 So we talked in semester one about models, about how different models incorporate and explain different features of things and are useful for different things. We specifically looked at models of the atom and how they've developed through time as scientists gained more information about the atom. The point of having a model is that it explains what we observe and it generates useful predictions about new systems. We also use models to describe different aspects of bonding. For instance, last semester we talked about metallic bonding in terms of a lattice of positive ions surrounded by a sea of electrons. This is a model, a way of thinking about metallic bonding that explains a lot of what we see. But if you study chemistry further at university, you'll go into more complex models that more accurately and precisely describe what holds metal atoms together. When we looked at covalent bonding, you learned how to draw Lewis structures as a way of representing how molecules could be formed from covalent bonds. Lewis structures give you information about which atoms are joined to each other in a molecule. However, they don't give you much information about what the actual shape of the molecule would be in 3D. And this is where VSEPR theory comes in. For instance, a child's drawing of a house involves structural relationships. It shows that the house has a roof and walls and a door and windows, that the roof is attached to the walls and that a door is in the wall. A more advanced model might include spatial relationships like the 3D nature of a house. Both of these models generate predictions. For instance, you could predict from the first that the roof will be on top of the walls and that there will be three windows in the wall. For the second, you could predict what the house would look like if you stood directly in front or to the side. And these are testable predictions, which is what you need from a model. These two drawing models correspond quite nicely to the difference between a Lewis structure and a VSEPR structure for the same molecule. Lewis structures tell you how the atoms are joined together, while VSEPR structures take the Lewis information and add information about how the atoms are arranged in space.