An isomer in chemistry is a chemical that has the same molecular formula as another, but it has a different arrangement of atoms in space. They are molecules with the same chemical formula but different chemical structures. Different isomers have different chemical properties (that is, they may look, or smell, or taste different from each other). That is, isomers contain the same number of atoms of each element but have different arrangements of their atoms in space. It is important to be able to recognize isomers because they can have different chemical, physical properties, and biological properties.
A simple example of isomerism is given by propanol: It has the formula C3 H7OH and occurs as two isomers: propan-1-ol (n-propyl alcohol; I) and propan-2-ol (isopropyl alcohol; II).
In organic chemistry, isomers are molecules with the same molecular formula (i.e. the same number of atoms of each element), but different structural or spatial arrangements of the atoms within the molecule. Isomerization is the process by which one molecule is converted into another molecule with the identical atoms. When atoms can assume different configurations, the phenomenon is termed isomerism. This may occur spontaneously or a reaction may be required to achieve this effect.
There are two main categories: stereoisomers and structural isomers. Structural isomers are isomers with the atoms making up the molecule joined together in different ways. They differ with regards to the specific attachment of atoms and functional groups. Types of structural isomers include chain isomers, position isomers, which differ based on the positioning of a functional group on the chain.
Stereoisomers have different positions of atoms in space and are much more subtle than structural isomers. Types of stereoisomers consist of enantiomers, diastereomers, and conformational isomers. See enantiomer for stereoisomers that are related to each other by a reflection: they are mirror images of each other that are non-superimposable.