The shapes of PF5 (phosphorus pentafluoride) and PCl6 (phosphorus hexachloride) can be understood through Valence Shell Electron Pair Repulsion (VSEPR) theory, which predicts the geometry of molecules based on the repulsion between electron pairs around a central atom.
In PF5, phosphorus (P) has five fluorine (F) atoms bonded to it. According to VSEPR theory, these five bonded pairs of electrons will arrange themselves to minimize repulsion, resulting in a trigonal bipyramidal shape. Here, three fluorine atoms are in the equatorial positions, while the other two are in the axial positions. This arrangement allows for a balanced distribution of electron density around the phosphorus atom, creating a stable and symmetrical molecule.
On the other hand, PCl6 has six chlorine (Cl) atoms bonded to the phosphorus atom. In this case, having six bonded pairs of electrons leads to a different geometric arrangement. VSEPR theory predicts that these six pairs will orient themselves in an octahedral shape, where the chlorine atoms are positioned at the corners of an octahedron. This configuration also minimizes the repulsion between the electron pairs effectively.
In summary, the key difference in the shapes of PF5 and PCl6 arises from the number of bonded atoms and the geometry they adopt according to VSEPR theory: PF5 adopts a trigonal bipyramidal shape, while PCl6 forms an octahedral structure.