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isomerism



  1. The existence of two or more chemical compounds with the same molecular formula but having different properties owing to a different arrangement of atoms within the molecule. E.g., ammonium cyanate, NH4CNO, and urea, CO(NH2)2, are isomers. See also tautomerism.

       • ORGANIC CHEMISTRY


  2. In nuclear physics, nuclei having the same atomic number, but which exist in different energy states are said to be isomeric. E.g., a nucleus in its ground state and a nucleus in a metastable excited state are isomers. Nuclear isomers are formed by bombardment or in a radioactive decay chain. They usually have very short half-lives and decay by emitting gamma rays.

       • ATOMIC AND NUCLEAR PHYSICS