GARY BROWN / SCIENCE PHOTO LIBRARY GARY BROWN / SCIENCE PHOTO LIBRARY
Caricature of the German organic chemist August Kekule (1829-1896). In 1858 Kekule proposed that the valence of a carbon atom (the fixed number of other atoms or groups it can bond with) is four. At the same time, he proposed that carbon atoms could be linked to form stable chains, and that the molecular structure of these chains could be given diagrammatically, with lines representing chemical bonds. This 'structural theory' allowed chemists to deduce the structure of an organic molecule from its physical and chemical properties. In 1865 Kekule discovered the ring structure of the molecule benzene, another major advance in organic chemistry.
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