Advanced Theories of
Chemical Bonding
Chapter 9
Atomic Orbitals
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Molecules
1
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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2
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.
Linus Pauling, 1901-1994
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3
Sigma Bond Formation by
Orbital Overlap
Two s orbitals
overlap
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Sigma Bond Formation
5
Two s
orbitals
overlap
Two p
orbitals
overlap
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Using VB Theory
Bonding in BF3
•• ••
F ••
Boron configuration
B

 
•••
••••
F
F• 1s
2p
2s
••
••
planar triangle
angle = 120o
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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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Why Hybridize?
Just looking at valence electrons:
Be should form no covalent bonds
8
But…
B should form one covalent bond
BeF2, BF3 and
CF4
C should form 2 covalent bonds
Exist! HOW?
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Hybrid Orbitals: Why?
• To explain the bonding in molecules like BeF2,
BF3 and CF4, Linus Pauling proposed that
orbitals become ‘hybridized’
– Hybrid orbitals are orbitals created by
mixing the s, p or d orbitals of an atom.
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10
Hybrid Orbitals: The Rules
1. The number or hybrid orbitals is ALWAYS
equal to the number of atomic orbitals that
are combined to make the hybrid set
2. Hybrid orbital sets are always built by
combining an s orbital with as many p or d
orbitals necessary to accommodate the
bonding and lone pairs on the central atom
(Remember Electron Pair Geometry?)
3. The Hybrid Orbitals are directed TOWARDS
the terminal atoms
•
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This results in a better orbital overlap AND stronger
bonds between the central and terminal atoms
sp Hybrid Orbitals
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Mix an s
orbital with a
p orbital to
create two sp
orbitals
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sp2 Hybrid Orbitals
Mix an s
orbital with 2
p orbitals to
create three
sp2 orbitals
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sp3 Hybrid Orbitals
Mix an s
orbital with 3
p orbitals to
create four
sp3 orbitals
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sp3 Hybrid Orbitals: Examples
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15
Bonding in BF3
2p
2s
hydridize orbs.
2
rearrange electrons
three sp
hybrid orbitals
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unused p
orbital
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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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BF3, Planar Trigonal
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19
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.
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Bonding in a Tetrahedron —
Formation of Hybrid Atomic Orbitals
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4 C atom orbitals
hybridize to form
four equivalent sp3
hybrid atomic
orbitals.
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22
Bonding in CH4
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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
25
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
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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
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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
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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
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Orbital Hybridization
2 e- clouds
3 e- clouds
4 e- clouds
5 e- clouds
6 e- clouds
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31
Multiple Bonds
Consider ethylene, C2H4
H
H
120°
C
H
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sp
C
H
2
32
Sigma Bonds in C2H4
H
H
120°
C
H
sp
C
2
H
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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.

2s
 
2p
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
 
2
3 sp
hybrid
orbitals

p
orb.
for š
bond
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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.
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Multiple Bonding
in C2H4
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 and π Bonding in C2H4
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 and π Bonding in CH2O
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 and π Bonding in C2H2
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 and π Bonding in C2H2
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Consequences of Multiple
Bonding
There is restricted rotation around C=C bond.
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Consequences of Multiple
Bonding
Restricted rotation around C=C bond.
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42
Consequences of Multiple Bonding
Formation of Isomers
One isomer may have biological
activity while the other may not
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Double Bonds and Vision
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