Sulfur covalent bonding, octet rule

It is well known that the octet rule explains most of the chemical bonding of typical elements. Sulfur has, however, a variety of valences, as in such molecules as SF2, SF4 and SF5. As the SF2 molecule has two electron-lone-pairs and two covalent bonds, it satisfies the octet rule. While the valence shell of the SF4 molecule consists of decet electrons the sulfur atom of the SF6 molecule is surrounded by dodecet electrons. Therefore sulfur compounds do not always follow the octet rule. It has been said that the sulfur 3d orbital plays an important role in chemical bonding, and many researches have tried to clarify the nature of sulfur chemical bonding. 

A doubly bonded oxygen needs 4 more electrons to satisfy the octet rule. A sulfur with one covalent bond needs six more electrons to satisfy the octet rule. This gives the S a formal charge of -1. The resonance structure has the double bond to the S and the negative formal charge on the O. The situation is very similar to acetate anion (see problem 1.10). 

The number of covalent bonds formed by an atom is termed its COVALENCY. The covalency of an atom is equal to the number of electrons the atom needs to become isoelectronic with a noble gas. Some of the more common elements have the following covalencies when they follow the octet rule and also have no charge hydrogen and the halogens, 1 oxygen and sulfur, 2 nitrogen and phosphorus, 3 carbon and silicon, 4. LEWIS ELECTRONIC FORMULAS, in which bonds and unshared electrons are shown, are given for a few typical compounds of these elements ... 

---