Lawrence Berkeley Laboratory UNIVERSITY of CALIFORNIA

LBL-9074 :!"4 Preprint Lawrence Berkeley Laboratory UNIVERSITY OF CALIFORNIA

Published in Chemical and Engineering News, Vol. 57, pp. 46-52. April 16, 1979

THE PERIODIC TABLE TORTUOUS PATH TO MAN-MADE ELEMENTS

Glenn T. Seaborg rhE Y L.OATQ!Y AUG 2E 1979 April 1979 L1BR/RY 1.ND DOCUMENTS SEc.iN

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THE PERIODIC TABLE Tortuous path to man-made elements

Glenn T. Seaborg

Reprinted from CHEMICAL & ENGINEERING NEWS, Vol. 57, April 16, 1979, pp. 46-52 Copyright 9 1979 by the American Chemical Society and reprinted by permission of the copyright owner Priestley Medal Address

THE PERIODIC TABLE Tortuous path to man-made elements

Glenn T. Seaborg, University of California, Berkeley

This article is based on the Priestley Medal Ad- or simply to the fact that certain elements had not yet dress presented by Glenn T. Seaborg of the Uni- been discovered. Indeed, Mendeleev's claim to priority in the discovery of the periodic system was not completely versity of California, Berkeley, on April 2 during accepted until his predictions of missing elements were the ACS/CSJ Chemical Congress in Honolulu. proved by experimental evidence in later years. Sea borg, winner of the Nobel Prize for Chemistry The periodic table Mendeleev published in 1871, which (with E. M. McMillan) in 1951 and president of incorporates improvements made in the original version ACS in 1976, received the medal, ACS's highest of 1869, predicts the existence and properties of the elements with atomic weights of 44, 68, and 72. These cor- award, in recognition of his distinguished services respond to scandium, gallium, and germanium, as we to chemistry. know them. These three elements were actually found in nature during the period from 1875 to 1886. Many other experimental proofs of Mendeleev's "law" were made in the years that followed. The periodic table of the chemical elements has been As time progressed, adjustments had to be made to the the key to the discovery of many elements since its for- periodic table to accommodate the rapidly expanding mulation as a guiding principle almost exactly 110 years knowledge of the properties of the elements and their ago. Occasionally, when improperly used, it has misled atomic and nuclear structures. Also, additional elements investigators into temporary excursions along erroneous were discovered during the late 19th century and the first routes to new elements. However, even these tortuous part of the 20th century which required some recon- paths eventually have led to the correct destination. I struction of the Mendeleev periodic system. The most shall describe the part that the periodic table has played significant changes were the addition of another vertical in the discovery of the man-made elements, especially the row, or group, of elements now known as the noble gases, transuranium elements, and its possible future role. and the substitution of a series of elements—the rare The story begins, of course, on March 6, 1869, when earths, or lanthanides—in the place of a single element Dmitri Ivanovich Mendeleev and his associate, Nikolai (placed between barium and hafnium). Menshutkin, presented a paper to the Russian Chemical By the end of the first decade of the 20th century the Society in St. Petersburg (now Leningrad) which postu- total number of elements had increased to 85, and soon lated that the elements showed a periodicity in their thereafter the concept of atomic number, as the funda- chemical properties when they were arranged in the order mental basis for the ordering of the elements in the pe- of their atomic weights. This in itself was not novel. riodic table, was established. During the next 25 years, Several chemists in other countries had observed some three more elements were discovered, leaving below kind of orderliness in the elements then known, the most uranium (element 92, the heaviest element) those having prominent being the German Johan W. Dobereiner and atomic number 43, 61, 85, and 87 as the missing elements. his triads (1829), the Frenchman A. E. Béguyer de Even these properly empty places in the periodic table Chancourtois and his "telluric screw" (1862), and the were filled under names such as masurium for element Englishman John A. R. Newlands and his "law of oc- 43, illinium for element 61, alabamine for element 85, and taves" (1864). virginium for element 87. These "discoveries," however, It is not generally known that some American chemists were erroneous. The state of the understanding of the of the 19th century also proposed various forms of peri- atomic nucleus was such in the 1930's that it could be odic classifications, including Oliver W. Gibbs in 1845 and shown that the missing elements were all radioactive, Josiah P. Cooke Jr. in 1854. The only real challenge to the with such short half-lives that their existence in appre- generally accepted validity of Mendeleev's originality, ciable concentration on earth was not possible. however, has come from the work of Lothar Meyer in The periodic table, as it looked before World War II Germany, who in 1870 produced independently a gen- when scientists first tried to produce elements beyond eralization almost identical to that of Mendeleev. uranium, includes elements 43, technetium (Tc); 61, The reason for the general acceptance of Mendeleev's promethium (Pm); 85, astatine (At); and 87, francium pre-eminence is straightforward: Not only did he show (Fr), although they were actually given their names later, that periodicity existed in the properties of the elements and some were first synthesized or discovered at a later then known, but he had the courage and the vision to date. state that his method of classification constituted a Thoughts on the position in the periodic table of the fundamental law of nature, and that where there ap- heaviest elements varied considerably before the final peared to be deficiencies in his periodic table, they were recognition that another series of elements, resulting from due to gross errors in the measurement of atomic weights the addition of electrons to an inner shell (5!), should

46 C&EN April 16, 1979 I'pynna 1 1'pynna II I'pynria III Fpynn.i IV [pynila V Fpyiiva VI rpynna Vii

TRnM'IoeKue 0=Ifi F=l9 aJleMeHTL.i L17 Be9,4 B=ll C=12 N=l4

PA.Z 1 Na=23 \Ig==24 Al=27,3 Si=2. )'=31 S=32 CI=35,5 Fe=56, to=59, - 241 K=39 Ca==40 —=11 Ti=.14.' V=)l Cr=2 'NI I I = 55 Ni=59, Cu=6

- 3 (Cu63) Zn=r65 —=6- —=72 .s=75 Se=7S I.Ir=80 - 4-u Rb=85 Sr=7 (?), t='') Z1=90 NIi=91 y10=96 —=l() Hiu= 104, Rh=104 Pd= 104, Ag= UP

- 5-il (Ag=I0) Cd==112 In==113 Sri = 118 Sb=122 Tc=128? J 1 27

- 6-9 Cs 13.,1 Ba=137 ---137 Ce=136? - - - - - - - - - U - 8-fi Ta= I2 V = I$4 - Os=199', Jr= 19 Pt=197?,Au=Ri7

- 94 (Au = 197) Hg=200 TI=204 Pb=207 Bi=20S - - — 104 Th=232 - Ur= 240 -

R20b LIMeruaR coiu 1120 11 202 110 H?W 11205 B206 RO HOl uan OlCUch itiu HO ii.-in 1102 11.1H H03

Bu.jenuee 8o210- (Rli') RH 4 RH 3 RH 2 RH - pojwue coe)u- Heue Mendeleev's periodic fable of 1871 predicted three (hen-unknown elements occur somewhere in the heavy-element region. This new The synthesis and identification (i.e., discovery) of an family of elements would be similar to the 14-member element with atomic number higher than 92, neptunium, rare-earth or lanthanide (chemically similar to lantha- at the University of California, Berkeley, came in 1940 num) series of elements which results from the addition as a result of the work of Edwin M. McMillan and Philip of inner 4f electrons. H. Abelson. This was followed shortly by the discovery Even until World War II, however, the three heaviest of plutonium by McMillan, Joseph W. Kennedy, Arthur known elements—thorium, protactinium, and ura- C. Wahi, and me in late 1940, also at the University of nium—were believed to be related to hafnium, tantalum, California, Berkeley. The tracer chemical experiments and tungsten, respectively. The next element—number with neptunium (atomic number 93) and plutonium 93—was thus expected to have chemical properties re- (atomic number 94) showed that their chemical proper- sembling those of rhenium. Similarly, elements 94 to 100 ties were much like those of uranium and not at all like were expected to fit neatly into the periodic table. those of rhenium and osmium! The pre-World War II The first attempts to produce elements beyond ura- periodic table had misled Fermi and Hahn and their co- nium were made by Enrico Fermi, Emilio G. Segré, and workers, but the ultimate result was the monumental coworkers, who bombarded uranium with neutrons in discovery of nuclear fission. Italy in 1934. They actually found a number of interesting For a few years following this, uranium, neptunium, radioactive products. The radioactive products of the and plutonium were considered to be sort of "cousins" neutron bombardment of uranium were the object of in the periodic table, but the family relationship was not chemical investigations during the following years by clear. It was thought that elements 95 and 96 should be Otto Hahn, Lise Meitner, and Fritz S. Strassmann, in much like them in their chemical properties. Thus it was Germany, and by numerous other scientists. On the basis thought that these and the following elements formed a of incomplete tracer studies, some of these activities "uranide" (chemically similar to uranium) group. seemed to have chemical properties such as would be The periodic table of 1944 therefore implied that the expected for "transuranium" elements with an atomic chemical properties of elements 95 and 96 should be very number such as 94 or 96—properties similar to those of much like those of neptunium and plutonium. These elements such as osmium and platinum listed directly assumptions proved to be wrong, and the experiments above elements 94 and 96 in the periodic table of that directed toward the discovery of elements 95 and 96 on time. this basis failed. Again, the undiscovered elements 95 and Subsequent work—especially the discovery of nuclear 96 apparently refused to fit the pattern indicated by the fission by Hahn and Strassmann and their coworkers late periodic table of 1944. in 1938—showed that this interpretation was not correct. Then, in 1944, I conceived the idea that perhaps all the This subsequent work revealed that these products of known elements heavier than actinium were misplaced uranium bombardments with neutrons actually were on the periodic table. The theory advanced was that these radioactive isotopes of lighter elements and thus were elements heavier than actinium might constitute a second fission product elements such as barium, lanthanum, series similar to the series of "rare-earth" or "lanthanide" iodine, tellurium, or molybdenum. elements. The lanthanides are chethically very similar to

April 16, 1979 C&EN 47 How the periodic table has evolved during past 40 years

I 2 H H.

3 4 $ - 6 7 8 9 10 Li Be BC N 0 F No

Il 62 13 14 15 16 67 68 Na Mg Al Si P S Cl Ar

19 20 26 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Go As S. Or Kr

37 38 39 40 41 -42 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo (43) Ru-Rh PdAg Cd In Sn Sb To I Xe

55 5771 72 56 V 73 74 75 76 77 78 79 80 81 82 83 84 86 Cs Ba La-Hf To W Re Os Ir Pt Au Hg TI Pb Si Po (85) Rn Lu 88 89 90 91 1 92 (87) Ra Ac Th Pa U (93) (94) (95) (96) (97) (98) (99) (100)

57 58 59 60 62 63 64 65 66 67 68 69 70 7) La Ce Pr Nd(61) Sm Eu Gd Tb Dy No Er Tm Yb Lu

Pre- World War Il periodic fable predicted erroneous positions for transuranic elements

By 1944, two new transuranium 55 54 5771 72 73 7 75 76 77 70 elements had been placed Cs Ba La- Hf Ta W Re Os Ir Pt in an "uranide "group Lu 87 86 89 90 9) 92-606 Fr Ra Ac Th Pa U • (106)

;3 94 No!e: In the accompanying periodic tables, atomic numbers of undiscovered (95) (96) elements are in parentheses. L

An acfinjde series of heavy elements revised the neriodic table in 1945 - I 1 2 H . . H He - LOU 4.003 3 4 5 6 7 8 9 Li Be B C N 0 F- Ne 4.940 0.02 10.82 12.010 14.00* 18.000 19.00 20.183 12 11 13 13 14 15 26 27 18 No Mg Al Al Si P S Cl A 22.997 24.32 26.97 26.97 24.06 30.90 32.06 35.417 39.944 29 21 29 22 23 24 25 26 27 29 30 31 32 33 34 35 34 K Ca Sc Ti .V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 09096 4000 4510 499 30M flI 5493 2282 58.94 5809 63.57 60i0 81 fl 71.9) 7896 79.916 83.1 37 38 39 10 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Cb Mo Ru Rh Pd Ag Cd In Sn Sb Te I Xe 82.46 87.63 81.22 01.92 92.91 9595 101.7 102.91 606.7 107.880 112.41 614.76 118.70 121.76 127.61 126.92 131.3 V 55 56 37 58-71 12 73 74 75 76 71 S.. 18 79 80 81 82 83 84 85 86 Cs Ba 6.. L. Hf To W Re Os Ir Pt - Au Hg TI Pb Bi Po Rn 132.81 637.26 34.92 670.6 .1.. 180.8$ 183.92 186.3) 190.2 93.1 695.33 197.2 200.6) 204.39 207.26 209.00 222 I 81 88 89 s.. 90 I 91 I 92 93 I I Ra A U ThiPal INPPUI M

Isil 58 1 59 60 62 62 63 64 65 66 67 68 LANTHANIDE I J F69 10 71 SERIES La I Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu 640.13 140.92 14-4.21 I I 50.43 652.0 116.9 159.2 162.46 663.5 161.2 113.04 674.99

ACTINIDE 90 91 92 93 94 95 96 Th Pa I:_AMC j U Np Pu 232.12 321 238.07 237

48 C&EN April 16, 1979

2 F He 34 5 6 1 8 9 10 LiBe BC 1J OF Ne II 12 13 14 IS 16 Ii $8 NaMg Al SiP S CI Ar 19 20 21 22 23 24 25, 26 27 28 29 30 31 32 33 34 35 36 K Co Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo Ic Ru Rh Pd Ag Cd In Sn Sb Te I Xe 55 56 1 51 72 73 14 15 76 77 78 79 80 81 82 83 84 85 86 Cs Ba La Hf To W Re Os Ir Pt Au Hg TI Pb Bi Po At Rn 87 88 r a - - - 106 (I01)I(108){(109)(II0).1 (IIl)1(II 2 )(II 3 )1(lI 4 )(II5)(II6) ( 111 - 11I1 ± llJ )1(118)1

58 59 1 60 1 61 1 62 1 63 1 64 1 65 1 66 1 67 68 1 69 1 10 1 71 LANTHAN IDES Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu I

ACTIN IDES 90 91 92 93 94 95 96 91 98 99 100 $01 102 103 Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Modern periodic table fits in all elements through number 118

- Futuristic periodic table finds places for elements extending to number 168 - I 2 H He 3 . 4 . 5 6 1 8 9 10 Li Be B C N 0 F Ne II $2 13 14 15 16 Ii 18 NaMg Al Si P S Cl Ar 19 20 21. 22 23 24 25 26 21 28 29 30 31 32 33 34 35 36 K Co I Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge I As I Se Br Kr 31 38 39 40 41 42 43 44 45 46 41 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe. 55 56 57 72 73 74 15 76 71 18 19 80 81 82 83 84 85 86 Cs Ba La Hf To W Re Os Ir Pt Au Hg TI Pb Bi Po At Rn 87 1 J 106 j(107),.(108)j'(109)j'(110)(IH)(iI21I3)(u4)i5)(ll6){(ufl1uI8) (.w . ! (119) (120) (121) 1 (154)(155) (l56) ' (I51)(I58)(I59)(l60)(I6I) (162) (163)(164)(165)1(166)(l61)(168) L - - I - - .1 — - I - - I - - _i - _i_ _1_ _I_ - L - - I - - I - _i_ 1. - - I - - L - - L - _J

LANTHAN IDES 58 1 59 1 60 1 61 62 1 83 64 1 65 66 67 68 69 70 . 7I Ce Pr Nd Pm Sm Eu Gd Tb 1 Dy 1 Ho 1 Er 1 Tm Yb Lu

90 1 91 1 92 93 94 1 95 96 91 98 1 99 1 100 101 1 102 1 103 ACTIN IDES Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

S(.JPER - ------if------{(122)1(,23)1(124)k125):( 26)1 1(153)1 (97 and 98) in 1949 and 1950, einsteinium and fermium (99 and 100) in 1952 and 1953, mendelevium (101) in 1955, and nobelium (102) in 1958. It also signaled the end of the actinide series at lawrencium (103) when this ele- nent was discovered in 1961. Since all the elements beyond actinium through lawrencium, element 103, belong to the actinide group, the elements thorium, protactinium, and uranium have been removed from the positions they occupied in the periodic table before World War II and placed in this transition family. Elements 104, 105, and 106 have taken over the positions previously held by thorium, protactinium, and uranium. Rutherfordium (104), hahnium (105), and element 106 (as yet not named) were synthesized and identified during the next 13 years by Albert Ghiorso and coworkers at Berkeley. (It now seems clear that competing claims to the discovery of these elements by G. N. Flerov and coworkers at the Dubna Laboratory in the Soviet Union cannot be substantiated.) And we can confidently place all of the undiscovered elements through number 118 in their expected places in the periodic table. The elements beyond the actinides in the periodic table can be termed the "transactinides." These begin with the element with atomic number 104 and extend, in principle, indefinitely. Although only three such elements—num- Quiz Kids Sheila Conlan (center) and Robert Burke (right) ap- bers 104, 105, and 106—are definitely known at this time, peared with Glenn Seaborg when he informally announced dis- there are some prospects for the discovery of a number covery of elements 95 and 96 on radio show in 1945 of additional elements. Such elements may be synthesized and identified in the region just beyond number 106 or may be synthesized in the region of much larger atomic each other and usually are listed in a separate row below numbers. They would be synthesized by the bombard- the main part of the periodic table. This would mean that ment of heavy nuclides with heavy ions. The early all these heavier elements really belong with actin- transactinide elements find their place back in the main ium—directly after radium in the periodic table—just body of the modern periodic table. as the known "lanthanides" fit in with lanthanum be- Study of the chemical properties of element 104 has tween barium and hafnium. confirmed that it is indeed homologous to hafnium, as The revised periodic table, then, listed the heaviest demanded by its position in the periodic table (2). No elements as a second "rare-earth" series. These heaviest definitive chemical studies have been made for element elements (including undiscovered elements), with the 105 and no chemical studies have been made for element name "actinide" elements, were paired off with those in 106. Such studies are very difficult because the longest- the already-known lanthanide rare-earth series in a table lived isotope of 104 (261104) has a half-life of only about (1) published in C&EN, Dec. 10, 1945. one minute, of 105 (262105) a half-life of about 40 seconds, The new concept meant that elements 95 and 96 should and of 106 (263106) a half-life of about one second. have some properties in common with actinium and some On the basis of the simplest projections, it is expected in common with their rare-earth "sisters," europium and that the half-lives of the elements beyond element 106 gadolinium, especially with respect to the difficulty of will become shorter and shorter as the atomic number is oxidation above the III state. When experiments were increased, and this is true even for the isotopes with the designed according to this new concept, elements 95 and longest half-life for each element. Thus, if this rate of 96 were soon discovered at the wartime Metallurgical decrease could be extrapolated to ever-increasing atomic Laboratory at the University of Chicago—that is, they numbers, we would expect half-lives of the order of 10° were synthesized and chemically identified. second for the longest-lived isotope of element 110, and Quite by chance, the discovery of elements 95 and 96 10_20 second for element 115, and so forth, with decay by was revealed informally on a nationally broadcast radio spontaneous fission becoming of dominating importance program, the Quiz Kids, in which I appeared as guest on beginning with element 106. To make the situation even Nov. 11, 1945. The discovery information had already worse, predictions indicate that the yields from the nu- been declassified (removed from the "Secret" category) clear production reactions will fall off drastically with for presentation at an American Chemical Society atomic number as we proceed beyond element 106. The meeting to be held at Northwestern University the fol- observed yields for the production of element 106 are lowing Friday. Participating on the program with me down to the level of one atom per hour. were Quiz Kids Sheila and Patrick Conlan, Robert Burke, These considerations would present somewhat dismal Harvey Fishman, and Richard Williams. When Richard future prospects for heavier transuranium elements, but asked me if any new elements had been discovered in the other factors have entered the picture in recent years. course of research on transuranium elements during the These led to an increased optimism concerning the war, I revealed the discovery of elements 95 and 96. Ap- prospects for the synthesis and identification, or even the parently, many kids in America told their teachers about discovery in nature, of elements well beyond the observed it the next day and, judging from some of the letters I upper limit of the periodic table—elements that have received from such youngsters, they were not entirely come to be referred to as "superheavy" elements. An successful in convincing their teachers. excellent review article on the superheavy elements was Not only did this new understanding lead to the ele- written by S. G. Thompson and C. F. Tsang (3) in ments americium and curium (95 and 96), but to the 1972. synthesis and identification of berkelium and californium Complicated theoretical calculations, based on filled

50 C&EN April 16, 1979 0' 0

Ct 0) 0))

The "island of stability" for superheavy elements may have rather than the more gentle slopes of Mt. Diablo, which sharply dropping slopes, like Half Dome in Yosemite... is located in the San Francisco Bay area

shell (magic number) and other nuclear stability con- alkaline earth metal, respectively. Next the calculations siderations, led to these extrapolations to the far trans- point to the filling, after the addition of a 7d electron at uranium region. These suggested the existence of closed element 121, of the inner 5g and 6/ subshells, 32 places nucleon shells at Z = 114 and N = 184 that exhibit great in all, which I have termed the "superactinide" elements resistance to decay by spontaneous fission, the main and which terminate at element 153. This is followed by cause of instability for the heaviest elements. Earlier the filling of the 7d subshell (elements 154 through 162) considerations had suggested a closed shell at Z = 126, by and 8p subshell (elements 163 through 168). analogy to the known shell at N = 126, and some calcu- Actually, more careful calculations have indicated that lations suggest a closed shell at N = 228. Although Z = the picture is not this simple. The calculations indicate 164 and N = 318 may represent additional hypothetical that other electrons (8p and 7d) in addition to those points of stability, they correspond to a much more ex- identified in the above discussion enter the picture as tensive extrapolation and any predictions are therefore early as element 121 (or even element 104), thus further much more uncertain. complicating the picture. These and other perturbations, Enhancing the prospects for the actual synthesis and caused by spin-orbit splitting, lead to predictions of identification of superheavy nuclei was the fact that the chemical properties that are not consistent, element by calculations showed the doubly magic nucleus 298114 not element, with those suggested by the modern and to be the single long-lived specimen but to be merely the futuristic periodic tables. Here again we are in danger of center of a rather large "island of stability" in a "sea of making wrong predictions about the chemical properties spontaneous fission." of undiscovered elements by using the periodic table in- More recent calculations suggest that the shell effect correctly (5). of Z = 114 is not very large. Although these calculations For example, the six 7p electrons, because of relativ- confirm the existence of a closed shell at N = 184 they istic effects, are predicted to be split into two subshells, suggest a lesser stability for the region with N < 184. Thus four 7P:/9 and two 7P1/2 electrons, with a separation of the island of stability would have a sharp dropoff for N energies such that the filled 7P1/2 orbital will act as a <184—a structure like Half Dome in Yosemite, steeply closed shell and additional 7P3/2 electrons will act as falling into the sea of instability, and not like the more electrons outside a closed shell. Thus element 115 (eka- gentle slopes of my favorite Mt. Diablo in the San Fran- bismuth) is predicted to have its valence electrons in the cisco Bay area. configuration 7P1122 7P3/2 with a consequent stable I If these later considerations are correct, it will become oxidation state in contrast to the stable III oxidation state much more difficult to synthesize and detect the su- of its homolog bismuth. Another predicted result of rel- perheavy elements, and their observation in terrestrial ativistic effects is that element 112 (eka-mercury) and matter or any object whose age significantly exceeds 10 5 element 114 (eka-lead) may be very noble, that is, liquids years will be precluded. A recent review article (4) de- or volatile gases. These considerations raise the exciting scribes the present situation with respect to superheavy possibility of studying "relativity in a test tube." They elements taking into account these more recent consid- become especially significant beyond the superactinide erations. series, a region far beyond the region of expected nuclear Turning to consideration of electronic structure, upon stability, which is the only region where it might be pos- which chemical properties must be based, modern high- sible to synthesize or find superheavy elements. speed computers have made possible the calculation of Simple considerations indicate that, as a first ap- such structures (5). The calculations show that elements proximation, elements in the island of stability can be 104 through 112 are formed by filling the 6d electron predicted to have chemical properties as follows. Element subshell, which makes them, as expected, homologous in 114 should be a homolog of lead, that is, should be eka- chemical properties with the elements hafnium (72) lead, element 112 should be eka-mercury, element 110 through mercury (80). Elements 113 through 118 result should be eka-platinum, and so on. If there is an island from the filling of the 7p subshell and are expected to be of stability at element 126, this superactinide element and similar to the elements thallium (81) through radon (86). its neighbors should have chemical properties like those Thus these calculations are consistent with the modern of the actinide and lanthanide elements. periodic table. Attempts to synthesize and identify superheavy ele- The calculations indicate the 8s subshell should fill at ments, through bombardments of a wide range of heavy elements 119 and 120, thus making these an alkali and nuclides with a wide range of heavy ions, have so far been

April 16, 1979 C&EN 51 Unfortunately, even these experiments have all led to Proton number, Z negative results so far. A possibility for improving the survival probability of superheavy nuclei formed in this 120 Nuclear reaction landing sites fusion reaction is to use a more neutron-rich target, such ® 4'Ca + °'°Cm as 250Cm, which would allow a closer approach to the 48Ca + °'°Cm 110 desired N = 184.Unfortunately, the availability of 250Cm is very 1imidad-quantities sufficient to undertake an experiment could only become available through ex- 100 pensive recovery from the debris of an old or a future Half-life in years undergrounl nuclear weapons test. • Stable Anoth'er poss 0bility (4) for the synthesis and identifi- 10'T,,2 • cation of superheavy elements, not yet so extensively 90 • 100 T112 < 10° studiedris by the use of the "deep inelastic transfer" re- • 105 Tj/ < 100 action, in which there is a massive energy and nucleon fl 10-" o T < 10-° transfer between projectile and target. Success of this 80 approach apparently depends on the use of the heaviest available projectile and target. Experiments at the Ge- 120 130 140 150 160 170 180 190 sellschaft für Schwerionenforschung Laboratory in Neutron number, N Darmstadt,. West Germany, where uranium ions are already available as projectiles, have been performed with the reaction 2 U + 238U. Products as heavy as 255Fm, Topological map of the heavy elements shows a peninsula of corresponding to a net transfer of eight protons and nine stability for known elements and an island of predicted stability neutrons, have been observed there. Although this leaves near Z = 114 and N = 184 in a surrounding sea of instability or a long way to go to reach the island of stability, there is spontaneous fission. The fusion reactions 48 Ca + 248 Cm and hope that the expected barriers for fission will enhance + 250 CM are two possible ways to form nuclei that would the yields in this region. Pursuit of this route using re- approach the island of stability actions such as 248Cm (target) + 244Pu (projectile), or even 249Cf or 254Es (target available in only very small unsuccessful (4). Also, every attempt to find evidence, quantity) + 244Pu might lead to positive results. direct or indirect, of the superheavy elements in nature Additional attempts might be along the lines of using associated with the island of stability centered on element higher-intensity heavy ion beams, improved methods of 114 has led to negative results or has not been conclusive detection, and more emphasis on detecting short half- enough to give a clear-cut answer. lives (one second down to iO second or less). Numerous attempts (4) have been made to synthesize It will be frustrating if we are left with the strong and detect the superheavy elements through the fusion feeling that the island of stability must be there but we of heavy ions (atomic number Z1) with heavy target nuclei are unable to devise means of reaching it. Chemists would (atomic number Z2) to produce compound superheavy like the challenge of once again testing the role of the nuclei (atomic number Z1 + Z2). Relatively strong nuclear periodic table in predicting the chemical properties of shells, with their accompanying high potential barriers new chemical elements. They hope to travel once again for nuclear fission, are required for two reasons: to enable the tortuous path to man-made elements. 0 any excited superheavy nucleus formed in a nuclear fusion reaction to survive destruction by fission during its de-excitation process, and to enable any "cold" su- References perheavy nucleus that survived its de-excitation to live long enough to be detected through its alpha particle or Seaborg, G. 1., "Chemical and Radioactive Properties of the Heavy Elements," Chem. Eng. News, 23, 2190 (1945). spontaneous fission decay. Silva, R., Harris, J., Nurmia, M., Eskola, K., Ghiorso, A., 'Chemical To maximize the probability for the compound nuclei Separation of Rutherfordium," lnorg. NucI. Chem. Lett., 6 (12), 871 to escape destruction by fission during their de-excita- (1970). tion, they should be produced with the lowest possible Thompson, S. G., Tsang, C. F., "Superheavy Elements," Science, 178, excitation energy. In other words, they should be syn- 1047 (1972). Seaborg, G. T., Loveland, W., Morrissey, D. J., "Superheavy Ele- thesized with a bombarding energy for the projectile ments—A Crossroads," Science, 203, 711(1979). heavy ion which just barely exceeds the Coulomb barrier. Keller, 0. L., Jr., Seaborg, G. 1., 'Chemistry of the Transactinide El- Unfortunately, more energy than this is apparently re- ements," Ann. Rev. NucI. Sci., 139 (1977). quired for the two participating nuclei to fuse to form a compound system. Thus we appear to be caught on the horns of a dilem: Suggestions for additional reading ma. If the bombarding energy is low, the reacting nuclei don't fuse; if the bombarding energy is high enough to Van Spronsen, J. W., "The Periodic System of Chemicai Elements: A History of the First Hundred Years," Elsevier Publishing Co., Amsterdam, produce fusion, the product nuclei don't survive. To London, New York, 1969. further exacerbate the situation, projectile nuclei with Seaborg, G. T., "The Transuranium Elements," Yale University Press, Z greater than about 25 do not seem to fuse with very 1958. heavy target nuclei to form a compound system at any Seaborg, G. 1., "Transuranium Elements: Products of Modern Alchemy," bombarding energy. Dowden, Hutchinson & Ross Inc., Stroudsburg, Pa., 1978. (Compilation The reaction 48Ca + 248Cm to form the compound of reprints of original papers.) nucleus 296116 has been studied extensively because of several favorable factors: The projectile 48Ca is light enough presumably to allow some fusion to take place, Reprints of this C&EN special report will be available at $2.00 its double shell closure (high binding energy) reduces the per copy. For 10 or more copies, $1.25 per copy. Send requests excitation energy of the product, and its relative neutron to: C&EN Reprint Department, American Chemical Society, excess allows an approach toward the closed shell at N 1155-16th St., N.W., Washington, D.C. 20036. On orders of = 184 (which needs to be attained for best results, but $20 or less, please send check or money order with request. unfortunately is not quite reached even in this case).

52 C&EN April 16, 1979 This report was done with support from the Department of Energy. Any conclusions or opinions expressed in this report represent solely those of the author(s) and not necessarily those of The Regents of the University of California, the Lawrence Berkeley Laboratory or the Department of Energy. Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Department of Energy to the exclusion of others that may be suitable. bj

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