Final answer:
The first ionization energy of sulfur is lower than that of phosphorus due to electron-electron repulsion caused by the additional electron in sulfur's 3p subshell.
Step-by-step explanation:
The first ionization energy of sulfur is less than that of phosphorus because of the electron configuration and electron-electron repulsion within the atoms. Sulfur's electron configuration is [Ne]3s²3p4, while phosphorus's electron configuration is [Ne]3s²3p³. The additional electron in sulfur's 3p subshell leads to electron-electron repulsion, making sulfur slightly less stable and easier to remove an electron compared to phosphorus. This results in a lower first ionization energy for sulfur.