In organic chemistry, isomerism is 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.
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.