Some of my Favorite Things for VSEPR

Teaching VSEPR theory presents some unique challenges for students - particularly picturing molecules in 3-D space. I have found manipulatives to be a powerful tool to help students visualize 3-D molecules. Here are some of my favorite manipulatives for teaching VSEPR and cheap at-home modifications for virtual learners.

 

Molecular Kits


Figure 1: Molecular model kit

Molecular Kits (Figure 1) are the classic tool for building VSEPR molecules. Once students learn how to select the correct plastic ”atom” for the central atom based on electron domains, they will be observing the correct VSEPR shape and angles in no time. This year I needed to purchase more molecule kits to comply with district COVID protocols. While I love my MolyMod Molecular Kits (), I purchased Old Nobby Molecular Kits () this time around as they were about the same price with over four times the number of pieces. I am happy with the Old Nobby kits. I can tell they are not quite MolyMod quality, but they get the job done. They contain 3-6 hole central atoms and durable bonds. I would purchase again without hesitation. My cheap at-home alternative is creating the VSEPR molecules with balloons. Students twist the ends of blown-up balloons together to make the various VSEPR shapes. When using balloons, the shapes have the correct angles, much like using the molecule kits. If you have a student with a latex allergy or do not have access to balloons, the PhET Molecular Shapes Simulation () works just as well. Similar to a molecular kit, every molecule the student builds will contain the correct angle and can be rotated 360 degrees.

 

Magz Magnetic Building Set

Figure 2: Magz Magnetic Building Set

I love introducing VSEPR with Magz Magnetic Building Sets (). I give each student one metal ball (central atom) and 6 magnetic sticks (bonds). First, I ask them to place two bonds/sticks on the central atom ball in the same way they think two bonds would align themselves around a central atom. At least one person places the bonds 180 degrees apart. As a class, we discuss why they selected 180 degrees. My goal as a facilitator is to get someone to explain bonds contain shared electrons and two negatively-charged areas/domains will repel each other. Once the class understands that bonds will repel each other, I challenge them to add in another bond and show me how to minimize the repulsion while staying bonded to the central atom. They easily create 120-degree angles between the three bonds. Then I have them add in a fourth bond and do the same thing. Most students show me 90 degrees between the four bonds. I challenge them to think outside the box to get me a larger angle. Eventually, one person starts creating something like a tetrahedral. I have the student show their tetrahedral shape to the class. We discuss how an angle larger than 90 is possible when thinking in three dimensions. I continue this process with 5 and 6 bonds. I love how the bonds in the Magz Magnetic Building set easily slide around and stay in place. It helps me lead a great inquiry where students “discover” the VSEPR shapes based on the simple idea “like charges repel.” It is especially powerful for the tetrahedral shape. Before using the Magz Magnetic Building Sets in class, I struggled to get some students to see that while the angles between the bonds in CH4 might look like 90 degrees on paper, larger angles (109.5 degrees) are possible in 3-D space. I like to use the Molecular Geometry POGIL (in ) after playing with the Magz Building set as it emphasizes the difference between a Lewis Diagram and VSEPR Model. My cheap, at-home alternative is marshmallows and toothpicks. Students can switch out marshmallows for gumdrops, Playdoh, clay, and even balled-up bread. Students can switch out toothpicks for spaghetti sticks, pine needles, and straws. Students have to decide what angle to put the bonds at, similar to the Magz Magnetic Building Set.  

 

Resistance Bands

Figure 3: Resistance Bands with Clips

My students have not taken physics before my chemistry class and have not been exposed to vectors and force diagrams. Therefore, they often struggle with how dipole forces can cancel or not cancel in a molecule. In class, I introduce the concept of electronegativity and bond dipoles with a tug-of-war match in the gym. Later on, during VSEPR, I use Resistance Bands with Clips (). The resistance bands are made out of rubber and students pull on their resistance bands to “feel” how bond dipole forces cancel and not cancel in different molecule configurations. I give each student a resistance band. I have them pair up with another student and clip their resistance bands together.  They pull at 180 degrees and feel their forces cancel. Then they add another person to their group. They clip three bands together and pull at 120 degrees apart (Figure 3). They feel how equal and opposite forces at the same angle apart cancel out. I ask one person to gently let go of their resistance band to simulate a lone pair on the central atom. When one student lets go, each group feels how a lone pair can cause forces to not cancel out. I continue adding students and resistance bands until 6 bands are clipped together in an octahedral shape. I have 7 sets of 6 similar strength resistance bands. Sometimes I have one person switch groups and join a group with a different strength resistance band. Then they can model situations in which the central atom is bonded to different axial atoms (like CH3Cl). I reference this activity many times as the class discusses polarity. My cheap, at-home alternative is six rubber bands tied together with string (Figure 4). Students can essentially do the same thing as with the resistance bands but on a smaller scale. Students will have to grab multiple rubber bands at one time to simulate 5 or fewer electron domains, but it is not a big deal. It is tough to do this activity by yourself, so if students are at home, they will have an easier time if they can get family members to help pull on the different rubber bands. For students with latex allergies, I swap out the rubber bands with string. I also like using the PhET Polarity Simulation () to follow up the resistance bands or rubber bands to drive home how electronegativity and angle/shape determine if bond dipoles cancel in a molecule.

Figure 4: Six Rubber Bands with String Alternative

Do you have any favorite manipulatives for VSEPR? Share your favorites by commenting below!

Concepts: 

NGSS

Modeling in 9–12 builds on K–8 and progresses to using, synthesizing, and developing models to predict and show relationships among variables between systems and their components in the natural and designed worlds.

Summary:

Modeling in 9–12 builds on K–8 and progresses to using, synthesizing, and developing models to predict and show relationships among variables between systems and their components in the natural and designed worlds. Use a model to predict the relationships between systems or between components of a system.

Assessment Boundary:
Clarification: