Question

In the compound PCl5 phosphorus has 10 valence electrons. Please explain this.

Answer 1

Phosphorus pentachloride: In the PCl5 molecule, the central phosphorus atom is bonded to five Cl atoms, thus having 10 bonding electrons and violating the octet rule.
The octet rule is based on the number of valence electrons available for bonding, and in the case of carbon, nitrogen, and oxygen, the electrons come from the outermost s orbital, and outermost p orbital. The s orbital contains 2, and the p orbital contains 6, for a grand total of 8 electrons maximum. For molecules like Phosphorus and Sulfur, the d orbital becomes available (notice there is no 2d orbital, but there is a 3d orbital). The 3d orbital may be empty in the case of Phosphorous and Sulfur, but they can be used to accommodate extra electrons.