Chemical Education Today
Reports from Other Journals
Learning about Atoms, Molecules, and Chemical Bonds: A Case Study of Multiple-Model Use by William R. Robinson
As they progress through their study of chemistry, students encounter a variety of theories, models, and analogies, some that appear mutually exclusive and others that appear based on questionable assumptions. They find still other theories that appear unrelated, although they describe similar aspects of the same system. Derek Smith makes the following points concerning such multiple theories in the introductory chapter of his inorganic chemistry text (1): The fact that a theory “works” to a useful extent does not prove that it is literally true, i.e. that it presents a physically-realistic description of the system. Conversely, the fact that some of the underlying assumptions in a theory can be shown to be unsound does not, per se, require the theory to be discarded. A simplistic approach can serve us well provided we understand its limitations and do not take it too literally. It is quite permissible to skip from one theory to another while discussing the same problem.
Although Smith speaks specifically of theories, the same ideas can be extended to models and analogies. The ability to accept and use different theories and models for the same system generally develops slowly, if at all, in students. I suspect that many of us have seen dismay, disbelief, and disappointment in our students’ eyes when they discover that the first theory they learned is not completely correct and they need to learn another. Upon introducing a second theory of bonding in a sophomore inorganic class, I have heard students say “Then why did we bother with the first one if it wasn’t true?” Harrison and Treagust (2) have considered the issues that make it difficult for students to understand and use multiple or competing models, models such as the valence bond theory and molecular orbital theory, for example. In their paper “Learning about Atoms, Molecules, and Chemical Bonds: A Case Study of Multiple-Model Use in Grade 11 Chemistry” these authors provide an excellent, extensive literature review that connects ideas from several different areas in addressing the reasons why students have difficulties learning to work with and understand multiple models. Then they thoroughly describe one of several case studies that support their conclusions about the successful introduction and use of models in the classroom. Harrison and Treagust are particularly concerned with analogical models—the physical objects, pictures, equations, and graphs that depict objects, theories, and relationships. Analogical models are particularly attractive to teachers and students because they explain abstract concepts in concrete ways that are often familiar and are generally relatively comprehensible. However, one needs to remember that students prefer to think about abstract processes in concrete terms and 1110
can be reluctant to give up concrete analogical models (such as …why did we bother a disorder model of entropy) with the first [model] for more appropriate models as their knowledge advances. if it wasn’t true? Because models are ways of thinking about, representing, and manipulating ideas, Grosslight (3) identifies models as epistemologies—beliefs about the nature of knowledge, about what we can know, and about how we can know it. He places model users in three categories. At level 1, model users believe there is a one-to-one correspondence between models and reality and that models should be correct. These users believe that a model may be an incomplete copy of reality, but only because the modeler wanted it that way. In other respects the model is simply a representation of reality and level 1 users do not look for ideas or purposes in a model’s form. At level 2, users retain the idea that models are realworld objects as opposed to representations of ideas, but they accept the idea that the main purpose of the model is communication of ideas. However, they have not reached the level where they can use models for the exploration of these ideas. Level 3 users accept multiple models and can manipulate these models to assist their thinking about the concept modeled. According to Grosslight, some students can reach as high as a borderline 2/3 classification, but only experts reach a level 3 capability. Grosslight’s levels of modeling roughly parallel Perry’s taxonomy of levels of students’ intellectual development (4, 5). Model-based instruction that ignores a student’s beliefs about the nature of the knowledge available from a model (his or her epistemological status) will likely be ineffective. On the basis of their study reported in ref 2 and other studies referenced therein, Harrison and Treagust provide five recommendations for teaching with analogical models: 1. Because scientists and teachers design most models, students may not be familiar with the specific analogies involved with the model. Instructors should check the way that their students visualize any model or analogy found in a text or used in class. 2. Students should not be expected to have interpreted correctly or completely models with which they have limited experience. Before a model or analogy is used in a second context, instructors should check students’ understanding, even when they claim to know about it. Students should be allowed to explore the differences between their model and the accepted model in order to comprehend the differences and come to a better understanding of the model. (Educators sometimes call this “negotiating meaning”. Chemists should
Journal of Chemical Education • Vol. 77 No. 9 September 2000 • JChemEd.chem.wisc.edu
Chemical Education Today
not take this to imply that the meaning of a model is up for negotiation, but rather that it is not effective to simply state the meaning for the students.) 3. Remember that students generally have the naive view that models presented by experts are “true” and that it may be difficult for them to move beyond this level of belief. Consequently, the arbitrary nature of models and their usefulness as tools for thinking about a concept or problem need to be discussed each time a model is used in a different context. 4. Modeling is a skill that cannot be learned in the same way that facts can be; modeling must be practiced. Introduce the utility of multiple models as early as possible and allow students to see that the use of multiple models is both legitimate and useful. Allow students to work explicitly with different models so they can begin to develop the ability to use different but appropriate models for the same systems. 5. Remain aware of your students’ developing conceptions. Three techniques that can be used are assign-
ments that require written qualitative explanation of the meaning of model; model-based problem solving; and building, manipulating, and exploring models.
With these techniques instructors can help their students come to a better understanding of the nature of models and their use. Literature Cited 1. Smith, D. W. Inorganic Substances, A Prelude to the Study of Descriptive Inorganic Chemistry; Cambridge University Press: Cambridge, 1990; p 5. 2. Harrison, A. G.; Treagust, D. Sci. Educ. 2000, 84, 352–381. 3. Grosslight, L.; Unger, C.; Jay, E.; Smith, C. J. Res. Sci. Teach. 1991, 28, 799–822. 4. Finster, D. C. J. Chem. Educ. 1989, 68, 659–661. 5. Finster, D. C. J. Chem. Educ. 1991, 70, 752–756.
William R. Robinson is in the Department of Chemistry, Purdue University, West Lafayette, IN 47907;
[email protected].
JChemEd.chem.wisc.edu • Vol. 77 No. 9 September 2000 • Journal of Chemical Education
1111
