Uses (and Abuses) of Models in Teaching Chemistry Henry A. Bent North Carolina State University, Raleigh, NC 27695
We think only through the medium of words [and models]. Abbe de Condilloc [augmented] as quoted by Louoisier. Introduction: Models in General and CH4 in Particular Chemistry's models are not like model airplanes: small versions of visible things. Chemistry's tangible models are lame mechanical metanhors of small. invisible.. auantum mechanical things. Take. for examde. a hall-and-stick model of methane. G d heave&! thinks ahokhemist. What's going on here? Methane doesn't look like that! Methane is a colorless eas. Indeed, to be useful, a model must be wrong, in some res~ects-else it would be the thine itself. The trick is to seedith the help of a teacher-where i t S right. Clearly chemical models don't render the visible. Rather, they express, in the words of the artist Paul Klee, "our secret visions and insights" into Nature's nature: methane's elementary composition, for example; and how many dichloro derivatives methane has--only one: hence its molecules can't he planar; they must he tetrdidral; and, correspondingly, by joining tip to tip the tetrahedrally directed sticks of balland-stick models of two carbon atoms to form single, double, and triple honds, one sees that molecules of ethylene are planar,*cetylene linear; and, introducing lone pairs, one sees that molecules of ammonia are pvramidal, water bent. Observe that in the banana- o; bent-bond modelsofdouhle and triple bonds (Paulina has remiirked that "twnt-hmds lire best"ihest for the easy, efficient description of multiple honds) we haven't even mentioned electrons, or orbitals, or hydridization, or the designations s , p , . . . and o,rr, . . . , or electron spin and the Exclusion Principle. We don't even have to think i f quantum physics here, just pure chemistry. T o a chemist, the entire stereochemistry of octet-rule atoms is latent in the model of methane. And the entire history of civilization, it's been said, is embedded in the formula for sulfuric acid.' Evidently there's more to seeing than meets the eye. T o see what a chemist sees one needs to know what a chemist knows. Knowine what a single sentence means means understanding & entire language. Knowing what model-talk means means understanding an entire science.
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Teaching Uses of Models in Chemistry How, then, can we teach students the meaning of modeltalk? How can we teach students who know little chemistry
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Presented at a workshoo for hiah school chemistn, teachers at the 35th Southeast Regional hieeting of the American chemical Society. Charlone, NC, November 11. 1983. ' Creation of the symbolic form "H2S04" implies an enormous infra-structure:the human brain, language, mathematics, the practical arts upon which chemisby is based, a culture capable of providing the leisue time necessary for punuit of pue science, enlightened curiosity reaardina Nature-in short. a modern civilization. " Students- in an averaoe first are, ex- - ~ - course ~-~ - - in colleae ~- --- chemistn, .~ . - pected to assimilate 6000 to 6750 units of informarion. states Dr. Mary B ~ d Row d IThis Journal, 60.954 (t983)I.That is more new language. adds Or. Row. than is usually found in the first year of [conventional] foreign language study. ~~~~
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the uses of models in chemistrv? In the same wav we teach young children the meanings of words, sentences, and other abstract models of the real world: by pointing and telling. Children don't learn a language-and chemistry (and its models) is nothing if not a language-from formal definitions. Children who have no language a t all for describing the "big, hloomine. -. buzzine confusion" in the world about them have no use for dictionaries-or quantum physics. Dictionaries (and quantum physics, even watered down quantum physics) are useful only if we know already a t least one language for talking about the things heing talked about. "But if a nerson has not vet . eot .. the contents. . .I shall teach him to use
