Representation of Periodic Properties of the Elements THOMAS H. HAZLEHURST and FRANK J. FORNOFF Lehigh University, Bethlehem, Pennsylvania
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RAPHICAL representations of the physical and for or must be joined by some sort of dotted lie. If chemical . properties of the elements are almost every family is treated in this way, the diagram is so indispensable in teaching inorganic chemistry. The crowded with lines as to be practically useless. Furusual type of graph, in which values are plotted against thermore, the embarassing lack of data and consequent atomic number as a series of points, leaves something absence of many points makes the construction of a to be desired in two respects. The charts should be complete curve impossible, and interpolation is not most useful in tracing trends in families of elements always desirable. Moeller (I) has recently suggested an improvement but, while it is easy to pick out a family of elements at the peaks or a t the minima of curves in the chart, upon the usual practice. He breaks the curve a t the points for members of other families must be searched atomic numbers of the inert gases and puts the various
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FIG- 1.-DENSITYOF THE SOLID ELEMENTS Scale 0 to 25; one square = 0.25 g./cc. The values have been selected from those listed in Landolt-Bornsteinand in the International Critical Tables. For elements which are solid at 20PC.,this temperature was used. For the others, the density is that at. the melting point.
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FIGURE 2.--OXIDATION STATESOP THE ELEMENTS Heavy horizontal line = 0 ; squares above represent stater +l to +8; those below, states -5 to -1. Oxidation states represented by well-known compounds are filled in solidly with a "wing line" an each side. Others are indicated simply by wing lines. Certain values have been purposely omitted, e. g., -1 for Re, and the many values which can be assigned to C in organic compounds.
portions under one another more or less in the manner of a long-style periodic table. We have found a further modification very helpful. The values of a given property are plotted for the elements in separate boxes, the boxes being arranged in the form of the usual iong-style periodic table. In order to represent the value with any accuracy, there must be a fairly long "axis" upon which to plot it. It is hardly feasible to make the box for each element very tall, and indeed this would defeat the main purpose, which is to keep the chart as nearly as possible in the familiar form of the periodic table. However, by putting in each box a half-inch square of cross-section paper, ruled, as usual, in twentieths of an inch, we have a scale consisting of 100 ruled units (10 X 10 little squares) large enough so that tenths of the units may be estimated. In effect, there is a scale of zero to 1000. Starting a t the lower left, the 100 small squares are counted from left to right along each horizontal row. After the completion of one row, the next higher one is begun a t the left. For example, if the whole
scale represented 1000, a value of 123 would be plotted by filling in the 10 squares of the bottom row, the first two squares from the left of the next-to-the-bottom row, and an estimated 0.3 of the third square in this row. A typical chart of the density of the solid elements is shown in Figure 1. For reproduction as lantern slides or for publication, black-on-white graph paper is most satisfactory. The squares are filled in with black drawing ink. When an element exists in more than one allotropic modification there should be some provision for plotting more than one value in a box. The method we have used is to fill in squares as usual up to the lower value and then to put diagonals in the squares included between the lower and upper values as illustrated in Figure 1 in the boxes for C, P, S, and Sn. In this way it is possible to plot with reasonable accuracy all of the usual properties associated with periodicity and to have the families of elements in their accustomed positions, so that horizontal and vertical
trends are immediately observable. Elements for which data are not available show up as blanks with no necessity for attempted interpolation. The form of the periodic table here used, the long style with the rare earth elements relegated to a special row below the main table, is justified by the following considerations: First, horizontal trends in the long periods are obscured in the more compact forms of the table. Second, it is difficult in the compact table to distinguish clearly the subfamilies in a given column. Third, the slight gain in logical completeness which would be secured by inserting the fourteen rare earth elements in the sixth period would be more than offset by the unwieldy shape and size of the table so produced and by the necessity of breaking the preceding periods of eighteen with a broad empty region and further enlarging the existing gap in the periods of eight. The short or compact form of the table can, of course, be used if preferred. Figure 2 shows a chart of oxidation state (valence number), a property for which a scale of 100 is not only unnecessary but inconvenient. Figure 3 is a chart of
the isotopes of the elements obtained by plotting the. value of W - 22, where W is isotopic weight and Z is atomic number. Aston's general rules about the occurrence of isotopes are clearly illustrated. Variations of the details may prove helpful in special cases, but the symbols for the elements should never be so prominent as to distract attention from the sequence of values of the quantity plotted. The preparation of such charts is a valuabl~student exercise, particularly if the property plotted is'a little less common than density, atomic volume, or melting point. The trends are far more obvious than they would be from a set of numbers placed in the boxes, and any normally interested student is likely to be intrigued into speculation about the reason for a trend or for apparent exceptions. A fairly complete set of such charts is a most valuable source of ideas and illustrations for the teacher. (1) M~ELLER, J. &EM. EouC., 17, 441-2' (1940). (2) SEABORG, Chem. Rm.. 27, 211-14 (1940). (3) Tanon AND GLASSTONE, "A treatise on physical chemistry," D. Von Nostrand, 1942, Vol. 1, pp. 4 1 4 .
FIGURE 3.-ISOTOPES O P TAE ELEMENTS Values of W - 22, where W = isotopic weight and Z = atomic number. Last square in bottom row = 0, fist square in second row = +1, etc. Isotopes listed on the basis of information in reference3 2 and 3.
