The modern definition of for compounds was realized gradually over the course of the last three centuries.
The concept of compound is probably as old as the concept of "element". Robert Boyle, the key figure in the transition from alchemy to chemistry, was one was one of the first to try to distinguish "compounds" from other types of matter. In the Sceptical Chymist (1661), Boyle refers to chemical compounds as 'perfectly mixt' elements. The term 'perfectly mixt' was meant to distinguish compounds from 'imperfect' mechanical mixtures. Boyle was familiar with atomism and believed that compounds involved chemical combination of atoms, an idea adopted by John Dalton about 150 years later:
Antoine Lavoisier defined compounds in essentially the same way in his Traité Elémentaire de Chimie (Elementary Treatise on Chemistry), published in 1789. Compounds were substances that could be decomposed into elements; elements being "the last point which analysis is capable of reaching". His table of 33 elements included a few compounds which could not be separated with late 18th century technology (e. g. chalk, alumina, and silica).
Definite composition is an essential characteristic of compounds included in the modern definition. Many quantitative studies of metallic compounds like silver chloride in the 1700's lead to the recognition that these compounds contained fixed, characteristic percentages of metal. French chemist Joseph Louis Proust painstakingly determined the elemental composition of a series of metal oxides, hydroxides, and sulfides. His results were remarkably consistent. In 1797, he wrote:
Proust's "invisible hand" was evident in many quantitative analyses and syntheses of compounds made by other 18th century chemists. The statement that when elements combine to form compounds, they do so in fixed proportions by weight was soon referred to as "the law of definite proportions".
Proust's new law was immediately challenged by contemporary Claude Berthollet. Berthollet found that when metals like copper or tin were heated in air, they formed oxides with variable composition. Proust performed a series of analyses on Berthollet's copper oxides that proved that they were in fact mixtures of CuO with Cu2O. He also showed that the "variable composition" tin oxide was a mixture of SnO and SnO2.
The law of definite proportions was later explained by John Dalton. Dalton assumed that compounds are formed from combining atoms. If atoms of an element have a characteristic weight, then the weight ratios of elements in compounds arise from the ratios of atoms combined to make the compound. For example, if carbon monoxide is CO, and each carbon weighs 12 units and each oxygen weighs 16 units, the weight ratio of carbon to oxygen should be 12:16. That implies that converting 12 g of carbon into carbon monoxide will require exactly 16 g of oxygen, and will produce exactly 28 g of carbon monoxide. This is observed. Similarly, the fact that 12 g of C consumes 32 g of oxygen to produce 44 g of carbon dioxide can be explained by proposing that there are 2 oxygens for every carbon atom in this compound. The different carbon-to-oxygen ratios makes carbon dioxide and carbon monoxide two entirely different compounds.
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