What we have just gone through has been the outline of a very successful theory of molecular structure and bonding. What has not been covered is the quantitative calculation of shapes and energies of molecular orbitals that makes MO theory even more attractive. We began by abandoning the concept of a "bond" and looking at the entire molecule at once. It is perhaps fortunate for our computational sanity that such a procedure usually is excessive with larger molecules, and that the approximation of localized molecular orbitals and two-atom bonds can be used instead. Adding the concepts of hybridization and single, double, and triple bonds provides the framework of a theory that can explain most molecules. One important class of molecules that cannot be explained in terms of two-atom bonds is the organic aromatic molecules, mentioned briefly in Chapter 4. Benzene, CH, is the most familiar example. It has six carbon atoms in a regular hexagonal ring, with all carbon-carbon bonds 1.39 Å long, which is intermediate between single and double bond lengths. Each carbon has one C-H bond of normal length. The skeleton of benzene is shown.