Unit 2 Atoms and Bonding
2.87 Resonance
Textbook ch 8.6
Big Idea 2: Chemical and physical properties of materials can be
explained by the structure and the arrangement of atoms, ions, or
molecules and the forces between them. Students will be able to
demonstrate understanding by laboratory investigation, analysis of data
and creation of models.
Learning Objectives
:
SWBAT:
• Recognize molecules where resonance
structures are needed to describe the bonding
and draw the dominant resonance structures.
This is the
Lewis
structure we
would draw
for ozone, O3.
Resonance
But could you
flip it?... Yes
Would the
formal charges
be same?... Yes
How do you know which one is “right”? Neither,
actually
Resonance
• In fact… the Lewis
structure does not agree
with observed structure
of ozone, in which…
– …both O—O bonds are
the same length
according .
– …both outer oxygens
have a formal charge
of 1/2 if you measure
it in lab.
• One Lewis structure cannot accurately
depict a molecule such as ozone. We use
multiple structures, resonance
structures, to describe the molecule.
How we represent resonance structures symbolically
Need brackets
Need double sided arrows in
between resonance
structures
Resonance
blended
Just as green is a
blend of blue and
yellow…
…ozone is a blend
of these two
resonance
structures.
Resonance
Resonance is invoked when more than one valid Lewis
structure can be written for a particular molecule.
H
Benzene, C6H6
H
H
H
H
H
H
H
H
H
H
H
The actual structure is an blend of the resonance
structures.
The bond lengths in the ring are identical, and
in between lengths of single and double bonds.
References
Our textbook: Brown, Lemay et all. AP edition
chemistry, 13th edition, 2015
I modified the information to fit our needs in AP
Chemistry needs.
Resonance Bond Length and Bond Energy
Resonance bonds are shorter and stronger
than single bonds.
H
H
H
H
H
H
H
H
H
H
H
H
Resonance bonds are longer and weaker than double
bonds.
References
Our textbook: Brown, Lemay et all. AP edition
chemistry, 13th edition, 2015
I modified the information to fit our needs in AP
Chemistry needs.