Some molecules and ions cannot be described using a single Lewis structure. This is because of resonance, the bonding in molecules and ions that result in more than one Lewis structure. Resonance is a feature of the valence-bond theory. In the valence-bond theory, Schrödinger's equation is used to determine the structure of molecules. If more than one solution to Schrödinger's equation is possible, then more than one molecular structure must be possible. According to the valence bond theory, various combinations of the solutions to Schrödinger's equations depict possible molecular structures. These different structures are called resonance structures or contributing structures. The actual structure of the molecule is called the resonance hybrid, a structure which represents the average of each of the resonance structures. It was originally believed that a molecule resonated between the different possible structures, much like a plucked guitar string...