One very important concept in chemistry is that different atoms interact with one another in very specific proportions. For example, two hydrogen atoms interacting with one oxygen atom lead to the water molecule, HO. This relationship is known as the "Law of constant proportions" and leads to the idea of "stoichiometry", a term that refers to the ratios of different atoms in chemical compounds. For example, in water, there are always exactly 2 hydrogen atoms to 1 oxygen atom. In carbon dioxide, there are exactly 2 oxygen atoms for 1 carbon atom. These relationships are described using chemical formulas such as HO (two hydrogen atoms and one oxygen atom) and CO (one carbon atom and two oxygen atoms).