In chemistry, the law of definite proportions and also the elements states that a chemical compound always contains exactly the same proportion of elements by mass. An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have the same elemental composition. For example, oxygen makes up ⁸/₉ of the mass of any sample of pure water, while hydrogen makes up the remaining ¹/₉ of the mass. Along with the law of multiple proportions, the law of definite proportions forms the basis of stoichiometry.^[1] For other uses, see Chemistry (disambiguation). ... Look up chemical compound in Wiktionary, the free dictionary. ... The periodic table of the chemical elements A chemical element, or element, is a type of atom that is defined by its atomic number; that is, by the number of protons in its nucleus. ... This article is about the chemical element and its most stable form, or dioxygen. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... This article is about the chemistry of hydrogen. ... In chemistry, the law of multiple proportions is one of the most basic laws of stoichiometry. ... Stoichiometry (sometimes called reaction stoichiometry to distinguish it from composition stoichiometry) is the calculation of quantitative (measurable) relationships of the reactants and products in chemical reactions (chemical equations). ...

History

This observation was first made by the French chemist Joseph Proust based on several experiments conducted between 1797 and 1804.^{[citation needed]} Based on such observations, Proust made statements like this one, in 1806: Joseph Louis Proust (September 26, 1754 - July 5, 1826) was a French chemist. ... 1797 (MDCCXCVII) was a common year starting on Sunday (see link for calendar) of the Gregorian calendar (or a common year starting on Wednesday of the 11-day-slower Julian calendar). ... 1804 was a leap year starting on Sunday (see link for calendar). ...

"I shall conclude by deducing from these experiments the principle I have established at the commencement of this memoir, viz. that iron like many other metals is subject to the law of nature which presides at every true combination, that is to say, that it unites with two constant proportions of oxygen. In this respect it does not differ from tin, mercury, and lead, and, in a word, almost every known combustible."

The law of definite proportions might seem obvious to the modern chemist, inherent in the very definition of a chemical compound. At the end of the 18th century, however, when the concept of a chemical compound had not yet been fully developed, the law was novel. In fact, when first proposed, it was a controversial statement and was opposed by other chemists, most notably Proust's fellow Frenchman Claude Louis Berthollet, who argued that the elements could combine in any proportion.^{[citation needed]} The very existence of this debate underscores that at the time, the distinction between pure chemical compounds and mixtures had not yet been fully developed. (17th century - 18th century - 19th century - more centuries) As a means of recording the passage of time, the 18th century refers to the century that lasted from 1701 through 1800. ... Claude Louis Berthollet. ... Look up chemical compound in Wiktionary, the free dictionary. ... In chemistry, a mixture is a substance made by combining two or more different materials in such a way that no chemical reaction occurs. ...

The law of definite proportions contributed to, and was placed on a firm theoretical basis by, the atomic theory that John Dalton promoted beginning in 1803, which explained matter as consisting of discrete atoms, that there was one type of atom for each element, and that the compounds were made of combinations of different types of atoms in fixed proportions.^{[citation needed]} This article focuses on the historical models of the atom. ... John Dalton John Dalton (September 6, 1766 – July 27, 1844) was an English chemist and physicist, born at Eaglesfield, near Cockermouth in Cumberland. ... 1803 was a common year starting on Saturday (see link for calendar). ... For other uses, see Atom (disambiguation). ...

Non-stoichiometric compounds

It may be noted that although very useful in the foundation of modern chemistry, the laws of definite proportions is not universally true. There exist non-stoichiometric compounds whose elemental composition can vary from sample to sample. 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. In general, Proust's measurements were not accurate enough to detect such variations. Non-stoichiometric compounds are chemical compounds with an elemental composition that cannot be represented by a ratio of well-defined natural numbers, and are therefore in violation of the law of definite proportions. ... Iron oxide pigment There are a number of iron oxides: Iron oxides Iron(II) oxide or ferrous oxide (FeO) The black-coloured powder in particular can cause explosions as it readily ignites. ... Wüstite (FeO) is a rare mineral form of iron(II) oxide found with meteorites and native iron. ...

In addition, the isotopic composition of an element can vary depending on its source, hence its weight in a pure stoichiometric compound may vary. This fact 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 measurable with modern instrumentation. An additional note: many natural polymers vary in composition (for instance RNA, 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. The word isotopic has a number of different meanings, including: In the physical sciences, to do with chemical isotopes; In mathematics, to do with a relation called isotopism. ... For other uses, see Astronomy (disambiguation). ... Atmosphere may refer to: a celestial body atmosphere, e. ... Look up Oceanic in Wiktionary, the free dictionary. ... Earth cutaway from core to exosphere. ... This article is about Earth as a planet. ... This article is about the chemistry of hydrogen. ... A polymer is a long, repeating chain of atoms, formed through the linkage of many molecules called monomers. ... For other uses, see RNA (disambiguation). ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Carbohydrates (literally hydrates of carbon) are chemical compounds that act as the primary biological means of storing or consuming energy, other forms being fat and protein. ...

References

1. ^ Zumdahl, S. S. “Chemistry” Heath, 1986: Lexington, MA. ISBN: 0-669--04529-2.