CHEMISTRY COMMUNITY

What is the conceptual difference between a Lewis Structure and a Resonance Structure?

Resonance structures are just different lewis structures. Lewis structures are the individual structures within the resonance structures. They are not two different things, they are integrated together.

A resonance structure is a version of a lewis structure.

Resonance structures are lewis structures which can have more than one possibility of forming the structure. An example of this would be by using different locations for double bonds for a structure.

One way to think of it is this:

Lewis structures are used to show how a molecule may look. However, when you draw a single Lewis structure, you may not be showing all of the possibilities (a double bond could be somewhere else, etc.). So, the Lewis structure you drew would be just a "generalized" version. Resonance structures are Lewis structures that show multiple ways one molecule could be configured (so there might be 3 resonance structures for one molecule, instead of just a single Lewis structure that is overgeneralized/correct for only some situations). Since electrons are not just in one spot, a a molecule in nature can be a resonance hybrid.

I found this image online. I think it does a good job of explaining things. The first three Lewis structures are all Lewis structures that show a single molecule (so they are resonance structures). The last structure shows how structures can be expressed when they are a resonance hybrid.

I think resonance structures also give a better understanding of a molecule's stability. My PLF mentioned that a molecule with many resonance structures has greater stability and is therefore less reactive than a molecule that has fewer or no resonance structures.

Are we required to use both? Or are we allowed to choose?