A compound that exists in forms having different arrangements of atoms but the same molecular weight.
Compound of same elements in same proportions by weight as another, but having different structure and properties.
Chemical compound having the same molecular composition and mass as another, but with different physical or chemical properties owing to the different structural arrangement of its constituent atoms. For example, the organic compounds butane (CH3(CH2)2CH3) and methyl propane (CH3CH(CH3)CH3) are isomers, each possessing four carbon atoms and ten hydrogen atoms but differing in the way that these are arranged with respect to each other.
Structural isomers have obviously different constructions, but geometrical and optical isomers must be drawn or modeled in order to appreciate the difference in their three-dimensional arrangement. Geometrical isomers have a plane of symmetry and arise because of the restricted rotation of atoms around a bond; optical isomers are mirror images of each other. For instance, 1,1-dichloroethene (CH2=CCl2) and 1,2-dichloroethene (CHCl=CHCl) are structural isomers, but there are two possible geometric isomers of the latter (depending on whether the chlorine atoms are on the same side or on opposite sides of the plane of the carbon-carbon double bond).
1. Atomsko jezgro u pobuđenom stanju.
2. Jedinjenje koje ima s nekim isti hemijski sastav, ali različitu strukturu formule.