VSEPR Theory and the shape of molecules
So now you've drawn dozens of LDS's including simple ones and complex ones, ones that obey the rules and ones that violate them, ones the LDS's represent well and ones for which LDS's fail miserably. But, in the end, why does anyone care. Why does it matter that water has one oxygen with two hydrogens attached? Even more, why does it matter that the oxygen in water has two other pairs of electrons that are not involved in bonding?
The answer to those questions (and those I raised at the beginning of the chapter) can be answered by looking at the shapes of the molecules and the polarity of the bonds in those molecules.
The shapes of molecules can be predicted by the VSEPR (pronounced ves-per) theory. VSEPR stands for Valence Shell Electron Pair Repulsion. In English, this means simply that the bonding and lone pairs of electrons (the ones we've been drawing in LDS's) repel each other and will try to get as far away from each other as they can. This repulsion, and subsequent avoidance, determines the shape of molecules.
For the sake of the discussion that follows I am going to define several common words in very specific terms.
Things – bonded atoms or lone pairs of electrons. We generally care about the number of things that surround a central atom. For example, each of the following LDS's have three things around the central atom.
In the first case (BH3) there are three atoms bonded to the central B atom. In the second case there are also three atoms bonded to the central C atom. Note that we do not care that one of the bonds is a double bond and that the others are singles. In the third case, the central N is surrounded by two other atoms and a lone pair (the un-bonded electron pair on the nitrogen). It is also important to notice that lone pairs on OUTER atoms do NOT matter. Only those things attached to the central atom are important in VSEPR theory.
Structure – the arrangement of all of the things in three dimensional space. Using the definition of things above, this means that arrangement of all atoms and lone pairs in a molecule.
Shape – the arrangement of the atoms in a molecule.
The difference between structure and shape will be made more clear as we work our way through the following examples.
If you already have some experience with VSEPR theory, you can see a summary of the theory here.