Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 113–120, 2002

113

The Chemistry of Transactinide Elements — Experimental Achievements and Perspectives
M. Schädel∗Gesellschaft für Schwerionenforschung mbH, Planckstr. 1, D-64291 Darmstadt, Germany
Received: March 24, 2002; In Final Form: May 3, 2002
The chemistry of transactinides and superheavy elements has reached element 108. Preparations are under way toleap to element 112 and beyond. This development, its current status and future perspectives are reviewed from anexperimental point of view. The atom-at-a-time situation of transactinide chemistry is briefly outlined. Experimental
techniques and important results enlightening the chemical properties of elements 104 through 108 are presented in
an exemplary way with emphasis on the aqueous chemistry of the lighter ones. From the results of these experiments
it is justified to place these elements in the Periodic Table of the Elements into groups 4 through 8, respectively.
However, strongly due to the influence of relativistic effects, it is no longer possible to deduce detailed chemical
properties of these superheavy elements from this position. Perspectives for future research programs are given.

1. Introduction
Today we know of 112 chemical elements.1 The discoveriesof elements 1142–4 and 1165, 6 are currently waiting to be confirmed. Element 104, rutherfordium, Rf, marks the beginning ofa remarkable series of chemical elements: From a nuclear pointof view they can be called superheavy elements — as they only
live because of their microscopic shell stabilization, see e.g. References 1,7, — and from a chemical point of view they are transactinide elements — as the series of actinides ends with element
103.8 One of the most important and most interesting questions
for a chemist is the one about the position of the transactinides
in the Periodic Table of the Elements, PTE, see Figure 1.
From atomic calculations, see e.g. References 9–11, one expects the filling of the 6d electron shell for the transactinides,
and, consequently, a chemical behavior similar to the lightertransition metals in the fifth and sixth periods. However, it isby no means trivial to put rutherfordium into group 4 of the Periodic Table — and the heavier ones into successive groups — if
one accepts that the Periodic Table as an ordering scheme also
reflects the chemical property of an element at a given position.
Modern relativistic atomic and molecular calculations10–12
clearly show the very large influence of direct and indirect relativistic effects on the energetic position and the sequence ofelectronic orbitals. This is also associated with changes in theirradial distributions. All these changes are so pronounced that,
if compared with non-relativistic calculations, significantly different chemical properties for the transactinides would not be
surprising. Therefore, it is of great interest to study chemical
properties of the transactinides in detail and to compare thesewith properties deduced from extrapolations and from modern
relativistic molecular calculations in combination with empirical models. First generation experiments on rutherfordium13–15
and element 105, dubnium, Db,16, 17 rendered enough justification to place Rf and Db into group 4 and 5 of the Periodic Table.
The chemistry of transactinide elements has now reached element 108. Preparations are under way to reach out for element 112 and beyond. These experiments have concentrated on
the question how the chemical properties of the transactinides
compare with the ones of their respective lighter homologs andhow well the Periodic Table accommodates the transactinide elements as transition metals in the seventh period.
This contribution is focusing on recent key experiments ( i )to unravel detailed chemical properties of elements 104 and 105
in aqueous solution, ( ii ) first survey experiments on the aqueous chemistry of element 106, seaborgium, Sg, and ( iii ) the
first, successful experiments on element 107, bohrium, Bh, and∗Corresponding author. E-mail: m.schaedel@gsi.de. FAX: +496159-71-2903.

element 108, hassium, Hs, performed in the gas phase. It includes aspects of nuclear reactions, in an exemplary way experimental techniques, important experimental results enlightening the chemical properties of these elements, and perspectives. For comprehensive reviews of earlier and additional results, especially on the Rf, Db, and Sg chemistry, see, e.g., References 18–21.
2. Experimental Techniques
2.1. Nuclear Synthesis and Decay. Actinide target based
heavy-ion reactions, frequently termed “hot-fusion” reactions,
are used to synthesize the most neutron-rich and relatively longlived isotopes of transactinides; see Table 1 for a compilation.
Cross sections are in the range from a few nanobarns to about
10 nb for 78-s 261 Rf, 34-s 262 Db, and 27-s 263 Db, about 240 pb
for 7.4-s 265 Sg, 70 pb for 17-s 267 Bh, and 6 pb for 9-s 269 Hs(Ref. 22–27). See Reference 28 for a recent, comprehensive
discussion of 261 Rf and 262 Db cross sections. Except for themost recent Hs experiment, see below, setups were used, thattolerate beam intensities of approximately 3 ×1012 heavy ions
per second on targets of about 0.8 mg/cm2 thickness. This
yields maximum production rates from about two or three atoms
per minute for Rf and Db isotopes to five atoms per hour for265
Sg and even less for 267 Bh and heavier nuclides. Therefore, all chemical separations on transactinides are performed
1

18

He
H 2
13
14
15
16
17 2
1
B
C
N
O
F
Ne
Li
Be
3
4
5
6
7
8
9
10
Mg
Al
Si
P
S
Cl
Na
3
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18Ar
11
12
K 20Ca 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br 36Kr
19
Rb
Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn 51Sb 52Te 53 I 54Xe
37
38
* Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Cs
Ba
La
55
56
57
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
+ Rf Db Sg Bh HsFrRaAc
Mt 110 111112
114
116
87
88
89
104
105
106
107
108
109

+ Actinides

Th Pa
90
91

110

111

114

112

116

Transactinides = Superheavy Elements

U Np

92

93

Pu Am Cm Bk Cf Es Fm Md No Lr

94

95

96

97

98

99

100

101

102

103

* Lanthanides 58Ce 59Pr 60Nd 61Pm 62Sm 63Eu 64Gd 65Tb 66Dy 67Ho 68Er 69Tm 70Yb 71Lu
Figure 1. Periodic Table of the Elements. The known transactinide
elements 104 through 112 shall take the positions of the seventh periodtransition metals below Hf in group 4 and Hg in group 12. While chemical studies have justified placing the elements Rf through Hs into the
Periodic Table, the chemically “unknown” heavier elements still need
to be investigated. The arrangement of the actinides reflects that the firstactinide elements still resemble, to a decreasing extent, the chemistry ofd-elements: Th below group 4 elements Zr and Hf, Pa below Nb andTa, and U below the group 6 elements Mo and W.

c 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 6/30/2002

114 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

SchädelTABLE 1: Nuclides from “hot-fusion” nuclear reactions used in transactinide chemistry.

TargetProjectile
Evap.
σ
Prod. rated
Comment
248
18
Cm
O
5n
≈10 nb2 min−1
a
244
22
Pu
Ne
5n
4 nb
1 min−1
c
262
249
18
Db
34 s
Bk
O
5n
6 nb
2 min−1
a
248
19
Cm
F
5n
1 nb
0.5 min−1
b
263
249
18
Db
27 s
Bk
O
4n
10 nb
3 min−1
b
265
248
22
Sg
7.4 s
Cm
Ne
5n
≈240 pb
5 h−1
a
266
248
22
Sg
21 s
Cm
Ne
4n
≈25 pb
0.5 h−1
b
266
249
22
Bh
≈1 s
Bk
Ne
5n
25–250 pb
b
266
249
22
Bh
17 s
Bk
Ne
4n
≈70 pb
1.5 h−1
a
269
248
26
Hs
9s
Cm
Mg
5n
≈6 pb
3 d−1
a
270
248
26
Hs
2–7 s
Cm
Mg
4n
≈4 pb
2 d−1
b
a
Reaction normally used in chemistry experiments.
b
Reaction occasionally used or nuclide observed as a “by-product” or discovered in a radiochemical experiment.
cMeasured with recoil separator.
dAssuming typical values of 0.8 mg/cm2 target thickness and 3 ×1012 s−1 beam particles.
Nuclide261m
Rf

Half-life
78 swith single atoms on an “atom-at-a-time”31 scale. Characteristic
α decays, and, more significantly, time correlated α(mother)α(daughter)-decay chains, are used to identify these isotopes
in specific fractions or at characteristic positions after chemical
separation.
2.2. Schematic for On-line Experiments
2.2.1. Production and Transport. The atom-at-a-time situation of chemical separations with transactinides requires an optimization of the following parameters:( i ) Choice of the nuclear reaction — normally the one with thehighest cross section.
( ii ) High production rates — ion-source, accelerator, and target
technology are cornerstones.
( iii ) Fast transport and fast chemical separation — to avoid severe nuclear decay losses.
( iv ) High yields in transport and in chemical separation.
( v ) Efficient detection and identification.
( vi ) A chemical separation procedure which fulfills these needs
and, at the same time, provides the desired information about
the chemical behavior of the transactinide element.
Many details of various experimental setups and detectors aredescribed in References 18–20, 32.
A schematic presentation of components for automated online experiments with transactinides is shown in Figure 2.
Heavy-ion beams are passing through a vacuum isolation window and target-backing before interacting with the actinide target material. Characteristic to all recent experiments on Rf
through Bh was the use of aerosols (“clusters”) as the carrier

Accelerator

TargetJet
gas

Cluster
generator

material for nuclear reaction products to be transported in the
He-gas jet from the recoil chamber — where they are stopped in
the gas — to the chemistry apparatus. Routinely, KCl is used as
an aerosol material for separations carried out from the aqueous
phase and carbon for gas chromatographic separations. Transport times of typically two to five seconds were achieved.
2.2.2. Liquid Phase Chemistry Setups. So far, all liquidphase separations to study the chemical behavior of transactinides were performed in a discontinuous, batch-wise operation with large numbers of cyclic repetitions. While in several experiments on Rf and Db manual procedures were used,33
most transactinide separations were carried out with automatedinstruments as schematically shown in the upper part of Figure 3. The implementation of the Automated Rapid Chemistry
Apparatus, ARCA,34 yielded the predominant share of today’s
knowledge about the chemical behavior of the transactinide elements Rf through Sg in aqueous solution. ARCA II is microcomputer controlled and allows fast, repetitive, and reproducible
chromatographic separations in a miniaturized and chemicallyinert liquid-chromatography system (8 mm long micro-columnsof 1.6 mm i.d.) with typical cycle times between 45 s and 90 s.
Depending on the chemistry, the columns were filled with cationor anion exchange resin or an organic extractant on an inert support material.
Common to all batch-wise separations are time consuming
evaporation steps to prepare one or more samples for α spectroscopy and to measure spontaneous-fission fragments. Samples are dried by intense IR light and a stream of hot He gas.
While separation times are normally between 5 s and 10 s, evaporating the aqueous phase requires about 20 s.
A breakthrough concerning the automatization of the samLiquid-phase chemistry
Solution
JET
Collect

atom-at-a-time

Dissolve +complexformation

Recoil chamber

continuous

Cluster jet
transport

Gas-phase chemistry
JET

Chemistry

Miniaturized
liquid chromatography

Figure 2. Schematic presentation of components of recent on-line
experiments with the transactinides Rf to Bh.

Samplepreparation

Reactionzone(isothermal)
chromatographic zone

high Ta, SFx, g

Recluster
Jet
Transport

or direct
coupling
continuous, » 10 s

Measurement( (cyclic, » 45 - 90 s

Reactive gas

Detector

Reference
22, 2829
23
28, 3023
24
24
25, 2625, 2627
27

Catcher(wheel, tape)withdetectors( a, SF, x, g )cyclic, ³ 1 s

Figure 3. Schematic flow chart of components for an automated online chemistry from liquid phase (upper part) and from gas phase (lowerpart).

The Chemistry of Transactinide Elements

J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

ple preparation was achieved with the Automated Ion Exchange
Apparatus Coupled with Detection System for Alpha Spectroscopy, AIDA.35, 36 In addition, AIDA includes the automatedtransport of samples to and from the vacuum chamber for nuclear spectroscopy. AIDA, very similar to ARCA in its part asa chemical separation apparatus — with improvements in somedetails —, has recently been applied to detailed studies of the Rfchemistry.
Perspectives of continuously working liquid-phase separation
systems are discussed in the last section of this contribution.
2.2.3. Gas Phase Chemistry Setups. Continuously operating gas-phase separations,37 like the On-Line Gas chromatographic Apparatus, OLGA,38 were extremely instrumental to
study the halide- and oxide-compound formation of the transactinides Rf through Bh and to investigate their characteristic
retention time — a measure very often expressed as a “volatility”. The lower part of Figure 3 shows a flow scheme of such
type of chromatographic separation. Common to all of theseexperiments is the use of the known nuclide half-life to determine a “retention-time-equivalent” in gas-chromatographic experiments on an atom-at-a-time scale — its the 50% yield valueof a break-through curve measured as a function of the isothermal temperature. The temperature corresponding to the 50%
yield at the exit of the chromatography column is equal to thetemperature at which, in classical gas-chromatographic separations, the retention time would be equal to the half-life of theinvestigated nuclide. Products leaving the chromatography column are usually attached to new aerosols in a so-called recluster
process and are transported in a gas-jet to a detector system likethe ROtating wheel Multidetector Analyzer, ROMA.39 Heresamples are assayed for time correlated, characteristic α decays
and for spontaneous-fission fragments.
More details are discussed in the context of the bohrium
chemistry. The very unique experimental technique used for the
first chemical identification and investigation of hassium is outlined in the next section.

the behavior of other elements investigated in the same experiments, and only if all investigated nuclides have about the samehalf-life. In the interpretation of these results, e.g., beyond thepure analogy to the lighter homologs, assumptions about the oxidation state or the type of compound formed enter. Empirical model assumptions are needed, e.g., to calculate physicochemical quantities like adsorption — or sublimation enthalpies.
The step towards the interpretation of these results in terms of
relativistic effects establishes an even more sophisticated taskcomplicated by other effects in the atoms and molecules likespin-orbit coupling, configuration mixing, and shell effects.10, 11
In this section, exemplary results of the chemical behavior ofRf, Db, and Sg in aqueous solution are presented and discussed.
First experiments successfully performed for Bh and Hs in thegas phase are briefly outlined. A more comprehensive compilation of recent results from chemical separations in the gas-phasecan be found in References 19, 20.
3.1. Element 104, Rutherfordium. Already the pioneering experiments of the Rf behavior in acidic, aqueous solutions14, 15 demonstrated that Rf behaves different from trivalentactinides and as expected for a member of group 4 of the PTE
with Zr and Hf as the lighter homologs. Many attempts havebeen made to study the behavior of Rf in comparison withgroup-4 elements and with tetravalent Th and Pu ions as pseudohomologs, see, e.g., References 18,19,21,33,40 for summaries.
However, some of these experiments yielded conflicting results
and some were plagued with adsorption problems, see discussion in Reference 19.
The key issue of most of these experiments concentrated on
the competing strength of hydrolysis and complex formation. To
shed more light on this question additional data were determinedwith ARCA to study the behavior of Rf in comparison withlighter homologs and pseudo-homologs. Differences in the F− -Th off-line
Hf off-line
Zr off-line

Kd - values

80

Rf,Zr, Hf,Ln, An

70

100 %
105Db
Ta

Pa, Nb

60
50
40
30

102

20

101

10

100
10-1

4 M HCl / 6 M HNO3 /0.02 M HF 0.015 M HF

90

Eluted Activity / %

104
103

12 M HCl /0.02 M HF

100

3. Experimental Results and Discussion
Experimental results presented in this section yield important
information on the chemical behavior of these elusive elements— for a chemist a value in itself. Discussing these properties
in the context of other elements properties, the structure of the
Periodic Table or even the manifestation of relativistic effects is
an increasingly difficult task. Here one should keep in mind thatthe measured quantities — deposition temperatures or retention
times in thermo- or gas-chromatography and elution positions
or distribution coefficients in liquid chromatography or extraction experiments — can straight forward only be compared with115

Rf on-line
Hf on-line

10-5

10-4

0
10-3

10-2

10-1

100

101

[HF] in 0.1 M HNO3
Figure 4. Sorption of Zr, Hf, Th, and Rf on the cation exchange resin
Aminex A6 (CIX) from 0.1 M HNO3 at various HF concentrations. As
indicated, some data were obtained in off-line and some in on-line experiments. Revised version of data from Reference 41.

0

5

10

15

20

25

30

35

Time / sFigure 5. Elution curve for pentavalent Nb- and Pa-ion tracers with
4 M HCl/0.02 M HF from TIOA/Voltalef columns with ARCA.47 Db
was eluted together with Nb and Pa and did not appear in the Ta strip
fraction (6 M HNO3 /0.015 M HF). The first fraction represents loading
and washing steps with 12 M HCl/0.02 M HF to separate unwanted
trivalent (Ln, An) and tetravalent (Zr, Hf, Rf) species.

116 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002
complexation of the group-4 elements Zr, Hf, and Rf, and thepseudo-homolog Th were obtained in mixed 0.1 M HNO3 /HF
solutions on cation exchange resin (CIX; see Figure 4) and onanion exchange resins (AIX). Data shown in Figure 4 are reevaluated data from Reference 41. Lower limit values and large
error bars for some of the Rf distribution coefficients, Kd , mainly
result from low statistics of observed Rf α-decays in one of thetwo fractions assayed.
Kd values drop for Zr and Hf on CIX between 10−4 M and−2
10 M HF. For Rf this decrease is observed at about one orderof magnitude higher HF concentrations, and even higher for Th,
i.e., the transition from cationic to neutral and anionic species
requires higher HF concentrations for Rf compared with Zr andHf but lower than the ones needed for Th. This established thefollowing sequence of F− -complex formation strength at low
HF concentrations: Zr ≥ Hf > Rf > Th.
This result is in agreement with data obtained with AIDA35
and with theoretical expectations.12 A complete and quantitative understanding of the Rf behavior requires further quantumchemical calculations and a theoretical treatment of complex formation and hydrolysis of these tetravalent ions. However, using the ionic radii of Zr (0.072 nm), Hf (0.071 nm), Rf(0.078 nm), and Th (0.094 nm)11, 42 it is suggestive to apply the
HSAB (Hard Soft Acid Base) concept43 for an explanation of
the differences observed on the CIX. In the HSAB concept one
assumes that the hard F− -anion interacts stronger with small(hard) cations. From this, one would expect, in agreement withthe observation, a weaker F− -complexation of Rf as comparedwith Zr and Hf.
On AIX, a rise in Kd values from about 10 to more than 100 isobserved between 10−3 M and 10−1 M HF for Zr and Hf measured off-line in batch extraction experiments. This is consistent
with the continuation of the trend observed on CIX. For Th offline data, and for the Hf and Rf on-line data, no significant riseof the Kd values was observed on AIX between 10−3 M HF andconcentrations of up to 1 M HF. While this is expected for Th,
which does not form fluoride complexes, it comes as surprisefor Hf and Rf. How much this is affected by the 0.1 M HNO3Pa

Db

in solution remains unclear. Presently, these data reflect the best
knowledge from these experiments. However, it is obvious that
there are still unresolved problems with the consistency between
off-line and on-line data. To solve this puzzle, kinetic effects arepresently considered. Therefore, one shall view the AIX data as
under discussion.
Earlier experimental results suggest that Rf forms anionic F− complexes in pure 0.2 M HF44 and in mixed 0.27 M HF/0.1 M
and 0.2 M HNO3 solutions.45 It is interesting to note that Rf
shows an intermediate behavior between Zr and Hf, with sometendency to be closer to Hf, in the formation of neutral chloride
complexes probed with tributylphosphate at 8 M HCl.46 The formation of anionic chloride complexes of Zr, Hf, and Rf above
8 M HCl was shown in experiments with AIDA.36 This clearly
indicates the group-4 properties of Rf. A hypothetically Th-likeor Pu-like behavior of Rf was probed with an anion exchange
resin and 8 M HNO3 in experiments with AIDA. While Th andPu form anionic complexes and are adsorbed Rf remains in solution36 as expected for a typical group-4 element with non Th-like— and non Pu-like properties.
3.2. Element 105, Dubnium. In its first aqueous chemistryelement 105, dubnium, Db, (called hahnium, Ha, in many of theearly publications) was adsorbed onto glass surfaces from HCl
and HNO3 solutions,17 a behavior very characteristic for group5 elements. An attempt to extract Db fluoride complexes failed
under conditions in which Ta extracts but Nb does not.17
The first detailed comparison between Db, its lighter homologs Nb and Ta, and the pseudo-homolog Pa was carriedout from solutions at different HCl concentrations with small
amounts of HF added. They were performed as liquid-liquid
extraction chromatography experiments with triisooctyl amine,
TIOA, as a stationary phase on an inert support in ARCA. In
these experiments Db showed a striking non Ta-like behavior,e.g., see Figure 5. At 0.5 M HCl/0.01 M HF, Db even showed
Pa-like properties.47, 48
The interpretation of these results is hampered by the useof the mixed HCl/HF solution that did not allow clearly distinguishing which complex was formed. In contrast to theexperimentally observed extraction sequence from HCl solutions with small amounts of HF added, the inverse orderPa  Nb ≥ Db > Ta was theoretically predicted49 for the extraction from pure HCl solutions. This work considered the competition between hydrolysis50 and chloride complex formation.
Recent experimental studies performed in the pure F− , Cl− , andBr− system51 are in excellent agreement with the theoretical predictions which include relativistic effects.49, 52
For the system Aliquat 336(Cl− ) and (pure) 6 M HCl an extraction sequence of Pa > Nb ≥ Db > Ta was determined, see
60

Nb

Ta

Eluted Activity / %KD Value105

Schädel

Sg

50
40

Me

6+

30
2+

3+

20

no Me , Me

10

no Me , UO 22+

4+

0
2

HCl / mol/lFigure 6. Coefficients for the extraction of Ta (small circles, solid line),
Nb (squares, long-dashed line), and Pa (triangles, short-dashed line)
tracers from pure HCl solutions at various concentrations into Aliquat
336(Cl− ). The Kd of Db at 6 M HCl (large circle) is plotted with error
bars encompassing 68% confidence limits. Data from Reference 51.

4

6

8

10

12

14

16

18

20

Time / sFigure 7. Elution curve for W-tracer (Me6+ ), modeling the Sg separation on ARCA in 0.1 M HNO3 /5 ×10−4 M HF, together with “lower
limit” (lower right arrow) for the elution of di-, tri-, tetravalent ions, andUO2+
2 . Elutions were performed at room temperature with a flow rate of
1 mL/min from 1.6 × 8 mm columns filled with the 17.5 ± 2 µm particle
size cation exchange resin Aminex A6. From Reference 54.

The Chemistry of Transactinide Elements

J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

Figure 6, which — in agreement of the theoretical prediction49, 52— is inverse to that in HF containing HCl solution. Kd valuesof 1440, 683, 438+532−166 , and 22 were measured for the Pa, Nb,
Db, and Ta, respectively, in series of off-line and on-line experiments. More information on other, previously performedexperiments is given in References 18, 19, 21, 33.
3.3. Element 106, Seaborgium. The first chemical separations of element 106, seaborgium, Sg, in an aqueous solution
were performed from 0.1 M HNO3 /5 ×10−4 M HF on a cationexchange resin in ARCA.53, 54 Seaborgium, detected throughthree correlated α-decays of the Rf- and No-daughter nuclei,
elutes together with the hexavalent Mo- and W-ions; see Figure 7 for a chromatogram of the W-tracer studies. Hexavalent
U-ions, present as UO2+
2 cations, are strongly retained on thecolumn as di-, tri-, and tetravalent ions are. This result showedthat the most stable oxidation state of Sg in aqueous solution
is +6, and that, like its homologs Mo and W, Sg forms neutral or anionic oxo- or oxohalide-compounds. Under the given
condition, Sg exhibits properties very characteristic of group 6elements, and does not show U-like properties.
A second experiment was performed with pure 0.1 M HNO3
to probe the influence of the F− -anions on the complex formation.55 Contrary to the lighter homologous elements Mo and W,
seaborgium was not eluted from the CIX in pure 0.1 M HNO3 .
From this it can be concluded that F− -anions significantly contributed to the complex formation in the first experiment. This
rules out that Sg was eluted as [SgO4 ]2− in the first experiment
while the formation of the neutral complex SgO2 F2 or the anionic complex [SgO2 F3 ]− seems to be likely. The non-tungsten
like behavior of Sg in pure HNO3 may be attributed to its weakertendency to hydrolyse.55, 56 While Mo and W can reach theneutral species MO2 (OH)2 (M = Mo, W) for Sg hydrolysis presumably stops at [Sg(OH)5 (H2 O)]+ (sometimes characterized as[SgO(OH)3 ]+ )56 or already even at [Sg(OH)4 (H2 O)2 ]2+ .
Results and discussions from studies of halide-oxide compounds and oxide-hydroxide compounds of Sg in gas chromatographic experiments can be found in References 57–59.
3.4. Element 107, Bohrium. So far, the only informationon the chemical properties of bohrium was obtained in gas chromatographic experiments26 with an experimental setup schematically shown in Figure 8. The investigation of volatile Me(VII)halides-oxides was an obvious way to study Bh in comparison
with the lighter group-7 homologous elements Tc and Re according to their intermediate position between the lighter transactinides, which form volatile halides, and the highly volatilegroup-8 tetroxides.
Nuclear reaction products were carried with C-aerosols as acluster material from the recoil chamber to the On-Line Gas
chromatographic Apparatus, OLGA,38 see Figure 8. The reactive gases HCl and O2 were added in front of the high temperature zone of the reaction oven where C-aerosols were burned
and where the compounds MeO3 Cl (Me = Tc, Re, Bh) wereformed. Relative yields of the breaking-through compounds
were measured as a function of the isothermal temperaturein the quartz chromatography column in the detection system
ROtating wheel Multidetector Analyzer, ROMA.39 CsCl was
He

Carbonaerosols

117

used as a re-cluster material in the Bh experiments to transport
those compounds, which leave the chromatography column, to
the ROMA.
From the observation of four 267 Bh nuclear-decay chainsat 180 ◦ C (isothermal chromatography temperature), two at
150 ◦ C, and zero at 75 ◦ C a BhO3 Cl break-through curve wasmodeled and was compared with known curves from tracer studies with lighter homologous elements.26 The characteristic 50%
yield of the BhO3 Cl curve is located at a higher temperaturecompared with the ReO3 Cl and the TcO3 Cl behavior.
Qualitatively, this results shows that Bh behaves like a member of group 7 of the PTE and forms a volatile halide-oxide —presumably BhO3 Cl as inferred from its properties — which isless volatile than the compound of the lighter homologous elements.26 More quantitatively, the deduced BhO3 Cl adsorption
enthalpy is in good agreement with a theoretical prediction including relativistic effects.60 This result coincides with the value
expected from empirical correlations of thermochemical properties and Bh positioned in group 7 of the periodic table.26
3.5. Element 108, Hassium. Very recently, the first chemical separation and identification of a Hs compound wasachieved.27, 61 From its expected chemical properties as a member of group 8 of the Periodic Table it is most attractive to investigate the highly volatile tetroxide — a unique property only
known for group 8 transition metals. The experiment was performed at the UNILAC accelerator at GSI with a setup schematically shown in Figure 9. This experiment was unique, in a number of aspects, and different from recent gas-chromatographic
experiments:( i ) A rotating target wheel, ARTESIA, see Figure 9, for 248 Cmtargets in combination with a gas-jet transport system was applied for the first time in transactinide chemistry to accept higher
beam intensities.
( ii ) The chemical reaction with the reactive gas O2 was
performed “in-situ” in the recoil chamber IVO, In situ
Volatilization and On-line Detection.62 An oven attached to
the recoil chamber provided a fast and efficient oxidation of the
stopped recoils. This allowed transporting highly volatile compounds without any cluster material in a dry He/O2 gas mixture
over 10 m in a Teflon capillary to the detection system. It should
be noted that already early work on the halide volatility of Rf
used a chemical reaction in or immediately behind the recoilchamber directly attached to a thermo-chromatography column,
see Reference 63 and references therein.
( iii ) An on-line thermo-chromatography separator and detector
setup, COLD, Cryo On-line Detector,61 operated at low temperatures was mainly used. This is an improved version of thepreviously developed Cryo-Thermochromatography Separator,
CTS,64 which was also used during this beam time. COLD consists of 36 pairs of silicon PIN-photodiodes coupled to a support
which provides the negative temperature gradient between about
room temperature at the entrance side and about 100 K at the gas
exit.
Nuclear decays of the known isotope 269 Hs and of the isotope
270
Hs, which was discovered in the course of this experiment,
were observed27 in a narrow peak61 along the temperature gradient. From the observation of HsO4 and its adsorption position,in comparison with the known one of OsO4 , one concludes thatcarbon aerosols
reaction
Reaction
products
products
249Bk
Target22
BEAM
118MeV

HCl

O2

Recoilchamber

Chromatography
columnto detection system
ROMA

Reaction
product

Capillary

248Cm

Reactionreaction
oven

Ar

CsCl
Aerosols

Heat

N
N22

Cool

Thermochromatography
and detection system

Targetwheel

He

Figure 8. Schematic view of the Bh gas-chromatography experiment.
Adapted from Reference 26.

Reaction
oven

BEAM

O2

Figure 9.
Schematic view of the low temperature thermochromatography experiment to investigate HsO4 . Adapted from Reference 61.

118 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002
Hs forms a relatively stable, volatile tetroxide — as expected fora typical member of group 8.12, 65 However, the exact adsorption
position is at a surprisingly high temperature,61 i.e., HsO4 exhibits an unexpected low volatility or, in other terms, a high,negative adsorption enthalpy.
4. Summary
As has already been observed in earlier experiments, the results from these more recent studies also justify positioning of
the transactinides — or superheavy elements —, beginning withelement 104, into the seventh period of the Periodic Table, see
Figure 1. So far chemical studies on their behavior have been
performed with Rf, Db, and Sg in aqueous phase. Studies in
the gas phase — also performed for Rf, Db, and Sg — haverecently been extended to Bh and Hs. All experimental results
yield properties which place these elements into their respective
group of the Periodic Table — Rf, Db, Sg, Bh, Hs into group 4,
5, 6, 7, and 8, respectively. This demonstrates that the Periodic
Table still remains an appropriate ordering scheme.
However, a closer and more subtle look reveals that all themore detailed chemical properties of these elements — in comparison with their lighter homologs — are no longer reliably
predictable by simple extrapolations in the Periodic Table. But
modern relativistic molecular calculations in combination with
empirical models allow for quantitative, or semi-quantitative,
comparisons of experimental and theoretical results, and theyshow excellent agreement in a number of cases. From this one
can deduce that relativistic effects strongly influence the chemical properties of the transactinides and they can give indications
what facets of the chemical properties are predominately due torelativistic effects.
5. Perspectives
The prospects to study chemical properties of transactinides
beyond element 106 in aqueous phase experiments mainly depend on the development of methods to cope with production
rates of less than one atom per hour. This situation will remain even after further developments to increase production
rates from higher heavy-ion beam intensities and developmentsof new target design which can tolerate these intensities. But
for all of that, only continuously running separation processes
with an on-line detection or with detection of long-lived daughter isotopes are applicable as long as the half-lives for these superheavy element nuclides will be in the range of minutes or
shorter.
Fascinating and challenging prospects are ahead with halflives of the order of about 10 s for isotopes of element 107,
Bohrium, Bh, — for which a gas-chromatographic separationand characterization was already achieved26 — and element 108,
Hassium, Hs, located at or slightly below the N = 162 neutronshell. Even more interesting and more challenging will be future attempts to extend the aqueous chemistry of superheavy elements all the way to element 114. The most recent results fromthe reaction of 48 Ca ions with 238 U and 242,244 Pu targets2, 3 indicate the existence of even longer-lived nuclides of elements 108
to 114 between neutron number N = 169 and N = 174.
In addition to the fascinating chemistry aspects of these superheavy elements, nuclear chemistry experiments are vital tools
for a clear identification of the atomic number (element identification) of relatively long-lived spontaneously-fissioning nuclides at the end of α-decay chains.
Two types of continuously operating liquid-chemical separation processes are presently emerging for transactinides. The
one with a truly continuous separation and detection technique isthe fast, automated, and miniaturized centrifuge system SISAK
III66–68 which allows separating nuclides with half-lives of theorder of 1 s. Separations are based on liquid-liquid extractions
with subsequent phase separation and detection of the α de-Schädelcay in the organic phase with an extractive liquid scintillator.69
Most recent improvements of the detection, data acquisition, andpulse shape discrimination system were tested in Rf-chemistry
experiments and yielded very promising aspects for the futureapplication of the SISAK system in the transactinide chemistrywith short-lived nuclides.
A different approach to continuously separate transactinide
elements and to establish their chemical behavior from the observation of long-lived daughter isotopes in one or anotherfraction was introduced with the three- or multi-column technique.44, 45, 70 First experiments have been performed with Rf to
study the F− -complexation44, 45 and with Db for nuclear studies in the RACHEL device.30 Rf and Db, transported with the
He(KCl)-jet to the chemistry apparatus, were continuously dissolved, and this solution was passed through three consecutive
ion-exchange columns. Primary produced di- and trivalent actinides were “filtered” out on the first cation-exchange column.
In the next anion-exchange column anionic species were retained for some time, while the following cation-exchange column adsorbed cationic species — the long-lived α-decay products of Rf. These were eluted after the end of irradiation andwere detected by off-line spectroscopy. So far, the Rf experiments showed only a group-4 behavior of Rf and were not able
to determine subtle differences within the chemical propertiesof group-4 elements.
One of the disadvantages of this technique is its limited rangeof half-lives and Kd values. Its big advantage over batch-wise
techniques like ARCA is its potential to study short-lived isotopes of, e.g., a few seconds half-life. In addition, because of its
continuous and not manpower consuming operation, it may allow extending these studies to nuclides with cross sections well
below the nanobarn level. Preparations are under way to perform such studies to determine differences in the hydrolysis andcomplex formation of Mo, W, and Sg.
Existing separation techniques, like ARCA II,34 will remain
essential tools to shed more light on the diverse and often unexpected behavior of the lightest transactinide elements. With therelatively long half-lives and rather high cross sections one hasa chance to reveal properties in more complex chemical systems
which may not be applicable for heavier elements.
Well developed gas-phase chemistry techniques are at handto deepen our insights into many unresolved questions of compound formation, volatility, and adsorption behavior of group
4 through 7 halides, oxides, and mixed compounds. A challenge for the future will be the gas-phase studies of the metallic
transactinides beyond group 8. Element 112 has a very uniqueposition in this row with Hg as its lighter homolog in group 12.
Here first experiments are under way71 to look into the fascinating question whether element 112 will exhibit a Hg-like behavior or, possibly, a much more inert Rn-like one.
Acknowledgement. The author thanks all colleagues fromBerkeley, Bern, Darmstadt, Dresden, Dubna, Göteborg, Livermore, Mainz, Niigata, Oslo, Kassel, Krakow, Tokai, Tokyo,
and Villigen with whom he enjoyed the collaboration on thechemistry of transactinide elements. Special thanks goes to W.
Brüchle, J. V. Kratz, and V. Pershina for continuous helpful discussions. I acknowledge the help of Ch. Düllmann, R. Eichler,
H. Gäggeler, H. Haba, Y. Nagame, J. P. Omtvedt, A. Türler, andA. Yakushev who provided me with most recent, partially unpublished information. I especially like to thank B. Schausten
for her work on all graphics.
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