Isomerism

Isomerism is the existence of compounds with the same molecular formula but different structures or spatial arrangements. Isomers can therefore have different physical, chemical, or biological properties.

Molecular formula: A notation specifying the number of atoms of each element in a molecule. Isomers share this formula even when their atoms are connected or arranged differently.
Conformational isomer: One of multiple spatial arrangements of a molecule interconvertible by rotation around single bonds. Conformations arise from bond rotation without breaking covalent bonds.
Thalidomide: A drug whose enantiomers interconvert in the body and whose use became linked to severe birth defects. Its history shows why assigning different effects to isolated enantiomers can be misleading.
Resonance: A representation of a molecule as a hybrid of contributing structures that differ only in electron placement. Resonance forms are not distinct compounds with different atom arrangements.
Structural isomer: A compound whose atoms are connected differently from those of another compound with the same molecular formula. Structural isomers differ in connectivity, the defining basis of one major branch of isomerism.
Configurational isomer: A stereoisomer that cannot interconvert without breaking and reforming covalent bonds. Unlike conformers, these arrangements are separated by a barrier requiring bond changes.
Cisplatin: A platinum-based chemotherapy drug with two chloride ligands arranged adjacent to one another. Its square-planar geometry distinguishes the active cis complex from the trans isomer.
Isotopomer: A molecular variant that differs from another in the positions of isotopes among its atoms. Isotopomers can share connectivity while differing in isotopic composition, not merely structure.
Stereoisomer: A compound whose atoms have the same connectivity as another but differ in their spatial arrangement. Stereoisomers retain connectivity while differing in three-dimensional arrangement.
Cis–trans isomerism: Geometric isomerism in which groups occupy the same or opposite sides of a bond or ring. Restricted rotation can preserve distinct cis and trans arrangements.

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