Sometimes simple wins. This is one of those times. Try using marshmallows to introduce VSEPR theory and have the kids use inquiry to discover the electron domain geometries. It is simple and fun, plus the kids actually have to think about the real three-dimensional outcomes of electron domains repelling each other. There are enough breakthroughs that happen in a 50-minute period, it is totally worth your time.
Introduce VSEPR
I take literally about 3 minutes to introduce the theory. I ask a pretty simple series of questions. What are bonds made out of? What do things of like charge do to each other? Therefore, where will bonds form around a central atom? That’s it. I help them recognize that the domains surrounding that atom are made of electrons that will repel each other. Then I set them to work.
Let them play
I give each group enough marshmallows to have 5 central atoms and then enough to make molecules with 2, 3, 4, 5, and 6 domains (about 25 marshmallows) and enough toothpicks to connect them. I also give each group a protractor. Their job is to build 5 models with the greatest separation of the domains. Then they record the bond angle between the domains. Two domains is easy. 180 degrees. So is three. 120 degrees. Almost all kids start building their four-domain structure the same way. They make a “plus” sign. 90 degrees between each domain. I will generally let them believe they have done a good job for a minute, and then ask each group the same question. What if I pry this domain up just a little? What happened to the bond angle? Shockingly, it goes up above 90 degrees. All of a sudden, they look at their molecules less like the drawings on a paper and more like the space-filling molecules that they actually are. This starts a flurry of chatter and action. They are bending the bonds around, trying to use the protractor to see what the angles would be. They don’t always get the perfect 109.5 degree angles recorded, but it is way better than 90.
Make them name their shapes
This sounds silly, but I make them write a descriptive name for the shape they have created. No, they can’t name it Jim. That isn’t descriptive. This sounds like an extra step since we will be teaching them the “real names” of the molecular geometries very soon. But what I have found is that the kids find it easier to palette the “scientific names” if they have already thought about naming it themselves. Tetrahedral is often called “the pyramid”. That is great. They are recognizing that the domains make a particular, recognizable shape. So, when you tell them that the real name is tetrahedral, it’s not really a stretch in their minds anymore.
The entire process of introducing VSEPR, having the students build, measure, and name their models can be completed in a standard 50-minute period. The next day, they are ready to dive headlong into the theory and all the terminology associated with it. At the end of the day, they know something about VSEPR theory, and you become the teacher that let them play with marshmallows. Sounds like a win-win.
You can download the simple handout for this activity on our resources page. Hope it helps you.
Looking for additional ways to engage your students? Try using curriculum-minded chemistry games. Kids have amazing chemsitry-centric conversations when playing Stoich Decks games. Try Up & Atom to intro the mole. CHeMgO helps with writing ionic formulas. Trendy allows kids to quickly compare periodic trends. Check them out today and join schools across the country and around the globe using games to engage their students.
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