In chemistry, one of two or more Compounds having the same molecular formula (i.e., the same number of Atoms of each Element in a Molecule) but different structures (arrangements of atoms in the molecule). Isomers
have the same number of atoms of each element in them and the same atomic weight
but differ in other properties. Structural isomers, e.g., ETHANOL (CH3CH2OH) and dimethyl ether (CH3OCH3), differ in the way the atoms are joined together in their molecules.
Stereoisomers have the same basic arrangement of atoms in their molecules but differ
in the way the atoms are arranged in space. Geometric isomers, which are
stereoisomers that differ in the positioning of groups about a double bond or some
other feature that gives the molecule a certain amount of structural rigidity,
differ in physical properties such as melting and boiling points (see Melting Point, Boiling Point). Optical isomers, also called enantiomers or chiral molecules, are
stereoisomers in which the two molecules are mirror images of each other and, each being
asymmetrical, cannot be superposed on each other; optical isomers differ in
the direction in which they rotate light passed through the molecules and can
have different properties. Spearmint leaves and caraway seeds get their
distinctive odors from different optical isomers of the compound carvone, and it has been
theorized that one optical isomer of Thalidomide produced sedation and the
other caused birth defects.