As with all analogies there are limitations if the student carries the comparison too far or if something is read into the analogy that was not intended by the teacher, ( I ) . For the analogies to the liquid and solid state it should he pointed out that the fundamental particles actually are touchine and do not have soace between them as eould be inferredfrom the pictures. it illno should be noted that the ~icturesillustrate samoles of three different substances of three different amou&. For example, a sample of solid iron; a slightly larger sample of liquid water; and a much smaller sample of gaseous chlorine. If the student infers that the figures represent the same size sample of the same substance (as for instance ice, water, and water vapor), then the fact that there are different numbers of people in the three pictures eould lead them to an incorrect conclusion in violation of the law of consenration of matter. Since 1977 analogies that my students have found useful have been converted into cartoon drawings by the Media Services Department a t Wright State for use as slides and overhead transparencies. The collection now numbers more than 100, and they have been organized into sets for this series of papers. Future articles will feature pictorial analogies useful for the teaching of topics ranging from acids and bases to wave function plots. The level of the analogies covers a spectrum from very fundamental concepts suitable for students in junior high to advanced topics useful for college students. Anyone wanting copies of the illustrations in this article, and future articles as they are published, will be sent on request dry photocopies of the correct size for making overhead transparencies by photocopiers or thermofax.
Figure 1. Analogy for a molecular solid.
Acknowledgment I wish to thank Bruce Stiver of the graphics section of the Media Services Department at Wright State University, whose talent converted these ideas into cartwns. I also wish to acknowledge funds from the chemistry department that paid for these services. Presented as part of a paper a t the 195th National ACS Meeting and 3rd Chemical Congress of North America; Toronto, Canada, June 9,1988. Literature Cited 1. Thiele. R. B.: Tre~regust,D. E Ausfral. Sci lhoch. J 1981.37, No. 2, pp. 1&14.
Pictorial Analogies II: Types of Solids John J. Fortman Wright State University Dayton, OH 45435
In part I of this series of pietorial analogies, a solid was related to an army unit a t rigid attention with regular distances between soldiers standine with fixed orientation~. In this article analogies are given that relate to the four different tvpes of solids on a fiubmicrosco~icbasis. It should be noted to students that in the real submicroscopic model the individual units are actually touching their nearest neighbors, but to have the units &at close together in the pictures would make it difficult toobserve the differences in the units. In molecular solids, units are individual, small molecules with strong covalent bonding within the molecules, hut weak polar, induced polar, or hydrogen bonding forces between molecules. Examples are sugar, sulfur, ice, and
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PaR of a presentation at the 195th National ACS Meeting and 3rd Chemical Congress of North America; Toronto, Canada. June 9. 1988.
Figure 2. Analogy for an ionic solid dry ice. An analogy to this is a regular arrangement of eouples that represent the covalent molecules. This is depicted in Figure 1. The second class of solids are ionic solids with a regular structure of alternating positive and negative ions that have strong forces of attraction, making these solids harder and higher melting than molecular solids. An example is sodium chloride. Figure 2 gives an analogy to this in the form of a regular array of single men (cations) and women (anions) with what one can describe as strong romantic attractive forces between them. The third type of solids are macromolecular with a continuous lattice of covalent bonds that makes the whole Volume 70 Number 1 January 1993
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Macromolecular Solids
All Units are Bonded Together Figure 3. Analogy for a macromolecular solid.
Figure 4. Analogy for a metallic solid.
solid piece effectively one big molecule. Examples are sand and diamond that are even harder and higher melting than salts. Figure 3 illustrates an analogy to this as a continuous lattice of individuals, all holding on to four nearest neighbors. Figure 4 gives an analogy for metalethe fourth type of solids. It presents a lattice of men (cations)with bees (electrons) flying among them. Metals are good conductors with these mobile electrons, and their properties range from
so&to hard, and from low melting to moderately high melting. As with other sets of illustrations in this series of papers, copies suitable for making transparencies will be sent on request.
58
Journal of Chemical Education
Acknowledgment The work of Bruce Stiver of WSU Media Services and funding from the chemistry department are appreciated.
