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Isomers have the same number of atoms of each element in them and the same atomic weight but differ in other properties. For example, there are two compounds with the molecular formula C2H6O. One is ethanol (also called ethyl alcohol), CH3CH2OH, a colorless liquid alcohol; the other is dimethyl ether, CH3OCH3, a colorless gaseous ether. Among their different properties, ethanol has a boiling point of 78.5°C and a freezing point of -117°C; dimethyl ether has a boiling point of -25°C and a freezing point of -138°C. Ethanol and dimethyl ether are isomers because they differ in the way the atoms are joined together in their molecules:
Isomers are classified as structural isomers, which have the same number of atoms of each element and molecular weight but different bonding patterns (see chemical bond), or as stereoisomers, which have the same number of atoms of each element, molecular weight, and bonding pattern but in which the atoms have different spatial relationships. Tautomers are structural isomers that readily convert from one isomeric form to another and therefore exist in equilibrium.
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