Final answer:
A neutral phosphorus atom has three unpaired electrons, which are found in the 3p subshell of its electron configuration. These unpaired electrons play a crucial role in forming covalent bonds in compounds like phosphorus pentachloride (PCl₅).
Step-by-step explanation:
A phosphorus atom has three unpaired electrons. The atomic number of phosphorus is 15, which means it has 15 electrons. The electron configuration of a neutral phosphorus atom is 1s²2s²2p⁶3s²3p³. In this configuration, the first two energy levels (1s, 2s, and 2p orbitals) are completely filled with paired electrons. However, in the third energy level, while the 3s subshell is filled with two electrons, the 3p subshell contains three electrons, which are unpaired.
This corresponds to the valence electrons of phosphorus, which are important for chemical bonding. In molecules like phosphorus pentachloride (PCl₅), these unpaired electrons are used to form bonds with chlorine atoms. Each of the unpaired electrons pairs up with an electron from a chlorine atom to form a covalent bond.