Which type of isomer must have a non-superimposable mirror image?

Enantiomers are a specific type of stereoisomer that are characterized by their ability to exist as non-superimposable mirror images of one another. This property arises due to the presence of chiral centers in the molecules. A chiral center typically refers to a carbon atom that is bonded to four different substituents, leading to two distinct configurations that cannot be aligned with one another, much like a left and right hand.

This feature is crucial because it impacts how these molecules interact with polarized light and biological systems, and thus it is a key concept in fields such as pharmaceuticals and organic chemistry. While both structural isomers and stereoisomers encompass a broader category of molecular variations, only enantiomers specifically require the non-superimposability condition for their mirror images.

Geometric isomers, on the other hand, involve different spatial arrangements of groups around a double bond or a ring structure and do not necessarily exhibit the non-superimposable nature characteristic of enantiomers. Therefore, the defining feature of enantiomers lies in their unique structural and spatial arrangements that lead to this essential chiral property.