WARPING the PERIODIC TABLE* .
SIDNEY J. FRENCH Colgate University, Hamilton, New York
een elements, and (3 the electronic structure type, used effectively by Deming, in which there are eighteen columns to provide for the so-called transition metals. Both types of tables have their adherents, and both * Presented before the Division of Chemical Education at the present much valuable information. On the other ninety-fourth meeting of the A. C. S., Rochester, New York, hand, both tend to emphasize some unnatural relationships. There never has been, and perhaps never will September 6, 1937.
M
OST of the periodic tables in common use today are based fundamentally on two types of classification: (1) the Mendelbeff type in which long periods are presented as double rows containing eight-
be, prepared a table which can bring out all of the normal relationships of the elements. A fundamental weakness of all tables is emphasized by placing the elements of a family or group in the same vertical column. This emphasizes structural and va-
by presenting the diagonal relationships in the second and third periods as is shown. LiBe
B C
N O F
\ \ \ \ \ \
Na Mg A1 Si P
S C1
However, such a relationship cannot be continued beyond the third period because the gradation in families becomes less ~urthe&ore, such relations h i ~ sare considerablv exazperated: silicon. for instance, resembles neither carbon nor boron in chemical properties but might be said to lie between the two. To make these cross relationships clearer, a simple device of offsetting can be used as follows.
--
LiBe
lence relationships a t the expense of chemical properties and chemical behavior. There is, for instance, a very marked gradation in the chemical behavior of the elements of the nitrogen family. Yet, unless this is carefully pointed out to h i , the beginning student is left with the impression that nitrogen and bismuth are much alike. In chemical behavior, bismuth bears a much greater resemblance to silicon and boron than to nitrogen. Even in valence, it resembles boron more than it does nitrogen. This difliculty in interpretation is sometimes lessened
B C N
O F
Such a device cannot be used in the later periods unless the transition metals are omitted, and their omission is not advisable since many important metals are included in these series. The difficulty can be avoided by laying the table out in the form of a graph and plotting each element horizontally according to atomic number and vertically according to chemical behavior. In such a graph one is forced to decide what property or set of properties shall be used in locating the element. For the main group elements both the physical properties of the elements themselves and the acid- or baseforming nature of the hydroxides can be used. In the case of the transition metals, however, acid- and baseforming properties vary with valence states and hence cannot be used as criteria in locating the element. The physical properties of the metal itself (hardness, ductility, malleability, and electrical conductivity) will serve to locate the metal reasonably well. Figure 1shows a graph of the elements constructed in this manner. It will be noted that the entire table is skewed to the left as base-forming or metallic properties become more predominant in heavier elements. However, the family relationships are definitely retained. Chemical relationships are found along the vertical lines, i. e., elements directly below one another are much alike often in physical properties and generally in cbemical behavior. Obviously, these relationships are neither exact nor quantitative, but they do throw together elements of similar behavior. The variable behavior of the transition metals in their different valence states is indicated by dotted lines. Just where the dividing lines should come between basic, amphoteric, and acidic elements is, of course, a matter of some conjecture. In the opinion of the writer, a table of this type has several distinct advantages over the older types of classification. 1. It minimizes somewhat the emphasis usually given to family relationships. 2. It emphasizes gradations within families.
3. It throws together elements which have similar chemical properties. 4. It permits more effective prediction of acidic or basic natures of the elements or the hydroxides. It is, of course, evident that such a table does not eliminate all of the diEculties encountered in other tables and no such claim is made for it. It would appear complicated to a beginning student, but if first approached through the medium of simple diagonal relationships in the short periods and the effect of sizes of atoms on properties of the elements, its significance is quickly and easily grasped even by beginners. By coloring the chart, blue on the left, red on the right, and an intermediate shade between, the acidic and basic properties of the elements are emphasized. The most difficult task is that of determining just what criteria shall be used in locating the elements horizontally upon such a table. For all hut the transition metals and the rare earths, chief reliance has been placed upon the acid- or base-forming natures of the hydroxides. Such evidence is, a t best, highly qualitative, and there may be other criteria, such as ionizing potentials, which will be found to he more effective. Incidentally, such a classification leads to another problem. We need a simple word to describe the acidic or basic natures of the elements, and we need some kind of a scale to measure these values. The writer suggests the use of the word "bascid," a contraction of base and acid to replace the awkward terminology of acid- and base-forming properties. To simplify the matter still further, a simple bascid scale might be used, ranging, say, from 1, for the strongest base-forming element, to 7, for the strongest acid-forming element. Figure 2 shows such a scale. On this scale
elements forming amphoteric hydroxides would have bascid values close to 4. Hydroxides and elements could be classed on the same scale. Obviously, such a scale would be only qualitative and bears no relationship to a pH scale. To place all of the elements and their hydroxides on such a scale in their relative positions is a task that the
I
I
ACIDIC
5 -7
-
0A5 CID SCALE
writer feels reluctant to attempt without the cooperation of others in the field who might regard such a proposal as worth while in clarifying some of our teaching concepts. Such a hascid scale would, of course, simplify the use of a periodic table similar to that described in this article, both by giving more definitecriteria for locating elements on the table and by describing the nature of the element more precisely. Neither table nor scale would eliminate other methods of approach to periodic classification but would merely supplement them.
