Structure

To illustrate these terms without recourse to molecules, consider structures that may be assembled from identical cubes by glueing them together face-to-face. Since a cube has six identical faces, up to six other cubes may be attached to a given cube. The composition of structures prepared in this manner is simply the number of cubes used in their construction. The more cubes we use, the more different structures we can assemble and the more complex some of the structures may be.
If we limit our attention to structures made up of four cubes, we will discover eight such assemblies. As shown in the following diagram, these isomeric structures may be divided into three constitutional groups. The first group consists of five structures (1 through 5) in which two cubes (colored blue) are each attached to two other cubes, and the remaining two cubes are each attached to one cube (these are shaded gray). The second constitutional group consists of two structures (6 & 7) in which one cube (colored green) is attached to three other cubes, and the remaining three (shaded gray as before) are attached only to the first. Finally one structure (8), representing a third constitution, is a ring of four identically bonded cubes, each attached to two others and colored blue.

The three groups identified here can be considered "constitutional isomers", since the cubes are attached to each other in different ways. The five structures in the first group differ from each other in their configuration (spatial orientation), and may therefore be considered "stereoisomers". Structures 4 & 5 are particularly interesting because they are non-identical mirror-image configurations. Stereoisomeric compounds having similar mirror-image molecular configurations are known, and are called "enantiomers". As shown, there are also two different spatial arrangements of the structures in constitutional group 2.