Isomers are molecules with the same molecular formula but different structural arrangements. Isomers can be further classified into constitutional isomers and stereoisomers. Constitutional isomers differ in the connectivity of their constituent atoms. For example, 2-butanol and diethyl ether are constitutional isomers, as they have the same chemical formula, C₄H₁₀O, but differ in the connectivity of the carbon and oxygen atoms. Constitutional isomers have different physical and chemical properties.

Stereoisomers are molecules that have the same chemical formula and the same connectivity of their constituent atoms but differ in the spatial arrangement of their constituent atoms. The cis and trans isomers of a compound, such as cis-2-butene and trans-2-butene, are examples of stereoisomers. Here, the cis and trans molecules have the same chemical formula and connectivity but exhibit different spatial orientations.

Chiral molecules and their mirror images are also examples of stereoisomers, as they are non-superposable on each other and accordingly exhibit different spatial orientations. It should be noted that stereoisomers are different molecules and do not readily interconvert into each other. In contrast, different conformations of a molecule readily interconvert and are all the same molecule.