Molecular Shape-VSEPR
Objectives:
1. Understand and become proficient at using the VSEPR (Valence Shell Electron
Pair Repulsion) model to predict the geometries of simple molecules and ions.
2. Become proficient at predicting bond angles and polarity of simple molecules.
In the model, groups of valence electrons are considered occupy localized regions. (Some
textbooks use the term electron domains rather than electron groups.) The localized valence
electron groups arrange themselves around a central atom in such a way as to minimize
repulsions.
An electron group can be:
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 groups is the one that minimizes the
electrostatic repulsions between them.
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ELECTRON Geometries Predicted by VSEPR
Valence Shell Electron Pair Repulsion predicts the following molecular
geometries around a central atom.
linear
These are the FIVE BASE ELECTRON GROUP GEOMETRIES.
trigonal planar
tetrahedral
trigonal
bipyramidal
octahedral
These two geometries require
an Expanded Octet
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Example Electron Group Geometry
From the Lewis Structure we can count
electron groups around the central atom.
The number of electron groups determines
the basic arrangement of the electron
groups around the central atom.
In this case four electron groups (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!
What geometry do you predict for the
nitrate ion?
Tetrahedral
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Electron-group Arrangement vs Molecular Shape
Bond angles are the angles formed by the nuclei of two surrounding
atoms with the nucleus of a central atom. Each of the five basic
geometries has specific bond angles associated with it that you
must memorize. These “ideal” bond angles may be distorted by certain
conditions as we shall see later.
The electron-group arrangement is defined by both bonding and nonbonding
electron groups.
The molecular shape is the three-dimensional arrangement of nuclei joined by
the bonding groups. This is defined only by the relative positions of the nuclei.
The single molecular shape of the linear electron-group arrangement.
Examples:
CS2, HCN, BeF2
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AX2
A = central atom
X = surrounding atom
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Molecular Geometries Derived from Trigonal Planar
Electron Group Geometry
There are two molecular shapes derived from the trigonal
planar electron-group arrangement.
AX3
AX2E
Examples:
SO3, BF3,
NO3–, CO32–
Examples:
SO2, O3
A = central atom
X = surrounding atom
E = nonbonding valence-electron group
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Molecular Geometries Derived from Tetrahedral Electron
Group Geometry
There are three molecular shapes derived from the
tetrahedral electron-group arrangement.
AX4
Examples:
CH4, SO42–,
NH4+
AX3E
Examples:
NH3, PF3,
ClO3–
AX2E2
Examples:
H2O, OF2, SCl2
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Molecular Geometries Derived from Trigonal Bipyramidal
Electron Group Geometry
There are four molecular shapes derived from the
trigonal bipyramidal electron-group arrangement.
AX5
AX3E2
Examples:
PF5, SOF4
Examples:
ClF3, BrF3
Examples:
SF4, IF4+
AX4E
Examples:
XeF2, I3–
AX2E3
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Molecular Geometries Derived from Octahedral Electron
Group Geometry
There are three molecular shapes derived from the
octahedral electron-group arrangement.
AX6
Examples:
SF6, IOF5
AX5E
Examples:
BrF5, TeF5–,
AX4E2
Examples:
XeF4, ICl4–
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VSEPR Bond Angle Detail #1: “Real” Bond Angles
(Lone Pairs and Deviations from Ideal 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.
Less
repulsive
More
repulsive
Decrease in bond angle as lone pairs are added
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VSEPR Bond Angle Detail #2: “Real” Bond Angles
(Double Bonds and Deviations from Ideal Bond Angles)
A double bond on a central atom causes adjacent single bonding groups to move closer
together, decreasing the bond angle between them. Double bonds have greater electron
density than single bonds, and exert a greater repulsive force than single bonds.
For Repulsive Forces:
Lone Pair > Double Bond > Single Bond > Single e–
Give It Some Thought
One of the resonance structures of the nitrate
ion is shown here. The bond angles in this ion
are exactly 120°. Considering the CCl2O
example shown above, explain why all the
bond angles are the same for the nitrate ion.
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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.
Equatorial-equatorial repulsions
are weaker than axial-equatorial
repulsions.
1 lone pair
Equatorial
lone pairs
Seesaw Molecular Geometry
Note the deviations from “ideal” bond angles.
2 lone pairs
3 lone pairs
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Geometries of Larger Molecules
The VSEPR model can be extended to consider every central atom in a more complex, larger molecule.
What is the
chemical name of
this substance?
What is the
chemical name of
this substance?
DNA
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Molecular Shape and Molecular Polarity
Many molecules are polar. They have what’s called a
dipole moment (μ) and will align themselves in an applied
electric field. The units of dipole moment are debyes, D:
1D = 3.34x10–30 C•m. The larger the dipole moment, the
greater the polarity. Overall molecular polarity depends on
both shape and bond polarity.
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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. We can determine polarity of polyatomic molecules as follows:
• View bond dipoles and dipole moments as 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.
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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
offset (cancel) each other.
Polar bonds cancel
Polar bonds do not cancel
Give It Some Thought
Why is ozone (O3) a polar molecule?
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Polarity of Molecules
Dipole
molecules:
Text moments of some
Dipole
Moments of Some Molecules
Remember: The units of the dipole moment are debyes, D: 1D = 3.34x10–30 C•m
Give It Some Thought
Based on differences in electronegativites, the N–F bond is slightly more polar than the
N–H bond. Why then is NF3 a much less polar molecule compared to NH3?
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Microwave Ovens - How do they work?
The microwaves are oscillating
electric fields that cause the water
molecules to flip back and forth as
the wave passes through them.
Remember, water has a relatively
large large dipole moment to interact
with the electric field.
Each water molecule might flip a
billion times a second!
The friction from the flipping water
molecules heats your food.
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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
group 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 group.
2.
Determine the electron-group geometry by arranging the electron domains about the
central atom so that the repulsions among them are minimized.
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 groups. 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.
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Practice Problem: Acetonitrile
1. Determine the molecular geometry around each central atom. Draw the molecule,
showing the correct geometry.
2. State the indicated “ideal” bond angles.
3. Does this molecule have a net dipole moment? If it does, indicate the direction of the
dipole using an arrow.
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Practice Problem: Acetic Acid
1. Determine the molecular geometry around each central atom. Draw the molecule,
showing the correct geometry.
2. Indicate the “ideal” bond angles.
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Conceptual Questions
• An AB5 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?
• An AB3 molecule is described as having a trigonal-bipyramidal electron-domain geometry. How many
nonbonding domains are on atom A? Explain
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Conceptual Questions
• The three species NH2−, NH3, and NH4+, have H–N–H bond angles of 105.0°, 107.5°, and 109.5°,
respectively. Explain this variation in bond angles.
• Dichloroethylene (C2H2Cl2) 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 the sample?
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