Berzelius was a brilliant laboratory experimenter and chemical theorist. We owe to him the modern system of symbols used to represent the chemical formulas of compounds: Berzelius eschewed the various geometrical figures that had been employed previously to represent elements and compounds, and instead wrote chemical formulas in which each element was represented by one or two letters taken from its name -- for example, writing SO3 for sulfur trioxide (modern practice uses subscripts for the exponents instead of superscripts). In 1819, Berzelius published his Essai sur la Theorie des Proportions Chimiques, in which he summarized his extensive studies of the chemical composition of nearly 2000 compounds and his measurements of the atomic weights of the elements, obtaining results that are remarkably accurate even by today's standards. In his Essai, Berzelius stated his conviction that every chemical process was at the same time an electrical one. He arranged all the elements into an ordered list according to the charges they take up on contact with each other. In this classification, which is the basis of the modern concept of electronegativity, oxygen was the most strongly negative and potassium the most strongly positive. Berzelius was also the discoverer or co-discoverer of three elements: cerium, selenium, and thorium.
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