Recap 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: 
• The (one) actual structure is an average of possible
Lewis structures.
Example: HCO3-
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Recap Lecture 7
• Elements of Period 3 (eg P,
S, Cl) have their valence
electrons in the third shell
• The third shell can hold a
maximum of 18 electrons in
the s, p & d sub-shells
• Consequently we see
structures that may have
more than 8 electrons
associated with these atoms
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Example: POCl3
Total number of valence electrons: 5 + 6 + (3 x 7) = 32
Oxygen has a valency of 2 so, when uncharged, forms
two bonds.
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Example: POCl3
• P now has 10 electrons, but this is OK as they are in
the third shell (max 18 electrons).
• O has the two bonds expected from its valency.
• These are resonance structures (only position of the
electrons have moved) but not ‘equivalent’.
• Right hand structure is a better representation of the 4
real structure.
Valency and charge
• Valency works well for C, N, O & F in neutral molecules.
• For period 3 elements (and below) it is only part of the
story, eg S forms compounds with 2, 4 or 6 bonds.
• If a charge is present, the atom may have more/less
bonds than expected.
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Predicting the best structure
• Draw the Lewis structure, including lone pairs and
multiple bonds.
• Check for the presence of ‘equivalent’ resonance
structures.
• Check number of bonds on C, N, O & F as a prompt
for ‘non-equivalent’ resonance structures.
• Choose best structure and check again for resonance
structures.
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Chlorite ion, ClO2Total number of valence electrons: 7 + (2 x 6) + 1 = 20
Note: 8 electrons on oxygen remains fixed, Cl has 8, 10
or 12 electrons
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Sulfate ion, SO42Total number of valence electrons: 6 + (4 x 6) + 2 = 32
These are all resonance structure but the last one is ‘best’
so far.
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SO42-, possible structures:
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Sulfate ion, SO42There are also other resonance structures related to some
of these structures.
And the real structure is best represented by an average of
these six structures. All S-O bonds are the same length.10
Learning Outcomes:
• By the end of this lecture, you should:
− be able to construct Lewis structures of molecules
and polyatomic ions containing period 3 elements
− recognise situations where resonance occurs
− identify non-equivalent resonance structures
− be able to choose the resonance structure(s) that
are likely to most closely represent the real
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 NH2-, NH3 and NH4+ and
indicate how the number of bonds relates to the valency
of nitrogen.
2. Draw the Lewis structure of PCl3 and PCl5 and indicate
the number of electrons associated with the phosphorus
atom.
3. Draw the ‘best’ Lewis structure of SO2 and SO3 and give
a reason for your choice.
4. Draw the ‘best’ Lewis structure of HPO32- and give a
reason for your choice (note H is bonded to the P).
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