How do we know how much of each gas is in the atmosphere? We cannot see the individual atoms that make up the air and, even if we could, we would not be able to count all of the atoms to see which type of atom is most common. There are several ways to determine the percentage of oxygen in the atmosphere. In this Activity, you will take advantage of a chemical reaction—between iron and oxygen that forms rust—to find out just how much oxygen there is in the air. More specifically, you use the rusting of iron to remove the oxygen from the air. You will then see how much air is left after the oxygen has been removed.