Why use molecular models?

Why do chemists use molecular models?

Chemists use molecular models for a variety of reasons. Most of these reasons share a common theme: molecular models help us better to visualize the shapes of molecules and how those molecules behave. The material presented here purposes to show you how models can make it easier to understand even fairly simple chemical systems.

Paper is flat; molecules aren't.

Whereas molecules occupy three-dimensional space, the media most commonly used to present images of those molecules (pages of a book, computer screens) are two-dimensional. The obvious problem is how to project the image of a three-dimensional entity onto a two-dimensional surface without losing any information. Consider the following (ill-chosen) picture of a methane (CH₄) molecule.

Where is the fourth hydrogen? (Hidden behind the carbon atom.) Are the three hydrogens really in the same plane as the carbon atom? (No.) Are the H-C-H bond angles really 120° as implied by the picture? (No.)

A number of conventions have been developed whose intent is to minimize the ambiguities and loss of information that necessarily accompany any attempt to represent the shape of molecule using a two-dimensional medium. One such convention is illustrated here (a carbon atom is implied to be present at the center of the molecule).

If atoms are joined with a solid line, then both atoms lie in the plane that is parallel to that of the paper (or computer screen). That is, both atoms are equidistant from you, the observer. In the figure above, both the central C atom and the Br atom can be imagined to line in the plane of the paper.
If atoms are joined with a solid line that grows fatter at one end, the atom at the fat end of the bond is closer to you. Think of the bond as becoming foreshortened as it recedes. In the figure above, think of the H and I atoms both being in front of the plane of the paper, sticking out towards you.
If atoms are joined with a dashed line, then one of the two atoms lies further away from you than the other. Which atom is further away can be determined by considering the rest of the illustration. In the figure above, if the C atom lies in the plane of the paper, then the Cl atom lies further away from you, behind the plane of the paper.

So, if we have these conventions for illustrating molecules, why use models? Well, consider the illustration above -- can you determine, accurately, what the Br-C-Cl bond angle is? You could assume that the molecule is perfectly tetrahedral and calculate the angle, but can you be sure that the molecule is, in fact, perfectly tetrahedral? Can you tell from the illustration if the C-Br and C-H bonds are the same length? With a well constructed model, you could answer these questions.

Rotating molecules in my head makes my brain hurt.

Here's another reason for using models: most people have some difficulty, and some people have a very great deal of difficulty rotating three-dimensional objects in their heads. To demonstrate, consider the following two molecules. Do they represent different molecular species, or do they represent the same molecule just viewed from different angles?

To answer the question, you have to rotate one or both of the images in your head. Not so easy, is it? Both images do represent the same molecule, only the viewing angle is different. A model makes this kind of question much easier to answer. You just spin the model around in your hand until it matches the illustration, or until you convince yourself that it cannot match the illustration.

Computer models are an alternative to physical models. For certain tasks, they may not be as satisfying as a model that you can hold in your hand. But a computer model that you can manipulate on-screen is definitely better than a static illustration. Here are two more molecules. Once again, try to determine if they represent different molecular species or if they are just the same molecule viewed from different angles. This time, use your computer's pointing device to "grab hold" of the image on the right and rotate it about. Can you rotate it to a point where it exactly matches the illustration on the left? Can you rotate it to a point where you can see that there is no way, short of breaking bonds, to make the two models coincide?

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