Chapter 9.4-9.8
Bonding Theory
Complete Ch 9 problems # 29, 36, 38, 39, 42, 45,
49, 52, 55, 63, 75, 77, 85, 87
Valence Bond Theory
Molecular Orbital Theory
Molecular
Geometries
and Bonding
Let’s Focus on Valence Bond Theory
As we saw earlier, a single bond formed between two atoms is formed
with orbital overlap.
Molecular
Geometries
and Bonding
Let’s Focus on Valence Bond Theory
Polyatomic molecules
form via hybrid
orbitals.
Carbon, for example,
yields four sp3
hybrid orbitals,
allowing a molecule
like CH4 to have a
tetrahedral shape.
Molecular
Geometries
and Bonding
Hybrid Orbitals
As we have
discussed before,
once you know the
electron-domain
geometry, you know
the hybridization
state of the atom.
Molecular
Geometries
and Bonding
Valence Bond Theory designates two
bond types arising from orbital overlap
and hybrid orbitals.
Sigma Bonds (σ)
And
Pi Bonds (π)
Molecular
Geometries
and Bonding
Sigma () Bonds
• Sigma bonds are characterized by
 Head-to-head overlap.
 Cylindrical symmetry of electron density about the
internuclear axis. In other words, electron density
concentrated between the two bonded nuclei. Molecular
Geometries
and Bonding
Pi () Bonds
• Pi bonds are
characterized by
 Side-to-side overlap.
 Electron density
above and below the
internuclear axis.
Molecular
Geometries
and Bonding
Single Bonds
Single bonds are always  bonds, because 
overlap is greater, resulting in a stronger bond
and more energy lowering.
Molecular
Geometries
and Bonding
Multiple Bonds
In a multiple bond one of the bonds is a  bond
and the rest are  bonds.
Molecular
Geometries
and Bonding
Multiple Bonds
• In a molecule like
formaldehyde (shown
at left) an sp2 orbital
on carbon overlaps in
 fashion with the
corresponding orbital
on the oxygen.
• The unhybridized p
orbitals overlap in 
fashion.
Molecular
Geometries
and Bonding
Multiple Bonds
In triple bonds, as in
acetylene, two sp
orbitals form a 
bond between the
carbons, and two
pairs of p orbitals
overlap in  fashion
to form the two 
bonds.
Molecular
Geometries
and Bonding
Delocalized Electrons: Resonance
When writing Lewis structures for species like
the nitrate ion, we draw resonance structures to
more accurately reflect the structure of the
molecule or ion.
Molecular
Geometries
and Bonding
Delocalized Electrons: Resonance
• In reality, each of the four
atoms in the nitrate ion has a
p orbital.
• The p orbitals on all three
oxygens overlap with the p
orbital on the central nitrogen.
Molecular
Geometries
and Bonding
Delocalized Electrons: Resonance
This means the  electrons are
not localized between the
nitrogen and one of the
oxygens, but rather are
delocalized throughout the ion
and thus accounting for the
equal bond lengths observed
throughout the structure.
Molecular
Geometries
and Bonding
Resonance
The organic molecule
benzene has six 
bonds and a p orbital
on each carbon atom.
Molecular
Geometries
and Bonding
Resonance
• In reality the  electrons in benzene are not
localized, but delocalized.
• The even distribution of the  electrons in benzene
makes the molecule unusually stable.
Molecular
Geometries
and Bonding
Example
• Consider acetonitrile:
a. Predict the bond angles around each
carbon atom
b. Give the hybridization at each of the
carbon atoms
c. Determine the number of σ and π
bonds in the molecule
Molecular
Geometries
and Bonding
Example
Which of the following molecules or ions
will exhibit delocalized bonding? SO3,
SO32-, H2CO, O3, NH4+
Molecular
Geometries
and Bonding
Valence Bond Theory in a Nut Shell…
1. All covalent bonds are formed by the sharing
of two or more electrons. All bonds will have
at least one of these bond pair concentrated
in the space between the two interacting
nuclei forming a sigma bond.
2. The appropriate set of hybrid orbitals used to
form these sigma bonds is determined by the
geometry of the molecule.
3. Atoms that share more than one pair of
electrons will do so in pi bonds, which lie
above and below the nuclei.
4. Molecules with two or more resonance
structures can have pi bonds that extend
beyond the two bonded atoms, allowing
electrons to become delocalized.
Molecular
Geometries
and Bonding
Molecular Orbital (MO) Theory
Though valence bond
theory effectively conveys
most observed properties
of ions and molecules,
there are some concepts
better represented by
molecular orbitals.
Molecular
Geometries
and Bonding
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.
Molecular
Geometries
and Bonding
Molecular Orbital (MO) Theory
If waves interact
destructively, the
resulting orbital is
higher in energy: an
antibonding molecular
orbital.
Molecular
Geometries
and Bonding
MO Theory
• MO Theory begins with
the construction of
molecular orbitals.
• Whenever two atomic
orbitals overlap, two
molecular orbitals form.
• Let’s consider H2.
Notice, two orbitals will
overlap, and thus, two
molecular orbitals
formed.
Molecular
Geometries
and Bonding
Let’s turn this into a diagram…
Molecular
Geometries
and Bonding
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.
• Recall, bond order
indicates the stability of
Molecular
the bond formed.
Geometries
and Bonding
MO Theory
For hydrogen, with two
electrons in the bonding
MO and none in the
antibonding MO, the
bond order is
1
(2 - 0) = 1
2
Molecular
Geometries
and Bonding
MO Theory
• In the case of He2,
the bond order
would be
1
(2 - 2) = 0
2
• Therefore, He2
does not exist.
Molecular
Geometries
and Bonding
Example
Write the electron configuration in terms
of MOs and determine the bond order of
H2- and H2+.
Molecular
Geometries
and Bonding
Example
According to molecular orbital theory,
would either Be2 or Be2+ be expected to
exist? Explain.
Molecular
Geometries
and Bonding
MO Theory
• For atoms with both s
and p orbitals, there are
two types of
interactions:
 The s and the p orbitals
that face each other
overlap in  fashion.
 The other two sets of p
orbitals overlap in 
fashion.
Molecular
Geometries
and Bonding
MO Theory
• The resulting MO
diagram looks like this.
• There are both  and 
bonding molecular
orbitals and * and *
antibonding molecular
orbitals.
Molecular
Geometries
and Bonding
MO Theory
• The smaller p-block elements in
the second period have a
sizeable interaction between the
s and p orbitals.
• This flips the order of the s and p
molecular orbitals in these
elements.
Molecular
Geometries
and Bonding
Second-Row MO Diagrams
Molecular
Geometries
and Bonding
Paramagnetism vs. Diamagnetism
Molecular orbital theory allows one to
predict the behavior of a substance in
the presence of a magnetic field.
Molecules with one or more unpaired
electrons are attracted into a magnetic
field. This behavior is called
paramagnetism.
Substances with no unpaired electrons
are weakly repelled from a magnetic
field…they exhibit diamagnetism.
Molecular
Geometries
and Bonding
Example
Predict the magnetic properties and bond
order of the peroxide ion, O22- and the
acetylide ion, C22-.
Molecular
Geometries
and Bonding
Molecular Orbital Theory
• This model represents electrons in allowed energy
states called molecular orbitals which can spread out
across all the atoms of a molecule.
• MO diagrams represent the combination and relative
energies of molecular orbitals.
• The diagrams allow for easy calculation of bond
order, indicating the stability of the bond formed.
• Bonding and antibonding MOs formed by the
combination of s orbitals are called sigma molecular
orbitals while the combination of p orbitals are called
pi molecular orbitals.
• The MO model correctly predicts properties like
paramagnetism and diamagnetism.
Molecular
Geometries
and Bonding
SAMPLE INTEGRATIVE EXERCISE Putting Concepts Together
Elemental sulfur is a yellow solid that consists of S 8 molecules. The structure of the S8 molecule is a puckered
eight-membered ring (Figure 7.30). Heating elemental sulfur to high temperatures produces gaseous S 2
molecules:
(a) With respect to electronic structure, which element in the second row of the periodic table is most similar to
sulfur? (b) Use the VSEPR model to predict the S—S—S bond angles in S8 and the hybridization at S in S8. (c)
Use MO theory to predict the sulfur–sulfur bond order in S2. Is the molecule expected to be diamagnetic or
paramagnetic? (d) Use average bond enthalpies (Table 8.4) to estimate the enthalpy change for the reaction just
described. Is the reaction exothermic or endothermic?
Molecular
Geometries
and Bonding