Resonance

The purpose of the Lewis structure is to accurately predict the bonding in compounds.  Sometimes, when assembling a Lewis structure, one finds that a double bond can be placed in more than one location between two like atoms.  When this can occur, this is usually an indication that the actual molecular structure is some average structure existing in some state between the available extremes.  The average structure is said to exhibit resonance, where the multiple bond is “resonating” amongst the extremes.  Experimental evidence supports this theory showing that the bonds that undergo resonance each have a bond energy between the extreme (i.e., less than the strength of a double bond but greater than the strength of a single bond).   A worked example showing the resonance structures for SO₃ is shown in the following video:

Here is a quick self quiz.

For a molecule that has resonance structures in which single and double bonds are flipping, if we could do an experiment to measure the bond strength of those bonds...

A. 50% of the time we would measure a single bond and 50% of the time we would measure a double bond

B. every time the measurement would give the same bond strength close to the average of a single and double bond

The second answer is correct. The first answer is a huge misconception in chemistry. Try try try to put this idea out of your mind. It is simply wrong. The reason we draw resonance structures is not because the bonds in the molecule can't make up their minds, but rather because we insist on drawing structures with single and double bonds. The real molecules is not flipping back and forth between these two but is always somewhere in between.

The video below looks at the bond strengths of ozone (that exhibits resonance) and molecular oxygen.