Existence of sets of two or more substances with identical molecular formulas (see chemical formula) but different configurations and hence different properties. Jons Jacob Berzelius was the first to recognize and name it (1830). In constitutional (structural) isomerism, the molecular formula and molecular weight of the substances are the same, but their bonding differs. For example, CHO is the molecular formula for both ethanol (CHCHOH) and methyl ether (CHOCH). Constitutional isomers that can be readily converted from one to another are called tautomers (see tautomerism). In stereoisomerism, substances with the same atoms are bonded in the same ways but differ in their three-dimensional configurations. See also isomer.
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