Resonance

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Ask yourself an important question: What is resonance? If you're having trouble coming up with an answer for this question, you're not alone. We'll explain what resonace is and how to draw both simple resonance structures. A separate tutorial will be created for more complex forms of resonance pertaining to organic chemistry.

Resonance is a bookkeeping trick used to represent delocalization of electrons on paper. By delocalization, I mean electrons spread out throughout the molecule. Lewis structures show the electrons being fixed (localized) on certain atoms; but often times, molecules can have their electrons spread out (delocalized) onto many different atoms.

Let's draw the carbonate anion CO32- and its resonance forms

 

It looks like we just rotated the molecule and obtained the other two resonace forms, but this is NOT the case. Each of those oxygen atoms is different and the double bond lies between all three oxygen atoms.  Consider the color coded picture below:

 

The oxygen atoms are color coded so you can see they are different. Notice that the double bond is present between each oxygen atom. What does this mean? This shows us that the electrons are capable of moving all over the molecule. If we were to condense the three structures above into one structure, it would look like this

 

This structure says that the electrons are on all three of the oxygen atoms at all times. The dashed bonds show this "electron highway" where the electrons are free to move. Each resonace form only represents one piece of this "highway", but when you put them altogether, you get the complete electron highway. 

 


IMPORTANT: Resonance is a concept, not a process. This means that resonace is purely conceptual-it represents an electron highway and nothing else. When looking at the individual resonance forms, it's important to note that the electrons are not switching from one form to another, but simply represent a piece of the electron highway. The electrons actually exist in all 3 resonance forms at all instances in time, forming the superhighway.

Also note: ONLY pi electrons and lone pairs play the resonance game. Single bonds don't change-these electrons are locked between two atoms.

 


 

Exercises:

Draw resonance forms for the following: SO42-  ,   benzene,     HCN,     acetone,          styrene (google structure if you have to)

 

Message Kyle1990 for solutions