"Hey Mr. T, why is the I3- ion a linear shape?"
[I grab my MolyMod kit and build a model to show the student how the lone pairs orient around the center atom, creating a linear shape.]
"Hey Mr. T, why are H2O, NH3 and CH4 all sp3 hybridization even though their molecular geometries are different?"
[I grab my MolyMod kit again and build models of each, along with a model of sp3 hybrid orbitals.]
These two previous conversations have been pretty common within my IB Chemistry course every time we study chemical bonding and molecular geometry - or review before IB exams each spring. But last year's equipment order in June included two new items to use in my classroom that have gotten quite a bit of use lately.
One new item is a model of the crystal lattice of a salt crystal from 3-D Molecular Designs, using magnets to hold the ions together. I often use this for comparing the structure of the ionic compound to my big beaker of water molecules. I frequently joke with my kids about being thirsty and grabbing my water molecules, but it serves a few distinct purposes. The obvious aim of this beaker is to show students the idea of particles in a molecular compound versus how the particles (ions) arrange in an ionic compound. But even more than that, I often emphasize that my beaker is full of a chemical! It's a bit of an attempt to reduce chemophobia in some small manner.
Another new item is my collection of magnetic water molecules from 3-D Molecular Designs. These models allow me to discuss several concepts with students. First, I can show them hydrogen bonding within a collection of water molecules and the attraction between the partial positive charge of the hydrogen and the partial negative charge of the oxygen. But this kit comes with ethane and ethanol models as well. The water molecules will hydrogen bond with the oxygen and hydrogen of ethanol, but have no attraction with the ethane molecule. It's a bit of a simplification, as it ignores London dispersion forces between any two molecules, but I use it to highlight the influence of intermolecular forces on solubility.
The water molecules also can be used to show the hydration of ions by using them jointly with the ions from the salt crystal. The hydrogens of water surround the chloride ions, while the oxygens of water surround the sodium ions.
Just this week I have used some combination of these models with seniors in IB Chemistry reviewing hybridization and bonding in organic molecules, juniors in IB Chemistry looking at IMFs of organic molecules (comparing ethane and ethanol in water), and introductory chemistry students as they develop their understanding of polarity. As with any model, these can simplify a concept to make it easier for students to understand. But I also emphasize with my students that these are just models and have limitations as well. I keep my collection of models right near my desk so that any time a question comes up that can be aided by the models, I've got them right there and don't need to go searching. And rarely a week goes by when I don't use them for some discussion.
Do you have any fallback models you use with your students? I'd love to hear about them as I look to expand my collection of manipulatives.