What are Models?

Let's start with a seemingly straight-forward question: What is the thing in Figure 2.2.1?

If you haven't taken a chemistry course, it just looks a strange arrangement of lines, dots, two H's an O. There's little meaning here - it's actually useless.

If you have taken a chemistry course (and we do have chemistry prerequisites!), you will probably recognize it as a structural diagram for a water molecule, $\ce{H2O}$. This type of diagram is called a Lewis Structure, named after the American chemist Gilbert N. Lewis. The Lewis Structure is clearly is not the actual molecule itself, it is a representation of the molecule. It is a useful tool for reasoning about what atoms will bond with one another based on their valence electrons. In the case of $\ce{H2O}$, the diagram indicates that a pair of valence electrons are shared between each hydrogen atom and the oxygen atom (using a solid, connecting line). These shared electrons form a single bond. The representation also shows that the oxygen atom has two additional lone pairs of valence electrons, indicated by the blue dots.

A Lewis Structure is an example of a model and it was designed for a purpose: reasoning about chemical bonding in molecules. It is a powerful tool for this purpose for almost a century and is still used as an introductory model in most chemistry courses. There are also many things that Lewis Structures are not good for. This is completely fine and normal. All models are useful for some purposes and not others.

Here is a different kind of model - also from chemistry - a ball-and-stick model (Figure 2.2.2):

The ball-and-stick model has some similarities with Lewis Structures, but also some clear differences. One major difference in this case is that the ball-and-stick model includes a visual representation of the sizes of atoms, the length of bonds, and the fact the molecule occupies three-dimensional space. This can be useful for thinking about the physical behavior of the molecule. Ball-and-stick models can also be made physically as in the example in Figure 2.2.3.

A physical ball-and-stick model allows you to manipulate and rotate the model. This can be useful for gaining further intuition regarding the geometry of the molecule.

Another type of model used in chemistry is the space-filling model. Here is an example, again using a water molecule (Figure 2.2.4):

This particular example of a space filling model includes what is essentially a Lewis Structure but then also represents the approximate electronic charge distribtion around the atoms. One thing this representation tries to make clear is that bonds are not actually sticks or lines.

We are going to discuss models more broadly in the next section, but there are a few things already worth noting about scientific models that we can see from these examples:

1. A scientific model always has an external representation of some kind.
2. The representation is approximate. It emphasizes some information and neglects other information. Each representation is useful for some purposes but not others.
3. The representation is only a model if you know how to interpret it. If you don't know how to interpet Lewis Structures, then a Lewis Structure is not a model of a molecule. It is just letters, dots, and lines. All scientific models have a mental component which we discuss later as mental models.