Introduction to VSEPR Theory
PROFESSOR: A simple yet very effective method of describing and predicting the shapes of molecules is VSEPR theory, or vesper theory. Although the acronym doesn't actually spell out vesper, chemists usually say vesper because it's just easier to say. Anyway, VSEPR stands for Valence Shell Electron Pair Repulsion.
And the ideas behind VSEPR theory are very simple. If you had a good physics teacher, then you're probably familiar with Coulomb's law, which basically says that opposite charges attract and like charges repel one another. And VSEPR theory is basically just Coulomb's law applied to the electron groups around an atom.
Now these electron groups, they could be chemical bonds. They could be lone papers. They could be double bonds, triple bonds, whatever. According to VSEPR theory, these electron groups space themselves out as far apart from each other as possible to achieve maximum separation.
So the implications of VSEPR theory are that if I have a central atom, which I've called A, and I have two electrons groups around that atom A, then I'm going to get a shape that looks like this. The angle between the two electron groups is 180 degrees, and the shape is called linear. There was no way that you can move these electronic groups to get them any further apart from each other than they already are.
Let's look at VSEPR theory applied to three electron groups around an atom. If there are three electron groups and those electron groups space themselves out as far apart as possible, then I'll get a structure that looks like this. The angle between any two of these electron groups is 120 degrees.
So the angle between these two is the same as the angle between these two is the same as the angle between these two. All of those angles are 120 degrees. And this shape is what we call trigonal planar.
Basically, the electron groups point towards the corners of an equilateral triangle. Gets a little bit more interesting when we start getting involved with four or more electron pairs or, excuse me, four or more electron groups around atoms. So let's look at when there are four electron groups around one atom. That'll give rise to a shape that looks like this.
So basically, what I've done here is I have attempted to draw a three dimensional picture on a flat board. This electron group is no bigger than this one. It's just coming out toward you, and this one is going out away from you into the ward.
So the angle between any two of these electronic groups is 109.5 degrees. And this shape is what we call tetrahedral. So I've showed it to you on a flat board, but it might be worthwhile to see an actual three dimensional tetrahedron. So here you go.
This is what a tetrahedron looks like. Notice that all of the white atoms that are bonded to the black atom, they're all the same distance and angle apart from one another. So that means that this angle is 109 degrees. So is this angle, and so is this angle, and so is this angle and so forth.