John C. Kotz
Paul M. Treichel
John Townsend
http://academic.cengage.com/kotz
Chapter 9
Bonding and Molecular Structure:
Orbital Hybridization and Molecular Orbitals
John C. Kotz • State University of New York, College at Oneonta
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Advanced Theories of
Chemical Bonding
Chapter 9
Atomic Orbitals
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Molecules
3
Two Theories of Bonding
• MOLECULAR
ORBITAL THEORY —
Robert Mullikan (18961986)
• valence electrons are
delocalized
• valence electrons are
in orbitals (called
molecular orbitals)
spread over entire
molecule.
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4
Two Theories of Bonding
• VALENCE BOND THEORY
— Linus Pauling
• valence electrons are
localized between atoms
(or are lone pairs).
• half-filled atomic orbitals
overlap to form bonds.
• See Figures 9.1 and 9.2.
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Linus Pauling, 1901-1994
5
Sigma Bond Formation by
Orbital Overlap
Two s orbitals
overlap
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Sigma Bond Formation
Two s
orbitals
overlap
Two p
orbitals
overlap
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7
8
Using VB Theory
Bonding in BF3
•• ••
F ••
Boron configuration
B

 
•••
••••
F
F• 1s
2p
2s
••
••
planar triangle
angle = 120o
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9
Bonding in BF3
• How to account for 3 bonds 120o apart using
a spherical s orbital and p orbitals that are 90o
apart?
• Pauling said to modify VB approach with
ORBITAL HYBRIDIZATION
• — mix available orbitals to form a new
set of orbitals — HYBRID ORBITALS
— that will give the maximum overlap
in the correct geometry.
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10
Bonding in BF3
2p
2s
hydridize orbs.
2
rearrange electrons
three sp
hybrid orbitals
unused p
orbital
See Figure 9.8
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Bonding in BF3
•
The three hybrid orbitals are made
from 1 s orbital and 2 p orbitals  3 sp2
hybrids.
•
Now we have 3, half-filled HYBRID orbitals
that can be used to form B-F sigma bonds.
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Bonding in BF3
An orbital from each F overlaps one of the
sp2 hybrids to form a B-F  bond.
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12
BF3, Planar Trigonal
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13
14
Bonding in CH4
How do we account for 4
C—H sigma bonds
109o apart?
Need to use 4 atomic
orbitals — s, px, py, and
pz — to form 4 new
hybrid orbitals
pointing in the correct
direction.
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109o
Bonding in a Tetrahedron
Formation of Hybrid Atomic Orbitals
4 C atom orbitals
hybridize to form
four equivalent sp3
hybrid atomic
orbitals.
PLAY MOVIE
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15
Bonding in a Tetrahedron —
Formation of Hybrid Atomic Orbitals
4 C atom orbitals
hybridize to form
four equivalent sp3
hybrid atomic
orbitals.
PLAY MOVIE
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16
17
Bonding in CH4, See Figure 9.6
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Bonding in
Glycine
sp
3
H
O
C
H H
C
••
H N
sp
3
sp
••
O H
••
sp
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2
3
20
Bonding in
Glycine
sp
3
H
O
C
H H
C
••
H N
sp
3
sp
••
O H
••
sp
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2
3
21
Bonding in
Glycine
sp
3
H
O
C
H H
C
••
H N
sp
3
sp
••
O H
••
sp
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2
3
22
Bonding in
Glycine
sp
3
H
O
C
H H
C
••
H N
sp
3
sp
••
O H
••
sp
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2
3
23
Bonding in
Glycine
sp
3
H
O
C
H H
C
••
H N
sp
3
sp
••
O H
••
sp
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2
3
24
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25
Orbital Hybridization
See Figure 9.5
BONDS
SHAPE
2
linear
sp
2 p’s
3
trigonal
planar
sp2
1p
4
tetrahedral sp3
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HYBRID REMAIN
none
26
Multiple Bonds
Consider ethylene, C2H4
H
H
120Þ
C
H
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sp
C
H
2
27
Sigma Bonds in C2H4
H
H
120Þ
C
H
sp
C
2
H
PLAY MOVIE
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π Bonding in C2H4
The unused p orbital on
each C atom contains an
electron and this p orbital
overlaps the p orbital on
the neighboring atom to
form the π bond. (See Fig. 9.8)

2s
 
2p
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
 
3 sp 2
hybrid
orbitals

p
orb.
for š
bond
29
π Bonding in C2H4
The unused p orbital on each C atom contains
an electron and this p orbital overlaps the p
orbital on the neighboring atom to form the
π bond. (See Fig. 9.10)
PLAY MOVIE
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30
Multiple Bonding
in C2H4
PLAY MOVIE
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 and π Bonding in C2H4
See Figure 9.10
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 and π Bonding in CH2O
See Figure 9.11
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 and π Bonding in C2H2
See Figure 9.12
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 and π Bonding in C2H2
See Figure 9.12
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Consequences of Multiple
Bonding
There is restricted rotation around C=C bond.
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See Figure
9.13
35
36
Consequences of Multiple
Bonding
Restricted rotation around C=C bond.
PLAY MOVIE
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37
Double Bonds and Vision
PLAY MOVIE
See Screen 9.13, Molecular Orbitals and Vision
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Molecular Orbital Theory
• Valence electrons are
delocalized
• Valence electrons are in
orbitals (called
molecular orbitals)
spread over entire
molecule.
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The Paramagnetism of O2
PLAY MOVIE
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Molecular Orbital Theory
• Bonding and antibonding sigma MO’s are
formed from 1s orbitals on adjacent
orbitals.
PLAY MOVIE
PLAY MOVIE
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Molecular Orbital Theory
See Figure 9.16
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1. No. of MO’s = no. of
atomic orbitals used.
2. Bonding MO is
lower in energy than
atomic orbitals.
Antibonding MO is
higher.
3. Electrons assigned
to MO’s of higher and
higher energy.
Dihelium Molecule
Bond order = 1/2 [# e- in bonding MOs
- # e- in antibonding MOs]
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Sigma Bonding from p
Orbitals
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π Bonding from p Orbitals
Sideways overlap of atomic 2p orbitals that lie in the same
direction in space give π bonding and antibonding MOs.
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 & π Bonding from p Orbitals
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 & π Bonding
from p
Orbitals
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Note change in order of energy
of sigma and pi MOs on going
from N2 to O2.
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