ATOMIC MODELS FOR USE IN TEACHING INORGANIC CHEMISTRY * EWING C. SCOTT.SWEET
BRIAR
COLLEGE, ~
E
BRIAR, T V~GINIA
All atomic models are true, as far as they go. The fact that the LeuisLangmuir-Bury model has for certain purposes been superseded by a system of equutions does not diminish its extreme usefulness in teaching chemistry. A mechanical model of this atmn has been devised which i s unusually simple to construct and possesses other advantages. The most important feature is that the valence electrons are detachable, and that aery atom beyond hydrogen has spaces for the cmpletion of its valence octet. Only three diferent models are needed to show all the atoms of the first three periods of the periodic system. Actual gain and loss of electrons in the formation of polar compounds, and the sharing of electron pairs in single and double bonds can readily be shown. Most simple molecules can be constructed, as NH4C104, COz, and SrSzOr. Bury's explanation of the multiple valence of the metals of the first transition series can be demonstrated. These models have greatly aided in the teaching of both elementary and advanced students in the following subjects: electron theory of oxidation and reduction, odet theory of valence, polar and non-polar bonds, ionic theory, and the distinction between N a and Nu+, and multiple valence.
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It is rather fashionable these days to inveigh against the use of mecbanistic pictures to explain any of the data of chemistry or molecular physics. According to Dr. Saul Dushman, for example, "Thou shalt not make unto thyself any graven image of the atom or anything that is within the atom, nor bow thyself down before them nor worship them." We must forego their convenience for fear that we shall be hampered by their limitations and be blind to the possibility (or even to the observation) of new facts and relationships if there is in our model no analogy to correspond to them. The finger of warning points out the changes that have come over our old billiard-ball atoms, and the moral is drawn against putting our trust in any physical picture of an atom. We are even beginning to be told that we must abandon our beloved tetrahedral carbon atoms and trust ourselves to the tender mercies of substitution groups when we venture into the realms of stereochemistry. The fact remains, however, that we think more easily in terms of the concrete than of the abstract. Particularly, we learn more easily that way. If a familiar concrete illustration can be found which has a onefor-one correspondence with a new set of facts and relationships, the latter are already as good as learned. When the correspondence goes so far that the illustration enables new facts to be predicted and discovered, its useful-
* Presented before the Division of Chemical Education of the A. C. S., Indianapolis, Indiana, April 1, 1931. 1845
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ness is, of course, even greater. We have been warned that people tend to jump to the conclusion that a snccessfnl model is not merely an illnstration, but the ultimate reality; but I, for one, fail to be greatly alarmed. Minds capable of creative thought can hardly conceivably be fettered by the bonds of a rigid picture. The long succession of atomic models which have one after the other made their contributions to truth and been superseded by better ones should be sufficient proof of this. There are those of us who feel, then, that each modern theory should be made use of in its place and as long as it will serve; who do not wish to occupy the ground of the late Dr. Ostwald, who for years objected to the atomic theory because it might not be true, and to his dying day stood on the beach trying to order back the incoming tide of the ionic theory because it was not necessary in order to describe the facts. If a theory can be represented in an easily understood model, so much the better. I n particular, t h e modem pictures of the structure of the atom are useful to the teacher of inorganic chemFIGURE 1.-A FORXIATE ION, //O istry, and without any serious HC-O-danger even in the case of eleThis configuration shows a carbon atom attached by a single bond to one hydrogen and one mentary students if care is oxygen atom, and by a double bond to anothet taken to separate facts from oxygen atom. tbeorv and to make it olain a t the beginning, and all along the way, to what extent the model is merely a diagrammatic representation of the data. Do not ask the question, "Which model is the true one?" All the models of the atom have been true, and still are in their field. Kendall is very right in saying, "The acceptance of new views does not involve the rejec-
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tion of the old." Even the billiard-ball atom is still true, in all that mystic realm of thermodynamics based on the perfect gas law. In the early days of the nuclear atom there was war to the knife between the static atom of the chemist (such as Lewis' cubical one) and the dynamic atom of the physicist (such as Bohr's original p l a n e model) but as usual both were right, though both were trying to claim too much. They have gradually worked toward each other until they are essentially identical. The chemists are now willing to admit that what they call the position of an electron may be the aphelion of an elliptical orbit, and the physicists, notably Stoner, have adopted the division of the electrons into layers (although they call them groups of a given total quantum number) put forward by Langmuir and so much improved by Bury. True, the atom bas now fallen into the hands of the mathematicians, who threaten to leave us nothing but "an aura surrounding a void," but their adumbrations do not destroy one jot or tittle of the truth of the more concrete model associated with the names I have mentioned. The model which I have chosen to illustrate, which may properly be called the
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K 2 . 4 H1 LIIX.HCIII.RAN" A H K AIUM. THELAI.TERI S ALSU H . Lli, I < R - -A, N D R i + IONS A S D THE KERNEL O F 1SC GROUP ATOMS The small balls represent the nucleus of any atom, according to what charge is assigned. The large balls represent electrons.
FWIXF:~.--I\N SH/- ION, ALSO d CHI MOLECIILE, IF NO ISTO TORT ION IS ALLOWED FUR
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Bury atom, does not claim to be truer than several others. It does, however, exemplify a large amount of chemical truth, with a minimum of extra chemical complication. Especially valuable is Bury's idea of never putting more than eight electrons in the outside layer, and later, when a group of eight in the third or fourth layer is no longer on the outside, filling it up to eighteen or thirtytwo. This affords the only reasonable explanation of the variable valence metals and of the existence and number F~cuee 5. K n r ~ r o xATOM,ALSO S F ,HR-, K n , Snr+, AND YT+++IONS of the rare earths. This frame is the same as that in Figure 6, In view of the difficulty of except that since it provides places at the poles three-dimensional visualizafor the completion of the 3rd layer of 18 (shown in pure white); all atoms of the 3rd period intion, even when assisted (if stead of only those up to Ni++can be built on it. it is assistance) by the inFor ease in distinguishing, electrons of the different layers are shown in different shades. structor's attempts a t picturization on the blackboard, a set of actual physical models was made. In the design of these models certain new featureswere incorporated which are believed to be a considerable improvement over anything which has previously been used. The first of these is the skeleton structure. As a result of this the wires entering any one ball, representing an electron, are all in one plane, so that the plaster of Paris balls can be cast in place with a simple two-piece mold. variable'valency according-to the Bury theory. V04---, GO4-, MnO,i, and TiF, would be made Also, the models can be teleby substituting this structure for the central atom scoped for storage, with a in Figure 7. great saving in shelf space The other feature is that the valence electrons are detachable. The detachability of the valence electrons is an extremely valuable feature in teaching. It makes possible actual demonstrations of all the things
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pictured in the chapters on the octet theory common in modern texts. The electronegativity of the non-metals and the electropositivity of the metals, oxidation and reduction, polar valence, and ionization are all made plain by the simple removal of an electron from a sodium atom and its addition to a chlorine atom. Non-polar bonds, both single and double, can he shown with ease. It is 90ssible to show a triple bond by making use of the tetrahedral distortion theory (although I personally agree with G. N. Lewis that no such thing as a triple bond actually exists). The formation of an ammonium ion is made easily understandable, and i t becomes very plain how the ions of the acids of chlorine increase in oxygen content up to a maximum of four while the valence of the ion remains the same. The nature of Sugden's semi-polar bonds is made obvious, a n d lthe demonstration of the equivalence of the oxygen atoms in the common oxy-acids reveals the futility of such discussions as those in Mellor ( 1 ) as to whether sulfur in an alkylsulfinate ion is to be represented a s q u a d r i v a l e n t O\S/*
R/
or as hexavalent
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T h e v ~ l v n wlaver o f first ~ ~ c ~atom5 i t d i i rnadc thc wmt w c . of !hat of w c m l w r w t l ar#,rn%,in Crdrr to fartlmtt 4lowing nor&-polsrIwtlrl%
FIGURE 8.-A
CI-1, ~ ~ X E C U I)ISTI>R.I'ED L ~ ~ , INTO T~rm.mrmn,cFORM
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ion is
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-
O)S=O -0
or -O\SYO, Finally, by the use of a larger model, ON \ showing the empty spaces in the third layer and valence electrons in the fourth, it is possible to show the first transition series, and explain the multiple valence of the metals from titanium to copper. An incidental advantage of the use of movable electrons is that a single model, by having the proper charge ascribed to its nucleus, may be used for all the elements in any one period of the periodic table. In addition to the simple bit of wire which serves for a hydrogen atom, only three different models are needed to show all the atoms up to krypton, together with Rb+. Sr++. and Yt+++ ions. These models have been used in the instruction of classes in general and advanced inorganic chemistry for three years in the premedical school of the Pekinc ', Union Medical College, and for four years a t Sweet Briar College. It is believed that they have been a great help to both lecturer and students.
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Practical Details.-The hardest grade of drawn (not I k u n s I(l.-h C H I M O L ~ ~ C UACCORDING LL. TO cast, on account of welding TeE K R K U I FORMULA .~ difficulties) b r a s s wire, of This illustrates how double bonds permit about B and S gage NO. 10, flexion but not rotation and how single bonds permit both, if the bonding electrons are drawn is a convenient for together. making the frames. It can most easily be joined by spot-welding, though oxy-acetylene welding is also satisfactory and perhaps necessary if the jaws of the spot-welder are not conveniently shaped. The detachable electrons are cast on one-inch sections of brass tubing of such a size as to slip easily over the wire of the models. The ends of the tube can be crimped slightly with pliers in order to obtain the correct degree of friction. Molds for casting electrons can be made as follows. Miter four small pieces of wood and fasten them with a rubber band into the shape of a bottomless box. Set this on a glass plate, fill with a thick plaster of Paris mixture, and press a ball bearing of convenient size into the surface until half buried. Smooth off and flatten the surface. As soon as the plaster has set thoroughly remove the sides of the box, the glass plate, and the ball bewing. The surface of the mold may then be trued up with a coarse
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flat file if necessary, and the channels for the wires around which the ball is to be cast may be cut out with a pocket knife. After thorough drying give the mold a good soaking in melted vaseline and grease well with the same material before each use. Before use, molds should be paired by placing the ball bearing between two halves and trimming the edges to fit. A groove or two made across the edges on one side will serve to identify a pair and to enable the halves to be fitted together in the same way each time. A battery of eight or nine molds can conveniently be filled at one time with a mix of 45 g. of plaster of Paris to 25 g. of water. After all the halves are filled the first will probably be thick enough so that it will not spill when inverted for joining to its mate around the desired wire. Rubber hands hold the halves together. An hour is ample time to allow before unmolding.
Literature Cited (I)
MELLOR."A Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. X, Longmans, Green and Co., New York City, 1930, pp. 238-9.
