A NATURAL PERIODIC SYSTEM INCLUDING THE RARE EARTHS*.?
A periodic table i s presented and its arrangement i s discussed from the standpoint of degree of similarity i n physical properties of the elements. The system i s termed a natural one since the position of each element i s largely determined from physical relationships. The rare earths are arranged in a "V-shaped" group in which relations with each other as well as with other elements are supported by a t w l data. The Fe, Rh, and P t metals are accommodated by a similar grouping. Ever since the memorable work of Mendeleeff in 1870 chemists and physicists have sought improvements in the periodic system of the elements. With the increasing knowledge of the rare-earth metals in recent years, naturally more attention has been given to their position in the periodic
system, and many types of configurations have been presented, of which the more prominent are represented by spirals (I), ( 2 ) ;cylinders (3), (4); space models ( 5 ) , (6); spheres (7); and other geometrical figures (S), (9), (10). The writer's observation is that in previous tables, classification of the elements is based more on their similarity than on their degree of similarity in properties. In the following classification (Figure 2) an attempt is made
* Presented before the Kansas Academy of Science, April 24, 1931.
t Thanks are due Dr. Robert Taft for his helpful suggestions during this investigation.
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to show the degree of similarity between elements of main groups and subgroups, as well as to classify the rare earths. The system may be considered a natural one since the position of each element is largely determined by physical relationships. The arrangement in Figure 2 is an outgrowth of Figure 1 in which the elements are grouped into the well-known periods based on. atomic structure, the order being, of course, according t o Moseley's atomic numbers. Here (Figure 1) a simple division between metals and nonmetals is effected by the diagonal line through B, Si, As, Te, and (85), i. e., the border-line elements. The relationships between similar elements are represented by horizontal and vertical lines. That is, the inert gases are found a t the top of the diagram, followed by the halogens and so on, while the alkali metals are found on a zigzag line at the bottom. The rare earths are taken care of in the sixth period which, of course, consists of thirty-two elements. Ce and Th are subgroup members of Ti, Zr, Hf, just as the latter are subgroup members of the carbon elements. To prevent duplication, other relationships, especially those of the rare
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earths, will be discussed more appropriately in connection with Figure 2, which is essentially the same as Figure 1 except that the position of each element is determined by its degree of similarity to succeeding elements of the family. For example, in Figure 2, Na is so placed that its relation to K is shown more prominently than to Cu, and similarly CI is placed more nearly with Br than with Mn, and so on. The elements of main groups are connected by continuous lines which are also more nearly vertical than the subgroup broken lines. Relations between periods 2 and 3 consisting of eight elements each are represented by parallel vertical lines since no subgroups enter. This is also substantiated by their degree of similarity in physical properties (see Figure 4). Also periods 4 and 5 which contain eighteen elements each are related similarly, but between periods 3 and 4 a new group of elements enters and it must be taken into account. Na is more nearly like K than like Cu, both chemically and physically, although there are some physical differences (see Figure 3). Mg is somewhat more like Ca than Zn but the relation diminishes, a fact which is represented in Figure 2 by the decreasing slope of the lines from left to right until, after we pass Al which is almost as much like Ga as Sc, we come to Si which is slightly more like Ge than Ti. P is increasingly more like As than like V although the latter possesses all the common valences of P and in addition the common valence of two. The next element S is distinctly like Se while its similarity to Cr exists practically only in the maximum valence, that is, in compounds of the chromates and bichromates as compared with the sulfates and pyrdsulfates. Cl also is related to Mn in the maximum valence only, while it igvery much like the other halogens. When we come to argon we see hardly any resemblance to Fe, Co, and Ni. The inert gases have only a valence of zero but, theoretically, we assume that eight would be a probable valence if any other were exhibited, and eight is the maximum valence of Fe, Co, and Ni--octavalent compounds of which have been prepared by Goralivich (11). Octovalency for Ru and 0 s is well established. Along this line it might be pointed out that a theory has been proposed by Lansing (12) in connection with passivity of metals and inert gas structure. Thus Lansing suggests that under normal conditions the electrons of Fe, for example, are thought to be distributed among the energy levels as 2, 8, 14,2, but under strong oxidizing action the arrangement may become 2, 8, 8, 8, a structure analogous to that of the inert gases and the Fe becomes passive and diamagnetic like them. Passivity, however, does not limit itself to Fe, Co, and Ni but Cr and Mn are also commonly passive and their structure may be explained similarly. Finally, from the standpoint of coordination valences carbonyl compounds of Fe, Co, and Ni are supposed to form as Fe(CO)s, Co~(C0)o Ni(CO)4, rather than Fe(CO)4, Co(CO),, Ni(CO)4 in order to make the
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stable inert gas structure (13), (14), that is, if two electrons are associated with each CO group then five carbonyls will bring Fe to the Kr structure and four will bring Ni to the same. Here again carbonyls are not limited to Fe, Co, and Ni but Cr and Mn must also be considered. While these latter elements have not been studied sufficiently to determine conclusively their possible carhonyl compounds, Mo, a member of the Cr family, has been found to form the compound MO(CO)~ which again is nicely explained by coordination valences to have the inert gas structure. Such compounds as KzPtCle, Pt(NH&CL, &Fe(CN)e. Co(NH&Cls, and other diamagnetic compounds of the "V-shaped groups may be explained similarly (15). Of the many compounds formed by the "V-group" metals only the above types are diamagnetic; i. e., those which seem to have the inert gas structure and, since the inert gases are diamagnetic, a remote relationship between these elements probably exists.' In period 3 it will be noted that after A1 is passed the main group shifts from the left to the right side of the chart. Similarly in period 5 when we pass the A1 member Y, which is slightly more like La than Lu (explained later), we come to Zr which is less like Ce than Hf. Thus it seems that when a new group of elements appears it does so following the A1 member. For example, following Sc is the Ti-Ga subgroup; following Y is the Zr-In subgroup; and following La is the rare-earth group which apparently is just a secondary subgroup, for all the rest of the elements following the rare earths occupy positions similar to those of elements in period 5. In period 7 the appearance of another rare earth group does not present itself after the A1 member Ac is passed, and sincgthe elements Th, Pa, U classify themselves with Hf, Ta, W, it is assumed that not over eighteen elements can exist in period 7 and possibly less than this number. This assumption is also suggested from Figure 3. The rare-earth group, arranged practically as in Figure 1, is characterized by several peculiarities. Chemically speaking we readily classify La with Y and Ce with Zr, but decreasing similarity is again noted from left to right. Pr, which forms compounds with valences of 3, 4, and 5 (16) becomes a subgroup member of Cb. Nd has valences of 3 and 4, wbich are also common to Mo. It must be remembered here that the similarity is not great in every respect, but the same thing may be said about Cu and Na, or Mn and C1 in the former conventional grouping. I1 is thought to have valences of 3 and 4 wbich are also probably common to Ma, a member of the Mn family. Sm, with valences of 2 and 3 is somewhat like the group Ru, Rh, Pd which commonly possess these valences. Eu also has valences of 2 and 3 which unfortunately do not ordinarily appear with Ag but it will be recalled that a common valence of the Ag group is two for Cu and three for Au. Divalent Ag does probably exist, however, in the case of silver peroxide since its reactions do not warrant the HzOzstructure, and argentic
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nitrate, Ag(NO&, can be formed from it (17), (18). For Gd, only the valence of 3 has been reported but Spencer (19),after reviewing the properties of this element, concludes that the divalent form probably exists; thus, Gd might be expected to be remotely connected with the Cd group. Finally, the rest of the rare earths Tb, Dy, Ho, Er, Tm, Yb, Lu are grouped with In, a member of the first A1 subgroup. Starting with the right side of the chart, relations are apparent between Xe and Rn, I and (85), etc., up to Y which is connected to Lu but not quite as closely as to La, reasons for which will he brought out presently. Likewise Yb is associated with Sr, but with still less resemblance as can be observed by the decreasing slope of the lines from right to left. YbC1, has been prepared (20) and found to possess very nearly the same atomic volume as SrC12. With our present knowledge of Tm-compounds, however, no logical comparison can be drawn between Rb and Tm valence, although the oxides Rbz03and Tm201are known. In this classification, only fourteen elements are considered strictly as rare earths, namely, Ce to Lu, inclusive. La is considered the regular Al member and distinct from the other rare earths. This is supported by the fact that Sc, Y, and La do not regularly form isomorphous salts with the rest of the rare earths. These three elements are also characterized by their colorless salts, sparingly soluble double alkali sulfates (a distinction of Y from the so-called yttrium earths), single isotopes (21), lower paramagnetic susceptibilities, absence of absorption spectra in the visible, doublets in the spark spectra (ZZ), and bandgpectra of ScO, YO, and La0 (23). Also from the standpoint of basiaty Y should be followed by La more closely than by Lu because basicity increases downward for succeeding elements in a given column and, since the basicity of the rare earths decreases from left to right, Lu is less basic than Y, the value of the latter being between Nd and Sm (24), (25). Similar conclusions are indicated by the work of Rolla and Piccardi (26) on ionization potential data. The rare-earth group as a whole is characterized by isomorphons salt formation, such as Rz(S04)&H20, R ( B I O ~ ) ~ . ~ Hetc.; ~ O , by highly paramagnetic susceptibilities; and by highly colored ions. This characterization may be extended to the other "V-groups" of the elements-the group Cr, Mn, Fe, Co, Ni is highly paramagnetic compared to neighboring elements, the ions of these metals are colored, and isomorphism is known for such salts as RSOa.7Hz0, R(N0&.6HzO, R(NH&Cl2, etc. The same situation exists in the Mo and W "V-groups'' except that the magnetic susceptibility of these elements is somewhat lower, yet they are still strongly paramagnetic. As far as the author is aware the Cr "V-group" gives rise to all the ferromagnetic substances yet known. While the elements Fe, Co, Ni, as well as some of their compounds and alloys, areferromagnetic, only certain com-
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pounds and alloys of Cr and Mn show this remarkable property (27), (28). Mn fused in an atmosphere of hydrogen is ferromagnetic (29) but the process may involve the formation of the hydride. Mo and W are known to increase the magnetic hardness of ferromagnetic substances; thus the possibility of ferromagnetic properties existing in certain compounds and alloys of these "V-groups," as well as in the rare-earth group, is interesting. I t will now be of interest to point out a few more characteristics which support the "V-shaped" arrangement of the rare earths. J. D. Main Smith (30) points out that the rare-earth salts decrease in color from each end of the group down to Gd and Th. The tendency for molecular-complex formation decreases from La to Gd and again increases from Tb to Lu (31). Shelwood (32) notes that with increasing concentration of salt the absorption spectra of Nd, Pr, and Sm shift toward the red but only slightly for Sm. Gd shows no appreaable change while Ho and Er, on the other hand, shift the spectrum toward the blue. Yntema (33) shows that the number of lines in the visible absorption spectra of rare-earth salts goes through a minimum at Gd. Klemm, Meisel, and Vogel (34) divide the sesquisulfides into two series, La-Gd and Dy-Yb, on the basis of color and X-ray determinations. The first series also shows a regular change in molecular volume while the second is variable. Tb was not studied. In order to show relations between spectral multiplicity and chemical valence, Williams (35) divides the rare earths into two groups, La-Gd and Tb-Lu, each group being represented by an equati$n. The melting points of the anhydrous chlorides pass through a minimum at Tb (36), (37). James (38), in separating the rare earths, treated the chlorides with saturated Na2S04solution and obtained a precipitate (La-Gd) and a solution (TbLu) The solubility of the nitrates in nitric acid has a minimum at Gd (39). I t appears that the solubilities of the rare-earth salts have minima at Gd if the solvent is an aqueous solution of the anion acid or the alkalimetal salt of this acid. Solubilities of the isomorphous bromates in pure water, however, seem to have a minimum a t Eu (40),as do also the corresponding sulfates. But in general it is seen that the rare earths undergo a change in many of their properties at Gd and Tb-hence the arrangement in the table. Some of these properties may he extended to the other "Vgroups"; for example, the isomorphous sulfates, RS04.7H20, of the Cr group decrease in solubility on each side down to Fe (with the exception of CrSO4.7H2O)and also the color of the salts goes through a minimum a t Fe. To return to the general characteristics of the table, the heavy diagonal line on the right-hand side of the chart here again divides the metals from the non-metals. H is placed on the border line since its reactions and position in the electromotive series are characteristic of metals. Metals form compounds with complex anions, such as sulfates or nitrates, while
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non-metals do not. In this respect H is like the metals but its appearance and unusually high critical potential make it resemble a non-metal. Another test is that metals form bases while non-metals form acids. Here again we naturally think of H as acidic but it will be recalled that it is also an essential element in bases. And, finally, if we should consider a nonaqueous system such as SO2,in which H is not a constituent of the solvent, compounds of H would probably be salts rather than either acids or bases. Since the border-line elements are practically neutral from the standpoint of basicity, the slope of this diagonal line should serve qualitatively as a line of zero change in basic properties. That is, in considering any one family of elements a line between any two periods drawn from an element in the upper period parallel to the diagonal line shows whether succeeding members of the family should be expected to be more or less basic than the element under consideration, depending on whether the slope of the line connecting the two elements is more or less than the slope of the diagonal line. For example, such a line extended from Na clearly shows that Cu is less basic and K more basic than Na. Similarly Zn is seen to be less basic and Ca more basic than Mg. But Ga is slightly more basic and Sc decidedly more so than Al. Likewise As is markedly more basic than P, while V is still more so than As, etc. Succeeding elements in any family in period 3 are more basic than corresponding members in period 2. Also elements of period 5 exhibit a higher basicity than the corresponding elements of period 4. In the family Cn, Ag, Au a similar relation shows that Ag is more basic than Cu while Au is less so than Ag. Hg also is less basic than Cd while TI, Pb, and Bi are more basic than 18, Sn, and Sb. Likewise Y lies between La and Lu in basicity. The parallelism which exists between basicity and ionization potentials is pointed out by Rolla and Piccardi in the article already cited I t is interesting to note that these border-line elements (other than H) have nearly the same ionization potentials and, with the exception of Hg, the elements which have ionization potentials higher than 10 volts are non-metals while the others are metals. The other heavy diagonal line separates the paramagnetic elements on the left from the diamagnetic elements on the right. The exceptions, as obtained from the International Critical Tables are, on the left Zr, Sr, and Cs which are listed as diamagnetic, and on the right 0 2 which is paramagnetic. However, the oxygen atom in most compounds is diamagnetic (42). As the "V-groups" are strongly paramagnetic, the chart seems to divide the elements magnetically into three sections-the left, which is paramagnetic, the right, which is diamagnetic, and the "V-groups" which are ferro- or strongly paramagnetic. The salts of the ordinary paramagnetic elements are diamagnetic while those of the "V-groups" are strongly paramagnetic except where the structure is indicated to be that of the inert gases, as discussed previously.
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Another property in connection with this same diagonal line is that of density. The elements increase in density toward the line from each end of the period with hut few exceptions, which are Be, N, Pd, Ir, and Pt. Little is known about the densities of the rare-earth metals but their isomorphous sulfates, nitrates, and chlorides (43) increase in density throughout the group; therefore, it is assumed that the metals themselves will not present many exceptions to the rule. Furthermore, the dotted lines between periods 4 and 5 show the customarysuhgrouping. In addition, the group of metals Cr, Mn, and Fe is linked with Te. It will be recalled that these metals are quite similar in their di-, tri-, and hexa-valences and the latter valence is quite similar to the miximum valence of the Te group; i. e., the chromates,manganates, and ferrates resemble the sulfates, selenates, and tellurates. The shape of the chart as a whole is supported by certain 1 2 3 4 5 of which density is prohSquare Root of Density. ably the most interestFIGURE 3 ing. In Figure 3 the square root of the density of left-hand members of the chart is plotted against period numbers. The lines are approximately parallel between periods 2 and 3, and 4 and 5, while they diverge between periods 5 and 6 to give room for the rare earths and then converge again to period 7, a fact which makes it seem improbable that this last period also contains thirtytwo elements. This conclusion is further supported by ionization potential
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data. The dotted lines between periods 3 and 4 represent reversed slope since the densities of elements in period 3 beyond A1 likewise reverse. This, however, does not alter the slope of any of the continuous lines. In order to arrange the right-hand elements of the chart according to atomic number their densities are plotted in reverse order in Figure 4 since in this case the densities decrease successively with increasing atomic number; but the slope of each line with which we are mainly interested is natural. The effected decreasing similarity from left to right is apparent between elements of periods 3 and 4 and parallelism again exists between periods 2 and 3, and 4 and 5, the slope of the latter, however, being the greatest. The elements of period 6 crowd together as a result of the rare-earth group a t the other end of the period. The outline of Figure 2 is obtained by plotting densitiesof alkali metals and inert gases against their corresponding periods. Such a plot indicates that hydrogen is more like the alkali metalsthanlikethe halogens. This is true in the majority of cases. However, an important connection between hydrogen and the halogens Square Rcut of Density. is brought out clearly FIGURE 4 by Bardwell (&), who shows that hydrogen in the hydrides of Ca, Li, and K is quite similar to the halogens in the corresponding salts of these metals. This relation is expected from atomic strncture considerations since hydrogen should gain an extra electron almost as easily as it can lose the one it normally possesses. Many physical properties show relationships similar to those represented in Figure 3 and Figure 4, especially melting points and ionizdtion potentials. But in any case, i t is apparent that the physical relationships of various groups are not straight-line functions of the periods (see Figures 3 and 4), a fact which lends support to the arrangement presented in Figure 2. Al-
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though remote relationships might be shown between almost any two elements, the arrangement of the rare-earth group here presented affords a means of connecting these elements with the rest of the periodic system, as well as showing a division in their physical properties. They stand out b y themselves in the "V-group" as do the other smaller "V-groups," and all these groups have many characteristics in common. I n conclusion i t may be added that this scheme will accommodate the radioactive series as well as the isotopes of the elements, provided atomic weights of elements are plotted in the third dimension.
Literature Cited STEINMBTZ, C. P.,J. Am. Chem. Soc., 40, 733 (1918). PIWTTI, A. P., Gaza. chim. ital., 55, 746 (1925). HARKINS, 1. Am. Chem. Soc., 38, 169 (1916).
MONROE AND TURNER, J. CHEM. EDUC.,3, 1058 (1926). COURTLINE, M., ibid., 2, 107 (1925). SCHALTENBRAND, G., Z.Unorg. allgm. C h . , 112, 221 (1920). FRIEND,N., Chem. News, 130,196 (1924). VON ANTROPOPP, A,, Z. aflgm. Chem., 39, 722 (1926). MAIN SMITH,J. D., J. C k m . Soc.. 1927,2029. DAWN, W., Z. pkysik. chem. Unterricht, 40, 106 (1927). GORALIVICH, J. Russ. Pkys.-Chem. Soc.. 62, 1165 (1930). LANSING, Pkys. Rev., 29, 216 (1927). BLANCRARD AND GILLILAND, 1. Am. Chem. Soc., 48, 879 (1926). LOWRY, T., Chem. & I d . 42, 316 (1923). SIDGWICK,Trans. Chem. Soc., 123, 725 (1923). Roscoe m~ SCBORLEMMRR, "Treatise on-hemistry," Vol. 11, Macmillan & Co.. Ltd., London, 1911, p. 797. MELLOR. ''Treatise on Inorganicand Theoretical Chemistry." Vol. 111, Longman's Green & Co., New York City, 1922, p. 384. BARBIERI, Atli. accad. Lincei, 13, 882 (1931); Chem. Absb., 26, 663 (1932). SPENCER, J. Am. Chem. Soc., 50, 264 (1928). KLEMM, W., Z. enorg. allgem. Chem., 184,345 (1929); 187, 29 (1930). ASTON,Nature, 113, 856 (1924). HICKS,Phil. Trans., 212A, 58 (1912). MEGGERS AND WHEELER, Bur. Stand. J. Research, 6, 239 (1931). KREMERSAND QUILLS. Trans. Am. Electrochem. Soc., 55, 199 (1929). GUNTHER, KOTOWSKI, AND L E ~Z., anorg. allgm. Chem., 200, 287 (1931). ROLLA AND PICCARDS Phil. Mag. [71, 7 , 286 (1929). Duswmrr, Gen. Elec. Rm.,19, 826 (1916). POTTER,Phil. Mag., 12, 255 (1931). "International Critical Tables," Vol. 1, p. 408. MAINSMITH,J. D., Natuve. 120, 583 (1927). MELLOE,"Treatise on Inorganic and Theoretical Chemistry" [see ref. (17)l. Vol. 5, p. 617. SHELWOOD, J. Am. Chem. Soc.. 52, 4308 (1930). YNTEMA, ibid., 48, 1598 (1926). KLEMM, MEISEL.AND VOGEL, Z. anorg. allgem. Chem., 190, 123 (1930). WnLuMs, Compt. rend., 189. 1075 (1929).
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AND ALBER,Z. anorg. allgem. Chem., 185, 49 (1929). HOFEMANN "International Critical Tables." Vol. 1, B-Table. JAMES,I.Am. Chem. Soc., 34, 757 (1912). MELLOR,"Treatise on Inorganic and Theoretical Chemistry" [see ref. (17)], Vol. 5, p. 559. J n m s , I. Am. Chem. Soc., 36, 2060 (1914). JACKSON AND REINACEBR, I. Chem. Soc., 1930, 1687. Ann. chim. phys. [XI, 25, 329 (1912). PASCAL, HEVESY,Z. (11107g.allgem. Chem., 147, 217 (1925). I. Am. Chem. Soc., 44, 2499 (1922). BARDWELL,
PSYCHOLOGICAL ASSAYS No one would think of operating a mine unless the ore being excavakd were assayed frequently and carefully. At the present time our civilization is being conducted without evaluations of its human raw material. Millions of men are idle through an economic clogging of the wheels of industry. This is a severe indictment of the world we live in. Millions of people are discouraged through their inability to find work. Enforced idleness causes them to lose confidence in themselves. At such times as this assaying of human abilities is sorely needed. In addition to that essential minimum of food and shelter which a stable civilization must afford its human units, there should be given each individual the opportunity of discovering how he can serve the world most effectively. On an experimental scale a t Minneapolis, St. Paul, Duluth, Philadelphia, and Rochester, unemployed persons are being tested and evaluated by psychological methods in order that they may resume most effectively their productive places in the producing world when opportunity arises. When American armies were entering the World War psychologists went $to uniform and applied the famous Army "alpha" and "beta" tests to help place soldiers in the most effectiveplaces. The man with the outstanding military bearing was not always the best commander and the doughboy of high intelligence might be wasted digging ditches. So in this economic emergency, psychologists can perform an even greater service I t is also more difficult. Flags do not wave, bands do not play, crowds are more likely to growl than cheer. It is easier t o find people for jobs than johs for people. One part of the task is t o discover the kind of abilities that are necessary far various kinds of johs. Tests for general intelligence, special abilities, personality, and other human attributes, given to successful workers, allow the psychologists to write specifications for various kinds of workers. The person out of a job is put through an assaying process, not as a piece of human clay, but as a human being. Psychological tests reveal his abilities and shortcomings. How he spends his money and his time is discussed. His physical health is evaluated by a medical examination and his mental health is considered when that is necessary. The staff of experts individually considers each person and works out a recommended plan for his future endeavor. Sometimes he is advised t o change his occupation, often he needs intensive re-training. The unemployed person emerges from the assay with hope and greater prospects. Assaying human beings costs money and money is not easy t o find. The cost is about $10 to $15 per person. But individual diagnosis and training for all the unemployed would undoubtedly be a paying addition t o the mechanisms that will need t o be put into operation t o assure every one of the opportunity to work and live in this era of too much material goods and too few jobs.-WATSON DAVIS, Managing Edilor.
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