Here's why:

* Central atom: Phosphorus (P) is the central atom.

* Valence electrons: Phosphorus has 5 valence electrons, and each chlorine atom contributes 1 valence electron, for a total of 10 valence electrons.

* Electron pairs: These 10 electrons form 5 bonding pairs, all single bonds between phosphorus and the five chlorine atoms.

* Geometry: The five bonding pairs arrange themselves in a trigonal bipyramidal shape. This shape has three chlorine atoms in a triangular plane around the phosphorus, with the other two chlorine atoms located above and below this plane.

Important Note: The actual structure of PCl5 is more complex than a simple trigonal bipyramid. It's better described as a "distorted" trigonal bipyramid, where the axial P-Cl bonds are longer than the equatorial P-Cl bonds due to the greater repulsion between the axial chlorine atoms.