ACTIVITY 09-1
Valence Shell Electron Pair Repulsion Model
WHY?
Molecules adopt a shape that minimizes their energy. In many cases, it is possible to predict the
geometry of a molecule simply by considering the repulsive energy of electron pairs. You can
use this valence shell electron pair repulsion model (VSEPR) to predict shapes and determine
whether or not a molecule is polar. Scientists commonly use this model when they need to predict
or estimate the shape of a molecule.
LEARNING OBJECTIVE
•
Understand how molecular shape is predicted from the Lewis structure
SUCCESS CRITERION
•
Accuracy in predicting molecular shapes
PREREQUISITE
•
Activity 08-2: Lewis Model of Electronic Structure
INFORMATION
The terms Lewis structure, electronic structure, electron arrangement, and electron geometry are
used to describe how the bonding and nonbonding electron pairs are positioned in a molecule. The
terms molecular shape, molecular structure, and molecular geometry are used to describe how the
atoms are positioned relative to each other in a molecule.
MODEL 1: METHODOLOGY FOR DETERMINING MOLECULAR GEOMETRIES
(SHAPES OR STRUCTURES) FROM THE VSEPR MODEL
Methodology
Example
Step 1: Draw the Lewis electronic
structure.
For ozone:
O
Step 2: Count the number of bonds and
nonbonding electron pairs around the
central atom.
O
O
1 single + 1 double bond +
1 nonbonding pair = 3.
This number is called the steric number.
continued on following page
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Foundations of Chemistry
MODEL 1: METHODOLOGY FOR DETERMINING MOLECULAR GEOMETRIES
(SHAPES OR STRUCTURES) FROM THE VSEPR MODEL (CON’T)
Methodology
Example
Step 3: Molecules take a shape that minimizes
their energy. Arrange the bonds and
nonbonding electron pairs to maximize
their separation, which minimizes the
electron-electron repulsion energy.
A steric number of 3 in step 2 means
a trigonal planar electronic structure
minimizes the energy:
Step 4: Add the atoms in a way that is
consistent with how the electrons
are shared, and put the nonbonding
electron pairs as far apart as possible.
O
O
Step 5: Determine the molecular shape from
the position of the atoms.
O
The atoms are arranged in
a nonlinear or bent shape.
KEY QUESTIONS
1. In Step 1 in the preceding methodology, how do you determine the Lewis electronic structure?
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2. Why are bonds and nonbonding electron pairs (aka: lone pairs) spaced as far apart as possible in
the structure?
3. According to Step 4 in the methodology, if you have two lone pairs and bonds to four atoms around
a central atom, would you position the lone pairs at 90º or 180º to each other? Explain.
4. How would you describe the geometrical arrangement of the bonds and lone pairs around the
central oxygen in ozone?
5. How would you describe the shape of ozone?
6. Some triatomic molecules are linear. What feature of O3 leads to the bent geometry?
7. What three insights has your team gained about the shape of molecules by examining the model
and responding to the key questions?
Activity 09-1 Valence Shell Electron Pair Repulsion Model
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Foundations of Chemistry
EXERCISES
1. Complete the illustrations in the following table to show the arrangement of bonds and electron
lone pairs that minimizes the energy in each case. Your illustration represents the Lewis electronic
structure of the molecule. The number of bonds and lone pairs is called the steric number.
Table 2
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Number of Bonds
and Lone Pairs
Lewis Electronic
Structure
2
linear
3
trigonal planar
4
tetrahedral
5
trigonal bipyramidal
6
octahedral
Illustration of the Electronic Structure
Chapter 9: Molecular Structures
2. All of the possible molecular shapes for atoms arranged around a central atom are shown in the
Illustration column of Table 3. Each of these shapes is exemplified by one molecule from the
following list.
O3
I3–
IF6+
SbF5
COCl2
SeO32–
SiF4
KrF4
SF4
ICl3
BrF5
Use the VSEPR model to predict the shape of each molecule and draw its Lewis structure in the
left column of the row corresponding to its structure in the Table.
Table 3
Structure/Example
Activity 09-1 Valence Shell Electron Pair Repulsion Model
Illustration
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Structure/Example
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Illustration
Chapter 9: Molecular Structures
Structure/Example
Activity 09-1 Valence Shell Electron Pair Repulsion Model
Illustration
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Foundations of Chemistry
PROBLEMS
1. An article in a journal, Inorganic Chemistry, cites both BF3 and PF3 as examples of flat or planar
molecules with bond angles of 120°. Another article reports the FPF bond angle as 98°. Which
report is consistent with the VSEPR model? Explain.
2. Is the shape of OCS like that of CO2 or SO2? Identify which are linear and which are bent.
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