 Okay, one last thing to look at before we finish up here. For most elements we can assume that they're made from single individual atoms. So when you write down the formula for sodium it's simply Na, meaning a single Na atom. And if you write the formula for helium it's He, one helium atom. However, there are seven elements that are an exception to this rule because they always hang around as molecules rather than individual atoms. All seven of them form diatomic molecules, two atoms, diatomic, two atoms joined together. And they are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine. All of these seven are diatomic when you encounter them in the real world. On the periodic table they're shown the same as all the other elements because the periodic table is like a dictionary of all the possible atoms. It doesn't give you direct information about how the atoms react or what form they occur in naturally. Although as you become more familiar with the periodic table you'll be able to infer information like this from it. However, if you ever have to write the formula of any one of these seven elements as it actually exists in nature, for instance if you're writing a chemical equation, then these seven must be written to show that they are diatomic. Learn these seven and know them off by heart because they will turn up over and over again. Okay, so we've now distinguished pure substances and mixtures, atoms and molecules and elements and compounds. Your task for this video is to draw a flow chart to classify substances that uses all of those six terms. Pure substances, mixtures, atoms, molecules, elements and compounds. There are many ways of doing a flow chart. Try to come up with something that makes the relationships between the six terms as clear as possible. See you next time.