The First Ionization Energies of Phosphorus and of Sulfur Are

Multiple Choice

The first ionization energies of phosphorus and of sulfur are 1012 and 1000 kJ/mol.Why is IE₁ for S lower than that of P when the usual trend is for IE to increase across a row?

A) Z_eff decreases from left to right on the periodic table.
B) Sulfur is smaller than phosphorus.
C) More electrons in a subshell decrease orbital energy.
D) An electron from phosphorus must be removed from a lower-energy subshell.
E) Removing an electron from sulfur produces a half-filled subshell and reduces electron-electron repulsions.