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
________________.
Carbon, for example,
yields four sp3
hybrid orbitals,
allowing a molecule
like CH4 to have a
________________.
Molecular
Geometries
and Bonding
Hybrid Orbitals
As we have
discussed before,
once you know the
________________
_______________,
you know the
________________
state of the atom.
Molecular
Geometries
and Bonding
Valence Bond Theory designates two
bond types arising from orbital overlap
and hybrid orbitals.
Molecular
Geometries
and Bonding
Sigma () Bonds
• Sigma bonds are characterized by
 ________________________________________
 Cylindrical symmetry of electron density about the
internuclear axis. In other words,
________________________________________________
________________________________________________
Molecular
Geometries
and Bonding
Pi () Bonds
• Pi bonds are
characterized by
 ________________
overlap.
 Electron density is
concentrated
________________
the internuclear axis.
Molecular
Geometries
and Bonding
Single Bonds
Single bonds are always ________ bonds,
because ________ overlap is _________,
resulting in a ___________ 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 _____________
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
________________
________________
________________.
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
___________.
• The p orbitals on all three
oxygens overlap with the p
orbital on the
_________________.
Molecular
Geometries
and Bonding
Delocalized Electrons: Resonance
This means the __________
electrons are not __________
between the nitrogen and one of
the oxygens, but rather are
____________ throughout the
ion and thus accounting for the
_________________________
_________________________.
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 _______________________.
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
________________. All bonds will have at least
one of these bond pair concentrated in the space
________ the two interacting nuclei forming a
________ bond.
2. The appropriate set of hybrid orbitals used to form
these sigma bonds is determined by the
_______________________________________.
3. Atoms that share more than one pair of electrons
will do so in _____ bonds, which lie
____________________ the nuclei.
4. Molecules with two or more resonance structures
can have pi bonds that _______________ the
two bonded atoms, allowing electrons to become
_____________________.
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
Geometries
and Bonding
Molecular Orbital (MO) Theory
• In MO theory, we invoke
the
___________________
__________________.
• If waves interact
constructively, the
resulting orbital is lower
in energy:
__________________.
Molecular
Geometries
and Bonding
Molecular Orbital (MO) Theory
If waves interact
destructively, the
resulting orbital is
higher in energy: an
___________________
__________________.
Molecular
Geometries
and Bonding
MO Theory
• MO Theory begins with
the construction of
molecular orbitals.
• Whenever two atomic
orbitals overlap, _____
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
___________________.
• The bond order is one
half the difference
between the
____________________
___________________.
• Recall, bond order
indicates the ________
of the bond formed. Molecular
Geometries
and Bonding
MO Theory
For hydrogen, with two
electrons in the bonding
MO and none in the
antibonding MO, the
bond order is
Molecular
Geometries
and Bonding
MO Theory
• In the case of He2,
the bond order
would be
• Therefore, He2
_____________
____________.
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 _______________
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
_____________________
antibonding molecular
orbitals.
Molecular
Geometries
and Bonding
MO Theory
• The smaller p-block elements in
the second period have a
_____________________
between the s and p orbitals.
• This ________ 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 ____________________________.
Molecules with one or more unpaired
electrons are attracted into a magnetic
field. This behavior is called
______________.
Substances with no unpaired electrons
are weakly repelled from a magnetic
field…__________________________.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 ____________________________________.
• MO diagrams represent the combination and relative
energies of molecular orbitals.
• The diagrams allow for easy calculation of bond
order, _____________________________________
• 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 __________________ 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