Having the same chemical composition and molecular weight; having the same number of atoms of each kind in the molecule; -- said of chemical compounds with known chemical composition. This property used to be called metameric to distinguish it from other forms of isomerism.

Two or more substances that have identical molecular formulas but different molecular structures or configurations, and hence different properties, are called isomers. Isomers differ only in the arrangement of their component atoms.

(state): in which an atomic nucleus is "blocked" in an excited state (at an energy level above its fundamental state) for a given time, ranging from several billionths of a second to several billion years. A single nucleus can have several isomers

of or relating to or exhibiting isomerism