Multiple Lewis Dot Structures

1
Multiple Lewis Dot Structures
CO32-
24 total valence electrons
2
Resonance Structures
Resonance Structure
Resonance Structure
Resonance Structure
Experimentally, all three bonds are measured and
have the same bond length...
...longer than a double bond and shorter than a
single bond....
...i.e. the observed bonds have characteristics
of both single and double bonds.
Resonance Hybrid Representation of the average
resonance structures.
3
Resonance Structures Benzene (C6H6)
Resonance
Resonance Hybrid
4
Formal ChargesDetermining the most correct
Lewis Structure
CH4O
4 Valence e-
1 Valence e- ? 4
6 Valence e-
14 Valence e-
H
H
H
H
C
H
H
O
C
H
H
O
C
H
H
O
H
H
How do we decide which Lewis Dot Structure is
correct?
By determining the Formal Charge for each atom
and deciding which F.C. arrangement is most
reasonable.
5
Formal ChargesDetermining the most correct
Lewis Structure
Formal Charge (F.C.)
- Unshared e-
- Bonds
Valence e-
FCC
4 Ve-
- 4 Ue-
- 2 Bonds
-2
? 4
FCH
1 Ve-
- 0 Ue-
- 1 Bond
0
FCO
6 Ve-
- 0 Ue-
- 4 Bonds
2
0
The total of all F.C.s must equal the charge on
the molecule.
CORRECT!
6
Formal ChargesDetermining the most correct
Lewis Structure
Formal Charge (F.C.)
- Unshared e-
- Bonds
Valence e-
H
H
H
C
H
H
O
C
H
H
O
H
FCC
4 Ve-
- 0 Ue-
- 4 Bonds
0
FCC
4 Ve-
- 2 Ue-
- 3 Bonds
-1
? 4
? 4
FCH
1 Ve-
- 0 Ue-
- 1 Bond
0
FCH
1 Ve-
- 0 Ue-
- 1 Bond
0
FCO
6 Ve-
- 4 Ue-
- 2 Bonds
0
FCO
6 Ve-
- 2 Ue-
- 3 Bonds
1
0
0
7
Formal ChargesDetermining the most correct
Lewis Structure
Formal Charge (F.C.)
- Unshared e-
- Bonds
Valence e-
Best Structure!
Time to choose!

• Smaller formal charges are preferable to larger
  ones

• Like formal charges on neighboring atoms are
  not desirable

• A more negative formal charge should be
  associated with atoms having higher
  electronegativity.

8
Formal ChargesDetermining the most correct
Lewis Structure
Problem 10.17 Draw a Lewis structure for the
most important resonance form of the BrO3- ion,
showing formal charges and oxidation numbers of
the atoms (26 Valence e-)
-1
-1
O
Br
O
O
-1
-1
0
0
2
0
Too Large
Better!
Convert lone pair electrons into bonding
electrons to improve formal charge values.