For Future Reference

To be able to predict the 3-D geometry, one approach is the VSEPR theory.

VSEPR stands for Valence Electron Pair Repulsion. It relies on the idea that bonded atoms and electron pairs tend to repel one another, and the closer they are, the stronger the repulsion. It therefore follows that if you minimize the repulsion (maximize the distance) you have the most stable structure.

That idea is probably quite confusing, so we will go over a few examples in this gallery.

Let’s take water, for example.

Water’s Lewis Structures looks like this:

<Image 1>

So, if we want to minimize the repulsion between the two hydrogen atoms and the two lone pairs, what kind of structure are we left with?

One way of thinking about this problem is to imagine a ball with 4 sticks protruding from it, representing the bonds and lone pairs.

Now, we need to evenly distribute the sticks around the ball. To do this, we need to go back to our geometry. To have an even distribution, we will draw triangular planes between all adjacent points.

Now, we know we have a 3-D object with 4 triangular faces. What kind of object would that be when we make each face the same area? A regular triangular pyramid:

If we then assign two sticks to be the hydrogen atoms (represented by white balls) and the other two to be lone pairs (represented by the yellowish oblong ovals), we have the following structure:

When drawing only the atoms and bonds, you may leave lone-pair dots out of a simplified picture. The lone pairs still exist and affect electron-domain geometry and bond angles. VSEPR first counts all domains—bonding and nonbonding—then names the molecular geometry from the positions of the atoms. Water therefore has four domains around O and a bent molecular shape.

This structure is referred to a ‘bent’, due to the bent shape you see.

Instead of going through the derivation for each structure, you should instead learn the patterns outlined below.

Before we begin, you need to know how to draw 3-D structures, as you won’t be making 3-D models for every problem you encounter.

If we want to draw the water molecule (including the electron pairs), we pick one plane on the molecule to act as our paper, and then we draw the bonds that lie on that plane normally. When picking the plane, try to have as many bonds as possible. Next, you add in the bonds that would go into the paper using a dashed triangle instead of a simple line. The bonds that would be coming out of the paper should be solid triangles. This gallery shows what these will look like.

To determine the structure of any centralized molecule (i.e. one atom with multiple atoms bonded to it), we will utilize the AXE method.

The AXE method starts with the central atom: A, and then counts the number of bonds (double bonds and triple bonds only count as one bond for this case). This is X. Then E is the number of lone pairs of electrons on the central atom.

To represent Water, We look at the central atom, O, then see 2 bonds with Hydrogens, and 2 lone pairs. The AXE method shows $AX_2E_2$. The AXE method can let you use the following table to identify the structure:

Let’s introduce a concept called the Steric Number, which is the sum of the lone pairs and the bonds of the molecule. Every steric number has its own base structure.

This gallery shows the structure of Steric Numbers 2 - 6, with each component as a bond with Hydrogen, and the central atom X.

Now when we start adding bonded atoms to replace those lone pairs, they start taking shapes in a pattern that’s a lot more understandable. Time for a new table, where we have the steric number, the AXE number, the name of the shape, and finally a small image to help you visualize that shape: