isomer

One of two or more compounds that have the same chemical formula but different arrangements of the atoms within the molecules and that may have different physical/chemical properties.

1. Structural isomers have the same chemical formula but different structures; e.g. leucine and isoleucine. 2. Stereoisomers are different topological forms of an otherwise single chemical structure, due to changes in bond configurations about some axis or plane of symmetry; eg, D- and L-glucose or cis- and trans-cinnamic acid.

One of several nuclides with the same number of neutrons and protons capable of existing for a measurable time in different nuclear energy states.