Chapter 9
Molecular Geometries
and Bonding Theories
Multiple Bonds
Formation of two π bonds in acetylene
Fig 9.26
Describing σ and π bonds in a molecule
formaldehyde
σ
σ
σ
π
Fig 9.27 Formation of σ and π bonds in formaldehyde, CH2O
Sigma (s) and Pi Bonds (p)
1 sigma bond
Single bond
Double bond
1 sigma bond and 1 pi bond
Triple bond
1 sigma bond and 2 pi bonds
How many s and p bonds are in the acetic acid
(vinegar) molecule CH3COOH?
H
C
H
O
H
C
O
H
s bonds = 6 + 1 = 7
p bonds = 1
Molecular Orbital (MO) Theory
In MO theory, we invoke the wave nature of electrons
• If waves interact
constructively, the resulting
orbital is lower in energy: a
bonding molecular orbital.
• If waves interact destructively,
the resulting orbital is higher
in energy: an antibonding
molecular orbital.
MO Theory
 In H2 the two electrons go into
the bonding molecular orbital.
 The bond order is one half the
difference between the number
of bonding and antibonding
electrons:
Bond order = ½ (no. of bonding e− – no. of antibonding e−)
Here: ½ (2-0) = 1
MO Theory
 In the case of He2, the
bond order would be:
Here: ½ (2-2) = 0
• Therefore, MO
theory predicts that
He2 does not exist,
which we know to be
true.
Fig 9.35
MO Theory
He2+
• In the case of He2+, the
bond order would be:
½ (2-1) = 1/2
• Therefore, MO theory predicts that He2+ does exist and it will be
relatively stable
MO Theory – Second-Row Diatomics
Consider only homonuclear diatomic molecules
•
Number of MOs = number of AOs combined
•
AOs combine most effectively with other AOs of similar energy
•
The greater the overlap of AOs, the lower the energy of MO
•
Each MO can hold max of 2 electrons (Pauli exclusion)
•
Hund’s rule applies (same spin in degenerate orbitals)
MOs for Li2 and Be2
Fig 9.37 Energy-level diagram for the Li2 molecule
MOs from 2p Atomic Orbitals
Fig 9.38

For atoms with both s and p
orbitals, there are two types of
interactions:

The p orbitals that are head
to head overlap in s fashion.

The other two sets of p
orbitals overlap in p fashion.
MO Theory – Second-Row Diatomics
Fig 9.43
• There are both s and p
bonding molecular
orbitals and s* and p*
antibonding molecular
orbitals
• Diagram fits only O2
and F2
MO Theory
• The smaller p-block elements in the
second period have a sizeable
interaction between the s and p
orbitals:
Fig 9.44
• This flips the order of the s and p
molecular orbitals in O2 and F2
Fig 9.45
Fig 9.48 Paramagentism of O2
Fig 9.48 Paramagentism of O2
Figure 09.48