What are resonance structures and how can I understand them better?

[emily(:]

My teacher is explaining resonance structures in class, but I don't understand it that well. Can someone please help? (:

 

[zhermes]

http://en.wikipedia.org/wiki/Resonance_structure
What are you having trouble with?

 

[emily(:]

The setup of them, I just don't understand them, like at all. I know that they provide more information than the Lewis structure, however I just don't know how to set them up correctly, or when you would want to use them over the Lewis structure or the electron dot diagram.

 

[zhermes]

You still haven't been too helpful in giving me a place a start... so again, I recommend reading the Wikipedia article, and doing a google search. That being said, let me say a few general things.

If you have a molecule---lets say NO2 (the example in the wikipedia article)---which has two (or more) possible configurations, it will often be in a combination between the possible states. In the figure, NO2 can have a double bond with either the left oxygen, or the right oxygen. In nature, NO2 will actual have kind of a 1 and a half bond on each side---more accurately, it will have partial double bond character. It other words it will 'resonate' between single and double bonds on each side.

Thus, the electron dot depictions aren't very accurate. Resonance structures are a way of trying to better describe the actual situation.

The reason molecules 'resonate,' is becomes it makes them more stable. The NO2 molecule is more stable with the double bond electrons shared between both sides of the molecule, than it would be if they were attached to only one side.

Let me know if there is something in particular I (or the forums in general) can try to elaborate on, or explain better. Please try to be precise.

 

[DDTea]

Emily, I don't blame you: the idea of resonance is kind of nebulous.

What it comes down to is that "resonance" is an artifact of valence bond theory/electron dot drawings on sheets of paper. We simply cannot draw a single structure that describes the chemical reactivities of certain molecules. So we draw several "contributing structures" with the understanding that none of them individually represents the "complete" description of the molecule. Instead, the best description of the molecule is that it is ALL of the contributing structures at the same time--pretty weird, eh? It's called quantum superposition :)

It's important to remember that the electrons aren't "shifting" between two resonance structures, so the term "resonance" is actually a bit misleading. The term "contributing structures" is more appropriate because multiple structures, frozen in drawings on paper, are contributing to the overall description of the chemical behavior of the molecule.

 

[emily(:]

Thank you both for your responses. The description I gave is the best I way I could explain my problem.