<P> A related early idea was Prout's hypothesis, formulated by English chemist William Prout, who proposed that the hydrogen atom was the fundamental atomic unit . From this hypothesis was derived the whole number rule, which was the rule of thumb that atomic masses were whole number multiples of the mass of hydrogen . This was later rejected in the 1820s and 30s following more refined measurements of atomic mass, notably by Jöns Jacob Berzelius, which revealed in particular that the atomic mass of chlorine was 35.45, which was incompatible with the hypothesis . Since the 1920s this discrepancy has been explained by the presence of isotopes; the atomic mass of any isotope is very close to satisfying the whole number rule, with the mass defect caused by differing binding energies being significantly smaller . </P> <P> Although very useful in the foundation of modern chemistry, the law of definite proportions is not universally true . There exist non-stoichiometric compounds whose elemental composition can vary from sample to sample . Such compounds follow the law of multiple proportion . An example is the iron oxide wüstite, which can contain between 0.83 and 0.95 iron atoms for every oxygen atom, and thus contain anywhere between 23% and 25% oxygen by mass . The ideal formula is FeO, but due to crystallographic vacancies it is reduced to about Fe O. In general, Proust's measurements were not accurate enough to detect such variations . </P> <P> In addition, the isotopic composition of an element can vary depending on its source, hence its contribution to the mass of even a pure stoichiometric compound may vary . This variation is used in geochemical dating since astronomical, atmospheric, oceanic, crustal and deep Earth processes may concentrate lighter or heavier isotopes preferentially . With the exception of hydrogen and its isotopes, the effect is usually small, but is measurable with modern instrumentation . </P> <P> An additional note: many natural polymers vary in composition (for instance DNA, proteins, carbohydrates) even when "pure". Polymers are generally not considered "pure chemical compounds" except when their molecular weight is uniform (monodisperse) and their stoichiometry is constant . In this unusual case, they still may violate the law due to isotopic variations . </P>

3 laws that describe how compounds are made