A MODIFIED PERIODIC CLASSIFICATION OF ELEMENTS ADAPTED TO THE TEACHING OF ELEMENTARY CHEMISTRY* ROYCEH. LEROY,DOANECOLLEGE, CRETE, NEBRASKA
A modification of the Deming style periodic classification of elements i s presented which i s claimed by the author to be simple and easily adapted to the teaching of elementary chemistry. The "A" groups are all located together, as are all the "B" groups. The inert gases are assigned a nem relative position, between the halogens of V I I A and the alkali metals of I A and the name of the group has been changed to "transition" A to match Deming's nomenclature of group V I I I in his table. Pictorial methods are introduced by using narrow sficing for the "B" group elements cnd colored backgrounds for the different types of elements.
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There comes a time in every course of general chemistry when the periodic classification of elements is studied. The position in the course is not necessarily determined by the place allotted in the textbook used, nor is the significance and emphasis laid upon this subject by the instrnctor determined by the amount of material given in the text. It is possible that every teacher of inorganic chemistry teaches this subject and determines its importance in his course according to his own evaluation of it rather than by the treatment accorded by the text. It is the purpose of this paper to present the experiences of the author in teaching this subject and to present a model of periodic classification, evolved from t h s e experiences, that so far has proved to be simple and perfectly usable even by the beginning student. Let us assume that the class has followed an outline of study covering the following subjects: the fundamental laws, theories and definitions that usually introduce the elementary course; oxygen, hydrogen, atomic theory, and the gas laws; water, solutions, equations, and chemical calculations, chlorine and hydrochloric acid, acids and bases in general, and then the fundamental differences between the non-metallic type of element and the metallic type of element. With this preparation, the ground is ready for the introduction of the periodic classification of elements and the study of the structure of the atom. From the author's past experiences, i t is now deemed more logical that the study of the structure of the atom should accompany the study of the periodic classification of elements rather than precede or follow it. By developing the fundamental ideas of the structures of the first ten elements, pointing out the relation of electron arrangement to the group in which the element belongs and a t the same time placing emphasis on the relation of the "valence shell" structure of the atom to its chemical
* Presented before the Division of Chemical Education apolis,
Indiana, April 1, 1931. 2052
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properties, the periodic table becomes a living representation of the structure of the element. The treatment just outlined should, of course, be restricted to the "A" group elements. Then, by pointing out the significance of oxidation-reduction reactions, in terms of electron rearrangement or transfer, a study can be made of the "B" group elements and evidence introduced showing why the "B" group elements are more closely related to each other "horizontally" in the periodic classification than are the "A" group elements. Thus, the periodic classification of elements can be made to become a thing of vital importance in the study of chemistry and can serve as a useful tool in successful teaching. It is not a difficult thing to go on from this point into the mechanics of chemical combination and to formulate the conception of polar and non-polar valence. The theory of ionization then follows in natural sequence. It is suggested that now the study of the groups of elements may be undertaken by using a contrast method, viz., alternate a non-metallic group with a metallic g r o u p t h e halogen family, VII A, followed by the alkali metals, I A, then the sulfur group, VI A, followed by the alkaline earths, I1 .I. Frequent reference can thus be made both to the periodic classification and the atomic structure, and the student's mind may be kept cognizant of the existence of two contrasting types of elements, the metallic and the non-metallic. This method can be made very effective. I t is the belief of the author that nothing has been indicated in the foregoing outline that is decidedly individual or extremely revolutionary. Now a question arises concerning the type of periodic classification that is best adapted to teaching the elementary student. There are a variety available: the Hubbard (1) and the simple Mendel6eff types, Deming's modification of the Werner style (Z),the model described by Rodebush (3), and the various helix models, notably that of Monroe and Turner (4) which was recently published. The helix or spiral type is not usually considered adaptable to the use of the beginning student. Now, if we should take a simple Deming type chart (Figure 1) and with a pair of shears cut the chart along the line between groups I11 A and IV B and then fit the two pieces together again so that the group VII A is one column to the left of the inert gases, some interesting things are apparent (Figure 2). I. Now the " A group elements are all together, as are all the "B" group elements. 11. The numbered groups occupy the same relative positions, regardless of whether they are of the "A" or "B" groups. 111. Again, the inert gas gronp has always been called the "Zero" group, while gronp VIII has been more or less difficult to explain satisfactorily to the beginning and inquisitive student. In Deming's table,
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group VIII is called the "transition" group and serves very nicely to bridge the differences in the properties of VII B and I B. Now, the inert gas group occupies the same position between VII A and I A, and the nomenclature of Deming is used (Figure 2). To the student, the profound difference between the halogens and the alkali metals is nicely explained by his study of atomic structure and the inert gases truly occupy the "transition" position, just as the old group VIII occupies a "transition" position in the "B" groups. IV. Now the first indicator that the beginning student learns to use is litmus. Since acids turn litmus red, the area of the chart occupied by the non-metals, the type of element that forms acids, can he colored or painted red. Similarly, the area occupied by the alkali and alkaline earth metals can he colored blue. The sole object of this scheme is to enable the student to grasp immediately the principal chemical properties of an element by noting both the group position the element occupies and the color of its background. (See Figure 3.) Since the "B" group elements are mostly amphoteric and to distinguish still further between the " A group type and the "B" group type, the background of the "B" groups can be painted purple. This color, resulting when red and blue are properly blended will serve to denote the amphoteric properties. The rare earth type elements, including I11 A, have been colored a light shade of blue, to denote the metallic properties of this group, and will set off the group as a set of elements peculiar unto themselves as compared to the other types of elements. V. We say that the "B" group elements are more closely related in a horizontal direction than are the " A group elements and so i t is possible to represent pictorially that fact by placing the symbols of the "B" group elements in the narrower spaces (Figure 3). Of course, this is possible in the case of the Deming modification but not in the Mendeleeff model. VI. Looking over the chart again (Figure 2), we see that group I11 A occupies a position a t the extreme right of the chart. Lanthanum and the fourteen rare earth elements that usually are assigned to the same space with lanthanum are in group I11 A. Now the 10 extra elements of the long periods (extra in that there are 18 as compared to the 8 of the short periods) are classified as "B" group elements. Why not call the rare earth elements the " C groups? (Figure 3.) It has been suggested that the "transition" group of the "C"groups would have to contain seven elements, corresponding to three in the "transition" B group and one in "transition" A, if the grouping should he identical with that of the "A" and "B" groups. It is the purpose of this article only to suggest this possible modification rather than to justify it.
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A wall chart of this particular classification of elements has been used in connection with the elementary course in chemistry for the past three years a t Doane College, and an earlier modification was used the two preceding years (5). This year, and last, each student has been required to make a duplicate of the chart, notebook size, with colored backgrounds and spacing as shown in this chart. This, of course, is done because this chart does not appear in the textbook. The reactions from the students toward the work in the part of the course following the study of the periodic classification and the structure of the atom has been in most cases extremely favorable. In the cases where students go on with the study of chemistry, i t has been found that. they use the chart as a sort of catalog of chemical and physical properties. It must be emphasized that this type of chart is not the only one presented in the course. The history of the development of the periodic classification of elements is given in the course of the lectures, and quite some little time is spent on each type of chart previously mentioned, in order that the student shall not be given a one-sided view of this particular type. In the case of the better students who are interested, i t is demonstrated to them that this chart can be evolved from a helix model by cutting the spirals down the proper sides and then laying the model out flat on the table. Finally, it should be remembered that this classification or chart will lend itself to the same "informational treatment" that Hubhard and Deming provide for their respective charts. At present the value to the heginning student of such extensive treatment as Hubbard provides, for example, is questionable in the minds of many teachers of elementary chemistry. Believing that simplicity should he the keynote of a chart intended for use by the beginner, this treatment is not made use of in this modification. There is no doubt, however, that such extensive information is desirable for the student who is making an extended study of chemistry.
Literature Cited (1)
(2) (3) (4)
(5)
HUBBARD, "Periodic Chart of Atoms," W. M. Welch Scientific Co., Chicago, Ill., 1929. DEMING, "General Chemistry," John Wiley & Sons, Inc.. New York City, third edition, 1930. RODEBUSH. "A Compact Arrangement of the Periodic Table," J. CHEM.EDUC., 2, 3 8 1 3 (May, 1925). MONROE AND TURNBR, "A New Periodic Table of the Elements." itid., 3, 105865 (Sept., 1926). LEROY,"Teaching the Periodic Classification of Elements," Scb. Sci. & Math., 27. 793-9 (Nov., 1927).
without a historic setting for his work, a man is almost as hopeless as is the man I A M BISHOP who lacks a sense of ~ ~ ~ O ~ . - W I L LWARNER
