Chemical Bonding:
Valence Bond (VB) &
Molecular Orbital (MO)
Theories
Chapter 9 Section 4 through 6
of Jespersen 7TH ed)
Dr. C. Yau
Spring 2015
1
VSEPR Theory
• You had previously learned how to predict
the molecular geometry of a species from
examining its Lewis structure.
• It utilizes the concept of repulsion amongst
the charge clouds of the central atoms.
• This was the Valence Shell Electron Pair
Repulsion Theory (VSEPR Theory).
• It does not explain how a bond is formed
and how bonding relates to the s, p, d
orbitals.
2
Valence Bond Theory (VB Theory)
The VB Theory explains bonding as an
overlap of valence orbitals.
H2 bonds are due to the overlap of their 1s
valence orbitals.
1s
1s
A
B
A. Two separate H atoms
B. H2 molecule with covalent bond due to overlap of the 1s
orbitals.
3
Valence Bond Theory
Consider H2S. The valence electrons of S
are 3s2 3p4 
 

3s
3p
and the orbitals would be overlapping with
the 1s orbitals from the two H atoms.
4
Difficulties With VB Theory
• Most experimental bond angles do not support
those predicted by mere atomic orbital overlap.
• For example: C 1s22s22p2 and H 1s1
• One might expect C and H to form CH2 with a
bond angle of 90o. Why might one expect that?
• Experimental bond angles in methane CH4 are
in fact, 109.5° and all angles are the same
• p orbitals are 90° apart, and not all valence e- in
C are in the p orbitals
• Also,
how can multiple bonds form?
5
Hybridization in VB Theory
• Atomic orbitals are mixed to allow formation of
bonds that have realistic bond angles
• The newly mixed orbitals that result are called
“hybrid orbitals” with specified shapes:
Review: # charge clouds
2
3
4
5
6
Hybridization Bond Angles
sp
sp2
sp3
sp3d
sp3d2
180o
120o
109.5o
120o & 90o
90o & 180o
6
How are sp3 hybrid orbitals formed?
__ __ __
__
p
s
__ __ __ __
four sp3
hybrid orbitals
109.5o
If we take s and all three p
we form four sp3 hybrid.
These hybrid orbitals are “degenerate.”
That is, they are of the same E, higher
than s but lower than p.
Note that # orbitals is conserved.
7
How are sp2 hybrid orbitals formed?
__ __ __
__
p
s
__
__ __ __ p
three sp2
hybrid orbitals
If we take s and just two p orbitals,
we form three sp2 hybrid orbitals
leaving one pure p untouched.
These three sp2 hybrid orbitals are planar
with angles of 120o.
The un-hybridized p (pure p) are used in double and
triple bonds.
8
How are sp hybrid orbitals formed?
__ __ __
__
p
s
__ __
two p
__ __
two sp
hybrid orbitals
p
If we take s and just one p orbital,
we form two sp hybrid orbitals sp
leaving two pure p untouched.
These two sp hybrid orbitals are linear
with angles of 180o.
p
sp
9
How are hybrid orbitals formed?
When we run out of p orbitals, we start using
d orbitals, but this would not be possible
for elements smaller than Ne (Period 2
and smaller) because they do not have dorbitals.
Thus we have sp3d hybrid orbitals and sp3d2
hybrid orbitals (but only for elements
beyond Period 2).
10
H
Bonding in CH4
H
• The 4 hybrid orbitals
are evenly distributed
around the C
• The H s-orbitals
overlap the sp3 hybrid
orbitals to form the
bonds.
H
H
11
Bonding Types
• Two types of bonds result
from orbital overlap:
• sigma () bonds
– from head-on overlap
– lie along the bond axis
– account for the first bond
• pi ()bonds
– pi bonds are perpendicular
to bond axis
– account for the second and
third bonds in a multiple
bond
12
Sigma and Pi Bonding
• Please refer to lecture notes on  and 
bonding of C2H4 and C2H2.
• Here is what you should be able to do:
• Given the structural formula of a
compound, be able to specify the
hybridization of each atom, state the
bond angles and determine the # of
sigma and pi bonds.
13
KNOW THIS WELL!
X
X
X
X single bond =  bond
X double bond =  bond +  bond
X triple bond =  bond + two  bonds
14
: O:
H
..
H C C C O
.. C H
1
2
3
H
4
Always start with hybridization.
Give the hybridization of each C and each O.
Give the bond angles.
How many  bonds are there? 9  bonds
How many  bonds are there? 3  bonds
Ans. C1 = sp C2 = sp C3 =sp2 C4 = sp3 O = sp3
C1-C2-C3 = 180o
C2-C3-O = 120o
C2-C3=O = 120o
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O-C4-H = 109.5o
Molecular Orbital Theory (MO Theory)
Oh No! Yet another theory!
VSEPR and VB theories do not explain
everything.
MO Theory considers the molecule as an
entity rather than just a collection of
atoms.
Instead of considering atomic orbitals or
hybrid orbitals, the MO theory considers
molecular orbitals and molecular
energy levels.
16
MO Theory
• When two atomic orbitals overlap, their wave
functions interact by constructive and destructive
interference to form bonding and antibonding
orbitals.
• Electrons in bonding molecular orbitals stabilize
the molecule.
• Electrons in antibonding molecular orbitals destabilize the molecule.
• The Bond Order is # bonds, and calculated thus:
Bond order
= ½ (# bonding electrons - # antibonding electrons).
17
MO Energy Diagram of H2
Two 1s orbitals of H atoms combine to form
two molecular orbitals:
1s (bonding) and 1s* (antibonding).
H2 has 2 electrons, both in bonding MO orbitals.
bond order = 2/2 = 1 (a single bond)
18
MO Theory
See handout for application of MO theory on
diatomic molecules such as N2, O2 etc.
Know how to fill electrons into the MO
energy diagram and calculate bond order.
19