THE USE OF MODELS IN TEACHING POLARITY The Lewis-Langmuir hypothesis, which was 6rst applied to the extreme cases of electron transfer and equal sharing, has been extended to apply to that great intermediate field lying between. We now have a unified concept which removes the sharp line of distinction formerly thought to exist between ionized and non-ionized compounds. We have come to realize that all compounds cannot be divided into the two classes any more than all people can be classified as good or bad. The one type shades into the other. We are handicapped, howevef, by the fact that we have developed no methods of precision by which we can study polarity in this intermediate field, and are forced to rely on methods of inference. Nevertheless, our understanding of the mechanism of chemical reaction is growing rapidly, thanks to the octet hypothesis. The original theory of Arrhenius has become a historical incident in
the development of a more comprehensive viewpoint regarding the structure of matter. Viewed in the light of economy of time and economy of nervous energy, i t would seem to the writer that the beginning student should be first introduced to polarity through the unified concept presented by the extension of the octet theory to cover the entire field of polarity. He gains through this concept a clearer picture of valence and ionization through electron transfer, but best of all, he includes in the same picture cases of non-polarity and partial polarity. He can then view the theory of Arrhenius in retrospect. This more complete concept can he developed quite independently of the ionization theory of Arrhenius. One of the great difficulties the student faces is that of transferring his mental image of the octet atom, with its electrons $xed a t the corners, to a plane surface on which the electrons are shown in various positions about the symbol. Most of our elementary and advanced textbooks, in their projections of the atoms, show the electronsfixed on the corners of the cube and can thus only present the extreme cases of complete non-polarity or 2687
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complete transfer of electrons. From such projections, the student is forced to jump to electronic formulas in which electrons are shared unequally; i. e., being displaced toward one or the other of the atoms. We are forcing the student to conclude that electrons are not held on fixed corners, but are rather held in an elastic manner which permits them to he pulled away some distance, or completely, from the kernel of the atom. Because of the difficulty the student has in making this transition in his mind, the writer has taken the liberty of building an atom model which will bridge the gap and simplify this problem for the student. L The criticism may he justly made that such NaC ' C1a concept as presented I~rcuRESODIUM AND CHLORINEION$ here is too concrete, and does not align itself with the existing physicai evidence. Nevertheless, as long as the student is forced in his own thinking to p a p through such a concept, it might as well he done for him in the most concrete Na C1manner possible, so that FIGURE 4.-SHOWING THE ELECTROSTATIC ATTRACTION there will he no mistake BETWEEN SODIUM AND CHLORINE IONS in his understanding of the complete meaning of polarity. In the models used by the writer to develop the idea of polarity, the kernel of the atom is a painted block of wood. It is assumed that the student is already familiar with the modern theory of the structure of atomic nuclei. Therefore, only the valence shell of electrons is shown. A copper wire spring is attached at each corner of the cube. A small finishing nail +
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forms a short core for the spring, and acts as a guide when coiling the spring. The coil is made slightly larger than the diameter of the finishing nail. Electrons are made of modeling clay rolled in red lead to give color. They are merely stuck on the ends of the springs where desired. The affinity of each atomic kernel for its valence electrons is shown by the degree to which the spring is coiled or uncoiled. Thus the lone, loosely held electron of a sodium or lithium atom is shown a t the end of an uncoiled spring while the other seven springs remain tightly coiled. Satlium is thus concretely represented as a donor atom. The reverse case is shown in the chlorine atom, where the seven electrons are held on tightly coiled springs, with the one free spring reaching out in space for an electron. In the case of carbon, all springs are half extended, illustrating the balanced atom. Figure 1 shows the seven active elements of a short period.
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FIGURE~.-SHOWING THE RELATIVE POLARITIES OP AMMONIA, WATER,AND HYDROGEN CALORIDE
The transfer of an electron in the formation of sodium chloride is easily illustrated with these models. Figures 2 and 3 illustrate the transfer. After gaining its electron, the extended spring of the chlorine atom coils up, and the chlorine ion assumes a balanced position. The extended coil of the sodium ion likewise coils up. If desired, the electrostatic attraction between the sodium and chlorine ions can be indicated by leaving the one coil of the sodium ion extended toward the chlorine ion (Figure 4). As is noted in Figure 1, the chlorine atom holds its electrons closer to the kernel than oxygen; oxygen closer than nitrogen. Thus, electron pairs of nitrogen can be brought closer to the proton of an attached hydrogen and hold the latter in a non-ionizable union in ammonia. In the hydride of oxygen, the electrons are unable to hold attached the protons in as close a union as in the hydride of nitrogen for they are bound closer to the oxygen kernel; slight ionization is therefore possible. The electrons of chlorine
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are held so close to the kernel that they exert little influence on the proton of hydrogen in the hydride of chlorine. Therefore hydrochloric acid ionizes freely in a water medium, where the a t t a c h e d proton may come under the influence of the free electrons of a water molecule to give a hydrated ion. The relative polarities of these H t h e e compounds are FIGURE 6.-SHOWINGTRE A ~ O NOP AMMONIA shown in Figure 5. WITN WATER Figures 6, 7, and 8 show the reaction of ammonia with water in dissolving and forming ammonium hydroxide, and in ionizing into ammonium and hydroxide ions. Figure 6 shows a molecule of water, the proton of which is coming under the influence of the two free electrons F I Q ~ ESHOWING T m UNIONOB WATER WITH of an ammonia molecule. A-ONIA TO FORM AMMONIUMHYDROXIDE Figure 7 shows the union of a w a t e r a n d a n ammonia molecule. The attached proton is now more under the influence of the nitrogen electron pair than the oxygen pair. Also, hydrogen is temporarily given the unorthodox valence of two, and nitrogen an equally unorthodox valence of four. Figure 8 shows the separation of the molecule into ammonium and hydroxyl ions. The formation of ammonium chloride may he similarly shown.
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The action of a water molecule on a hydrogen chloride molecule is similar to the action of ammonia on water. The attached proton of hydrogen chloride readily comes under the influence of a free electron pair on the water molecule forming a hydrated hydrogen ion. Carbon may he considered a balanced atom since its tendency to give is balanced by its tendency to take. As a result it does neither completely. It merely shares electrons. The union between two carbon atoms is nonpolar when both of the carbon atoms are attached to identical groups. Ethane (Figure 9) shows such a non-polar union. Many other illustrations are, of course, possible in showing reactions of inorganic and organic nature. These suffice, however, to point out the use of such models as teaching aids. The student easily makes the transition from such models to electronic formulas on a plane surface. The older cubical form is used to represent the atomic kernel largely because most elementary textbooks still use cubical projec- tions in presenting the H H Lewis-Langmuir hypothH:C : C:H .. .. esis. However, the pairH H ing of electrons as well FIGURE9.-ETIIANE, S ~ I O T VA INON-POI,AR NG LINKADZ as the tetrahedral form for the atom can be readily shown by bringing the ends of adjacent springs close together. Carbon can thus be shown as a typical tetrahedral atom in which each electron is held by two springs. By using this form for the carbon atom the stability of a molecule of carbon monoxide can he better understood. However, the use of the cubical form makes i t difficult to show triple bonds. Undoubtedly, wooden balls would be superior to cubes for many reasons in representing the kernel of the atom. The springs could be attached to the halls in tetrahedrally distributed pairs. When the student has developed some ability to handle the concept mentally, this spherical model which lends itself more readily to the interpretation of modem theories could well be used. The use of such models makes it possible to teach polarity to beginning students, and thus start them out with a rational view of ionic and nonionic reactions, donor and acceptor atoms, acids and bases, solution and chemical action, hydration, crystal structure, and electrostatic forces. The models merely aid in making the concept, which is finally arrived a t by the advanced student, clear to the beginning student, so that he may omit many of the laborious steps now involved in arriving a t a unified view of the nature of chemical reaction through polarity.
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