A group of atoms joined together to make a compound is called a molecule. Some molecules are very simple: a molecule of water, for example, has just those three atoms. Other molecules, especially those in living bodies, have hundreds of atoms, all joined together in a very particular way. Indeed, it is the way they are joined together, as well as the type and number of atoms, that makes any particular molecule one compound and not another. You can also use the word ‘molecule’ to describe what you get when two or more of the same kind of atom join together. A molecule of oxygen, the gas we need
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