Chapter 10: Chemical Bonding II: Molecular Shapes, Valence Bond
Theory, and Molecular Orbital Theory
10.1 Artificial Sweeteners: Fooled by Molecular Shape (Suggested
Reading)
10.2 VSEPR Theory: The Five Basic Shapes [10.1]
10.3 VSEPR Theory: The Effect of Lone Pairs [10.1]
10.4 VSEPR Theory: Predicting Molecular Geometries [10.1]
10.5 Molecular Shape and Polarity [10.2]
10.6 Valence Bond Theory: Orbital Overlap as a Chemical Bond [10.3 &
10.4]
10.7 Valence Bond Theory: Hybridization of Atomic Orbitals [10.3 &
10.4]
Chemistry 1011 Y8Y,U Paul G. Mezey

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Chapter 10 Chemical Bonding II
Lewis dot structures only give an idea of the electron distribution in the species .
There is NO INFORMATION about the molecular geometry , which depends on the relative position of nuclei around the central atom .
?
Molecular Geometry and Chemical Bonding, Paul G. Mezey

Chapter 10 Chemical Bonding II
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One may connect the information of electron distribution in a Lewis dot structure to molecular geometry by using the
The essence of the VSEPR theory:
GROUPS of electrons repel each other , ending up as far from each other as possible .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

Chapter 10 Chemical Bonding II
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Some textbooks talk about repulsion of ELECTRON PAIRS .
The term repulsion of ELECTRON GROUPS is perhaps better, because multiple bonds are treated the same way as ONE PAIR of electrons in VSEPR theory even though in a multiple bond there are more than one
pair of electrons present .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

Chapter 10 Chemical Bonding II
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Considering directions around a central atom,
A lone pair is ONE GROUP of electrons
A single bond is ONE GROUP of electrons
A double bond is ONE GROUP of electrons
A triple bond is ONE GROUP of electrons
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Electron distribution vs. geometry
Electron distribution
Molecular geometry
The “shape” of electron group distribution
The “shape” of nuclear positions around the central atom
The “shape” of electron distribution INCLUDES all lone pairs
Lone pairs influence molecular geometry, but they are not part of this “shape”, since there are no terminal nuclei on lone pairs
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Electron distribution vs. geometry
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Figure of shapes (GROUPS)
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
n
The central atom A is bonded to n atoms or functional groups , denoted as X .
This notation ignores lone pairs , so it is suited for categorizing molecular geometries , which also ignore lone pairs .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Figure of shapes (2 to 4 GROUPS)
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Figure of shapes (5 GROUPS)
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Figure of shapes (6 GROUPS)
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Comment
With experience , we tend to start drawing Lewis dot structures with molecular geometry information included … instead of instead of
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Getting geometry information
1. Draw the Lewis dot structure
2. Determine the number of electron groups on the central atom to get electron group arrangement
3. Use the number of lone pairs and the arrangement to determine the molecular geometry
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Advanced geometry considerations
Lone pairs are the “biggest” electron groups
(best at repelling other electron groups).
Triple bonds are the next “biggest” groups .
Double bonds are “smaller” .
Single bonds are the “smallest” electron groups .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Tetrahedral arrangement (advanced)
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Trigonal planar arrangement (advanced)
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Problem
What is the arrangement of electron groups , and geometry around the central atom for the following molecules?
SF
2
H
3
O +
XeO
AsF
4
5
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Molecular dipole moments
If there are polar covalent bonds (partial charge separations) in a molecule, the molecule
MAY OR MAY NOT have a permanent dipole moment .
A permanent dipole moment means there are regions of the entire molecule that are permanently partially negative and permanently partially positive .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Molecular Geometry and Chemical Bonding, Paul G. Mezey

Chapter 10 Chemical Bonding II
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To add vectors , we chain vectors by putting the tail of the next vector on the head of the previous vector .
The resultant vector is then drawn from the tail of
the first vector to the head of the last vector in the chain .
This resultant vector is the permanent dipole moment .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
H -
 
Cl
 
:
δ  δ 
H -
 
Cl
 
:
We saw earlier that the diatomic molecule HCl has a polar covalent bond .
Since there is only one bond, this one vector of charge separation
ALSO describes the permanent dipole moment of HCl .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
The permanent dipole moment in water can be seen by adding together the charge separation vectors of the two polar covalent O-H bonds.
Lewis structure
Adding vectors
Permanent dipole moment
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
A molecule with more than one polar bond
MIGHT NOT have a permanent dipole moment when the charge separations are symmetrically distributed so that the resultant vector sums up to to zero .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Geometry and dipole moments
Molecular Geometry and Chemical Bonding, Paul G. Mezey

Chapter 10 Chemical Bonding II
Permanent dipole moments and molecular properties
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Ionic bonds are generally strong because of the
strong electrostatic attraction between positive and negative charges .
Molecules with permanent dipole moments have regions with partial positive and negative charges that attract the opposite regions on other molecules of the same type .
Such intermolecular forces affect the bulk properties of collections of molecules.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
H -
 
Cl
 
:
Dipole moment for
HCl is 1.08 D
Dipole moments measure the amount of charge separation (in
Coulombs) that occurs over the bond length (in meters) in a derived unit called a debye (D)
1 D = 3.34 x 10 -30 C
 m
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
a) The molecule BrF
3 has a dipole moment of 1.19 D . Which of the following geometries are possible: trigonal planar, trigonal pyramidal, or
T-shaped?
b) The molecule TeCl
4 has a dipole moment of 2.54 D.
Is the geometry tetrahedral, seesaw, or square planar?
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Bonds form between atoms when:
1. Orbitals (the “allowed” electron distributions) in the atoms overlap to create molecular bonding orbitals .
2.
Each molecular bonding orbital has
NO MORE THAN 2 electrons in it.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

Chapter 10 Chemical Bonding II
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Bond strength
Covalent bonds are strongest when there is maximum orbital overlap between atomic orbitals . This maximum overlap occurs in the same direction as the atomic orbitals point.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Hybrids occur when we mix two or more
different types of things from the same class.
The resultant hybrid shows similarities to the original things , but is distinctly different from them.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
The atomic orbitals of atoms can be mixed together (WHEN REQUIRED!) to form hybrid atomic orbitals that are different from the source orbitals.
Such hybrid orbitals are used to better explain molecular geometry and bonding.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Oxygen atom orbital diagram
We would expect water to have a 90
 angle between its bonds, based on the atomic orbitals on oxygen.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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plus
Chapter 10 Chemical Bonding II
Oxygen hybrid orbitals gives
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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plus
Chapter 10 Chemical Bonding II
Oxygen hybrid orbitals gives
One s and three p orbitals combine to give four sp 3 hybrid orbitals
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
In general:
A total of n atomic orbitals combine to give n hybrid orbitals of a given kind .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Hybrid orbitals
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Oxygen hybrid orbital diagram
We would expect water to have a ~ 109.5
 angle based on the hybrid sp 3 orbitals on oxygen.
Molecular Geometry and Chemical Bonding, Paul G. Mezey

Chapter 10 Chemical Bonding II
Determining hybrid orbitals diagrams
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1. Draw the Lewis dot structure
2. Use VSEPR theory to predict electron group arrangement
3. Use Table 10.2 to determine what hybrid orbitals have the same arrangement
4. Create the hybrid orbital diagram based on changing the ground state diagram
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
3
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Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Multiple bonds (double or triple bonds) are possible when more than one set of orbitals can overlap between two atoms.
The first bond is the sigma ( s
) bond , which occurs from orbital overlap on the axis between the atoms .
The second and third bonds are pi ( p
) bonds that occur from orbital overlap both above and below
the axis between the two atoms .
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
No orbital overlap between these p orbitals
Notice we’ve chosen to create sp 2 hybrid orbitals
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Notice we’ve chosen to create sp hybrid orbitals and not sp 3 or sp 2
Molecular Geometry and Chemical Bonding, Paul G. Mezey

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Chapter 10 Chemical Bonding II
Molecular Geometry and Chemical Bonding, Paul G. Mezey