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Transuranium elements

Those synthetic elements with atomic numbers larger than that of uranium (atomic number 92). They are the members of the actinide series, from neptunium (atomic number 93) through lawrencium (atomic number 103), and the transactinide elements (with atomic numbers higher than 103). Of these elements, plutonium, an explosive ingredient for nuclear weapons and a fuel for nuclear power because it is fissionable, has been prepared on the largest (ton) scale, while some of the others have been produced in kilograms (neptunium, americium, curium) and in much smaller quantities (berkelium, californium, and einsteinium).

The concept of atomic weight as applied to naturally occurring elements is not applicable to the transuranium elements, since the isotopic composition of any given sample depends on its source. In most cases the use of the mass number of the longest-lived isotope in combination with an evaluation of its availability has been adequate. Good choices at present are neptunium, 237; plutonium, 242; americium, 243; curium, 248; berkelium, 249; californium, 249; einsteinium, 254; fermium, 257; mendelevium, 258; nobelium, 259; lawrencium, 260; rutherfordium, 261; dubnium, 262; seaborgium, 266; bohrium 267; and hassium 269.

The actinide elements are chemically similar and have a strong chemical resemblance to the lanthanide, or rare-earth, elements (atomic numbers 57–71). The transactinide elements, with atomic numbers 104 to 118, should be placed in an expanded periodic table under the row of elements beginning with hafnium, number 72, and ending with radon, number 86. This arrangement allows prediction of the chemical properties of these elements and suggests that they will have an element-by-element chemical analogy with the elements that appear immediately above them in the periodic table. However, deviations from this analogy are expected and are observed in specific detailed chemical properties of transactinide elements.  See also: Actinide elements; Periodic table; Rare-earth elements

The transuranium elements up to and including fermium (atomic number 100) are produced in the largest quantity through the successive capture of neutrons in nuclear reactors. The yield decreases with increasing atomic number, and the heaviest to be produced in weighable quantity is einsteinium (number 99). Many additional isotopes are produced by bombardment of heavy target isotopes with charged atomic projectiles in accelerators; beyond fermium all elements are produced by bombardment with heavy ions. Brief descriptions of transuranium elements follow. They are listed according to increasing atomic number.

Neptunium

Neptunium (Np, atomic number 93, named after the planet Neptune) was the first transuranium element discovered. In 1940 E. M. McMillan and P. H. Abelson at the University of California, Berkeley, identified the isotope 239Np (half-life 2.35 days), which was produced by the bombardment of uranium with neutrons according to reaction (1).

The element as 237Np was first isolated as a pure compound, the oxide, in 1944 by L. G. Magnusson and T. J. La Chapelle. Neptunium in trace amounts is found in nature, and is produced in nuclear reactions in uranium ores caused by the neutrons present. Kilogram and larger quantities of 237Np (half-life 2.14 × 106 years), used for chemical and physical investigations, are produced as a by-product of the production of plutonium in nuclear reactors. Isotopes from mass number 227 to 244 have been synthesized by various nuclear reactions.  See also: Nuclear reaction

Neptunium displays five oxidation states in aqueous solution: Np3+ (pale purple), Np4+ (yellow-green), NpO2+ (green-blue), NpO22+ (pink), and NpO53− (green). The ion NpO2+, unlike corresponding ions of uranium, plutonium, and americium, can exist in aqueous solution at moderately high concentrations. The element forms tri- and tetrahalides such as NpF3, NpF4, NpCl3, NpCl4, NpBr3, NpI3, as well as NpF6 and oxides of various compositions such as those found in the uranium-oxygen system, including Np3O8 and NpO2.

Neptunium metal has a silvery appearance, is chemically reactive, and melts at 637°C (1179°F); it has at least three crystalline forms between room temperature and its melting point.  See also: Neptunium

Plutonium

Plutonium (Pu, atomic number 94, named after the planet Pluto) in the form of 238Pu was discovered in late 1940 and early 1941 by G. T. Seaborg, McMillan, J. W. Kennedy, and A. C. Wahl at the University of California, Berkeley. The element was produced in the bombardment of uranium with deuterons according to reaction (2).

The important isotope 239Pu was discovered by Kennedy, Seaborg, E. Segrè, and Wahl in 1941. Because of its property of being fissionable with neutrons, plutonium-239 (half-life 24,400 years) is used as the explosive ingredient in nuclear weapons and is a key material in the development of nuclear energy for industrial purposes. 1 lb (0.45 kg) of plutonium produced is equivalent to about 107 kWh of heat energy; plutonium is produced in ton quantities in nuclear reactors. The alpha radioactivity and physiological behavior of this isotope make it one of the most dangerous poisons known, but means for handling it safely have been devised. Plutonium as 239Pu was first isolated as a pure compound, the fluoride, in 1942 by B. B. Cunningham and L. B. Werner. Minute amounts of plutonium formed in much the same way as naturally occurring neptunium are present in nature. Much smaller quantities of the longer-lived isotope 244Pu (half-life 8.3 × 107 years) have been found in nature; in this case it may represent the small fraction remaining from a primordial source or it may be caused by cosmic rays. Isotopes of mass number 232–246 are known. The longer-lived isotopes 242Pu (half-life 390,000 years) and 244Pu, produced in nuclear reactors, are more suitable than 239Pu for chemical and physical investigation because of their longer half-lives and lower specific activities.

Plutonium has five oxidation states in aqueous solution: Pu3+ (blue to violet), Pu4+ (yellow-brown), PuO2+ (pink), PuO22+ (pink-orange), and PuO53− (blue-green). The ions Pu4+ and PuO2+ undergo extensive disproportionation to the ions of higher and lower oxidation states. Four oxidation states (III, IV, V, and VI) can exist simultaneously at appreciable concentrations in equilibrium with each other, an unusual situation that leads to complicated solution phenomena.

Plutonium forms binary compounds with oxygen (PuO, PuO2, and intermediate oxides of variable composition); with the halogens (PuF3, PuF4, PuF6, PuCl3, PuBr3, PuI3); with carbon, nitrogen, and silicon (including PuC, PuN, PuSi2); in addition, oxyhalides are well known (PuOCl, PuOBr, PuOI).

The metal is silvery in appearance, is chemically reactive, melts at 640°C (1184°F), and has six crystalline modifications between room temperature and its melting point.  See also: Plutonium

Americium

Americium (Am, atomic number 95, named after the Americas) was the fourth transuranium element discovered. The element as 241Am (half-life 433 years) was produced by the intense neutron bombardment of plutonium and was identified by Seaborg, R. A. James, L. O. Morgan, and A. Ghiorso in late 1944 and early 1945 at the wartime Metallurgical Laboratory at the University of Chicago. By using the isotope 241Am, the element was first isolated as a pure compound, the hydroxide, in 1945 by B. B. Cunningham. Isotopes of mass numbers 237–247 have been prepared. Kilogram quantities of 241Am are being produced in nuclear reactors. The less radioactive isotope 243Am (half-life 7400 years), also produced in nuclear reactors, is more suitable for use in chemical and physical investigation.

Americium exists in four oxidation states in aqueous solution: Am3+ (light salmon), AmO2+ (light tan), AmO22+ (light tan), and a fluoride complex of the IV state (pink). The trivalent state is highly stable and difficult to oxidize. AmO2+, like plutonium, is unstable with respect to disproportionation into Am3+ and AmO22+. The ion Am4+ may be stabilized in solution only in the presence of very high concentrations of fluoride ion, and tetravalent solid compounds are well known. Divalent americium has been prepared in solid compounds; this is consistent with the presence of seven 5f electrons in americium (enhanced stability of half-filled 5f electron shell) and is similar to the analogous lanthanide, europium, which can be reduced to the divalent state.

Americium dioxide, AmO2, is the important oxide; Am2O3 and, as with previous actinide elements, oxides of variable composition between AmO1.5 and AmO2 are known. The halides AmF2 (in CaF2), AmF3, AmF4, AmCl2 (in SrCl2), AmCl2, AmBr3, AmI2, and AmI3 have also been prepared.

Metallic americium is silvery-white in appearance, is chemically reactive, and has a melting point of 1176°C (2149°F). It has two crystalline forms between room temperature and its melting point.  See also: Americium

Curium

The third transuranium element to be discovered, curium (Cm, atomic number 96, named after Pierre and Marie Curie), as the isotope 242Cm, was identified by Seaborg, James, and Ghiorso in 1944 at the wartime Metallurgical Laboratory of the University of Chicago. This was produced by the helium-ion bombardment of 239Pu in the University of California 60-in. (152-cm) cyclotron. Curium was first isolated, using the isotope 242Cm, in the form of a pure compound, the hydroxide, in 1947 by L. B. Werner and I. Perlman. Isotopes of mass number 238–251 are known. Chemical investigations have been performed using 242Cm (half-life 163 days) and 244Cm (half-life 18 years), but the higher-mass isotopes 247Cm and 248Cm with much longer half-lives (1.6 × 107 and 3.5 × 105 years, respectively) are more satisfactory for this purpose; these are all produced by neutron irradiation in nuclear reactors.

Curium exists solely as Cm3+ (colorless to yellow) in the uncomplexed state in aqueous solution. This behavior is related to its position as the element in the actinide series in which the 5f electron shell is half filled; that is, it has the especially stable electronic configuration 5f7, analogous to its lanthanide homolog, gadolinium. A curium IV fluoride complex ion exists in aqueous solution. Solid compounds include Cm2O3, CmO2 (and oxides of intermediate composition), CmF3, CmF4, CmCl3, CmBr3, and CmI3.

The metal is silvery and shiny in appearance, is chemically reactive, melts at 1340°C (2444°F), and resembles americium metal in its two crystal modifications.  See also: Curium

Berkelium

Berkelium (Bk, atomic number 97, named after Berkeley, California) was produced and identified by S. G. Thompson, Ghiorso, and Seaborg in late 1949 at the University of California, Berkeley, and was the fifth transuranium element discovered. The isotope 243Bk (half-life 4.6 h) was synthesized by helium-ion bombardment of 241Am. The first isolation of berkelium in weighable amount, as 249Bk (half-life 314 days), produced by neutron irradiation, was accomplished in 1958 by Thompson and Cunningham; this isotope, produced in nuclear reactors, is used in the chemical and physical investigation of berkelium. Isotopes of mass number 242–251 are known.

Berkelium exhibits two ionic oxidation states in aqueous solution: Bk3+ (yellow-green) and somewhat unstable Bk4+ (yellow), as might be expected by analogy with its rare-earth homolog, terbium. Solid compounds include Bk2O3, BkO2 (and oxides of intermediate composition), BkF3, BkF4, BkCl3, BkBr3, and BkI3.

Berkelium metal is chemically reactive, exists in two crystal structure modifications, and melts at 986°C (1807°F).  See also: Berkelium

Californium

The sixth transuranium element to be discovered, californium (Cf, atomic number 98, named after the state and University of California, Berkeley), in the form of the isotope 245Cf (half-life 44 min), was first prepared by the helium-ion bombardment of microgram quantities of 242Cm by Thompson, K. Street, Jr., Ghiorso, and Seaborg at Berkeley early in 1950. Cunningham and Thompson, at Berkeley, isolated californium in weighable quantities for the first time in 1958 using a mixture of the isotopes 249Cf, 250Cf, 251Cf, and 252Cf, produced by neutron irradiation. Isotopes of mass number 239–256 are known. The best isotope for the investigation of the chemical and physical properties of californium is 249Cf (half-life 350 years), produced in pure form as the beta-particle decay product of 249Bk.

Californium exists mainly as Cf3+ in aqueous solution (emerald green), but it is the first of the actinide elements in the second half of the series to exhibit the II state, which becomes progressively more stable on proceeding through the heavier members of the series. It also exhibits the IV oxidation state in CfF4 and CfO2, which can be prepared under somewhat intensive oxidizing conditions. Solid compounds also include Cf2O3 (and higher intermediate oxides), CfF3, CfCl3, CfBr2, CfBr3, CfI2, and CfI3.

Californium metal is chemically reactive, is quite volatile, and can be distilled at temperature ranges of 1100–1200°C (2010–2190°F). It appears to exist in three different crystalline modifications between room temperature and its melting point, 900°C (1652°F).  See also: Californium

Einsteinium

The seventh transuranium element to be discovered, einsteinium (Es, atomic number 99, named after Albert Einstein), was found by Ghiorso and coworkers in the debris from the “Mike” thermonuclear explosion staged by the Los Alamos Scientific Laboratory in November 1952. Very heavy uranium isotopes were formed by the action of the intense neutron flux on the uranium in the device, and these decayed into isotopes of elements 99, 100, and other transuranium elements of lower atomic number. Chemical investigation of the debris in late 1952 by workers at the University of California Radiation Laboratory, Argonne National Laboratory, and Los Alamos Scientific Laboratory revealed the presence of element 99 as the isotope 253Es. Einsteinium was isolated in a macroscopic (weighable) quantity for the first time in 1961 by Cunningham, J. C. Wallman, L. Phillips, and R. C. Gatti at Berkeley; they used the isotope 253Es, produced in nuclear reactors, working with only a few hundredths of a microgram. The macroscopic property that they determined in this case was the magnetic susceptibility. Isotopes of mass number 243–256 have been synthesized. Einsteinium is the heaviest transuranium element to be isolated in weighable form. Most of the investigations have used the short-lived 253Es (half-life 20.5 days) because of its greater availability, but the use of 254Es (half-life 276 days) will increase as it becomes more available as the result of production in nuclear reactors.

Einsteinium exists in normal aqueous solution essentially as Es3+ (green), although Es2+ can be produced under strong reducing conditions. Solid compounds such as Es2O3, EsCl3, EsOCl, EsBr2, EsBr3, EsI2, and EsI3 have been made.

Einsteinium metal is chemically reactive, is quite volatile, and melts at 860°C (1580°F); one crystal structure is known.  See also: Einsteinium

Fermium

Fermium (Fm, atomic number 100, named after Enrico Fermi), the eighth transuranium element discovered, was isolated as the isotope 255Fm (half-life 20 h) from the heavy elements formed in the “Mike” thermonuclear explosion. The element was discovered in early 1953 by Ghiorso and coworkers during the same investigation which resulted in the discovery of element 99. Fermium isotopes of mass number 242–259 have been prepared.

No isotope of fermium has yet been isolated in weighable amounts, and thus all the investigations of this element have been done with tracer quantities. The longest-lived isotope is 257Fm (half-life about 100 days), whose production in high-neutron-flux reactors is extremely limited because of the very long sequence of neutron-capture reactions that is required.

Despite its very limited availability, fermium, in the form of the 3.24-h 254Fm isotope, has been identified in the “metallic” zero-valent state in an atomic-beam magnetic resonance experiment. This established the electron structure of elemental fermium in the ground state as 5f127s2 (beyond the radon structure).

Fermium exists in normal aqueous solution almost exclusively as Fm3+, but strong reducing conditions can produce Fm2+, which has greater stability than Es2+ and less stability than Md2+.  See also: Fermium

Mendelevium

Mendelevium (Md, atomic number 101, named after Dmitri Mendeleev), the ninth transuranium element discovered, was identified by Ghiorso, B. G. Harvey, G. R. Choppin, Thompson, and Seaborg at the University of California, Berkeley, in 1955. The element as 256Md (half-life 1.5 h) was produced by the bombardment of extremely small amounts (approximately 109 atoms) of 253Es with helium ions in the 60-in. (152-cm) cyclotron. The first identification of mendelevium was notable in that only one or two atoms per experiment were produced. (This served as the prototype for the discovery of all heavier transuranium elements, which have been first synthesized and identified on a one-atom-at-a-time basis.) Isotopes of mass numbers 247–259 are known. Although the isotope 258Md (half-life 56 days) is sufficiently long-lived, it cannot be produced in nuclear reactors, and hence it will be very difficult and perhaps impossible to isolate it in weighable amount.

The chemical properties have been investigated on the tracer scale, and the element behaves in aqueous solution as a typical tripositive actinide ion; it can be reduced to the II state with moderately strong reducing agents.  See also: Mendelevium

Nobelium

The discovery of nobelium (No, atomic number 102, named after Alfred Nobel), the tenth transuranium element to be discovered, has a complicated history. For the first time scientists from countries other than the United States embarked on serious efforts to compete in this field. The reported discovery of element 102 in 1957 by an international group of scientists working at the Nobel Institute for Physics in Stockholm, who suggested the name nobelium, has never been confirmed and must be considered to be erroneous. Working at the Kurchatov Institute of Atomic Energy in Moscow, G. N. Flerov and coworkers in 1958 reported a radioactivity that they thought might be attributed to element 102, but a wide range of half-lives was suggested and no chemistry was performed. As the result of more definitive work performed in 1958, Ghiorso, T. Sikkeland, J. R. Walton, and Seaborg reported an isotope of the element, produced by bombarding a mixture of curium isotopes with 12C ions in the then-new Heavy Ion Linear Accelerator (HILAC) at Berkeley. They described a novel “double recoil” technique that permitted identification by chemical means, one atom at a time, of any daughter isotope of element 102 that might have been formed. The isotope 250Fm was identified conclusively by this means, indicating that its parent should be the isotope of element 102 with mass number 254 produced by the reaction of 12C ions with 246Cm. However, another isotope of element 102, with half-life 3 s, also observed indirectly in 1958, and whose alpha particles were shown to have an energy of 8.3 MeV by Ghiorso and coworkers in 1959, was shown later by Flerov and coworkers (working at the Dubna Laboratory near Moscow) to be of an isotope of element 102 with mass number 252 rather than 254; in other words, two isotopes of element 102 were discovered by the Berkeley group in 1958, but the correct mass number assignments were not made until later. On the basis that they identified the atomic number correctly, the Berkeley scientists probably have the best claim to the discovery of element 102; they suggest the retention of nobelium as the name for this element.

All known isotopes (mass numbers 250–259) of nobelium are short-lived and are produced by the bombardment of lighter elements with charged particles (heavy ions); the longest-lived is 259No with a half-life of 58 min. All of the chemical investigations have been, and presumably must continue to be, done on the tracer scale. These have demonstrated the existence of No3+ and No2+ in aqueous solutions, with the latter much more stable than the former. The stability of No2+ is consistent with the expected presence of the completed shell of fourteen 5f electrons in this ion.  See also: Nobelium

Lawrencium

Lawrencium (Lr, atomic number 103, named after Ernest O. Lawrence) was discovered in 1961 by Ghiorso, Sikkeland, A. E. Larsh, and R. M. Latimer using the HILAC at the University of California, Berkeley. A few micrograms of a mixture of 249Cf, 250Cf, 251Cf, and 252Cf (produced in a nuclear reactor) were bombarded with 10B and 11B ions to produce single atoms of an isotope of element 103 with a half-life measured as 8 s and decaying by the emission of alpha particles of 8.6 MeV energy. Ghiorso and coworkers suggested at that time that this radioactivity might be assigned the mass number 257. G. N. Flerov and coworkers have disputed this discovery on the basis that their later work suggests a greatly different half-life for the isotope with the mass number 257. Subsequent work by Ghiorso and coworkers proves that the correct assignment of mass number to the isotope discovered in 1961 is 258, and this later work gives 4 s as a better value for the half-life.

All known isotopes of lawrencium (mass numbers 253–260) are short-lived and are produced by bombardment of lighter elements with charged particles (heavy ions); chemical investigations have been, and presumably must be, performed on the tracer scale. Work with 260Lr (half-life 3 min) has demonstrated that the normal oxidation state in aqueous solution is the III state, corresponding to the ion Lr3+, as would be expected for the last member of the actinide series.  See also: Lawrencium

Rutherfordium

Rutherfordium (Rf, atomic number 104, named after Lord Rutherford), the first transactinide element to be discovered, was probably first identified in a definitive manner by Ghiorso, M. Nurmia, J. Harris, K. Eskola, and P. Eskola in 1969 at Berkeley. Flerov and coworkers have suggested the name kurchatovium (named after Igor Kurchatov with symbol Ku) on the basis of an earlier claim to the discovery of this element. In 1964 they bombarded 242Pu with 22Ne ions in their cyclotron at the Joint Institute for Nuclear Research in Dubna and reported the production of an isotope, suggested to be 260Ku, which was held to decay by spontaneous fission with a half-life of 0.3 s. After finding it impossible to confirm this observation, Ghiorso and coworkers reported definitive proof of the production of alpha-particle-emitting 257Rf and 259Rf (half-lives 4.5 and 3 s, respectively), demonstrated by the identification of the previously known 253No and 255No as decay products, by means of the bombardment of 249Cf with 12C and 13C ions in the Berkeley HILAC.

All known isotopes of rutherfordium (mass numbers 253–262) are short-lived and are produced by bombardment of lighter elements with charged heavy-ion particles. The isotope 261Rf (half-life 78 s) has made it possible, by means of rapid chemical experiments, to demonstrate that the normal oxidation state of rutherfordium in aqueous solution is the IV state corresponding to the ion Rf4+. This is consistent with expectations for this first “transactinide” element which should be a homolog of hafnium, an element that is exclusively tetrapositive in aqueous solution. Gas chromatographic studies of volatile halides and halides oxides of rutherfordium demonstrate characteristics as expected for a group-4 element in the periodic table. However, some detailed chemical properties of rutherfordium compounds, studied in the aqueous phase and the gas phase, resemble more the behavior of its lighter homolog zirconium (atomic number 40) than hafnium (atomic number 72).  See also: Rutherfordium

Dubnium

Dubnium (Db, atomic number 105, named after the Dubna Laboratory), the second transactinide element to be discovered, was probably first identified in a definitive manner in 1970 by Ghiorso, Nurmia, K. Eskola, Harris, and P. Eskola at Berkeley. They reported the production of alpha-particle-emitting 260Db (half-life 1.6 s), demonstrated through the identification of the previously known 256Lr as the decay product, by bombardment of 249Cf with 15N ions in the Berkeley HILAC. Again the Berkeley claim to discovery is disputed by Flerov and coworkers, who earlier in 1970 reported the discovery of an isotope thought to be dubnium, decaying by the less definitive process of spontaneous fission, produced by the bombardment of 243Am with 22Ne ions in the Dubna cyclotron; in later work Flerov and coworkers may have also observed the alpha-particle-emitting isotope of dubnium reported by Ghiorso and coworkers.

The known isotopes of dubnium (mass numbers 256–263) are short-lived and are produced by bombardment of lighter elements with charged heavy-ion particles. Using rapid chemical techniques and the isotope 262Db (half-life 40 s), it is possible to study the chemical properties of dubnium. The results show that dubnium exhibits the V oxidation state like its homolog tantalum. A number of chemical studies demonstrate that, in specific chemical environments, dubnium behaves more like its lighter homolog niobium or sometimes like the pentavalent actinide element protactinium (atomic number 91).  See also: Dubnium

Seaborgium

The discovery of seaborgium (Sg, atomic number 106, named after Glenn T. Seaborg) took place in 1974 simultaneously as the result of experiments by Ghiorso and coworkers at Berkeley and Flerov, Y. T. Oganessian, and coworkers at Dubna. The Ghiorso group used the SuperHILAC (the rebuilt HILAC) to bombard a target of californium (the isotope 249Cf) with 18O ions. This resulted in the production and positive identification of the alpha-particle-emitting isotope 263Sg, which decays with a half-life of 0.9 ± 0.2 s by the emission of alpha particles of a principal energy of 9.06 MeV. The definitive identification consisted of the establishment of the genetic link between the seaborgium alpha-particle-emitting isotope (263Sg) and previously identified daughter (259Rf) and granddaughter (255No) nuclides, that is, the demonstration of the decay sequence: A total of 73 263Sg alpha particles and approximately the expected corresponding number of 259Rf daughter and 255No granddaughter alpha particles were recorded.

The Dubna group chose lead (atomic number 82) as their target because, they believed, its closed shells of protons and neutrons and consequent small relative mass leads to minimum excitation energy for the compound nucleus and therefore an enhancement in the cross section for the production of the desired product nuclide. They bombarded 207Pb and 208Pb with 54Cr ions (atomic number 24) in their cyclotron to find a product that decays by the spontaneous fission mechanism (a total of 51 events), with the very short half-life of 7 ms, which they assign to the isotope 259Sg. Later work at Dubna and the Gesellschaft für Schwerionenforschung (GSI) laboratory in Darmstadt, Germany, has shown that this assignment is not correct. 259Sg is an alpha emitter of 0.48 s. In 1984 the isotopes 261Sg and 260Sg were discovered at GSI, Germany.

Long-lived isotopes 266Sg (21 s) and 265Sg (22 s) were discovered in 1994 at Dubna bombarding 248Cm with 22Ne. In 2000, spontaneous fissioning isotopes 258Sg (2.9 ms) and 262Sg (6.9 ms) were synthesized at GSI using 290Bi- and 207Pb-based reactions. Thirty years after discovery of the element seaborgium, 8 isotopes are known.

Fast chemical separations, performed with the Automated Rapid Chemistry Apparatus (ARCA) and On-Line Gas Chromatography Apparatus (OLGA) on a one-atom-at-a-time scale of 265Sg, allowed investigating seaborgium in aqueous solution and in the gas phase. A series of experiments were done in international collaborations at GSI, Darmstadt. As expected from its projected position in the periodic table, seaborgium shows chemical properties similar to those extrapolated from its lighter group-6 homologs, molybdenum (atomic number 42) and tungsten (atomic number 74). Seaborgium is the heaviest element that has been studied in aqueous solution.  See also: Seaborgium

Bohrium

Bohrium (Bh, atomic number 107, named after Niels Bohr) was synthesized and identified by G. Münzenberg and coworkers at GSI-Darmstadt. The element has a long half-life in the millisecond range, indicating an unexpected relative stability in this region of high atomic numbers. In the discovery experiment, a target of 209Bi [located in the region of closed shells (N = 126)] was used, which when bombarded with heavy ions, led to a compound nucleus of minimum excitation energy (∼15 MeV), allowing for a cooling of the nucleus by emission of a single neutron. The element was identified by the alpha-particle-emitting isotope 262Bh, and the genetic links with its known alpha-particle-emitting descendants was established, as was done by Ghiorso and coworkers in their discovery of seaborgium. In 1981, Münzenberg and coworkers observed six atoms of 5 ms (the time interval for its decay) 262Bh, produced by the 209Bi (54Cr,n) reaction.  See also: Nuclear structure

The experiment was repeated, and in 1988 about 40 decay chains corroborated the discovery of bohrium. The isotope 261Bh was discovered in the same reaction by the 2-neutron emission channel.

Today the isotope 264Bh, a member of the decay-chain of roentgenium, is known and two isotopes, 266Bh and 267Bh, synthesized in the reaction of 22Ne with 249Bk, have been discovered. 265Bh with a half-life of 0.94 s was synthesized at the Institute of Modern Physics, Lanzhou, China, in 2004. All these isotopes manifest the trend towards larger half-lives in the region of seconds.

The one and so far only chemical separation and characterization of bohrium was done at the Paul-Scherrer Institute (PSI), Villigen, Switzerland, during gas chromatography of a chloride oxide compound of 267Bh. As expected, bohrium showed chemical properties similar to those extrapolated from its lighter group-7 homologs, technetium (atomic number 43) and rhenium (atomic number 75).  See also: Bohrium

Hassium

Hassium (Hs, atomic number 108, named after the German state of Hessen from the Latin word Hassias) was synthesized in 1984 at GSI, Darmstadt, Hessen. A target nucleus of 208Pb, an isotope with two closed nucleon shells (Z = 82, N = 126) was fused with 58Fe to synthesize 265Hs; see also bohrium for aspects of the hassium production mechanism. The element was identified by its links to known descendants. Three atoms of 1.8 ms 265Hs were produced by the 208Pb(58Fe,n) reaction. The reaction was confirmed in many laboratories and serves as a calibration for Pb- and Bi-based reactions. Later the isotope 264Hs was discovered by the same group in the reaction 207Pb(58Fe,n). It links the elements with even proton numbers via a bridge of an α-decay at 256Rf to lighter elements and establishes a connection of absolute binding energies up to element hassium.

The isotope 267Hs was discovered at Dubna in 1992. It was observed in a 5n-channel in the fusion of 34S and 238U. This isotope 269Hs was confirmed at GSI, being found as a descendant in the α-decay chain of 271Ds. The isotope 269Hs was observed first in 1996 at GSI in a decay chain of 277112 and has been reproduced since in the reaction 248Cm(26Mg,5n) by experiments at GSI, observing its decay after a chemical hassium separation. In the reaction of magnesium-26 with curium-248, the isotope 270Hs was synthesized. This is the first isotope with the closed nuclear shell N = 162. With increasing neutron number, a trend toward larger half-lives in the few-seconds range is observed also for hassium isotopes.

Since 2001, studies of the formation and the behavior of hassium tetroxide were done one-atom-at-a-time with the isotope 269Hs in three international collaborations at GSI. All the experiments showed chemical properties of hassium similar to those of osmium (atomic number 76), the lighter homolog in group-8 of the periodic table.  See also: Hassium

Meitnerium

Meitnerium (Mt, atomic number 109, named after Lise Meitner) was synthesized in 1982 at GSI. (For its production see also bohrium.) A target nucleus of 209Bi, as done before for bohrium, was fused with 58Fe to synthesize 266Mt. Again the element was identified by its genetic links to known descendants. In 1982, one atom of 1.7 ms 266Mt was produced by the 209Bi(58Fe,n) reaction. Since then, the discovery has been confirmed by about 10 more decay chains.

The isotopes 268Mt and 270Mt were observed as descendants in decay chains from heavier elements. 268Mt was discovered at GSI, in 1994 in the discovery experiment of the element roentgenium. 270Mt was reported from RIKEN, Japan, in 2004, together with the discovery of element 113 in the reaction 209Bi(70Zn,n). Half-lives stayed in the range below 1 second and the production cross section of 5 × 10−38 cm2 is very small (the smallest cross section known, to date).

Meitnerium is expected to have chemical properties similar to those of iridium, its lighter homolog in group 9 of the periodic table. No chemical experiments have been done on meitnerium.  See also: Meitnerium

Darmstadtium

Darmstadtium (Ds, atomic number 110, named after Darmstadt, Germany, the location of the GSI laboratory) was synthesized in 1994 at GSI.

Darmstadtium should be a heavy homolog of the elements platinum, palladium, and nickel. It is the eighth element in the 6d shell.

Research for this element began in 1985. Experiments at Dubna, Russia; at GSI, and at Lawrence Berkeley Laboratory, Berkeley, California, failed to provide reliable evidence for a successful synthesis. However, at GSI on November 9, 1994, a decay chain was observed that proved the existence of the isotope 269Ds (the isotope of darmstadtium with mass number 269). The isotopes were produced in a fusion reaction of a nickel-62 projectile with a lead-208 target nucleus. The fused system, with an excitation energy of 13 MeV, cooled down by emitting one neutron and forming 269Ds, which by sequential alpha decays transformed to 265Hs, 261Sg, 257Rf, and 253No (nobelium-253). All these daughter isotopes were already known, and four decay chains observed in the following 12 days corroborated without any doubt the discovery of the element. Illustration a shows the first decay chain observed, which ended in 257Rf. The isotope 269Ds has a half-life of 0.2 ms and is produced with a cross section of about 3 × 10−36 cm2.

A second isotope, 271Ds, was produced in a subsequent 12-day experiment by fusion of nickel-64 and lead-208. Nine atoms, with an excitation energy of 12 MeV, were produced. They were transformed by sequential alpha decay to the known isotopes 267Hs, 263Sg, 259Rf, and 255No (nobelium-255). The half-life of 271Ds is 1.1 ms, and its production cross section amounts to 1.5 × 10−35 cm2.

The methods used to produce element Ds were the same as those already used to synthesize the three preceding elements, Bh, Hs, and Mt. Improved beam intensity and quality, improvement of the detection efficiency, and a new detector system allowing nearly complete chain reconstruction made possible the discovery after an extensive search for the optimum bombarding energy. The total sensitivity for finding a new species was increased by a factor of 20.

Two additional isotopes, 270Ds and 273Ds, are known. 273Ds was discovered as a descendant in the α-decay chain of 277112 in 1996. In the fusion of 207Pb and 64Ni, the isotope 270Ds with a half-life of 0.1 ms was synthesized at GSI in 2001.  See also: Darmstadtium

Roentgenium

Roentgenium (Rg, atomic number 111, named after W. Roentgen) was synthesized in 1994 at GSI. Roentgenium should be a homolog of the elements gold, silver, and copper. It is the ninth element in the 6d shell.

The element was discovered on December 17, 1994, by detection of the isotope 272Rg, which was produced by fusion of a nickel-64 projectile and a bismuth-209 target nucleus after the fused system was cooled by emission of one neutron. The optimum bombarding energy for producing 272Rg corresponds to an excitation energy of 15 MeV for the fused system. Sequential alpha decays to 268Mt, 264Bh, 260Db, and 256Lr (lawrencium-256) allowed identification from the known decay properties of 260Db and 256Lr. In the decay chain in illus. b, the first three members are new isotopes. The isotope 272Rg has a half-life of 1.5 ms, and is produced with a cross section of 3.5 × 10−36 cm2. Altogether, three chains were observed during the 17 days of irradiation. The methods used to produce roentgenium were the same as those used in the discovery of darmstadtium.  See also: Roentgenium

Element 112

Element 112 should be a heavy homolog of the elements mercury, cadmium, and zinc. It is expected to be the last element in the 6d shell. The element was discovered on February 9, 1996, at GSI by detection of the isotope 277112, which was produced by fusion of a zinc-70 projectile and a lead-208 target nucleus following the cooling down of the fused system by emission of a single neutron. The fused system was observed at an excitation energy of 12 MeV. Sequential alpha decays to 273Ds, 269Hs, 265Sg, 261Rf, and 257No (nobelium-257) allowed unambiguous identification by using the known decay properties of the last three members of the chain. In the decay chain in illus. c, the first three members are new isotopes. The isotope 277112 has a half-life of 0.24 ms, and it is produced with a cross section of 0.5 × 10−36 cm2. The new isotopes of Ds and Hs are of special interest. Their half-lives and alpha energies are very different, as is characteristic of a closed-shell crossing. At the neutron number N = 162, a closed shell was theoretically predicted, and this closed shell is verified in the decay chain observed. The isotope 269Hs has a half-life of 9 s, which is long enough to allow studies on the chemistry of this element. The methods used to produce element 112 were the same as those used for the two preceding elements, Ds and Rg. The decay chain of the new element was observed in an irradiation time of about 3 weeks. The cross section measured is the smallest observed in the production of heavy elements.

The crossing of the neutron shell at N = 162 is an important achievement in the field of research on superheavy elements. The stabilization of superheavy elements is based on high fission barriers, which are due to corrections in the binding energies found near closed shells. The shell at N = 162 is the first such shell predicted, and is now verified. Next in line are the predicted shells at proton number Z = 114 and neutron number N = 184.  See also: Element 112

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