A resonance structure is a system of covalent bonds that contains at least two bonds intermediate in strength between classical single and double bonds.
Any given atom (most typically of carbon) can always have a single bond to one atom and a double bond to another. But often the second and third atoms are of the same chemical element, and those atoms in turn have the same types of bonds (or no covalent bonds at all) to other atoms. In such a case, no reasonable ground exists to assume that one atom is any more tightly bound than the other--and in fact, chemists have discovered that the overall bindings of the various atoms have equal strength.
Current theory holds that resonance takes place between the two alternative schemes for binding the atoms together. Resonance does not mean that the molecule, or polyatomic ion, oscillates between two binding schemes. Rather, the atoms will form bonds that are intermediate between single and double bonds. Such a scheme is a resonance structure, and accomplishes all the goals of a covalent bond.
- Resonance Structures Georgia Southern University, Allen E. Paulson College of Science and Technology department