In this lecture, we're going to begin a study of the geometries that molecules

adopt. In three dimensions, what their

structures are in three dimensions. And we're going to develop a model that

accounts for those geometries, called the electron domain theory, or otherwise

referred to as the valance shell electron pair repulsion model.

This material is from the 11th Concept Development Study on Molecular Geometry,

so I'm referring you to that, as we're going to follow it fairly closely.

Let's recall. At the end of our study of the structures

of solids, we examined the structure of solid diamond, carbon, and a network

lattice that is depicted here. We asked in passing, but never answered

the question. Why do the carbon atoms arrange

themselves in the way that they do? In order to understand that we need to

back all the way back to discussions of individual molecules and ask?

What are the three dimensional structure of molecules?

To think about that, let's go back to our simplest hydrocarbon, methane, which is

carbon surrounded by four hydrogen's that is the lowest structure for the methane

molecule. Drawn in this way we might predict that

it's experimentally we would see a player molecule in which each bond angel where a

bond angel is the angel between two adjacent bonds that we might expect that

the bond angles are all 90 degrees. In fact, experimentally when we study

methane, it turns out that the bond angle is 109.5 degrees, and that is true for

each of the bond angles between adjacent carbon-hydrogen bonds.

From that we can immediately tell that this is not a planar molecule.

If it were, each bond would have to be 90 degrees.

As it stands, we have more than 360 degrees in a circular angle.

That can't be. So this molecule doesn't exist in a

plane. What does it look like?

We'll examine that in just a moment. But let's consider a variation on methane

in which we have substituted. Two of the hydrogens, four chlorines.

Well, we might say that this would be the structure of that molecule.

If I draw it this way, and if the molecule is planar, then in fact I could

draw another version of dichloromethane in which the two chlorines appear to

adjacent from each other. Rather than across from each other.

But it turns out experimentally there's only a single molecule, which is

dichloride methane, and we even commented back in our structures about little

structures that these two types of structures are, in fact, identical to

each other. They are the same structure, even though

they appear different in two dimensions, it must be true that in three dimensions

the arrangements of the atoms are exactly the same.

We actually encountered that when we discussed water before.

And our comment about water was that it appeared to be the case that we could

draw two different drawings for water. They are both drawn here, and the thought

was then that perhaps the angle was either 90 degrees or 180 degrees.

In fact there's only one form of water, and the actual bond angle is neither 90

degrees or 180. But is instead 104.5 degrees.

Likewise, just in order to complete a set, if we examine ammonia, and ask what

the bond angles are in ammonia, it turns out that the bond angle is about a 107

degrees. And right away we notice that there's a

similarity amongst these bond angles. They are not a 120, they're not a 180,

they're not 90, but they are all very much in proximity to one another.

Question is, how do we account for what the three dimensional structures are?

We need to look at those and there are a couple of different places we can look.

One place we could look, I've highlighted here as a website that's really a

marvelous website I'm going to show in just a moment.

I'm going to be showing you some geometries based upon experimental data

from a different program. you can also actually get model kits, and

I would encourage you, that you'll understand these models better if you

have three-dimensional kits in front of you, but if you don't have access you can

use these web resources, the one I will show you in just a moment.

Here's what the geometry of methane actually looks like.

We look at this, we can see that it is in fact a three dimensional molecule, it is

not planar. If we rotate it about, it has enormous

symmetry. If we look down any one of the CH bond

axes, we can see that the other three CH bonds are arranged equally separated from

one another and each of those bond angles is 109.5.

None of them are different from the other.

If I can rotate and look down any of the CH bond angles, I wind up with

essentially exactly the same depiction. What if we push a little past methane and

ask, let's replace one of the hydrogens with a carbon.

Then we would have C2H6, and that would be Ethane.

Let's look and see what Ethane looks like.

Here's the geometry of Ethane, and we notice that the arrangement of the bonds

about the Carbon atom here at the bottom that I'm pointing to, are very much the

same as the arrangement of the bonds about the Carbon atom in Methane.

Go back and look at Methane, look at Ethane.

The arrangement of the Carbon-Hydrogens, and the other bonds about the Meth-,

about the Carbon in the middle of Meth-, of Ethane, are the same as they were in

Methane. Furthermore, the two ends of the

molecules are about the same. If I flip it over and ask about the

carbon on the other end, I immediately notice that this carbon also looks like

the bonds were arranged in the way that they were arranged around methane.