We now know enough about atoms and how they combine to investigate a number of different chemical reactions. We’ll even be able to represent those reactions with symbols that tell us how many of each kind of atom we begin with and how many of each kind we end up with. This is a powerful tool, and finally gets us to the point of improving upon Empedocles’ elements of earth, fire, air, and water. Before we begin, though, I want to impress upon you how much time and effort went into developing the models we’re going to use. In this chapter, I’m going to give you molecular formulas for all kinds of substances. Those formulas don’t just come out of thin air, but rather represent tons of experiments, replications of experiments, and intense thought as to how the atoms in the Periodic Table come together and how we can decide what kinds of atoms and molecules are present in a situation.