|Themes > Science > Chemistry > Inorganic Chemistry > More Information about Chemical Bonding > Chemical Bonding Index > Resonance|
The above structure would indicate that there are two different types of
bonds in the molecule: one single S-O bond and one double S=O bond. However,
experimental data shows that this is not correct: there is only one S-O bond
length, equal to about a bond and a half. This is an example where the basic
Lewis diagram breaks down: to indicate the fact that there is only one type of
bond, we draw a resonance structure
It cannot be overemphasized that the various structures in a resonance structure do NOT really exist. SO2 does NOT exist as two structures flipping back and forth, but only as the average of the two structures. Think of it as a mutt: a dog that is 1/2 doberman and 1/2 retriever is an average of the two breeds; it's not a doberman 1/2 the time and a retriever the other half.
It's possible to have more than two resonance forms: for example, the
SO3 molecule has three:
Resonance structures are typically needed when you can draw a molecule multiple different ways, but cannot distingush between them by formal charge.
Example: Draw the resonance Lewis structure for the formate ion COOH-. (Everything is bonded to the central carbon.)
Solution: If we simply write a Lewis
structure for the ion, we end up with