Resonance Structures
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What are resonance structures?
Resonance structures are used to communicate the delocalization of electrons in a 2D line
drawing of a molecule. Resonance is NOT a process!
How do you draw resonance structures?
Using electron-pushing arrows, look for opportunities to move electrons in these three ways:
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A lone pair to become a pi bond.
A pi bond to become a lone pair.
A pi bond to become another pi bond.
© Samantha Keller, PhD
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Non-equivalent resonance structures
Each resonance structure is a “contributor” to the reality of the molecule. In some molecules,
all of the contributors are equal. In other molecules, there are major contributors and minor
contributors. The reality is like a weighted average.
Sometimes, you are asked to rank or compare resonance contributors. Consider these
guidelines when deciding which structure is a major or minor contributor:
1. Better resonance structures have more bonds and don’t violate the octet rule.
2. Better resonance structures have fewer formal charges.
3. Better resonance structures distribute charge according to electronegativity.
© Samantha Keller, PhD
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Practice Problems
Draw all possible resonance structures for the following molecules? Which structures
contribute most significantly to the “hybrid” or the reality of the molecule? What do the
resonance structures tell us about how the molecule might react?
© Samantha Keller, PhD
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