Why are there 12 electrons on the...?

Question

... the outermost shell of S in H2SO4. I know that there should be 8.

Answer 1

sulphur is capable of a level of hybridization higher than that of, say, carbon

Answer 2

One way to answer your question is to draw the Lewis structure of the sulfate ion. You can draw it with four single bonds to give an octet of electrons around sulfur - but in this situation the sulfur has a +2 formal charge and the oxygen atoms all have a -1 formal charge. Because sulfur can have an expanded octet it is usually drawn with double bonds to two of the oxygen atom. This gives it a formal charge of 0 and two oxygens have a formal charge of 0 and two have -1. This is the preferred situation and gives 12 electrons around the sulfur. In sulfuric acid the hydrogens connect to the two oxygen atoms with single bonds.

We can also look at the electronic structure of sulfur to find the possible bonding patterns. Sulfur's valence electrons are 3s2, 3p4. Its ground state has two unpaired electrons so can form two covalent bonds with two lone pairs to satisfy the octet rule. If we promote one p electron to a 3d orbital there are now 4 unpaired electrons that can form 4 covalent bonds with one lone pair to give 10 electrons around sulfur. Finally if we promote one s electron there are 6 unpaired electrons that can form 6 covalent bonds with no unpaired electrons to give 12 electrons around sulfur. This last is the situation in sulfuric acid.

Answer 3

sulfur in sulfuric acid has 12 bonding electrons because it is capable of using the 3d subshell for hybridization purposes. Thus, there is 2 3s electrons, 6 3p electrons, making up the octet 8 electrons, but another 4 electrons from the 3d shell.

What you should know coming out of this example is that octet rule is often broken from the third row elements because they are capable of using the d orbital to satisfy the bonding condition. In most of these cases were there are more than 8 outer shell electrons, the atom is bonded to a more electronegative atom, thus forcing the atom to break the octet rule