The idea of resonance has brought clarity and unity into modern structural chcmistry, has led to the solution of many problems of valence theory, and has assisted in the correlation of the chcmical properties of substanccs with the information obtained about the structure of their molcculcs by physical methods.
The goal of a structure investigation of a system is the description of the system in terms of simpler entities. This description may be divided into two parts, the first relating to the material particlcs or bodies of which the system is considered to be composed, and the sccond to the ways in which these particlcs or bodies arc interrelated, that is, to their interactions and interconnections. In describing a system it is usually convenient to resolve it first into the next simpler parts, rather than into its ultimate constituents, and then to carry the resolution further and further in steps. Wc arc thoroughly accustomed to this way of describing the material constitution of substances. The use of the conccpt of resonance permits the extension of the procedure to include the discussion hot only of the next simpler constituent bodies but also of their interactions. Thus the material description of the benzene molcculc as containing carbon and hydrogen atoms, which themselves contain electrons and nuclci, is amplified by use of the resonance conccpt in the following way: The structure of the normal benzene molcculc corresponds to rcsonanccbctwccn the two Kckulc structures, with smaller contributions by other valence bond structures, and the molecule is stabilized and its other properties arc changcd somewhat by this resonance from those cxpcctcd for either Kckulc structure alone; each Kckulc structure consists of a ccrtain distribution of single and double bonds, with essentially the properties associated with these bonds in other molcculcs; cach bond represents a type of interaction between atoms that can be dissolved in terms of the resonance between structures differing in the interchange of electrons between atomic orbitals.