Expanded Octet in the PF5 Molecule

The electron structure of phosphorus ends with 3s2 3p3 3d°. As we saw previously, the phosphorus atom normally forms three bonds. However, it is able to undergo sp3d hybridization as shown below. 

When it is excited one of the 3s electrons is promoted to a 3d orbital. In this configuration, phosphorous has five half-filled orbitals, and therefore a bonding capacity of five. When these half-filled orbitals are filled with the unpaired electrons from five fluorine atoms, the PF5 molecule results. In this molecule, the phosphorous atom is surrounded by five pairs, or ten electrons. 

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