Delocalization has appeared several times in previous chapters, but only now can we give it an interpretation in terms of molecular orbitals. The benzene molecule was introduced in Chapter 4, and the carbonate and nitrate ions in Chapter 5. The various Lewis dot models for CO, which assigned single- or double-bond character to different combinations of the three C-0 bonds, were resonance structures for the carbonate ion, differing only in the positioning of electrons between atoms. The phosphate, sulfate, and perchlorate ions discussed in Chapter 6 also were examples of delocalization. In all of these examples, delocalization brought extra stability to the ion, and it is a good practical rule of thumb that the more resonance structures one can draw for a delocalized ion or molecule, the more stability this delocalization creates.