Final answer:
The valency of phosphorus can be either 3 or 5, and there are 5 valence electrons located in the 3s and 3p orbitals of a neutral phosphorus atom.
Step-by-step explanation:
The valency of phosphorus is typically 3 or 5, depending on the compound it is in. The valence electrons are the electrons in the outermost shell of an atom that are involved in forming bonds. A neutral phosphorus atom, with the atomic number 15, contains 15 electrons. Its electron configuration can be represented as 1s²2s²2p¶3s²3p³, which means in the outermost shell (3s and 3p orbitals), phosphorus has 5 valence electrons. These electrons are used in bonding when forming compounds such as phosphorus pentachloride (PCl₅), where the phosphorus atom can expand its valence shell to form five P-Cl bonds using sp³d hybridization.