Sulfur has a lower ionization energy than chlorine because 1. the effective nuclear charge of sulfur is less than that of chlorine. 2. sulfur has a greater ionization energy than chlorine. 3. the outer orbitals of sulfur are located farther from the nucleus than the outer orbitals of chlorine. 4. the effective nuclear charge of sulfur is greater than that of chlorine. 5. sulfur elevates one of its p electrons to a d orbital, thus making it easier to remove the electron.