Definition of a model:
A simplified description of a system or process to assist calculations and predictions.
A Lewis structure is a model that describes the bonding between atoms and the lone pairs of electrons. The key thing to remember though is that these are simplified descriptions. In reality, the electron distribution of some molecules cannot be accurately described by one Lewis structure alone. To more accurately represent the electron distribution, we draw multiple structures by rearranging pi and lone pairs of electrons. An example is given below.
Notice the arrow is NOT an equilibrium arrow. The molecule is not interconverting between these two resonance structures. Instead, it is a hybrid (a combination) of the two. The result is a more accurate description of the molecule.
Let’s think back to that definition of a model from the start of this post. Notice the purpose of a model is to “assist predictions”. Lewis structures, we said, give us a more accurate description of the structure and therefore electron distribution. Since mechanisms is all about mapping the flow of electrons during a reaction, Lewis structures help us predict where there are areas of high and low electron density, and thus show how electrons flow to make the reaction happen. For example, in the first resonance structure above, carbon has a full octet and a neutral charge, however in the second structure it is clear that carbon is actually electron poor. If a nucleophile is present, it would be reasonable to predict that a nucleophile will attack that carbon.
Learn more about resonance in the videos below. Then practice finding resonance structures. Remember, showing resonance structures is not a stand-alone activity, it is a tool you will use to solve more complicated mechanisms.