Review of Material
from General Chemistry I
Part II:
Resonance, Hybridization, VB Theory
(mostly from Chap. 9 & 10
of Jespersen 6th Ed)
Dr. C. Yau
Fall 2013
1
Resonance vs. Isomers
Resonance structures are ones that have
the same spatial arrangement of atoms
but different arrangement of electrons to
achieve the Octet.
In contrast, isomers are ones that have the
same molecular formula but different
spatial arrangement of atoms.
2
Isomers
H
O
H C
H
C
H
H H
H
H
C H
H C
O
H
H
Both have the molecular formula of C2H6O but
different structures, written as...
CH3CH2OH
CH3OCH3
3
Resonance
3 resonance structures of SO2
+
+
..
..
..
S ..
O
:O
..: _
..
.. S
O
O
..:
_: ..
:O
..
S
O
..:
Resonance hybrid of SO2

_
:O
..
..
S
+
.. 
O
..:
_
You are expected to know how to determine “formal charges”
and how to write resonance hybrids.
4
Quick Review: Hybridization
Hybridization of an atom is based on
number of "electron clouds" on the atom.
# of electron cloud =
# lone pairs + # atoms directly attached
(You do NOT count # of bonds!!!!)
..
: Cl
..
..
N C O
H
..
..
H
5
Electron Clouds & Hybridization
..
: Cl
..
..
N C O
H
..
..
H
•
•
•
•
•
Cl has 4 e-clouds
N has 3 e-clouds
C has 3 e-clouds
O has 4 e-clouds
H has 1 e-cloud
KNOW THIS WELL!
# e-cloud
Hybridization Geometry Bond Angle
4
sp3
tetrahedral
109.5o
3
sp2 (+ 1 p)
trigonal planar 120o
2
sp (+ 2 p)
linear
180o
1
s (not hybridized)
6
Hybridization & Geometry
..
: Cl
..
..
N C O
.. H
..
H
Lewis structure is not meant
to show the geometry.
This molecule is NOT linear!
What is the hybridization & bond
angle at each atom?
Cl? sp3 n/a
N?
sp2 120o
C?
sp2
O?
sp3 109.5o
120o
..
.. ..
N C
Cl
O H
H
7
Quick Review: Resonance
Draw the Lewis structure of the nitrate
ion.
What is the formula of nitrate?
What is the geometry of nitrate?
What is the bond angle?
What is the hybridization of the N
atom?
88
Quick Review: Resonance
• Why do we need to know about resonance
structures? Are the geometries of the
resonance
structures
different?
_
..
: O:
N ..
:
O
+
:O
.. _
..
: O:
.. _
: O:
.. N ..
_: O
..: _
.. + O
.. N
:
O
+
:O
.. _
..
• Are all the N-O bonds the same length?
Why or why not?
Single bonds are longer than double bonds.
Yet, N-O bonds in nitrate have the same
length. WHY?
99
Importance of Resonance Hybrid
.. _
: O:
N ..
:O
..: _
.. + O
: O:
.. _
: O:
.. N ..
_: O
..: _
.. + O
.. N
:O
..: _
.. + O
The resonance hybrid explains why
all N-O bond lengths are the same.
_

:O:

_
:O
..
N
+
1
The bond order = 1
3

O
..:
_
VB Theory
VB Theory = Valence Bond Theory
Covalent bonds are formed by the overlap of
atomic orbitals.
KNOW THIS WELL!!
single bond =
 bond
double bond = one  bond + one  bond
triple bond = one  bond + two  bond
..
: Cl
..
..
N C O
.. H
..
H
How many  bonds?  bonds?
11
VB Theory Applied to the Nitrate Ion
Sketch the hybrid orbitals in the nitrate ion,
showing the sigma and pi bonds.
Hint: Start with the hybridization of each
atom.
12
VB Theory Applied to Benzene
What is the structure of benzene (very
common organic solvent)?
Its molecular formula is C6H6.
Where are the C's and
H's??
13
Structure of Benzene
H
H
H
H
C
C
C
C
H
C
C
H
H
H
H
C
C
C
C
C
C
H
H
H
Draw the other resonance structure of benzene.
14
VB Theory Applied to Benzene
Benzene is often written as
Learn what this symbol means.
Know how to tell where the
atoms and bonds are.
15