Isomer and Enantiomer Definitions
In organic chemistry, many molecules can exist in one of several forms, or arrangements. These related molecules are called isomers. Some have completely different structures and are related only by having the same chemical formula. Others have the same basic structure, but have different arrangements in space. These variations are called stereoisomers.
Most types of stereoisomers have different chemical properties from each other because the arrangement of the atoms in a molecule affects its properties. Some stereoisomers, however, are mirror images of each other. These variant molecules are called enantiomers, and they have identical physical properties. Molecules that can exist as two enantiomers (there are always exactly two) are called chiral molecules, and the specific "form" an enantiomer takes is called its chirality.
Enantiomers are like hands. The left hand and right hand are identical, except that neither can be "superimposed" on the other; they are mirror images, and no matter how you rotate one hand, it will never look like the other hand. In fact, chirality is sometimes referred to as the "handedness" of a molecule.
A quantity of a compound that contains a mixture of two enantiomers is called a racemic mixture of that molecule. Many of the medications developed during the twentieth century are racemic mixtures. But even though the chemical properties of both "mirror images" are the same, sometimes the body does not recognize them as being the same. One enantiomer might be more active than the other; one may even be completely inactive.