Law of definite proportions
One of the fundamental observations of modern chemistry, made by Joseph Louis Proust (1754-1826), the law of definite proportions, states that, in a sample of a pure compound, the elements combine in definite proportions to each other by mass. This is considered one of the fundamental chemical laws. It is also known as the "law of constant composition", alluding to the fact in every sample of a pure chemical compound the elements combine in the same definite proportion — whether the sample was extracted from rock or earth, the ocean bed, magma, or synthesized in the laboratory.
For example, water is a pure compound, made up of hydrogen and oxygen. In any given sample of pure water, the mass ratio will always be 88.81% oxygen to 11.20% hydrogen.
The explanation for the law of definite proportions, i.e., the law of constant composition, awaited the development of the quantitative atomic theory introduced by John Dalton (1766-1844), wherein the unit particle of a compound was inferred to consist of atoms of different types having different atomic weights. In every sample of pure water, the unit particle, or molecule, of water, will always consist of two hydrogen atoms and one atom of oxygen. Since hydrogen and oxygen have different atomic weights, the fixed atomic ratio of 2-to-1 means that the mass ratio of hydrogen-to-oxygen in any bulk sample of water will be the same.
(This is not to say, however, that all compounds of hydrogen and oxygen combine in this proportion. Hydrogen peroxide, for example, is a compound of two hydrogen atoms for every two oxygen atoms. For more about this, see Law of multiple proportions.)
There is also a whole class of substances, called non-stoichiometric compounds (also called Berthollides), which do not follow this law. For these compounds, the ratio between the elements can vary continuously within certain limits. Ferrous oxide is an example of a berthollide. The ideal formula is FeO, but due to crystallographic vacancies it is reduced to about Fe0.95O.