What is the name of PCl5?

The substance PCl5 is named phosphorous pentachloride. This name is derived from the components present in the compound. The prefix "penta-" in the name indicates that there are five chlorine (Cl) atoms bonded to a single phosphorus (P) atom. In chemical nomenclature, the element with the higher electronegativity, in this case, chlorine, is named last, while the first element, phosphorus, is modified by its oxidation state or by a numeric prefix denoting its quantity in the compound.

Unlike the other options, the chosen name clearly follows the systematic rules for naming covalent compounds, which include using prefixes to denote the number of atoms of each element involved. Specifically, "phosphorous" reflects the specific oxidation state of phosphorus in this compound, which is +5, correlating with the presence of five chlorine atoms. The other answer choices either misuse oxidation state terminology or do not follow the correct format for naming the compound.