Chapter 9 - VSEPR

Bonding and Molecular Structure - PART 1 - VSEPR
Objectives:
1. Understand and become proficient at using VSEPR to predict the
geometries of simple molecules and ions.
2. Become proficient at predicting bond angles and polarity of simple
molecules.
The basis of this model is that valence electrons arrange themselves around a central atom
in such a way as to minimize repulsions.
Valence electrons are considered to be localized into regions called electron domains.
(Some textbooks use the term electron groups rather than electron domains.)
An electron domain around an atom is:
a single bond,
a double bond,
a triple bond,
a lone pair, or
a lone electron (recall that free radicals contain unpaired electrons).
The best arrangement of a given number of electron domains is the one that minimizes the
electrostatic repulsions between them.
Molecular Geometries and Bonding Theory 1
ELECTRON Geometries Predicted by VSEPR
Valence Shell Electron Pair Repulsion predicts the following molecular
geometries around a central atom.
These are the FIVE BASE ELECTRON GEOMETRIES.
These 5 geometries minimize the
repulsions between the electron
domains on each central atom.
These Last two 2 geometries requiring an Expanded Octet
Molecular Geometries and Bonding Theory 2

Example Electron Domain Geometry - Tetrahedral
From the Lewis Structure we can count
electron domains around the central atom.
The number of electron domains
determines the basic arrangement of the
electron domains around the central atom.
In this case four electron domains (4 single
bonds) gives the tetrahedral geometry
with bond angles of 109.5°
To determine the geometry around a
central atom you must first be able to draw
the LEWIS STRUCTURE!
Tetrahedral
Molecular Geometries and Bonding Theory 3
Details of the 5 Base Electron Domain Geometries from
VSEPR
Bond angles are the angles made by the lines
joining the nuclei of the atoms in a molecule.
Each of the five basic geometries has specific
bond angles associated with it that you must
memorize (see Table 9.1). These “ideal” bond
angles may be distorted by certain conditions as
we shall see later.
Molecular Geometries and Bonding Theory 4

The MOLECULAR GEOMETRY describes the spatial arrangement of ATOMS
around a central atom. This is a subset of the ELECTRON GEOMETRY
The arrangement of electron domains about a central atom is called the electron-domain
geometry (or electron-group geometry) as previously discussed.
The molecular geometry is the arrangement of only the atoms in a molecule or
polyatomic ion.
All Electron Domains counted
Molecular Geometries and Bonding Theory 5
Molecular Geometries and Bonding Theory 6
Only Bonds counted,
lone pairs ignored
Details of Molecular Geometries Derived from Linear and
Trigonal Planar Electron Domain Geometries

Details of Molecular Geometries Derived from a
Tetrahedral Electron Domain Geometry
Molecular Geometries and Bonding Theory 7
Details of Molecular Geometries Derived from a Trigonal
Bipyramidal Electron Domain Geometry
Molecular Geometries and Bonding Theory 8

Details of Molecular Geometries Derived from an
Octahedral Electron Domain Geometry
Molecular Geometries and Bonding Theory 9
VSEPR Bond Angle Detail #1:
Lone Pairs and Slight Changes to Bond Angles
Lone pairs on a central atom will cause the bonding groups to move closer
together, decreasing the bond angle. Lone pairs occupy more space and
are more repulsive than bonding pairs.
Decrease in bond angle as lone pairs are added
Molecular Geometries and Bonding Theory 10
Less
repulsive
More
repulsive

VSEPR Bond Angle Detail #2:
Double Bonds Change Bond Angles
A double bond on a central atom cause adjacent single bonding groups to move closer
together, decreasing the bond angle between them. Double bonds occupy more space and
are more repulsive than single bonds.
Give It Some Thought
One of the resonance structures of the nitrate ion, NO 3– , is
The bond angles in this ion are exactly 120°. Is this
observation consistent with the above discussion of the
effect of multiple bonds on bond angles
Conclusion for Repulsive Energies:
Lone Pair > Double Bond > Single Bond > Single e –
Molecular Geometries and Bonding Theory 11
More VSEPR Details:
5 Electron Groups and Axial vs. Equatorial Positions
When we form the trigonal bipyramidal electron domain
geometry we have inequivalent bonding positions, axial
and equatorial.
Lone pairs prefer the equatorial positions since they
minimize the strong 90° repulsions for the lone pairs.
Seesaw Geometry
Equatorial
lone pairs
Molecular Geometries and Bonding Theory 12
1 lone pair
2 lone pairs
3 lone pairs

Geometries of Larger Molecules
The VSEPR model can be extended to consider every central
atom in a more complex, larger molecule.
Consider Acetic Acid:
Molecular Geometries and Bonding Theory 13
Molecular Geometries of Complex Molecules - DNA
Molecular Geometries and Bonding Theory 14

Dipole Moments and Polar Molecules
Many molecules are polar. They have a dipole moment and will align
themselves in an applied electric field.
Polar molecule
No alignment
Alignment
Molecular Geometries and Bonding Theory 15
Dipole Moments for Polyatomic Molecules
For a molecule that consists of more than two atoms (a polyatomic molecule), the
dipole moment depends upon both the individual bond polarities and the molecular
geometry.
• Bond dipoles and dipole moments are vector quantities; that is they have both a
magnitude and a direction.
• The overall dipole moment of a polyatomic molecule is the vector sum of the bond
dipoles. Both the magnitudes and the directions of the bond dipoles must be considered.
(Molecular Geometry analysis is necessary!)
• It is possible to have a nonpolar molecule that contains polar bonds if the polar bond
dipoles are arranged in such a way as to “cancel” each other.
Molecular Geometries and Bonding Theory 16

Dipole Moment Depends on Bond Polarity and Electron Geometry
To have a dipole moment a molecule must have:
1. Polar bonds and/or lone pairs.
2. A molecular geometry where the polar bonds/lone pairs do not
cancel.
Polar bonds cancel Polar bonds do not cancel
Molecular Geometries and Bonding Theory 17
Polarity of Some Molecules
Give It Some Thought
The molecule OCS has a Lewis structure analogous to that of CO 2 and is a linear molecule.
Will it necessarily have a zero dipole moment like CO 2?
Molecular Geometries and Bonding Theory 18

Polarity of Molecules
Dipole Moments of Some Molecules
Molecular Geometries and Bonding Theory 19
Predicting Electron Domain Geometries, Molecular Geometries, Bond
Angles and Dipole Moments
We can generalize the steps we follow in using the VSEPR model to predict the electron
domain geometries, molecular geometries, bond angles and dipole moments. Use the
VSEPR worksheet to guide you as you learn this process.
1. Draw the Lewis structure of the molecule or ion, and count the total number of electron domains around the
central atom. Each nonbonding electron pair, each single bond, each double bond, and each triple bond counts
as an electron domain.
2. Determine the electron-domain geometry by arranging the electron domains about the central atom so that the
repulsions among them are minimized, as shown in Table 9.1.
3. Use the arrangement of the bonded atoms to determine the molecular geometry as shown in Tables 9.2 and 9.3.
4. Look at the arrangement and types of electron domains. Predict if any bond angles will vary from their “ideal
values”.
5. Determine if the molecule has a net dipole moment. Use the flow diagram on the VSEPR worksheet. Note: Since
IONS have a nonzero charge, dipole moments do not apply.
Molecular Geometries and Bonding Theory 20

Problem 1: Acrolein
1. Give the molecular geometry around each central atom.
2. Identify all bonds as polar or nonpolar.
3. Does this molecule have a net dipole moment?
Molecular Geometries and Bonding Theory 21
Problem 2: Acetonitrile
1. Give the molecular geometry around each central atom.
2. State the indicated bond angles.
3. Identify all bonds as polar or nonpolar.
4. Does this molecule have a net dipole moment?
Molecular Geometries and Bonding Theory 22

Problems
Text question 9.3: An AB 5 molecule adopts the geometry shown to the right.
(a) What is the name of this geometry?
(b) Do you think there are any nonbonding electron pairs on atom A? Why or why
not?
(c) Suppose the atoms B are halogen atoms. Can you determine uniquely to
which group in the periodic table atom A belongs?
Text question 9.18: The AB 3 molecule is described as having a trigonal-bipyramidal electron-domain
geometry. How many nonbonding domains are on atom A? Explain
Molecular Geometries and Bonding Theory 23
Problems
Text question 9.28: The three species NH 2 − , NH3 , and NH 4 + , have H–N–H bond angles of 105°, 107°,
and 109°, respectively. Explain this variation in bond angles.
Additional Question: Dichloroethylene (C 2 H 2 Cl 2 ) has three forms (isomers), each of which is a different
substance. A pure sample of one of these substances is found experimentally to have a dipole
moment of zero. Can we identify which of the three isomers was used?
Molecular Geometries and Bonding Theory
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