Table of Contents
Chapter 10: Structure and Bonding –
Valence Bond & Molecular Orbital Theories ..................................... 2
I.
Overview ......................................................................................... 2
Valence Bond Theory Overview .......................................................................................................... 2
Molecular Orbital Theory Overview ................................................................................................... 2
II. Valence Bond Theory .................................................................... 2
What is a bond? ....................................................................................................................... 2
Hybridization ........................................................................................................................... 4
What’s wrong with using pure atomic orbitals? ................................................................................. 4
Hybridization (sp3, sp2, & sp) ................................................................................................. 4
sp3 Hybridization................................................................................................................................. 4
sp2 Hybridization................................................................................................................................. 6
sp Hybridization .................................................................................................................................. 7
Sigma and Pi Bonding ............................................................................................................. 7
Cis-Trans Isomerism ........................................................................................................................... 8
Benzene: a special case of pi bonding. ............................................................................................... 9
Further Hybridization (sp3d and sp3d2) ................................................................................ 9
sp3d Hybridization............................................................................................................................... 9
sp3d Hybridization............................................................................................................................. 10
Hybridization Examples: ...................................................................................................... 10
III.
Molecular Orbital Theory ........................................................ 12
Four Principles of M.O. Theory........................................................................................... 12
M.O. Principle #1.............................................................................................................................. 12
M.O. Principle #2.............................................................................................................................. 14
M.O. Principle #3.............................................................................................................................. 14
M.O. Principle #4.............................................................................................................................. 15
Molecular Orbital Energy Levels ........................................................................................ 16
Molecular Orbital Electron Configuration:...................................................................................... 17
Molecular Orbitals VS Resonance ....................................................................................... 17
Page 1 of 17
Valence Bond and Molecular Orbital Theories
Chapter 10: Structure and Bonding – Valence Bond &
Molecular Orbital Theories
I. Overview
We will study two theories of chemical bonding.
Each has its merits and is useful in explaining certain attributes of
molecules.
Valence Bond Theory Overview
 Developed by __________________________________________
 It looks only at _________________________________________
______________________________
 Good for describing _____________________________________
Molecular Orbital Theory Overview
 Developed by __________________________________________
 Molecular Orbitals (M.O.s): spread out over _________________
 M.O.s are ____________________
(not confined to an individual bond as in VB theory.)
 Exact calculations difficult except for _______________________
 Good for understanding __________________________________
_____________________________________________________
II. Valence Bond Theory
What is a bond?
Up until now we have loosely talked about “bonds”, defining them
simply as _____________________________________ between atoms.
Now lets take a closer look!
Page 2 of 17
Valence Bond and Molecular Orbital Theories
A covalent bond is __________________________________________
__________________________________________________________
The covalent bond is _________________________________________
When the ___________________ on 2 separate atoms ___________
and _______________, a _____________ is formed.
As the two separate atoms approach, the attraction continues to get
_____________ as the distance between the (+) of one and the (-) of the
other gets _________________.
Eventually, the two (+) centers get too close and start to ____________
each other.
The bond length is determined by the point of
__________________________________________________________
The bond energy is determined by the loss in
__________________________________________________________
(This is the energy required to break the bond.)
Page 3 of 17
Valence Bond and Molecular Orbital Theories
Hybridization
What’s wrong with using pure atomic orbitals?
Let’s start by studying methane, CH4.
VSEPR predicts a __________________ geometry for CH4.
The C in CH4 has ____ valence electrons (___s & ___p).
 If pure atomic orbitals were used for bonding,
then the H’s attached to the ___ orbitals would be _____________
 Where would we put the H attached to the ___ orbital?
 Tetrahedral says that all bond angles are the same at ______.
Hybridization (sp3, sp2, & sp)
Linus Pauling proposed that molecules change the character of their
atomic orbitals when they are used for bonding.
The new
orbitals have a character based upon
_________________________________________________________ .
A hybrid is ________________________________________________
__________________________________________________________
sp3 Hybridization
For CH4, hybridization of ___ s and ___ p orbitals into _______
orbitals, all of _____________________________________________ .
Page 4 of 17
Valence Bond and Molecular Orbital Theories
The __________ of a hybrid orbital is different than that of any
of the constituent atomic orbitals.
 Each ______ hybrid orbital has ___ lobes of ________________ .
 The 4 _____________ lobes are oriented towards
_____________________________________________________
 This puts the largest electron density
_____________________________________________________
Page 5 of 17
Valence Bond and Molecular Orbital Theories
Rule: _____________________________________________________
__________________________________________________________
Other molecules exhibiting sp3 hybridization:
______________
______________
______________
sp2 Hybridization
 Created from ___ s and ___ p orbitals.
 Leads to _______________________ geometries.
 The extra p orbital (not used for  bonding) sticks up out of the
plane.
Note: ____ hybridization correlates to ___ VSEPR electron density
regions and that ____ hybridization correlates to ___ VSEPR electron
density regions.
Page 6 of 17
Valence Bond and Molecular Orbital Theories
sp Hybridization
 Created from ___ s and ___ p orbital.
 Leads to _______________ geometry.
 The 2 extra p orbitals (not used for  bonding), 1 sticks up out of
the plane and the other is at right angles to the first.
Note: ___ hybridization correlates to __ regions of electron density.
Sigma and Pi Bonding
Sigma Bonds () are formed from a ____________________________
of hybridized orbitals.
Pi Bonds () are formed from overlap ___________________________
OR ___________________________________________ the ___ bond
using __________________________ ___ orbitals.
Page 7 of 17
Valence Bond and Molecular Orbital Theories
For multiple bonds:
The first bond formed is a
______ bond
(direct overlap).
The second and/or third
bonds are always
______ bonds.
(sideways overlap).
Cis-Trans Isomerism
Isomers: __________________________________________________
__________________________________________________________
Because double bonds can’t spin , this leads to a
special type of isomer pairs: __________________________________ .
Trans isomers: _____________________________________________
__________________________________________________________
Cis isomers: _______________________________________________
_________________________________________________________ .
Page 8 of 17
Valence Bond and Molecular Orbital Theories
Benzene: a special case of pi bonding.
Benzene (C6H6) is the simplest of a group of compounds known as
__________________ compounds. This name originally came from the
fact that they usually had a notable aroma.
Each C is ____ hybridized and has ________________________ orbital
sticking up out of the ring. It forms a flat ring that is represented by the
following resonance structures. Each C-C bond order is _____
Valence Bond theory is very weak in explaining the pi bonding in
benzene since it only allows interaction between a single central atoms.
Further Hybridization (sp3d and sp3d2)
sp3d Hybridization
 Created from ___ s, ___ p and ___ d orbital.
 Leads to ____________________________________ geometries.
Page 9 of 17
Valence Bond and Molecular Orbital Theories
sp3d2 Hybridization
 Created from ___ s, ___ p and ___ d orbital.
 Leads to ________________________ geometries.
Hybridization Examples:
Determine the hybridization of the underlined in each of the following:
H2C=O
BH4-
XeOF4
Page 10 of 17
Valence Bond and Molecular Orbital Theories
SO3
BrO3-
Note: _____________________________________________________
__________________________________________________________
Page 11 of 17
Valence Bond and Molecular Orbital Theories
III. Molecular Orbital Theory
In contrast to Valence Bond Theory, which takes the electrons form
atomic orbitals and combines them into hybrid orbitals on a single
atom, M.O. Theory combines atomic orbitals to produce orbitals that
are spread out ______________________________________________
 The new orbitals are called _______________________________
 These are also considered ________________________________
 M.O. theory is more complex than V.B. theory, but it can better
account for certain known chemical and physical properties:
e.g. O2 is paramagnetic (has ______________ electrons)
This would not be predicted by VB theory and Lewis Dot
Structures:
Four Principles of M.O. Theory
M.O. Principle #1
The number of molecular orbitals is equal to ______________________
__________________________________________________________
 M.O.s have specific _________ levels and specific ___________ .
 They can each hold ___ electrons of opposite _______.
 Valence electrons from all constituent atoms are placed in these
M.O.s, _______________________________________________
_____________________________________________________
Page 12 of 17
Valence Bond and Molecular Orbital Theories
Let’s look at how Molecular Orbitals form:
+
+


________________________
Since orbitals are wave functions,
the ______________________ or
_____________________ can be
added and subtracted.
Interference
 A sigma bonding orbital is an area of _____________ probability
of electron density directly __________________ two atoms with
_______________ stability for the molecule. Same as VB Theory.
 There is no counterpart for _____________orbitals in VB Theory.
 Antibonding orbitals have a _____________ e- density between the
nuclei. The 2 nuclei then _______ each other which leads to
________________________.
Page 13 of 17
Valence Bond and Molecular Orbital Theories
M.O. Principle #2
A bonding molecular orbital has a __________ energy than the
parent atomic orbitals that produced it while an antibonding orbital has
a ______________ energy.
M.O. Principle #3
Electrons are assigned to M.O.s in order of increasing energy.
 Filling still follows:
_________________________(Lower energy levels are filled
first.)
_________________________(Orbitals or equal energy each get
one electron before they start pairing up.)
____________________________(If an orbital contains two
electrons, they must have opposite spins).
Page 14 of 17
Valence Bond and Molecular Orbital Theories
Why doesn’t He2 form?
 For a bond to exist the bond order needs to be greater than zero.
 For M.O. Theory
Bond Order = ___________________________________________
__________________________________________ )
He2 bond order = ______________________
H2 bond order = ______________________
M.O. Principle #4
Atomic Orbitals combine to form M.O.s most effectively from atomic
orbitals of similar energy.
P orbitals can approach each other in 2 different ways:
 They can approach each other end-to-end and form a ___ bond:
 They can approach each other side-to-side and form a ___ bond.
Page 15 of 17
Valence Bond and Molecular Orbital Theories
A second pair of pi bonds can form from the third set of p orbitals.
Three p orbitals on two bonded atoms can lead to a total of ___ MOs
 Three will be ____________________ orbitals with
_______________ energy than the parent atomic orbitals.
 Three will be ____________________ orbitals with
_______________ energy than the parent atomic orbitals.
Molecular Orbital Energy Levels
The early energy levels for diatomic molecules is represented below:
Picture from McMurry Fay
The 1s orbitals are
not shown. The
two MOs (one
bonding and one
antibonding)
cancel each other
out in terms of
bond order.
This is in keeping
with VB theory: core
electrons do not
affect bonding.
We will try to explain the paramagnetic behavior of oxygen now using
this type of energy level diagram.
 O2 has ___ core and ____ valence electrons.
 The last two electrons go into the _____ orbitals.
 These do not get paired up because of ___________ Rule.
These unpaired electrons account for the paramagnetic property of O2.
The BOND ORDER for oxygen = ____________________
Page 16 of 17
Valence Bond and Molecular Orbital Theories
Molecular Orbital Electron Configuration:
N2 =
B.O. = ____________
O2 =
B.O. = ____________
F2 =
B.O. = ____________
Picture from Mcmurry Fay
Molecular Orbitals VS Resonance
Example: Ozone, O3
Picture from McMurry Fay
Molecular  Orbital
VB Resonance Theory
Example: Benzene
M.O. Theory
predicts
Page 17 of 17