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Valence Shell Electron Pair Repulsion Model
Why?
Molecules adopt a shape that minimizes their energy. In most cases simply considering the repulsive energy of electron pairs is sufficient to predict molecular shape. You can use this valence shell electron pair repulsion model to predict the molecular shape and to determine whether a molecule is polar or not. Scientists who work with molecules commonly use this model when they need to know the shape of a molecule.
Learning Objectives
λ Understand how molecular shape is predicted from the Lewis structure. λ Identify polar and nonpolar molecules.
Success Criteria
λ Ability to distinguish between the Lewis structure and the molecular shape. λ Accuracy in determining molecular shapes and identifying polar molecules.
Resources
Olmsted and Williams ( Chemistry 3/e , Wiley, 2002) pp. 351-374.
Prerequisites
Lewis electronic structures, polar and nonpolar molecules
New Concepts
VSEPR model, polar and nonpolar molecules, molecular shape
Definitions
In your own words, write definitions of the above concept terms.
Information
The terms Lewis structure, electronic structure, electron arrangement, and electron geometry all 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 all are used to describe how the atoms are positioned relative to each other in a molecule.
Model: Methodology for Determining Molecular Geometries (or Structures) from the VSEPR Model
Methodology Example
Step 1: Draw the Lewis electronic structure.
For sulfur dioxide -
O S O
Step 2: Count the number of bonds and nonbonding electron pairs around the central atom.
1 single + 1 double bond + 1 nonbonding pair = 3. This number is called the steric number.
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
Step 4: Add the atoms in a way consistent with how the electrons are shared and space the nonbonding electron pairs as far apart as possible. First, minimize the number of interactions between
nonbonding electron pairs at 90o
to each other, and then at 90o to bonding pairs because such interactions increase the energy of the structure significantly.
O O
S
Step 5: Determine the molecular shape from the position of the atoms.
The atoms are arranged in a nonlinear or bent shape.
Step 6: Identify whether the molecule is polar or not.
Oxygen is more electronegative than sulfur so SO2 is polar since the center of negative charge (halfway between the two oxygen atoms) is displaced from the center of positive charge (the sulfur atom).
Exercises
- 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. Number of Bonds and Lone Pairs
Lewis Electronic Structure
Illustration of the Electronic Structure
2 linear
3 trigonal planar
4 tetrahedral
5 trigonal bipyramid
6 octahedral
- Use the VSEPR model to predict the shape of each of the following molecules and then sketch the molecule in the first column and the appropriate row of the table below. Label the molecule as polar or nonpolar. O 3 I3-^ IF 6 +^ SbF 5 COCl2 SeO 32 -^ SiF 4 KrF 4 SF 4 ICl 3 BrF 5
Structure/Example Illustration linear
bent
trigonal planar
octahedral
square planar
square pyramidal
see-saw
tee
bent, linear, octahedral, trigonal bipyramidal, trigonal planar, trigonal pyramidal, tetrahedral, square planar, see-saw, tee, square pyramidal
Problems
- An article in a journal, Inorganic Chemistry , cites both BF 3 and PF 3 as examples of flat or planar molecules with bond angles of 120o. Another article reports the FPF bond angle as 98o. Which report is consistent with the VESPR model? Explain.
- Is the shape of OCS like that of CO 2 or SO 2? Identify which are linear and which are bent.
