A material that can be represented by a chemical formula is either a mixture or a pure substance. Mixtures, or compounds, can be represented by a chemical formula to denote which atoms make up their composition. Pure substances, such as water, can also be represented by chemical formulas.
Chemical formulas include all of the atoms that comprise a certain mixture or pure substance. In chemical formulas, atoms appear in the abbreviated forms that are found on the periodic table. The amount of each atom in a chemical formula is then denoted by a subscript number next to the atom's abbreviated name. Atoms can appear in any order in a chemical formula.