One of the most important challenges in teaching students chemistry is in conveying the fact that chemistry is essentially a milieu of models. Too often students can misinterpret the conceptual devices taught to them as "real" entities. While models seem to perpetually and cruelly banish the concept of "reality" itself to fanciful speculation at best, the real beauty of chemistry is in how the simplest of models can explain a vast range of diverse chemical phenomena. Students' understanding can only be enriched by communicating to them the value of models as a window into our world. How can we achieve this?
We can start by emphasizing the very fact. Very few of my chemistry teachers even mentioned the word "model" in their discourse, let alone emphasized the preponderance of models used in chemistry. One can claim that all of chemistry is in fact a model. The reason for this is not hard to grasp: models come to our aid when the world gets too complex. The complex nature of chemical systems wherein one cannot describe them using first principles lends especially this 'central science' to modeling.
You can start with the simplest fact taught in freshman chemistry class- the structure of methane as it is drawn on paper. The methane molecule of course exists in real life, but that does not mean that you can actually see four bonds growing out tetrahedrally from a central carbon. Recent advancements in techniques like scanning tunneling microscopy have brought an astoundingly real feel to molecules, but what you see is still diffuse electron density and not actual bonds. The tetrahedral representation of methane that we draw on paper is very much a model.
Once students realize that even their simple representations of molecules are models, the road ahead becomes easier. Since we are talking about methane, we will inevitably talk about hybridization and describe how the carbon is sp3 hybridized. But of course hybridization is merely a mathematical and conceptual device- and a very powerful one at that- and this needs to become clear. Hybridization in methane leads to discussion about hybridization in other molecules. This is usually followed by one of the most conceptually simple and useful models in chemistry where you can make back-of-the-envelope calculations to get real and useful results- VSEPR. VSEPR is a great example of a simple model that works in a great number of cases; asking whether it is "real" is futile. Thus, one can drive home the importance of modeling even in the first few sessions of chemistry 101.
Once these facts become clear, the floodgates can open. Students can cease to think of the world as real and still be happy. Think the famous Van der Waals "12-6" curve is real? Think again. It does a marvelous job of representing in simple terms an incredibly complex and delicate tension between attraction and repulsion engendered by point charges, dipoles and higher order terms, and it's no more than that. But it works! It's a disarmingly simple model that's even incorporated in popular molecular modeling programs. How about crystal field theory? Another fantastic model that does a great job of explaining the properties of transition metal complexes without being real. Of course, let's not even get started on that ubiquitous act that initiates a newbie into the world of organic chemistry- arrow pushing. That's the very epitome of modeling for you. And after this onslaught, students should have little trouble understanding that those ephemeral, seductive twin forms of benzene that seem to interconvert into each other on paper are pure fiction.
Want a book that teaches chemistry through models? You are in luck. One of the best books that conveys the reality of chemistry as model building also turns out to be one of the most influential scientific books of the 20th century- Linus Pauling's "The Nature of the Chemical Bond". In this book Pauling introduces dozens of ideas like polarization, hybridization, ionic and covalent character of bonds, resonance and hydrogen bonding. All of these are enshrined in his Valence Bond Theory. And all are models. If conveying the importance of models to students gives us an opportunity to introduce them to this classic text, the effort would already have been worthwhile.
So would students turn fatalistic and despondent once they have been convinced that the world is not real but is a model? Not at all. The singular fact that snatches hope from the jaws of defeat is the very fact that we can in fact build such models and understand the world. Think about it; we build models that are almost laughingly simplistic representations of a hideously complex reality that's probably going to remain out of our reach forever. And yet these apparent embarrassments help us understand protein folding, design new drugs against cancer, build solar cells, bake a cake and capture the smell of a rose in a bottle.
What more could we want.
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