Oswald U. Anders The Dow Chemical Co. Midland, Michigan
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The Place of Isotopes in the Periodic Table The 50th anniversary o f the Fajans-Soddy displacement laws
The 22nd of May 1964 marked the 50th anniversary of the final act of the dramatic events surrounding the clarification of the place of the radioelements in the periodic table (7). Upon this foundation has been built our nuclear physics and a much more complete understanding of chemistry. The final piece of evidence was supplied by American experimental science, when Theodore W. Richards and Max E. Lembert, after their careful investigation of the atomic weights of lead samples obtained from different minerals, proved without doubt, that different samples can have different atomic weights, although they appear to be identical in all other respects (I, %).
the Harvard techniques and testing whether the atomic weights of lead samples from different sources do vary. This was to verify once and for all the theory proposed by Fajans a t the meeting of the Karlsruhe Chemical Society on January 10, 1913 (j),and within a month later by Frederick Soddy (6) that the stable end products of the three radioactive series and common lead are practically inseparable from each other, occupy the same place in the periodic table but have different atofnic weights. After careful purification of the lead samples obtained from various sources and duplicating the results of previous atomic weight determinations of ordinary lead by Baxter and Wilson to prove their technique, Richards and Lembert determined the atomic weights of lead from samples stemming from various radioactive minerals. "The result is amazing," they write (I) in the report of their results. They had found atomic weights differing from that of ordinary lead by as much as 0.75 atomic weight units, a value many times their experimental errors. They purified and repurified the samples, checked them spectrographically, but found the samples to be as pure lead as any that was lcnnwn. They had to admit: This amazing outcome is contrary to Harvard experience with several other elements notably copper, silver, iron, sodium and
chlorine, each of which seem to give a constant atomic weight, no matter what the geographic source might have been ( I ) .
Theodore W. Richards of Harvard University had devoted much of his lifetime research to the question, "Are the supposed constant magnitudes to be measured in chemistry really variable?" Believing that "every variation must have a cause and that cause must be one of profound effect throughout the physical universe" (3, 4), Richards and his co-workers devoted their efforts of many yean to the determination of the atomic weights with ever increasing accuracy. For many years the possibility that samples of a given element from different sources might have different atomic weights had been considered and investigated, but never before (1914) with a positive outcome (I).
To this great American scientist and his world famous school a t the Wolcott Gibbs Memorial Laboratory a t Harvard came Max E. Lembert in the fall of 1913. He was sent to Richards by Xasimir Fajans, then 26 years old and Privatdozent a t the Technische Hochschule Karlsruhe, Germany, with the expressed task of learning 522
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Their result was first announced by Dr. Fajans on May 22, 1914 (7) a t the meeting of the Bunsen Gesellschaft in Leipzig, Germany. I t immediately stimulated further search for lead samples differing even more in ato~nicweights. The lowest values down to 206.05 were found almost immediately by Hijnigschmid and Horovitz in samples from crysta1he uranium minerals (8) and the highest value 207.90, a. few years later for the lead from a thorite mineral (9). The availahility of weighable quantities of different isotopic species made it possible to test experimentally the equivalence of their physical and chemical properties. In an early theoretical study Fajans (10) showed that these properties cannot be identical, but that larger differences in atomic weights than then known would he necessary to demonstrate isotope effects not directly dependent on mass. It might he worthwhile on this occasion to go back and look a t the events which led t o this dramatic conclusion. The Periodic Table
There existed two bodies of facts which appeared seemingly contradictory. On the one hand was the periodic table set up in 1869 by Dimitri I. Mendeleev.
In their studies it became apparent that some of the newlv -- " discovered materials had similar. if not identical. chemical properties and could not be separated from each other chemically (50, 51). Their characteristic radioactive behavior indicated, however, that they are distinguishable rn d i e r e n t substances. Von Hevesy found in 1912, after extensive and frustrating attempts to separate radiolead (RaD) from ordinary lead, that this task is impossible by ordinary chemical means (32). As the three radioactive decay series, starting with uranium, thorium, and actinium, were evolving in more and more detail, certain generalizations were proposed to correlate the type of radioactive decay and position of the parent substance in the periodic table with the chemical behavior of the daughter product. Such rules were first proposed by Von Lerch (35) and Lucas (54), as well as by Soddy (35). These rules, however, appeared to be not universally applicable and seemed thus only partially true. This was recognized by Fajans (36) and Hevesy (37) in 1912. In "The Chemistry of the Radioelements" of 1911, Soddy (85) writes: . . . The disintegration series affords a. most remarkable picture ~
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of the actual process of the production of elements from one another, of which the Periodic Law is, as it were, the consequence. Just as from an instantaneous photograph of 8, water-fall the movement of the apparently motionless water can be inferred, so from the Periodic Law the continuous transformation of the apparently unchangeable elements has been suspected . . In three separate instances we are tracing the successive transit of matter from group to group of the Periodic Table . . The loss of a helium atom or alpha-pmtiele appears to cause the change of the element, not into the next family hut into the next hut one . . the step being from the family of even valency into the next, the familv of odd valencv beine missed . In this connection it should be notad which in its or. that the bhium Drodueed bv these chanees. ,. d i n ~ r ystare appwm nmvxlmn~like argon, wrrirs, in the form of ive the radiant alpha-pbrtiele, two ntwnir d l d l ' g ~ 301 ~ ~ o s ~ ldwtririty, and ia therefore electrachemicdly divalent. The passage from group I1 (radium) to the zero group (emanation) is direct,
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transition appears ibbrupt ind no indication is afforded to intermediate connecting links. The "active deposits" thus represent a new region in the constitution of matter, of the existence of which there has so far been no evidence. This region bridges the two ends of the Table. The atom having suffered successive reduction of its valency to eero, passes to the electronegative end of the preceding period through a well-defined sequence of short-lived forms. According to v. Lerch's the process is accompanied by a regular increase in the electronegative character, the successive products being electrochemically "nobler" than the kwt.
Displacement Laws
It appears that a t a certain stage in the development of a problem a clarification becomes possible and forces itself upon science. It happens then that several investigators conceive the same idea almost simultaneously. One may require a few less facts to gain the insight than another who collects soole additional ex~erimental data. I n 1912 enough was known about the radioactive decay series that the as yet unsolved mystery underlying the relationship of the radioelements to the periodic table severely bothered the scientists working in the field. Among them was Dr. Kasimir Fajans, presently professor emeritus of the University of Michigan a t Ann Arbor. After having spent the academic year 1910-11 a t Rutherford's Laboratory in Manchester, he assumed an instructorship a t the Technische Hoch524
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schule in Karlsruhe, Germany. There on November 23, 1912, he enjoyed a performance of Tristan and Isolde at the State Opera. Nevertheless, he recalls, his mind began to wander over the various facts connected with the radioactive transformations and the electrochemical behavior of the radioelements. Suddenly things seemed to fall into plaoe. He understood why the van Lerch-Lucas rule is about as often correct as incorrect: In d l cases of beta-disinternation the resultine oroduct is indeed electrochemically morenegative than its pirent. Upon alpha-emission, however, the directly opposite always holds (58).
A few weeks later Fajans proposed this insight for the first time in his Habilitationsschrift dated December 1912. On December 31, 1912, he submitted in two papers the coinplete hypothesis of his "displacement laws" and their consequences for publication in Physikalische Zeitschrift (39). He writes them as follows: 1. Alpha particle emission is accompanied by a transition from right to left in a horizontal roon of the periodic table. The observation by Soddy that this proceeds by a jump to the neat but one group is assumed to be true in every cam. 2. In sinlilsr fashion it is derived for beta-disintegrations, that they causes transition to thenext higher group, i.e., from left t n right in a horizontal row.
In the first paper the two generalizations are shown to apply to all cases in which they could be tested electrochemically and are used to elucidate some not sufficiently clear examples. In the second paper Fajaus was able to arrange all known radioactive elements into the periodic table and assign to them the expected atomic weights. He was able to do this by strict application of the two displacement laws and the facts, found by Ramsay and Soddy (40) and Rutherford ( d l ) , that alpha particles are actually helium ions with an atomic weight 4, while the mass of a beta particle is. negligible. Resulting from this arrangement is the fact that several radioelements of different orisin, different radioactive properties, and thus different atomic weights must share the same place in the periodic table and form what Fajans called a little later, a "pleiad." Mthough the chemical nature of most members of the decay series which follow the emanations were still unknown, placing them with Po, Bi, Pb, and T1 led him to such a beautiful and consistent system that he was encouraged to predict the positions and chemical properties of yet-to-be-found radioelements as well as the stable end products of the radioactive series. He thus overcaoie an apparent lack of insight which had misled other workers in the field, and found the connection between the periods of Mendeleev's table a t the rare gases, which form both the beginning of a period and the end of the previous one. Thus there need be no gap between the periods of the table, as others had assumed, and no "new region in the constitution of matter" vas represented by the radioelements. The break in the continuity a t the rare gases was merely an artifact in writing the table. The speed of publication possible at that time and the concern of the scientific world with these developments may be illustrated by the following examples: Before Fajans' papers (5,39) appeared in print (Feh. 4 and 15, 1913) an article was published (Jan. 31, 1913)t
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in Chemical News (42) by A. S. Russell discussing the place of the radioelements in the periodic table and also giving some as-yet-unpublished data on the chemical properties of radioelements by Alexander Fleck. While the reported data agreed with the theories of both Russell and Fajans, these men differed in their predictions. Fleck, who a t Soddy's laboratory in Glasgow had studied the chemical behavior of a considerable number of radioelements and had reported some of his results on Jan. 22 (31)and Feb. 7 (45), reported his new results himself in a note submitted to Chemical News on February 13 (44). He mentioned in it Russell's paper (@) as well as the disagreement of a new experiment which he carried out to verify Russell's assignment of ThD (now ThC") to lead. He was apparently not yet acquainted with Fajans' theory assigning ThD to thallium. Soddy in turn interpreting Fleck's results a few days later (6), came to the same conclusions as Fajans, whose paper he mentioned. He submitted his findings to Chemical News on Feb. 18, 1913, and coined for elements which share the same place in the periodic table the very appropriate name "isotopic elements." Isolation of "Brevium" (Protactinium)
Fajans immediately went to work to support the new theories experimentally and searched for the postulated missing l i i in the radioactive decay series. From his displacement rules he was able to predict their electrochemical behavior. The first place where the new theory could be proved, was in the decay of uranium-X. Uranium-I was known to decay by alpha emission to uranium-X, which Fajans placed with thorium. Uranium-X, in turn, decayed by beta emission into uranium-11. The displacement law postulated a double beta emission with an intermediate radioactive element whose place should be in Group V. From the correlation in the periodic table he predicted for this radioelement, in analogy to tantalum, that it should be separable electrochemically from uranium-X by plating it out on lead. In a series of ingenious experiments carried out together with his student Oswald H. Gohring, uranium-Xz, the first known isotope of the new element, was discovered as a 1.15-min half life beta emitter by plating it out from uranium-X solutions on freshlv cleaned lead dishes (45). These experiments proved simultaneously the validity of the displacement laws, the usefulness of the electrochemical generalizations, and the existence of a new element belonging to a previously unassigned place in the periodic table. The further chemical identification of the new activity fit the characteristics to be expected for this place in the table. Realizing that they had discovered a new element Fajans and Gohring called it "brevium," because of its short life-time. This work was overshadowed by the later important discovery of the parent of actinium by Hahn and Meitner (46). Although the concept "element" should now be applied to the entire pleiad and not to the individual isotopes making up the element, the name of element 91 was adopted from that of its longest lived isotope, protactinium, rather than the first known, characterized "brevium." A further corollary of the new theories was the different atomic weights of the stable end-products of
the three radioactive decay serias, which Fajans had placed together in the proposed pleiad of lead. In order to test this assumption, atomic weight determinations artre intended an lead and bismuth samples which will be obtained from uranium minerals free of thorium and thorium minerds free of uranium (39).
They should yield diierent results. It was with this in mind that Fajans sent Max Lembert to Richards in the fall of 1913. He was able to see his theories verified by their "amazing" results, less than a year later (47). Acknowledgment
The author thanks Prof. K. Faians for discussions and suggestions clarifying the hist&ieal developments as well as for providing the illustrations. Literature Cited (1) RICHARDS, T. W., AND LEMBERT, M. E., J. Am. Chem. Soc., 36, 1329 (July 1914). M. E., Sciaee, 39, 831 (2) RICUDS, T. W., AND LEMBERT, (June 1914). T. W., Sciace, 26, 562 (1907). (3) RICHARDS, T. W., Die Umschau, 13, 542 (19W). (4) RICHARDS, (5) FUANS,K., Chemiker-Ztg., 37, 151 (Feb. 4, 1913). (6) Sonny, F., C h a . News, 107.97 (Feb. 28, 1913). (7) FNANS,K., Z. ElektrodLem., 20, 319, 449 (1914). 0.AND Honovr~e,S., Ber. 123, (11%)2407 (8) Hb~~ascwrno, (191.1L \----,
(9) H ~ N I G S CO., ~ DZ., Elektrochemie 25, 91 (1919). (10) FAJANS,K., "Elster-Geitel Festschrift," Vieweg & S o h , Braunschweig, 1915, pp. 623-43. H., Compt. mtd., 122, 420 (1896). (11) BECQUEREL, (12) BECQUEREL, H., Compt. r d . , 123, 855 (1896). H., Compt. rend., 124, 438, 800 (1897). (13) BECQUEREL, E., Phil. Mag., 47, 109 (1899). (14) RUTHERFORD, (15) RUTHERFORD, E., Phil. Mag., 5, 95 (1903). (16) SCHMIDT, G. C., Ann. Physik, 65, 141 (1898). (17) CURIE,MARYAS., Compt. end., 126, 1101 (1898). (18) CURIE,P., AND CURIE,M. S., Compt. rend., 127,175 (1898). G., Compt. rend., (19) CURIE, P., C ~ EM., S., AND BEMONT, 127, 1215 (1898). rend., 127, 1218 (1898); 129, 716 (20) D E M ~ ~ ~E., A Compt. Y, (1899). (21) D~BIERNE, A,, Compt. rend., 129, 593 (1899). (22) ELSTER,J., AND GEITEL,H., Ann. Physik, 69, 87 (1899). E., AND OWENS,R. B., Trans. Roy. Soe. Can(23) RUTHERFORD, ada, S3, 2, 9 (1899). W., PTOC. Roy. Soe., 66, 409 (1900). (24) CROOKES, E., AND SODDY, F., Phil. Mag., 4, 370, 569 (25) RUTHERFORD, . Soe., 81, 321, 837 (1902). (1902); T ~ a n sChem. E., AND SODDY, F., Phil. Mag., 5,561 (1903). (26) RUTHERFORD, E., Phil. Mag., 49, l(1900). (27) RUTHERFORD, E., AND SODDY, F., Phil. Mag., 5,576 (1903). (28) RUTHERFORD, (29) PEERIN,J., Rev. Sei., April 13 (1901). (30) Sonny, F., J. Chem. Soe., 99,72 (1911). (31) FLECK,A., J. Chem. Soe., 103, 381 (Jm. 22, 1913). G. v., Maalsh. Chem., 34, 1393 (32) PANETH, F., AND HEVESY, (1913). (33) LEECH,F. v., Ann. Physik, 20, 345 (1906). (34) L u c ~ s R., , Phys. Z., 7, 340 (1906). (35) SODDY, F., "The Chemistry of the Radioelements," Green Co.. London. 1911. and ~~-~~ FUANS,K., Le ~ a d i u m9,239 , (1912). HBYESY, G. Y., Phw. Z., 13, 672 (1912). FAIANS,K., Vwhandl. Na1urhist.-Med. Vwein, Heidelberg, 12, 173 (Dec. 1912). FAJANS,K., Phys. Z., 14, 131, 136 (Feb. 15, 1913). W., AND SODDY, F., Nature, 68, 246 (1903). RAMSAY, RUTEERFORD, E., Phil. Mag., 12, 348 (1906). RUSSELL, A. S., Chem. News, 107, 49 (Jan. 31, 1913). FLECK,A,, C h a . News, 107, 68 (Feb. 7, 1913). FLECK,A,, Chem. News, 107.95 (Feb. 21, 1913). , (1913); FAJANS,K., AND GBBRING,O., N ~ u ~ s1,. 339 Phys. Z . , 14, 877 (1913). L., Phys. Z . , 19,208 (1918). HAEN,O., AND MEITNER, K., Phys. Z., 15, 935 (1914). FAJANS, ~
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