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Valence Bond Theory
Chemical Bonding 4.8
• Combines Lewis’ theory of filling octets by
sharing pairs of electrons with the electron
configuration of atomic orbitals.
• Valence Bond Theory states that bonding occurs
when atomic orbital overlap.
Valence Bond Theory
Hybrid Orbital Theory
Multiple Bonds
Building BF3 with
Valence Bond Theory
B:
Building BF3 with
Valence Bond Theory
B:
1s
2s
2p
1s
2s
2p
1s
2s
2p
F:
Boron enters an excited state where an
electron from the 2s orbital is promoted to
the 2py orbital.
Valence Bond Theory has
problems with the shape
2py
F
2px
B
F
2s
Valence Bond Theory
Views bonding as an
overlapping of atomic
orbitals.
• Valence Bond Theory says 90o and a random
angle for the fluorine bonded to the s-orbital.
• VSEPR theory says 120o
Solution (Hybrid Orbitals)
Gets the bond
angle wrong
F
Problems with Valence Bond Theory
and BF3
Problem (the bond angle is wrong)
• When the electron is promoted, the 2s, 2px,
and 2py orbitals of boron morph into three
separate sp2 hybrid orbitals that are identical
in shape and size.
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sp2 Hybrid Orbitals
sp2 Hybrid Orbitals
sp2
F
s
sp2
+
+
p
=
p
sp2
sp2
sp2
F
F
sp2
The 2s and two 2p orbitals morph into three identical
sp2 hybrid orbitals
sp3 Hybrid Orbitals (e.g. CH4)
H:
sp3 Hybrid Orbitals (e.g. CH4)
H:
1s
1s
C:
C:
1s
2s
2p
1s
2s
2p
Hybridization of Carbon
C:
1s
sp3
sp3 Hybrid Orbitals (e.g. CH4)
H
Bond Angle
= 109.5o
sp3
H
sp3
sp3
H
sp3
H
Valence Bond Theory and Lone Pairs
(e.g. H2O)
Building H2O with Valence Bond Theory
H:
H:
1s
1s
O:
1s
2s
2p
Oxygen could accept one electron from one
Hydrogen in its py orbital and another from
the other Hydrogen in its pz orbital.
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Problems with Valence Bond
Theory in the H2O Example
Valence Bond Theory and Lone Pairs
(e.g. H2O)
• Valence Bond Theory views bonding as an
overlapping of atomic orbitals.
H
py
pz
py
px
pz
px
Valence
Bond Theory
get the angle
wrong again!
H
• The bond angle is wrong.
• Valence Bond Theory predicts 90o
• VSEPR Theory predicts 104.5o
• The orbital shape must be wrong.
• Shared electrons are not spending enough
time with the Hydrogen.
sp3 Hybrid Orbitals (H2O)
Hybrid Orbital Theory and Lone Pairs
(e.g. H2O)
H:
H:
1s
sp3
1s
sp3
sp3
O:
1s
sp3
Electrons are not promoted here, but every
orbital in the n = 2 energy level becomes
hybridized.
Hybrid Orbital Theory and
5 Charge Clouds! (e.g. PCl5)
Four sp3
hybrid orbitals
are formed
sp3
H
Bond Angle
= 104.5o
H
sp3d Hybrid Orbitals
Atomic Orbitals
P:
..
:Cl
..
..
:Cl:
P
:Cl:
..
:Cl:
..
..
Cl:
..
3s
3p
Promotion
P:
3s
3p
Hybridization
P:
sp3d
3d
3d
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Hybrid Orbital Theory and
6 Charge Clouds! (e.g. SF6)
sp3d2 Hybrid Orbitals
Atomic Orbitals
S:
..
:F:
..
:F
..
..
F:
..
S
:F:
..
:F:
..
:F:
..
3s
3p
Promotion
S:
3s
3p
Hybridization
S:
sp3d2
Summary
Charge Clouds
2
3
4
5
6
3d
3d
Double Bonds (e.g. C2H4)
Hybridization
sp
sp2
sp3
sp3d
sp3d2
H
H
C
C
H
H
Each carbon has 3 charge clouds.
sp2 hybrid orbitals
Double Bond (C2H4)
Double Bonds (e.g. C2H4)
Atomic Orbitals
C:
2s
Promotion
C:
2s
Hybridization
C:
sp2
p
H
2p
sp2
sp2
sp2
H
σ Bond
2p
2p
H
sp2
sp2
sp2
p
H
π Bond
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Triple Bonds (e.g. C2H2)
Triple Bonds (e.g. C2H2)
Atomic Orbitals
C:
H
C
C
H
Each carbon has two charge clouds.
sp hybrid orbitals
2s
Promotion
C:
2s
Hybridization
C:
sp
2p
2p
2p
Triple Bond (C2H2)
p
sp
p
σ Bond
sp
sp
p
p
sp
Two π Bonds
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