Definite Proportions, Law of

Definite Proportions, Law of 

(also law of definite composition), one of the basic laws in chemistry: every given chemical compound, irrespective of its methods of preparation, contains the same elements in the same fixed proportions by weight; the relative atomic numbers of the elements are expressed as integers. For example, water contains (by weight) 11.19 percent H and 88.81 percent O, and its molecular weight is 18.016; this satisfies the formula H₂O, in which there is one O atom for every two atoms of H.

The law of definite proportions is strictly applied only in relation to gaseous and liquid chemical compounds. The composition of crystalline compounds can either be constant or variable, and therefore not in conformance with the integral ratios of the atoms. Substances of varying composition are called berthollides, or nonstoichiometric compounds, whereas the compounds that clearly obey the law of definite proportions are known as daltonides. The vast majority of chemical compounds, and nearly all organic substances in particular, are daltonides. Berthollides include hydrides, carbides, nitrides, oxides, sulfides, and other crystalline compounds, especially intermetallic compounds. The formation of berthollides is closely related to the geometric characteristics of crystal structures in compounds, the sizes of atoms or atomic groups, and the conditions of preparation.

Chemists of the late 18th and early 19th centuries, including T. Bergman and A. Lavoisier, without realizing it, made use of the law of definite proportions. The law was finally established by J. L. Proust after a prolonged debate (1801-08) with C. Berthollet.