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Chemistry Solutions
September 2017 | Resource Feature
Increasing Student Comprehension of VSEPR Theory
By Jennifer Douglass
Classroom Activities, Modeling
When first presented with the valence shell electron pair repulsion (VSEPR) theory, students are told about the preferred arrangements of electron pair domains that minimize the repulsion between domains. This works well for the small number of students who can think spatially and visualize such points as:
- how a tetrahedral geometry maximizes the distance between four electron pair domains
- why CH_{3}Cl is a polar molecule
- why a nonbonding pair of electrons in a molecule with five electron pair domains around a central atom will assume one of the equatorial vertices
- why the bond angles in CH_{4} are not 90°, even when represented that way in a structural formula
Many other students, however, have difficulty building a mental model of what is happening when electron pair domains exert repulsive forces on one another. They particularly struggle with transitioning from a two-dimensional model on a piece of paper to a three-dimensional concept, and resort to memorizing a list of shapes they really do not understand. Molecular model kits can help, but since they come with pre-drilled holes, students still miss how and the why particular arrangements are adopted, and they wrestle with concepts like those that are listed above. This is particularly evident when they try to articulate an explanation of one of these concepts.
I developed the exercise described in this model to combat two issues. The first issue was concept retention. I cover VSEPR theory early in the academic year, and had noticed that when I reintroduced the topic later in the year, students had not retained the concepts, and would have to consult their notes. The second issue was concept application. I noticed that students were able to answer questions correctly about simple molecules (with one central atom), but then would not “see” how the geometries would transfer to larger molecules. They were memorizing without visualizing.
In this exercise, students construct physical models of molecular shapes. However, students are not told what the preferred arrangements of electron pair domains are. Instead, they derive the arrangements on their own (with some prompting). Students are given the opportunity to conceptualize what is happening when one electron pair domain acts upon another, and to understand how those interactions result in the molecular geometries predicted by VSEPR theory.
As an outcome of examining the physical basis of the VSEPR model, students should have a much better grasp of the implications of electron pair repulsions on molecular shape, and should be better able to understand, communicate, and apply that understanding.
Since implementing this exercise into my instructional planning, I have seen very clear evidence that students have internalized this information. When required to apply VSEPR, the students will sit back, look into the air, and “hold” their (invisible) model with their hands. They no longer have any trouble visualizing the three-dimensional structure of the molecules, and can provide clear and cogent answers when asked to explain or justify an answer using VSEPR theory. As a result, students are well prepared for VSEPR-related questions on the AP exam.
Note: Before beginning, have students complete the first three columns on the VSEPR handout (Lewis Structure, Electron Pair Domains, Nonbonding Pairs). They can then complete the remaining columns as you move through this exercise.
Materials
- Quilting pins (or any other pin topped by a colored ball)
- Modeling clay*
*If permanent models are desired, use an oven bake clay such as Fimo or Sculpey. Bake models in an over-proof pan at the lowest oven temperature possible for approximately 30 minutes to harden the clay. The plastic on the pins will not melt at this temperature. Before baking, line the pan with wax paper if it will be used for baking food afterward.
Part 1: Model Construction
Give each student a lump of clay and 20 pins. Have them form five balls ½ inch in diameter with the clay. These represent the central atoms. The pins represent electron pair domains.
- Instruct students to begin with one ball and two pins representing the central atom and two electron pair domains. Have them arrange the two electron pair domains in such a manner as to minimize electron pair repulsion by maximizing the distance between them. Students will quickly come up with the structure depicted in Figure 1. Do not discuss terminology or bond angles yet. Set this model aside.
Figure 1. Two electron pair domains (terminal atoms or nonbonding electron pairs) assume a linear structure.
- Instruct students to use a second ball to recreate the structure they just made. Then tell them to add a third electron pair domain (Figure 2). Ask students how the addition of this third domain will affect the spatial arrangement of the first two domains. Students will realize that the addition of the third electron pair domain will cause the first two to shift away from it, resulting in a trigonal planar configuration. Again, do not discuss terminology or bond angles at this point.
Figure 2. A third electron pair domain requires a “downward” shift.
- Once students have the correct shape, instruct them to set that model aside, and to reconstruct the three-electron pair domains structure. Then have them add a 4th electron pair domain, again maximizing the distance between electron pair domains. At this point, students will be able to “see” that the addition of a new electron pair domain causes a shift in the positions of the others (Figure 3). Repeat the progression for the remaining two models (Figures 4 and 5).
Figure 3. A fourth electron pair domain requires the planar domains to shift “downward.
Figure 4. A fifth electron pair domain allows three electron domains to “return” to an equatorial configuration.
Figure 5. A sixth domain requires that the equatorial domains shift from a trigonal to a square configuration
- The key is that the students see this as a progression: each additional electron pair domain causes a shift in the structure that preceded it. They will be physically manipulating their models to represent the repulsive force of the valence shell electron pairs. Students should be better able to make sense out of the molecular shapes predicted by the VSEPR theory. However, students should not be left with the impression that molecules actually form in this sequential manner, one domain at a time.
Part 2: Naming Molecular Shapes and Determining Bond Angles
Models in hand, the students will now be able to come very close to the actual names of the shapes with only a little bit of prompting. This should allow students to realize that the molecular shape names are simply descriptions of the actual shape (one would think this would be intuitive, but for many students it is not). For example, with the three-electron pair domains model, the discussion might go something like this:
Students: “It’s a triangle.”
You: “What kind of triangle?” (as you move it back and forth in a plane)
Students: “A flat triangle.”
You: “What is the math word for flat?”
Students: “Plane”
You: “Yes, this is trigonal planar.”
Students can also use their models to determine the bond angles for each shape. For example, when holding the octahedral model, you can ask students to identify the angles they see:
You: What angles does this molecule have?
Students: 90 degrees! (They always see this one first.)
You: Are there any other angles?
Students: Oh! There are 180-degree angles here – and here – and here (as they begin to rotate their models and “see” it three-dimensionally).
You: Now compare it to the molecule with five electron pair domains. What you notice?
Students: There is still a 180-degree angle – but only between these two pairs.
You: Those are called “axial” pairs. Can you see why they are called axial and the others are called equatorial? What happens when you rotate this molecule? (etc.)
Please refer to the table at the end of this article for more examples of expected student answers and prompts.Part 3: Relating Molecular Geometry to Electron Pair Geometry
In a subsequent lesson, introduce nonbonding electron pairs into the discussion. You might begin by asking students what would happen if one of the terminal atoms in a molecule with tetrahedral geometry were replaced by a lone pair of electrons. Would the molecule become trigonal planar? Encourage students to use their fingers to cover up one of the pins. They will be able to see that the nonbonding pair is still there and is still occupying one of the domains; the electron pair geometry has not changed. The shape of the molecule, however, is different because of the “missing” terminal atom. Students will now be able to make the correlation between underlying geometry, nonbonding pairs, and molecular shape. Use the example described in Part 2 to elicit shape names for these molecules with lone pairs of electrons (see Table 1).
This would also be an appropriate time to point out to
students the effect of a nonbonding pair of electrons on the bond angles within
a molecule (Figure 6).
Figure 6. A nonbonding pair of electrons occupies a slightly larger domain and exerts a greater repulsive force on electrons in adjacent domains, thereby compressing the bond angle between terminal atoms to an angle of approximately 107°.
On the models with five and six electron pair domains, students will have more than one choice for where the nonbonding electrons pairs will be, and may need a few hints. Since nonbonding pairs exert a greater repulsive force on neighboring domains, they will occupy the position that maximizes their distance from adjacent domains. For example, in the five-electron pair domains model, the nonbonding pair of electrons can best maximize its distance from other electron pairs when it occupies an equatorial, rather than axial, position. (There are only two bonded electron pairs at a 90° angle from the lone pair when it is in an equatorial position, as opposed to three pairs at a 90° angle when it is in an axial position.)
A possible extension of this activity would be for students to use their models to visualize the polarity of the molecules with nonbonding electron pair domains, or ones with different terminal atoms. For example, students can substitute a pin of a different color for one of the terminal atoms (or again, use their fingers to cover up one of the pins). This will help them visualize the unequal pull that results.
Summary
This exercise has been very effective in helping my students internalize, retain, and apply VSEPR concepts. Since students are actively involved in constructing a model that best represents a set of given conditions (i.e., “three pairs of electrons as far apart as possible”), VSEPR makes sense to them. Students spend time holding and manipulating their models, and are readily able to recall the information later in the course.
An additional benefit of this activity is that is helps APChemistry students clearly articulate their understanding of VSEPR andintermolecular forces. Students who have a solid understanding of VSEPR are better able to “describe the relationships between the structural features of polar molecules and the forces of attraction between the particles” (LO 2.13) and are better able “to use Lewis diagrams and VSEPR to predict the geometry of molecules, identify hybridization, and make predictions about polarity” (LO 2.21). After the AP exam, students report feeling very confident about VSEPR-related questions. Additionally, evidence from the instructional planning report indicates that this is typically one of the highest (if not the highest) performance areas for my students.Table 1. Typical student responses and suggested teacher prompts.
EP Domains |
NB Pairs |
Typical student response to “What shape is this?” |
Shape |
2 |
0 |
Students: It’s a line. |
Linear |
3 |
0 |
Students: It’s a triangle. |
Trigonal planar |
3 |
1 |
(Cover a pin) |
Bent |
4 |
0 |
Students: ??? |
Tetrahedral |
4 |
1 |
(Cover a pin) |
Trigonal pyramidal |
4 |
2 |
(Cover two pins) |
Bent |
5 |
0 |
(Cover up one of the axial pins for a moment) |
Trigonal bipyramidal |
5 |
1 |
(Covering up one of equatorial pins, and rocking the structure back and forth) |
See-saw |
5 |
2 |
(Covering up 2 equatorial pins) |
T-shaped |
5 |
3 |
It’s a straight line again! It’s linear! |
Linear |
6 |
0 |
You: How many faces this time? |
Octahedral |
6 |
1 |
(Cover a pin) |
Square pyramidal |
6 |
2 |
(Cover two pins, move back and forth in plane). |
Square planar |