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E. Frankland (1852) by examining series of formulae such as those given above found that "no matter what the characters of the uniting atoms may be, the combining power of the attracting element... is always satisfied by the same number of these atoms," and that the formula of a compound depends on this. Kekule in 1858 showed that carbon atoms in organic compounds are quadrivalent and are able to combine with one another to form chains. Van't Hoff and Le Bel in 1874 showed that, if the four valencies of a carbon atom point in space towards the corners of a regular tetrahedron, this would explain optical activity. Werner from 1893 onwards extended the theory of the structure of inorganic compounds, and the electrical theory of the atom has given a satisfactory picture of the nature of valency.
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