Phosphorus pentachloride is an ionic solid consisting of PCl4
cations and PCl6
anions; but at 160 C, it vaporizes to a gas of PCl5 molecules. The Lewis structures
of the polyatomic ions and the molecule are shown in (23) and (24). In the PCl6
anion, the P atom has expanded its valence shell to 12 electrons, by making use of
two of its 3d-orbitals. In PCl5, the P atom has expanded its valence shell to 10
electrons by using one 3d-orbital. Phosphorus pentachloride is therefore an example
of a hypervalent compound in both its solid and its gaseous forms.