Models

An even better way of perceiving the shape of these molecules is to use models. If you are in the lab get a model kit and continue. If not, be sure to do this when you are in the lab.

We have several kinds of model kits, and you may have any one of them. Carbon is almost always represented by black. Hydrogen might be yellow, light blue or white depending on the kit. Other atoms are different colors, depending on which one you want to use. For right now, carbon and hydrogen are the only ones we will be working with.
One of the advantages of models is that you can move them around to get different perspectives on them, as you will see when you work with them.

When you have the opportunity, make a model of the compound shown above. It is CH₄ with carbon in the center and four hydrogens around it. This compound is named methane. We will go into the naming of these compounds later in this lesson. Take a look at the model you have in your hand and become familiar with that shape. It is called the tetrahedral shape. Sometimes it is referred to as a tetrahedron. Here is a short language lesson for you. "Tetrahedron" is a noun and "tetrahedral" is an adjective. That is the reason for two different spellings.

I think you can see that the model and what is written on paper look a lot different.

Rotational Shapes

Another important aspect of molecules that can be demonstrated with models is that their shape can be changed by rotation around single bonds. When you have a model kit handy, make a model of this structural formula CH₃CH₂CH₂CH₃.

Notice that the shape of the model is not exactly like what is shown in the structural formula. Because of the angles between the carbon atoms in the model, it actually looks a little more like this.

Butane model and structural formula.[61mod04.JPG (8312 bytes)]

Also, notice that the bonds in the model can be rotated to make the model change shape. The point of this is to show you that the structure of a molecule can assume different shapes.

Space-filling models of butane showing different shapes of the molecule. [61modbut.jpg]

Also, the structural formulas can be drawn in different ways. If it is not apparent to you that the three structures shown here (and in example 4 in your workbook) are all the same, then make a model of each structure and compare the models to each other. You should be able to see that they are all the same. Thus, all three of these structural formulas all represent the same molecule.

H     H
|     |
H - C žž C - H
|     |
H-C-H H-C-H
|     |
H     H

This particular model points out three things.

	Even though the shape of the bonds and atoms around each individual carbon atom is a nice simple tetrahedron, the composite shape of the molecule as a whole is not simple and we do not have a name for it.
	Because of the rotations possible, the composite shape of the molecule can and does change. There is not just one shape.
	There are a variety of ways to draw structural formulas for a given molecule. The fact that the structural formula can be drawn differently does not mean that we are dealing with different compounds. Notice that in each case the atom-to-atom connections are the same.

Isomers

Now compare these models: the one on the left that we were just looking at (example 4 in your workbook) and the one on the right (also found in example 5 in your workbook). They do not match.

They are not the same. There is no amount of rotating and twisting in the model that will make it fit this structural formula. They are different compounds even though they both contain four carbon atoms and ten hydrogen atoms. They both have the same number and kind of atoms, but they are bonded together differently. They have the same molecular formula, C₄H₁₀, but have different structures. We call such compounds structural isomers.

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E-mail instructor: Eden Francis

Clackamas Community College
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