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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
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
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
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
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
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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