======================================================================
= Dubnium =
======================================================================
Introduction
======================================================================
Dubnium is a synthetic chemical element; it has symbol Db and atomic
number 105. It is highly radioactive: the most stable known isotope,
dubnium-268, has a half-life of about 16 hours. This greatly limits
extended research on the element.
Dubnium does not occur naturally on Earth and is produced
artificially. The Soviet Joint Institute for Nuclear Research (JINR)
claimed the first discovery of the element in 1968, followed by the
American Lawrence Berkeley Laboratory in 1970. Both teams proposed
their names for the new element and used them without formal approval.
The long-standing dispute was resolved in 1993 by an official
investigation of the discovery claims by the Transfermium Working
Group, formed by the International Union of Pure and Applied Chemistry
and the International Union of Pure and Applied Physics, resulting in
credit for the discovery being officially shared between both teams.
The element was formally named 'dubnium' in 1997 after the town of
Dubna, the site of the JINR.
Theoretical research establishes dubnium as a member of group 5 in the
6d series of transition metals, placing it under vanadium, niobium,
and tantalum. Dubnium should share most properties, such as its
valence electron configuration and having a dominant +5 oxidation
state, with the other group 5 elements, with a few anomalies due to
relativistic effects. A limited investigation of dubnium chemistry has
confirmed this.
Background
============
Uranium, element 92, is the heaviest element to occur in significant
quantities in nature; heavier elements can only be practically
produced by synthesis. The first synthesis of a new
element--neptunium, element 93--was achieved in 1940 by a team of
researchers in the United States. In the following years, American
scientists synthesized the elements up to mendelevium, element 101,
which was synthesized in 1955. From element 102, the priority of
discoveries was contested between American and Soviet physicists.
Their rivalry resulted in a race for new elements and credit for their
discoveries, later named the Transfermium Wars.
Reports
=========
The first report of the discovery of element 105 came from the Joint
Institute for Nuclear Research (JINR) in Dubna, Moscow Oblast, Soviet
Union, in April 1968. The scientists bombarded 243Am with a beam of
22Ne ions, and reported 9.4 MeV (with a half-life of 0.1-3 seconds)
and 9.7 MeV ('t'1/2 > 0.05 s) alpha activities followed by alpha
activities similar to those of either 256103 or 257103. Based on prior
theoretical predictions, the two activity lines were assigned to
261105 and 260105, respectively.
: + → 265−'x'105 + 'x' ('x' = 4, 5)
After observing the alpha decays of element 105, the researchers aimed
to observe spontaneous fission (SF) of the element and study the
resulting fission fragments. They published a paper in February 1970,
reporting multiple examples of two such activities, with half-lives of
14 ms and . They assigned the former activity to 242mfAm and ascribed
the latter activity to an isotope of element 105. They suggested that
it was unlikely that this activity could come from a transfer reaction
instead of element 105, because the yield ratio for this reaction was
significantly lower than that of the 242mfAm-producing transfer
reaction, in accordance with theoretical predictions. To establish
that this activity was not from a (22Ne,'x'n) reaction, the
researchers bombarded a 243Am target with 18O ions; reactions
producing 256103 and 257103 showed very little SF activity (matching
the established data), and the reaction producing heavier 258103 and
259103 produced no SF activity at all, in line with theoretical data.
The researchers concluded that the activities observed came from SF of
element 105.
In April 1970, a team at Lawrence Berkeley Laboratory (LBL), in
Berkeley, California, United States, claimed to have synthesized
element 105 by bombarding californium-249 with nitrogen-15 ions, with
an alpha activity of 9.1 MeV. To ensure this activity was not from a
different reaction, the team attempted other reactions: bombarding
249Cf with 14N, Pb with 15N, and Hg with 15N. They stated no such
activity was found in those reactions. The characteristics of the
daughter nuclei matched those of 256103, implying that the parent
nuclei were of 260105.
: + → 260105 + 4
These results did not confirm the JINR findings regarding the 9.4 MeV
or 9.7 MeV alpha decay of 260105, leaving only 261105 as a possibly
produced isotope.
JINR then attempted another experiment to create element 105,
published in a report in May 1970. They claimed that they had
synthesized more nuclei of element 105 and that the experiment
confirmed their previous work. According to the paper, the isotope
produced by JINR was probably 261105, or possibly 260105. This report
included an initial chemical examination: the thermal gradient version
of the gas-chromatography method was applied to demonstrate that the
chloride of what had formed from the SF activity nearly matched that
of niobium pentachloride, rather than hafnium tetrachloride. The team
identified a 2.2-second SF activity in a volatile chloride portraying
eka-tantalum properties, and inferred that the source of the SF
activity must have been element 105.
In June 1970, JINR made improvements on their first experiment, using
a purer target and reducing the intensity of transfer reactions by
installing a collimator before the catcher. This time, they were able
to find 9.1 MeV alpha activities with daughter isotopes identifiable
as either 256103 or 257103, implying that the original isotope was
either 260105 or 261105.
Naming controversy
====================
JINR did not propose a name after their first report claiming
synthesis of element 105, which would have been the usual practice.
This led LBL to believe that JINR did not have enough experimental
data to back their claim. After collecting more data, JINR proposed
the name 'bohrium' (Bo) in honor of the Danish nuclear physicist Niels
Bohr, a founder of the theories of atomic structure and quantum
theory; they soon changed their proposal to 'nielsbohrium' (Ns) to
avoid confusion with boron. Another proposed name was 'dubnium'. When
LBL first announced their synthesis of element 105, they proposed that
the new element be named 'hahnium' (Ha) after the German chemist Otto
Hahn, the "father of nuclear chemistry", thus creating an element
naming controversy.
In the early 1970s, both teams reported synthesis of the next element,
element 106, but did not suggest names. JINR suggested establishing an
international committee to clarify the discovery criteria. This
proposal was accepted in 1974 and a neutral joint group formed.
Neither team showed interest in resolving the conflict through a third
party, so the leading scientists of LBL--Albert Ghiorso and Glenn
Seaborg--traveled to Dubna in 1975 and met with the leading scientists
of JINR--Georgy Flerov, Yuri Oganessian, and others--to try to resolve
the conflict internally and render the neutral joint group
unnecessary; after two hours of discussions, this failed. The joint
neutral group never assembled to assess the claims, and the conflict
remained unresolved. In 1979, IUPAC suggested systematic element names
to be used as placeholders until permanent names were established;
under it, element 105 would be 'unnilpentium', from the Latin roots
'un-' and 'nil-' and the Greek root 'pent-' (meaning "one", "zero",
and "five", respectively, the digits of the atomic number). Both teams
ignored it as they did not wish to weaken their outstanding claims.
In 1981, the Gesellschaft für Schwerionenforschung (GSI; 'Society for
Heavy Ion Research') in Darmstadt, Hesse, West Germany, claimed
synthesis of element 107; their report came out five years after the
first report from JINR but with greater precision, making a more solid
claim on discovery. GSI acknowledged JINR's efforts by suggesting the
name 'nielsbohrium' for the new element. JINR did not suggest a new
name for element 105, stating it was more important to determine its
discoverers first.
In 1985, the International Union of Pure and Applied Chemistry (IUPAC)
and the International Union of Pure and Applied Physics (IUPAP) formed
a Transfermium Working Group (TWG) to assess discoveries and establish
final names for the controversial elements. The party held meetings
with delegates from the three competing institutes; in 1990, they
established criteria on recognition of an element, and in 1991, they
finished the work on assessing discoveries and disbanded. These
results were published in 1993. According to the report, the first
definitely successful experiment was the April 1970 LBL experiment,
closely followed by the June 1970 JINR experiment, so credit for the
discovery of the element should be shared between the two teams.
LBL said that the input from JINR was overrated in the review. They
claimed JINR was only able to unambiguously demonstrate the synthesis
of element 105 a year after they did. JINR and GSI endorsed the
report.
In 1994, IUPAC published a recommendation on naming the disputed
elements. For element 105, they proposed 'joliotium' (Jl) after the
French physicist Frédéric Joliot-Curie, a contributor to the
development of nuclear physics and chemistry; this name was originally
proposed by the Soviet team for element 102, which by then had long
been called nobelium. This recommendation was criticized by the
American scientists for several reasons. Firstly, their suggestions
were scrambled: the names 'rutherfordium' and 'hahnium', originally
suggested by Berkeley for elements 104 and 105, were respectively
reassigned to elements 106 and 108. Secondly, elements 104 and 105
were given names favored by JINR, despite earlier recognition of LBL
as an equal co-discoverer for both of them. Thirdly and most
importantly, IUPAC rejected the name 'seaborgium' for element 106,
having just approved a rule that an element could not be named after a
living person, even though the 1993 report had given the LBL team the
sole credit for its discovery.
In 1995, IUPAC abandoned the controversial rule and established a
committee of national representatives aimed at finding a compromise.
They suggested 'seaborgium' for element 106 in exchange for the
removal of all the other American proposals, except for the
established name 'lawrencium' for element 103. The equally entrenched
name 'nobelium' for element 102 was replaced by 'flerovium' after
Georgy Flerov, following the recognition by the 1993 report that that
element had been first synthesized in Dubna. This was rejected by
American scientists and the decision was retracted. The name
'flerovium' was later used for element 114.
In 1996, IUPAC held another meeting, reconsidered all names in hand,
and accepted another set of recommendations; it was approved and
published in 1997. Element 105 was named 'dubnium' (Db), after Dubna
in Russia, the location of the JINR; the American suggestions were
used for elements 102, 103, 104, and 106. The name 'dubnium' had been
used for element 104 in the previous IUPAC recommendation. The
American scientists "reluctantly" approved this decision. IUPAC
pointed out that the Berkeley laboratory had already been recognized
several times, in the naming of berkelium, californium, and americium,
and that the acceptance of the names 'rutherfordium' and 'seaborgium'
for elements 104 and 106 should be offset by recognizing JINR's
contributions to the discovery of elements 104, 105, and 106.
Even after 1997, LBL still sometimes used the name 'hahnium' for
element 105 in their own material, doing so as recently as 2014.
However, the problem was resolved in the literature as Jens Volker
Kratz, editor of 'Radiochimica Acta', refused to accept papers not
using the 1997 IUPAC nomenclature.
Isotopes
======================================================================
Dubnium, having an atomic number of 105, is a superheavy element; like
all elements with such high atomic numbers, it is very unstable. The
longest-lasting known isotope of dubnium, 268Db, has a half-life of
around a day. No stable isotopes have been seen, and a 2012
calculation by JINR suggested that the half-lives of all dubnium
isotopes would not significantly exceed a day. Dubnium can only be
obtained by artificial production.
The short half-life of dubnium limits experimentation. This is
exacerbated by the fact that the most stable isotopes are the hardest
to synthesize. Elements with a lower atomic number have stable
isotopes with a lower neutron-proton ratio than those with higher
atomic number, meaning that the target and beam nuclei that could be
employed to create the superheavy element have fewer neutrons than
needed to form these most stable isotopes. (Different techniques based
on rapid neutron capture and transfer reactions are being considered
as of the 2010s, but those based on the collision of a large and small
nucleus still dominate research in the area.)
Only a few atoms of 268Db can be produced in each experiment, and thus
the measured lifetimes vary significantly during the process. As of
2022, following additional experiments performed at the JINR's
Superheavy Element Factory (which started operations in 2019), the
half-life of 268Db is measured to be hours. The second most stable
isotope, 270Db, has been produced in even smaller quantities: three
atoms in total, with lifetimes of 33.4 h, 1.3 h, and 1.6 h. These two
are the heaviest isotopes of dubnium to date, and both were produced
as a result of decay of the heavier nuclei 288Mc and 294Ts rather than
directly, because the experiments that yielded them were originally
designed in Dubna for 48Ca beams. For its mass, 48Ca has by far the
greatest neutron excess of all practically stable nuclei, both
quantitative and relative, which correspondingly helps synthesize
superheavy nuclei with more neutrons, but this gain is compensated by
the decreased likelihood of fusion for high atomic numbers.
Predicted properties
======================================================================
According to the periodic law, dubnium should belong to group 5, with
vanadium, niobium, and tantalum. Several studies have investigated the
properties of element 105 and found that they generally agreed with
the predictions of the periodic law. Significant deviations may
nevertheless occur, due to relativistic effects, which dramatically
change physical properties on both atomic and macroscopic scales.
These properties have remained challenging to measure for several
reasons: the difficulties of production of superheavy atoms, the low
rates of production, which only allows for microscopic scales,
requirements for a radiochemistry laboratory to test the atoms, short
half-lives of those atoms, and the presence of many unwanted
activities apart from those of synthesis of superheavy atoms. So far,
studies have only been performed on single atoms.
Atomic and physical
=====================
A direct relativistic effect is that as the atomic numbers of elements
increase, the innermost electrons begin to revolve faster around the
nucleus as a result of an increase of electromagnetic attraction
between an electron and a nucleus. Similar effects have been found for
the outermost s orbitals (and p1/2 ones, though in dubnium they are
not occupied): for example, the 7s orbital contracts by 25% in size
and is stabilized by 2.6 eV.
A more indirect effect is that the contracted s and p1/2 orbitals
shield the charge of the nucleus more effectively, leaving less for
the outer d and f electrons, which therefore move in larger orbitals.
Dubnium is greatly affected by this: unlike the previous group 5
members, its 7s electrons are slightly more difficult to extract than
its 6d electrons.
Another effect is the spin-orbit interaction, particularly spin-orbit
splitting, which splits the 6d subshell--the azimuthal quantum number
ℓ of a d shell is 2--into two subshells, with four of the ten orbitals
having their ℓ lowered to 3/2 and six raised to 5/2. All ten energy
levels are raised; four of them are lower than the other six. (The
three 6d electrons normally occupy the lowest energy levels, 6d3/2.)
A singly ionized atom of dubnium (Db+) should lose a 6d electron
compared to a neutral atom; the doubly (Db2+) or triply (Db3+) ionized
atoms of dubnium should eliminate 7s electrons, unlike its lighter
homologs. Despite the changes, dubnium is still expected to have five
valence electrons. As the 6d orbitals of dubnium are more destabilized
than the 5d ones of tantalum, and Db3+ is expected to have two 6d,
rather than 7s, electrons remaining, the resulting +3 oxidation state
is expected to be unstable and even rarer than that of tantalum. The
ionization potential of dubnium in its maximum +5 oxidation state
should be slightly lower than that of tantalum and the ionic radius of
dubnium should increase compared to tantalum; this has a significant
effect on dubnium's chemistry.
Atoms of dubnium in the solid state should arrange themselves in a
body-centered cubic configuration, like the previous group 5 elements.
The predicted density of dubnium is 21.6 g/cm3.
Chemical
==========
Computational chemistry is simplest in gas-phase chemistry, in which
interactions between molecules may be ignored as negligible. Multiple
authors have researched dubnium pentachloride; calculations show it to
be consistent with the periodic laws by exhibiting the properties of a
compound of a group 5 element. For example, the molecular orbital
levels indicate that dubnium uses three 6d electron levels as
expected. Compared to its tantalum analog, dubnium pentachloride is
expected to show increased covalent character: a decrease in the
effective charge on an atom and an increase in the overlap population
(between orbitals of dubnium and chlorine).
Calculations of solution chemistry indicate that the maximum oxidation
state of dubnium, +5, will be more stable than those of niobium and
tantalum and the +3 and +4 states will be less stable. The tendency
towards hydrolysis of cations with the highest oxidation state should
continue to decrease within group 5 but is still expected to be quite
rapid. Complexation of dubnium is expected to follow group 5 trends in
its richness. Calculations for hydroxo-chlorido- complexes have shown
a reversal in the trends of complex formation and extraction of group
5 elements, with dubnium being more prone to do so than tantalum.
Experimental chemistry
======================================================================
Experimental results of the chemistry of dubnium date back to 1974 and
1976. JINR researchers used a thermochromatographic system and
concluded that the volatility of dubnium bromide was less than that of
niobium bromide and about the same as that of hafnium bromide. It is
not certain that the detected fission products confirmed that the
parent was indeed element 105. These results may imply that dubnium
behaves more like hafnium than niobium.
The next studies on the chemistry of dubnium were conducted in 1988,
in Berkeley. They examined whether the most stable oxidation state of
dubnium in aqueous solution was +5. Dubnium was fumed twice and washed
with concentrated nitric acid; sorption of dubnium on glass cover
slips was then compared with that of the group 5 elements niobium and
tantalum and the group 4 elements zirconium and hafnium produced under
similar conditions. The group 5 elements are known to sorb on glass
surfaces; the group 4 elements do not. Dubnium was confirmed as a
group 5 member. Surprisingly, the behavior on extraction from mixed
nitric and hydrofluoric acid solution into methyl isobutyl ketone
differed between dubnium, tantalum, and niobium. Dubnium did not
extract and its behavior resembled niobium more closely than tantalum,
indicating that complexing behavior could not be predicted purely from
simple extrapolations of trends within a group in the periodic table.
This prompted further exploration of the chemical behavior of
complexes of dubnium. Various labs jointly conducted thousands of
repetitive chromatographic experiments between 1988 and 1993. All
group 5 elements and protactinium were extracted from concentrated
hydrochloric acid; after mixing with lower concentrations of hydrogen
chloride, small amounts of hydrogen fluoride were added to start
selective re-extraction. Dubnium showed behavior different from that
of tantalum but similar to that of niobium and its pseudohomolog
protactinium at concentrations of hydrogen chloride below 12 moles per
liter. This similarity to the two elements suggested that the formed
complex was either or . After extraction experiments of dubnium from
hydrogen bromide into diisobutyl carbinol (2,6-dimethylheptan-4-ol), a
specific extractant for protactinium, with subsequent elutions with
the hydrogen chloride/hydrogen fluoride mix as well as hydrogen
chloride, dubnium was found to be less prone to extraction than either
protactinium or niobium. This was explained as an increasing tendency
to form non‐extractable complexes of multiple negative charges.
Further experiments in 1992 confirmed the stability of the +5 state:
Db(V) was shown to be extractable from cation‐exchange columns with
α‐hydroxyisobutyrate, like the group 5 elements and protactinium;
Db(III) and Db(IV) were not. In 1998 and 1999, new predictions
suggested that dubnium would extract nearly as well as niobium and
better than tantalum from halide solutions, which was later confirmed.
The first isothermal gas chromatography experiments were performed in
1992 with 262Db (half-life 35 seconds). The volatilities for niobium
and tantalum were similar within error limits, but dubnium appeared to
be significantly less volatile. It was postulated that traces of
oxygen in the system might have led to formation of , which was
predicted to be less volatile than . Later experiments in 1996 showed
that group 5 chlorides were more volatile than the corresponding
bromides, with the exception of tantalum, presumably due to formation
of . Later volatility studies of chlorides of dubnium and niobium as a
function of controlled partial pressures of oxygen showed that
formation of oxychlorides and general volatility are dependent on
concentrations of oxygen. The oxychlorides were shown to be less
volatile than the chlorides.
In 2004-05, researchers from Dubna and Livermore identified a new
dubnium isotope, 268Db, as a fivefold alpha decay product of the newly
created element 115. This new isotope proved to be long-lived enough
to allow further chemical experimentation, with a half-life of over a
day. In the 2004 experiment, a thin layer with dubnium was removed
from the surface of the target and dissolved in aqua regia with
tracers and a lanthanum carrier, from which various +3, +4, and +5
species were precipitated on adding ammonium hydroxide. The
precipitate was washed and dissolved in hydrochloric acid, where it
converted to nitrate form and was then dried on a film and counted.
Mostly containing a +5 species, which was immediately assigned to
dubnium, it also had a +4 species; based on that result, the team
decided that additional chemical separation was needed. In 2005, the
experiment was repeated, with the final product being hydroxide rather
than nitrate precipitate, which was processed further in both
Livermore (based on reverse phase chromatography) and Dubna (based on
anion exchange chromatography). The +5 species was effectively
isolated; dubnium appeared three times in tantalum-only fractions and
never in niobium-only fractions. It was noted that these experiments
were insufficient to draw conclusions about the general chemical
profile of dubnium.
In 2009, at the JAEA tandem accelerator in Japan, dubnium was
processed in nitric and hydrofluoric acid solution, at concentrations
where niobium forms and tantalum forms . Dubnium's behavior was close
to that of niobium but not tantalum; it was thus deduced that dubnium
formed . From the available information, it was concluded that dubnium
often behaved like niobium, sometimes like protactinium, but rarely
like tantalum.
In 2021, the volatile heavy group 5 oxychlorides MOCl3 (M = Nb, Ta,
Db) were experimentally studied at the JAEA tandem accelerator. The
trend in volatilities was found to be NbOCl3 > TaOCl3 ≥ DbOCl3, so
that dubnium behaves in line with periodic trends.
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Dubnium
