Isomer

Isomer, one of two or more molecules, having identical chemical compositions, but differing arrangements of atoms. Two isomeric substances may differ in their physical, chemical, and biological properties: one may be a useful drug, the other ineffective; one harmless, the other explosive. Isomers are classified as structural isomers, in which the sequence of the atoms differs, or as stereoisomers, in which the atoms are attached in the same order but are oriented differently in space. A further type is cis-trans isomerism (geometrical isomerism) caused by a molecule constrained to one of many possible arrangements by limited bond rotation. See Chemistry, Organic.

A nuclear isomer is one of two or more kinds of atomic nuclei with the same numbers of protons and neutrons, but differing in such physical properties as half-life, or manner of radioactive decay. Nuclear isomers are created by reactions such as the bombarding of nuclei by subatomic particles, or as decay products of radioactive nuclei. See Radioactivity.