Recap Lewis Structures
1. Arrange the atoms.
2. Count the total number of valence electrons.
3. Allocate two electrons between each pair of atoms
which are assumed to be covalently bonded.
4. Use remaining valence electrons to form lone pairs.
5. Check if the central atom has an octet (or more).
Example: OH-
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Molecules with multiple bonds
O2
Total number of valence electrons: 2 x 6 = 12
Each oxygen has 8 electrons.
N2
Total number of valence electrons: 2 x 5 = 10
Each nitrogen has 8 electrons.
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Ozone
O3
Total number of valence electrons: 3 x 6 = 18
Small rings are highly ‘strained’ and do not
normally form.
Try a different arrangement of oxygen atoms
Another valid Lewis structure would be
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Resonance
• When two or more Lewis structures are possible for a
molecule it is said to exhibit resonance.
• This is indicated by double headed arrows: 
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Resonance
• This does not mean the molecule flips back and forth
between the possible structures – the true structure is
actually an average of the possible resonance structures.
• Bonds are neither single nor double – they are
intermediate between the two. They have intermediate
length and strength.
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Resonance
• Benzene: C6H6
Benzene is best described as being a resonance hybrid
of these two bonding arrangements.
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Resonance
• Benzene: C6H6
Bond Length
Bond Strength
C–C
154 pm
356 kJ mol-1
C=C
133 pm
636 kJ mol-1
benzene
139 pm
518 kJ mol-1
• Benzene is also about 140 kJ mol-1 more stable than
predicted for 1,3,5-cyclohexatriene (the structure with
localised double bonds).
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Resonance – polyatomic ions
• Nitrate, NO3-
Total number of valence electrons:
5 + (3 x 6) + 1 = 24
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NO3- Nitrate ion
• All N-O bond lengths the same and intermediate
between N-O and N=O
• Negative charge distributed evenly across the three
oxygen atoms (1/3 on each oxygen)
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Resonance
If resonance occurs…
• The actual structure is an average of all possible Lewis
structures.
• The actual structure is a single structure.
• The actual structure is more stable than predicted from
the Lewis structure.
• There is no easy way of drawing the actual structure!
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Learning Outcomes:
• By the end of this lecture, you should:
− be able to construct Lewis structures of molecules
and polyatomic ions containing C, N, O or F
− recognise situations where resonance occurs
− appreciate that resonance results in a structure
that is an average of Lewis structures
− appreciate that resonance results in a more stable
molecule then predicted by a single Lewis
structure
− be able to complete the worksheet (if you haven’t
already done so…)
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Questions to complete for next lecture:
1. Draw the Lewis structures of the following molecules and
ions, including resonance structures where appropriate.
a) CO2
b) CO
c) HNCO
d) NO2-
2. Which pair of structures are resonance structures?
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