Resonance structure
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Resonance structures are diagrammatic tools in organic chemistry to symbolize resonant bonds between atoms in molecules. Essentially, these resonance bonds are the movement of "delocalized" electrons through the molecule. Resonance structures all have the same chemical formula and same general shape. However, unshared pairs of electrons and electron pairs making double bonds will often move around between bonds and molecules. For simple molecules like benzene, there are only two resonance forms. For more complex molecules, there are hundreds of possible resonance forms.
For example, for benzene rings, the alternating structure of double bonds in the ring structure are often drawn as a circle.
It is important to note that resonance structures do not represent different structures at all. In the example of benzene, for example, there are two important resonance structures. However, this doesn't mean there are two types of benzene; rather, the true structure of benzene is somewhere in between, constantly in flux. This is why a benzene ring is often shown with a circle inside rather than alternating double bonds — the latter example is theoretically incorrect.
Resonance occurs because of the properties of certain bonds. Double bonds are made up of pi bonds, which connect 2p orbitals. The electrons in these pi bonds, however, are considered to be delocalized; if possible, they will move around. Unshared electrons can be moved about as well, but only if they're in a position to do so. Electrons MUST be in the pi bonds to be delocalized. Often, a sp2 hybrid orbital will be filled with a pair of unshared electrons. While it seems reasonable that they'd want to move about, it is geometrically impossible. The sp2 hybridized orbital is perpendicular to the 2p orbital the pi bonds will be interested in.
Resonance is essential to stability, and stability is a "goal" for all molecules. It would be incorrect to apply a human emotion to a molecule, but it is a reasonable analogy: molecules "want" to be as stable as possible. The movement of the electrons through the pi bonds increases this stability.
To be an important resonance structure, the atomic structure of the atom must be the same throughout all resonance forms. In more scientific terms, the sigma skeleton must remain the same (sigma bonds are single bonds). Furthermore, it must be a correct Lewis Structure, and it must have the same charge. Arbitrary separation of charge causes the importance of a resonance form to go down considerably. However, unimportant does not mean impossible — simply improbable.
Resonance is an important concept of organic chemistry. It will usually be tested for on the MCAT, and is essential to learning the basics of organic chem.
[diagrams soon to come]