Resonance Structures
Chemistry: IB Diploma Higher Level, topic 2.3.4
Resonance Structures
In structures with multiple bonds and when more than one Lewis structure can be drawn for the same molecule, the true structure is best represented by a resonance hybrid.
Resonance
- Resonance shows the delocalisation of electrons in a molecule and this needs to be considered when more than one Lewis structure can be drawn for the same molecule.
- None of the Lewis structures drawn represents the true structure and the electron density is averaged over all positions.
- The delocalisation of electrons gives more space for the electrons to move and molecules with resonance structures are more stable as a consequence.
- This higher stability is known as resonance stability.
- Resonance occurs in structures with multiple bonds.
Example: Ozone
- The Lewis structure of ozone can be represented by the two diagrams above.
- The two structures can be interconverted by moving the non-bonding electron domain across from one oxygen to the other.
- Neither of these structures is the true structure and ozone is best represented by a resonance hybrid structure.
- This is supported by the bond lengths and strengths, which are consistent with a bond order of 1.5.
Example: Carbonate ion
- Three resonance structures can be drawn for the carbonate ion and the bond order is best described as 1.3.
Example: Nitrate ion
- Three resonance structures can be drawn for the nitrate ion and the bond order is best described as 1.3.
Example: Carboxylate ion
- On removing a proton from a carboxylic acid, a carboxylate ion is formed in which the two carbon-oxygen bonds become equivalent and can be considered to have a bond order of 1.5.
Example: Benzene
- An important example with delocalised electrons is benzene.
- In benzene, the π-bonds are shared equally between the six positions.
Graphene
Graphene is an important example where its structure, containing delocalised electrons, confers very useful properties.
Graphene
- Graphene is a recently discovered allotrope of carbon, which is related to the structure of graphite with just one layer of atoms.
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Graphene structure
- Each carbon atom bonds with three others to give a hexagon.
- Each carbon is sp2 hybridized.
- Each carbon atom has a spare electron, which is delocalised around the whole structure.
- This structure is often described as a honeycomb structure.
Properties
- Excellent conductor
- Very strong, but also very light
- Transparent
Applications
- Drug delivery in carbon nanotubes
- Electronics
- Touch screens
- Photovoltaics
Test yourself on Resonance Structures
Which of these statements is NOT true?
Which of these molecules does NOT have at least two resonance structures?
Which of these is NOT a potential application of graphene?