London Dispersion Forces (LDF) can be a little tricky to understand, especially for first year chemistry students. I started using this story to describe the polarizability of an electron cloud and it gives the students a mental picture of electron cloud. Maybe the story will spark some creativity in your thinking as well!
In middle school, we were bussed to the neighboring mountain town to attend school. I was the first kid on the bus which meant it was over an hour-long ride. Our bus driver was named Pete and had a horrific personality flaw; he desperately wanted to be liked by middle schoolers. He would do anything we told him.
On our route, the dirt road approached a pond and had to take two 90 degree turns to avoid it. The turns were near the end of the bus route, so the bus was pretty full. One day, a devious 8th grader (I promise it wasn’t me) schemed with all the kids. After the plan was in place, he yelled out to Pete, “How fast do you think you can take this corner?” Pete, sensing an opportunity to impress hormonal sub-adults, punched it. The bus picked up speed heading into the sharp corner.
The moment the bus started to go around the corner, the scheme was revealed. Everyone in the left row of seats jumped out onto to the right side. As the bus sped through the corner, and all the kids jumped to one side of the bus, the yellow-death rocket leaned up on two wheels, swerved a couple of times and just as it was about to roll into the pond, miraculously set back down on all four wheels. Everyone was screaming in terror that we almost died and victory as we had yet again convinced Pete to do our bidding.
This is where I connect the story to the idea of instantaneous dipole within atoms. The electrons are moving around the nucleus at extraordinary speed. There exists the possibility that for a split second, there could be more electrons on one side of the atom than the other. This creates an imbalance, a dipole. Once a dipole is made, it induces a dipole in the atom next to it, creating LDF. Then a split second later, the electrons rearrange, and the attraction turns off. The strength of the dipole is related to the number of electrons that can get off balance.
Number of electrons effect polarizability
Imagine that little, tiny me had hatched that plan when I was the only kid on the bus. Would there have been such a strong polarizing effect? No way. But as the number of kids increased in creating the lopsided nature of the bus, the dipole grew stronger. Therefore, the larger number of electrons in an electron cloud, the stronger the instantaneous dipole could be formed.
Have students explain why fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. They are all nonpolar and only possess LDF. Therefore, the number of electrons is directly proportional to the strength of the instantaneous dipole and therefore the strength of the intermolecular force. This causes iodine to become solid due to it’s increased LDF.
Shape of the molecule effects LDF
Once the kids have made the connection to the polarizability of atoms and the LDF, I add in how the shape of the molecule can effect the intermolecular force. I keep the same idea of kids jumping to one side and move them into a canoe or a raft. If 80 kids are in the world’s longest canoe, how many have to jump to one side to make it get imbalanced and have an “instantaneous dipole”? The answer is not many. Therefore, the LDF among molecules that are long, slender, and straight is high.
Even with molecules of similar molecular mass, the boiling point (which is one measure of the strength of intermolecular forces) can vary due to molecular shape. If 80 kids were riding in the world’s largest whitewater raft, how many would have to jump to one side to create a strong dipole? The answer is a lot. Therefore, the odds of a strong dipole happening in a molecule shaped like a round raft is less. Due to the difference in polarizability, the LDF will be lower as evidenced by a lower boiling point.
The picture of kids jumping to one side seems to help my students make a visual connection to instantaneous dipoles.
Sum of intermolecular forces is important
Why is iodine a solid at room temperature while water is a liquid? If the students were to list the strongest intermolecular force affecting the molecules, they would find that water has hydrogen bonding (much stronger force) than the LDF (much weaker force) in iodine. This question can trip kids up. Particularly, those that just want to memorize the nice clean list of the intermolecular forces in order.
It’s all about the sum of the forces as a whole, not the order of the individual forces. Years ago, there was a show called “Most Extreme Elimination Challenge” in which the producers used the video from a Japanese game show and dubbed mostly off-color English humor over the top. In one event, the contestants were dressed in Velcro suits from head to toe. They swung on a rope attempting to release from the rope, fly through the air, and stick to a giant Velcro target like a human projectile.
Velcro is pretty a pretty weak attractive force. That’s why we put it on shoes for the very young and the very old. Velcro is easy to rip. Unless your whole body is covered in it. Then the force can hold up an entire person.
Lots of a weak force, like LDF, can hold a substance together better than a little bit of a stronger force. The easiest evidence that we have for determining intermolecular forces is boiling point. Therefore, when students are looking at lists of boiling points to determine the strongest forces, they must keep in mind that the boiling point is controlled by the sum of all the forces, not just the strongest single force.
I hope these silly stories get your creativity flowing for helping students create mental images of how LDF works. These won’t be landing in a textbook any time soon, same as Pete won’t be landing any bus driving jobs.
Are you interesting in engaging your students in a whole new way in the classroom? Try using curriculum-driven Stoich Decks games! These aren’t fluffy, curriculum-adjacent games. They are manipulative tools made to help your students learn faster and can be part of the lessons that you are teaching. Try Up & Atom for moles. CHeMgO for ionic formulas. Trendy for periodic trends.
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