Stereoisomer

A stereoisomer is one of two or more molecules with the same atom connectivity but different spatial arrangements. Stereoisomers include forms that differ in handedness or in the arrangement of groups around restricted bonds.

Constitutional isomer: A molecule with the same molecular formula as another but different atom connectivity. Unlike stereoisomers, constitutional isomers differ in which atoms are bonded.
Enantiomer: One of a pair of stereoisomers that are nonsuperimposable mirror images. Enantiomers are the mirror-image class of stereoisomers.
Thalidomide: A drug associated with severe birth defects when used during pregnancy in the mid-twentieth century. Its stereochemical behavior became a cautionary case in drug development, though racemization complicates the enantiomer story.
Cis–trans isomerism: A form of geometric isomerism distinguished by whether groups lie on the same or opposite sides of a reference plane. It is a familiar subtype of stereoisomerism based on relative group positions.
Molecular geometry: The three-dimensional arrangement of atoms in a molecule. Stereoisomers share connectivity but differ in molecular geometry.
Diastereomer: A stereoisomer that is not the mirror image of its counterpart. Diastereomers capture stereoisomeric differences beyond mirror-image pairs.
Chiral resolution: The separation of enantiomers from a mixture into distinct samples. Separating stereoisomers is often necessary to study or use their different properties.
E–Z notation: A system for naming alkene stereochemistry using the priority of attached groups. It generalizes geometric descriptions when cis–trans labels are ambiguous.
Chirality: The property of an object or molecule that makes it distinguishable from its mirror image. Chirality gives rise to stereoisomers that are mirror images and cannot be superimposed.
Geometric isomerism: Stereoisomerism caused by different spatial arrangements around a restricted bond or ring. Restricted rotation creates distinct stereoisomers such as cis and trans forms.

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