Isomer

An isomer (/ˈaɪsəmər/; from Greek ἰσομερής, isomerès; isos = "equal", méros = "part") is a molecule with the same chemical formula as another molecule, but with a different chemical structure. That is, isomers contain the same number of atoms of each element, but have different arrangements of their atoms. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like positional isomers, cis-trans isomers and enantiomers, etc. (see chart below). There are two main forms of isomerism (/ˈaɪsəmərɪzm/ or /aɪˈsɒmərɪzm/): structural isomerism and stereoisomerism (spatial isomerism).

Structural isomers

In structural isomers, sometimes referred to as constitutional isomers, the atoms and functional groups are joined together in different ways. Structural isomers have different IUPAC names and may or may not belong to the same functional group. This group includes chain isomerism whereby hydrocarbon chains have variable amounts of branching; position isomerism, which deals with the position of a functional group on a chain; and functional group isomerism, in which one functional group is split up into different ones.