove and form an ion. D. Since sulfur has more electrons, it has more chances to form an ion.

First of all it is important to know that a half filled orbital is particularly stable. In phosphorus all the electrons occur singly in the 3p sublevel minimizing inter electronic repulsion hence it is more difficult to remove an electron from this energetically stable arrangement. In sulphur, electrons are paired in one of the 3p orbitals thereby lowering the energy of that level due to instability caused by interelectronic repulsion between two electrons in the same orbital.