From the Greek word enantios, which means “opposite.” Enantiomers are a pair of nonidentical, mirror image molecules. This means that both molecules are made up of the same atoms, that is, they have the same molecular formula, but the constituent groups that are attached to a carbon atom can be arranged in two different ways (forms) around the carbon atom. This gives rise to an asymmetric molecule that can exist in either of two mirror-image forms whose mirror images are not superimposable. A pair of these molecules is known as enantiomers. The four attached groups are all different from each other.