stereochemistry, study of the three-dimensional configuration of the atoms that make up a molecule and the ways in which this arrangement affects the physical and chemical properties of the molecule. It is a third aspect of chemical analysis, the first being the determination of which atoms are present in a molecule and the second being the determination of the interconnections between those atoms by chemical bonds. Central to stereochemistry is the concept of isomerism. Isomers are sets of chemical compounds having identical atomic composition but different structural properties. With geometric isomers, the differences arise from the atoms being bonded in different sequences or patterns. An example is ortho - and para -chlorobenzene; the former has chlorine atoms replacing adjacent carbon atoms in a benzene ring while the latter has chlorine atoms replacing opposing carbon atoms. Optical isomers are pairs of molecules that differ in the same way that a lefthand and righthand screw differ; i.e., they are mirror images of each other. Such molecules with a "handedness" typically rotate the plane of polarization of light that passes through them, but in opposite directions. The sugars glucose and dextrose are a pair of optical isomers; glucose rotates the plane of polarization to the left and dextrose to the right. Stereochemistry is particularly important in biochemistry and molecular biology.