Molecular tug-of-war: A hands-on demonstration of molecular polarity

I occasionally read through my teaching notebook to get a sense of what tools, techniques, examples, activities and strategies seemed to work well for my classes. This is not educational research (yet), but it is a collection of my observations and ideas that others may find useful when tackling new topics or trying to approach old, familiar topics from a different direction.

Last fall I taught polarity in a different order than I have before. In every previous class, I introduced polarity in the VSEPR unit: first bond polarity, and then molecular polarity immediately after. Instead, last semester, I introduced bond polarity after Lewis structures and tried to get the students in the habit of identifying polar bonds and drawing dipole moments for those bonds alongside their Lewis structures. Then, after working through VSEPR theory, we brought polarity back into the mix. Anecdotally, they seemed to have a better sense of which bonds were and were not polar in the 3D molecules than my previous classes. (I don't know that the order of topics improved their understanding. I suspect it's simply a matter of having had more time to "digest" the idea and practice it more in 2D before moving to 3D.)

Even though they were quite good at identifying polar bonds, the students still had trouble identifying polar molecules. Specifically, they used the simple rule of thumb that if the bonds were polar, the molecule had to be polar, which fails when the symmetry of a molecule is such that the polar bonds all cancel out (e.g. BF3 or CF4).

When I was an undergrad, I came to understand molecular polarity and dipole moment through vector math, so when my descriptions of polar B-F bonds canceling out in BF3 didn't help, I showed them how the dipoles canceled through vector addition, and then through vector decomposition. A small handful of my students caught on, but the rest were still as confused as ever: The "left" and "right" B-F bonds might cancel out, but surely the "top" B-F bond would still have more electron density on the F than the B, and you'd end up with a polar molecule!

That's when I had an idea. I went rummaging around in my bag of tricks1 and pulled out two wire twist ties, which I folded in half and looped together into a Y shape. I held both ends of one twist tie in my hand and offered each of the other two ends to two students in the front row. Then I asked them (and the students sitting behind them), what would happen to the center of the wire Y if we had a miniature tug-of-war and all three of us pulled on our twist tie ends equally and in the directions they were pointing.2 They knew the answer immediately and didn't even have to tug on the ends to find out. We must have spent 10-15 minutes of class going around and around about molecular polarity, but once they had the shape in their hands and thought about an electron-density tug-of-war,3 it all clicked. For group after group, in lecture and in recitation, handing them a model and asking which side of the tug-of-war would win dramatically improved their ability to explain the polarity of the molecule.

I did this demonstration with small groups of students in a class of about thirty, but I think it would scale well (assuming you have enough models) for groups of three or four students within larger classes: make a model, have each student pick an outer atom to push or pull from, and see what happens to the central atom. The direction the central atom moves is the direction of the dipole moment; no movement, no dipole. I think this demonstration would also work well in a high school classroom, since it is relatively cheap and quick to set up.

1: I should probably devote a post to the things I carry around in my lecture bag. I like to think of it like Mary Poppin's carpet bag: here's a little bit of everything in there, though it's not very big. You never know what little thing will give the students the spark of understanding they need.

2: Why didn't I hand them a piece from a model kit? We had been constructing models out of marshmallows earlier (more on that activity another time), so I'd left the plastic models in my office and I knew a marshmallow model wasn't going to hold up to tug-of-war. After that class, I did bring the plastic models.

3: The examples I've given so far all assume the central atom is less electronegative than the outer atoms, since that's the case for most of the basic Lewis structures. What do you do if the central atom is more electronegative than the atoms to which it's bonded? Push down the bond axis instead of pull, which again works better with the plastic model than my spur-of-the-moment wire model.4

4: I recognize that electron density isn't really a "push" or a "pull," but if we tossed out all our metaphors, it would be much harder to teach!