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NEW MOLECULAR MODELS SHOWING CHARGE DISTRIBUTION AND BOND POLARITY R. T. SANDERSON State University of Iowa, Iowa City, Iowa
INA new kind of molecular model, each component atom is colored to represent its partial charge. One color scale uses yellow to denote electrical neutrality, blue to indicate high negative charge, and red to indicate high positive charge. A range from neutral to high negative charge is represented by mixing iucreasingly large proportions of blue with yellow, going from yellow green through medium green and blue green to blue. Similarly, a range from neutral to high positive charge is represented by mixing increasingly large proportions of red with yellow, going from yellow orange through orange and red orange to red. Eight to ten hues from yellow to blue and an equal number from yellow to red are adequate to indicate the most significant differences in charges on combined atoms. A similar color scale, much less brilliant but offering less difficulty to persons of limited visual color sensitivity, uses white to represent neutrality, with pale to deep shades of blue indicating positive charge and pale gray to black indicating negative charge. Partial charges resulting from bond polarity arising solely from electronegativity differences can he calculated readily.' Models may be constructed in three dimensions, but two dimensions are adequate for most purposes if the illueion of three dimensions is created by depicting structure as accurately as pos~ible.~Two-dimensional models can be constructed from inexpensive colored paper mounted on cardboard. If backed with SANDERSON, R. T., J. CEEM.EDUC., 32, 140 (1955). For structural data, see A. F. WELLS,"StructuraI Inorganic Chemistry," 2nd ed., Oxford University Press, 1950; G. W. WHELAND, "Resonance in Organic Chemistry," John Wiley & Sons, New York, 1955; and recent literature on electron diffraction, and microwave spectroscopy. 2
VOLUME 34, NO. 4, APRIL, 1957
felt, they can then be displayed advantageously on a black flannel board.a The application of partial charge data to chemical interpretations useful in teaching chemistry has been described previously in some detail.& The new models illustrate such application uniquely, vividly, and effectively. For example, the change in the nature of / binary hydrogen compounds across the periodic table from LiH to H F is largely a reflection of the condition of hydrogen in these compounds. The models show not only the molecular structures, to scale, and the diminishing hydrogen radius from LiH t o HF, but also the differences in partial charge on hydrogen. Thus, in LiH it is blue, in BeHz green, in BzHs yellow green, in CH, yellow, in NHByellow orange, in Hz0 orange and in H F red orange. The author has found such models very helpful in teaching both elementary and advanced chemistry. Specifically, the models are especially effectivein aiding an understanding of periodicity, and in explaining why hydroxy compounds are acidic, amphoteric, or basic, why ammonia, water, and H F are much less volatile than their respective congeners, why different complex hydrides differ in stability, why eilane differs so greatly from both methane and germane, what happens to the carbon in the progressive oxidation of methane, how acid anions may influence the strength of the acids, etc. In fact, the models are helpful in explaining an extremely wide variety of chemical phenomena. The author will be glad to send further information on request, to any interested chemistry teacher. "his method of display was suggested by Professor LeRoy Eyring. SANDERSON, R. T.,J. CHEM.EDUC.,31,238 (1954).
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