| MadSci Network: Chemistry |
Dear Cam,
Normally, we follow the octet rule when building up molecules. However, this SF6 molecule is one of those exceptions The central S atom is obviously covalently bonded to 6 F atoms. This cannot be described by a Lewis structure unless more than 8 electrons are allowed around this S atom, a process known as valence shell explansion.
In the resulting dot diagram, you are going to have 6 single covalent bonds attached to the sulfur molecule, one electron from F, the other from S in each bond. The S atom now share a total of 12 electrons.
The need for this valence shell expansion is determined by the procedure for writing these Lewis structures when it is determined that the value for S(shared electrons present) = N(total number of electrons needed for each atom to have its own noble gas shell)- A(total number of electrons available):
S = N - A
The S value calaculated is not large enough to place a bonding pair between each pair of atoms where a bond is supposed to exist. In the compound in question A = 48, and N = 56, therefore S = 8. Four electron pairs are not enough to form all necessary bonds. Therefore, we have to use the two electrons from the 3s sublevel to make up for this deficiency.
If you can get hold of "Chemistry-Science of Change", published by Saunders College Publishing, ISBN #0-03-004814-1, you will find it an excellent reference for these problems. As a chem teacher for 10 years+ unless I have an AP chem class, I would not get stuck on this one topic. If you need anything else, e-mail me directly at:
Gary
Lots of luck!