Sulfur tetrafluoride (SF4) is four fluorine atoms surrounding a central sulfur atom. This is an example of a molecule that does not follow the octet rule.
Sulfur brings six valence electrons, and normally this means it has two unpaired electrons to share in covalent bonds. However, in sulfur tetrafluoride (SF4) there are four bonding pairs, and so four of those valence electrons are involved in single bonds with fluorine atoms.
How is it possible for sulfur to violate the octet rule? It’s because sulfur’s valence electrons on in the third energy level (shell). This means that the 3d orbitals can be involved. The 3s orbital, all three 3p orbitals AND one of the 3d orbitals all combine together to make five equal-energy (degenerate) sp3d hybrid orbitals.
This gives sulfur a hybridization of sp3d. The VSEPR notation here is AX4E, which corresponds to “sawhorse” geometry.