Final answer:
A phosphorus atom can form a maximum of five covalent bonds as seen in phosphorus pentachloride (PCl5), where it utilizes one 3s orbital, three 3p orbitals, and one 3d orbital to form five sp3d hybrid orbitals.
Step-by-step explanation:
The number of covalent bonds a phosphorus atom can form is influenced by its ability to use d orbitals in bonding, in addition to the s and p orbitals. In the case of phosphorus pentachloride (PCl5), phosphorus forms five P-Cl bonds utilizing its 3s orbital, three 3p orbitals, and one of the 3d orbitals to create a set of five sp3d hybrid orbitals. These orbitals are directed toward the corners of a trigonal bipyramid, which is why phosphorus can exceed the octet rule and form more than the typical three covalent bonds that it forms in molecules like PF3, where it only uses sp3 hybrids. Thus, at most, phosphorus can form five covalent bonds when it utilizes its d orbitals.