A hypervalent molecule (or expanded octet) is a molecule that contains one or more main group elements formally bearing more than eight electrons in their valence shells. Phosphorus pentachloride (PCl5), sulfur hexafluoride (SF6), chlorine trifluoride (ClF3), and the triiodide ion (I3−) are examples of hypervalent molecules.
Violation of the Octet Rule
The octet rule works best for the elements in the second period (Li through F) of the periodic table. Electrons, whether shared or not, must be contained in orbitals, and the energies of electrons in such orbitals must be relatively low. Otherwise, there would be no energetic advantage in forming a bond in the first place—the atoms would be better off by themselves.
For second- and third-period elements, the n=2 and n=3 s2p6 sets comprise the octet. Some of the third-period elements (Si, P, S, and Cl) can bond to more than four atoms, and thus need to involve more than the four pairs of electrons available in an s2p6 octet. This is possible because at n=3, an additional set of d orbitals can exist. Although their energies are higher (ordinarily higher than the 4s orbitals), they can participate in the valence shells of these atoms.
For atoms in the fourth period and beyond, higher d orbitals can sometimes be used to accommodate additional shared pairs beyond the octet. A diagram showing the relative energies of the different kinds of atomic orbitals, such as Figure 1, reveals that energy gaps become smaller as the principal quantum number increases. The energetic cost of using these higher orbitals becomes smaller.