Isomer

An isomer is one of two or more chemical species with the same molecular formula but different arrangements of atoms. Isomers can differ in connectivity or in the three-dimensional orientation of their atoms.

Molecular formula: A notation specifying the number of atoms of each element in a molecule. Isomers share this count even when their atoms are connected or arranged differently.
Structural isomerism: Isomerism in which compounds have the same molecular formula but different atom connectivity. It describes isomers produced by changing which atoms are connected.
Thalidomide: A drug used for multiple myeloma and leprosy-related complications, historically associated with severe birth defects. Its stereochemistry illustrates why isomer identity matters in drug development.
Enantiomer: One of a pair of stereoisomers that are nonsuperimposable mirror images. Enantiomers are a specific stereoisomer pair with mirror-image geometry.
Chemical bond: An interaction that holds atoms together in a molecule or solid. Differences in which atoms are bonded distinguish structural isomers.
Geometric isomerism: Stereoisomerism arising from restricted rotation, commonly around double bonds or within rings. Restricted rotation can preserve distinct spatial arrangements as separate isomers.
Limonene: A cyclic monoterpene found in citrus peels, with enantiomers associated with different odors. Its mirror-image forms show how isomers can have different sensory properties.
Diastereomer: A stereoisomer that is not the mirror image of another stereoisomer. Diastereomers contrast with enantiomers while remaining distinct isomers.
Molecular geometry: The three-dimensional arrangement of atoms in a molecule. Different spatial arrangements can produce stereoisomers without changing atom connectivity.
Conformational isomer: A molecular form differing from another through rotation around single bonds, without breaking bonds. Bond rotation creates interconverting spatial forms of the same molecule.

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