Structural isomers. Distinct chemical species which have the same molecular formula.

Constitutional isomers. Structural isomers differing in constitution (defined as sequential arrangement of atoms regardless of direction in space-i. e. the way the atoms are put together).

Stereoisomers. Structural isomers having identical constitutions and differing in the spatial arrangement of their atoms (several types possible including enantiomers, diastereomers, rotamers)

Enantiomers. Stereoisomers which possess the relationship of being non-superimposable mirror images (your hands are mirror images of each other, in a similar way a molecule can exist in a "left-handed" form or "right-handed" form, molecules are said to have "handedness"

*A molecule which possesses any of the following is considered achiral (i.e. nonchiral or identical to its mirror image)

1) A plane of symmetry that is a one fold axis of symmetry.

2) A center of symmetry that is a two fold axis of symmetry.

3) Any other alternating axes of symmetry (defined as rotation through 360[[ring]]/n followed by reflection perpendicular to initial axis of rotation)

Diastereomers. Stereoisomers which are not enantiomers also defined as non-superimposable, non-mirror images.

Classification of Isomeric Relationships