It's not uncommon to have a molecule where a Lewis structure does not perfectly describe that molecule. For example, consider the sulfur dioxide molecule, SO₂:

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



Here, we draw the two possible structures and a double headed arrow between them to indicate that the structure is an average of these two structures.

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



However, we can draw another valid structure with the double bond vertical



We can't decide which is correct based on formal charge: both are fine. In fact, this is a resonance structure

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