Resonance Structures
Which one of these three structures
is the correct one?
• As it turns out, laboratory measurements
show that all three bonds are equal.
• When more than one Lewis structure can
be drawn, the molecule or ion is said to
have resonance.
Resonance Structure
• Resonance structures are
simply alternate Lewis
structures for a given ion
or molecule.
• The structures are
changing so quickly that
all we see is an average
blur instead of being able
to detect individual
structures.
Resonance Structure
• We draw three Lewis structures because a
single structure is insufficient to describe
the real structure of the nitrate ion.
Resonance Structure
• ozone as a Lewis structure would be:
• We draw two Lewis structures because a
single structure is insufficient to describe
the real structure of ozone.
Bond Strength – Bond Length
Bond
# of
electrons
Bond
Order
Bond
Strength
Bond
Length
Single
2
1
Weakest
Longest
Double
4
2
Triple
6
3
Strongest
Shortest
Thus, triple bonds are stronger and shorter than
double and single bonds.
Bond Strength – Bond Length
• Laboratory analysis show all three of the bonds
in the nitrate ion to be the same strength and the
same length.
• They are longer and stronger than single bonds
but shorter and weaker than double bonds.
• What is the bond length between the N-O?
Bond Strength – Bond Length
• The ion behaves as if it were a blend of
the three resonance structures.
• The actual bond length for each N-O bond
is 1 and 1/3, because the double bond is
shared equally between all the N-O bonds.
Polarity of Molecules
• A polar bond has one end with a positive
charge and the other with a negative
chare.
– The charge difference is due to the difference
in electronegativity of the two bonded atoms.
• The direction of greatest electronegativity is
shown with an arrow called a dipole.
C---F
and C---H
Polarity of Molecules
• Non-polar molecules – the dipoles are all equal
and opposite. They cancel out and the molecule
is non-polar.
Cl---Be---Cl
• Polar molecules – the dipoles are not equal and
opposite so the molecule has a net dipole and
is polar.
H---C---N
Net Dipole
Examples
Draw all resonance structures, give the bond order
and determine the polarity of each ion.
1. NO22. ClO43. SO3
4. C2H4