What is the predicted order of first ionization energies from highest to lowest for aluminum, phosphorus, silicon, and sulfur?

First ionization energy generally increases from left to right across Period 3, so phosphorus and sulfur have higher values than silicon and aluminum. Phosphorus has a slightly higher ionization energy than sulfur because its three 3p electrons occupy separate orbitals, while sulfur has a paired 3p electron that is easier to remove. Thus the order is P > S > Si > Al, choice D.

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