Flexi Says: Ionization energy increases from left to right in a period. Hence, phosphorus is expected to have lower ionization energy compared to sulfur. However, sulfur has lower ionization energy than phosphorus. This is due to the fact that the 3p orbitals are all half-filled in phosphorus. Half-filled orbitals are relatively stable and make ionization difficult. Sulfur has one more electron in the 3p orbitals and is, therefore, more readily ionized than phosphorus due to its relatively unstable electron configuration.