The law of constant composition of compounds is a consequence of the atomic theory of matter . Atoms are indivisible, and a given atom either is or is not attached to another to form a compound. The empirical formula of a compound is the ratio of atoms of one type to atoms of another in the compound. Empirical formulae such as Fe 2 O 3 can be used interchangeably with molecular formulae in using the law of constant composition. For the oxides of iron and for many solid compounds, only empirical formulae are meaningful because the compounds do not exist as isolatable molecules but only in a stoichiometric crystal lattice.
Chemists always use the molecular formula, if there is one, in preference to an empirical formula.
The law of multiple proportions of elements in compounds is a consequence of the atomic theory of matter . Since atoms are indivisible, a compound can have only integral numbers of atoms of each type in its molecules.