Dubnium is a synthetic chemical element with symbol Db and atomic number 105. Dubnium is highly radioactive: the most stable known isotope, dubnium-268, has a half-life of just over a day. This greatly limits the extent of research on dubnium.
|Mass number||268 (most stable isotope)|
|Dubnium in the periodic table|
|Atomic number (Z)||105|
|Element category||transition metal|
|Electron configuration||[Rn] 5f14 6d3 7s2|
Electrons per shell
|2, 8, 18, 32, 32, 11, 2|
|Phase at STP||solid (predicted)|
|Density (near r.t.)||29.3 g/cm3 (predicted)|
|Oxidation states||5, (4), (3)
|Atomic radius||empirical: 139 pm (estimated)|
|Covalent radius||149 pm (estimated)|
|Crystal structure||body-centered cubic (bcc) (predicted)|
|Naming||after Dubna, Moscow Oblast, Russia, site of Joint Institute for Nuclear Research|
|Discovery||independently by the Lawrence Berkeley Laboratory and the Joint Institute for Nuclear Research (1970)|
|Main isotopes of dubnium|
Dubnium does not occur naturally on Earth and is produced artificially. The first discovery of the element was claimed by the Soviet Joint Institute for Nuclear Research (JINR) in 1968, followed in 1970 by the American Lawrence Berkeley Laboratory. 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 IUPAC/IUPAP Joint Working Party, resulting in credit for discovery being officially shared between both teams. The element was officially named dubnium in 1997 after the town of Dubna, the site of the JINR.
In the periodic table of the elements, dubnium is located in group 5 as the third member of the 6d series of transition metals. Limited investigation of dubnium chemistry has demonstrated that dubnium behaves as a typical group 5 element and the heavier homologue to tantalum; some deviations from periodic trends occur due to relativistic effects.
Uranium, element 92, is the heaviest of the elements that occur significantly in nature; heavier elements can only practically be 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 years thereafter, American scientists undoubtedly synthesized the elements up to mendelevium, element 101, which was created in 1955. However, from element 102, priority of discovery 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.
The first report of the discovery of element 105 came from the Joint Institute for Nuclear Research (JINR) in Dubna, Moscow Oblast, Russian SFSR, Soviet Union, in 1968. 243Am was bombarded by a beam of 22Ne ions. The scientists reported 9.4 MeV (with a reported half-life of 0.1–3 seconds) and 9.7 MeV (t1/2 > 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−x105 + 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. A subsequent paper, published in February 1970, reported multiple examples of two such activities, with half-lives of 14 ms and 2.2±0.5 s. The former activity was assigned to 242mfAm.[a] The latter activity was described as caused by an isotope of element 105; the researchers suggested that it was unlikely that the latter 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, which was in accordance with prior theoretical data. To establish that this activity was not from a (22Ne,xn) reaction, the researchers bombarded a 243Am target with 18O nuclides; 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, falling in line with theoretical data. The researchers thus concluded that the activities observed indeed came from SF of element 105.
In April 1970, a team at Lawrence Berkeley Laboratory (LBL), located in Berkeley, California, United States, stepped into the competition. They 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 correlated with 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.
In May 1970, JINR published another report on dubnium. In the follow-up research, the researchers conducted another experiment for creating element 105. 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, though the possibility of 260105 was not excluded. 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 the SF activity nearly matched that of niobium pentachloride, rather than hafnium tetrachloride. The team identified a 2.2-second SF activity contained within a volatile chloride portraying eka-tantalum properties; therefore, the source of the SF activity must have been element 105.
In June 1970, JINR made improvements on their first experiment, providing 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; thus, the original isotope was either 260105 or 261105.
JINR proposed the name nielsbohrium (Ns) in honor of the Danish nuclear physicist Niels Bohr, one of the founders of the theories of atomic structure and quantum theory, while the LBL proposed that the new element should be named hahnium (Ha), in honor of the German chemist Otto Hahn, the "father of nuclear chemistry", thus creating an element naming controversy.
In the late 1960s and 1970s, tensions somewhat simmered down. Both teams synthesized the next element, element 106, but decided not to suggest their names. In 1968, JINR presented a report that called recognition of other teams, including LBL, of the discovery of elements 102 and 103, "hasty". They later suggested establishing an international committee for elaborating the discovery criteria. This proposal was accepted in 1974 and a joint neutral group formed. Neither team showed interest in resolving the conflict through a third party, and 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—in an attempt to resolve the conflict internally and render the neutral joint group unnecessary; after two-hour-long discussions, that attempt failed. The newly formed joint neutral group never assembled to assess the claims and the conflict remained unsolved. In 1979, IUPAC published a new suggested system of systematic element names intended to be used as placeholders until permanent names were established; under it, element 105 would be named unnilpentium, from the Latin roots un- and nil- and the Greek root pent- (meaning "one", "zero", and "five", respectively, referencing the digits of the atomic number). Both teams ignored it as they did not wish to weaken their claims by adopting a neutral naming system rather than their own.
In 1981, a third major competitor joined the race for superheavy elements—Gesellschaft für Schwerionenforschung (GSI; English: Society for Heavy Ion Research) in Darmstadt, Hesse, West Germany. They claimed to have synthesized element 107; their report came out five years after the first report from JINR did but provided a greater level of precision, making a more solid claim on discovery. GSI joined with JINR in that it suggested the name nielsbohrium for the new element, believing Bohr did deserve an element named after him and hoping to ease the tension on the element 105 controversy. JINR did not suggest a new name for element 105, stating it was more important to first determine the discoverers of the element.
In 1985, the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP) formed a Joint Working Party (JWP) aimed at assessing discoveries and establishing final names for the elements in question. The party held numerous meetings with delegates from the three competing institutes; in 1990, they established criteria on recognizing an element and in 1991, they completed the work on assessing discoverer statuses 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 discovery of the element should be shared between the two teams.
LBL dismissed the report. In an open response, they proclaimed that in the review the input from JINR was overrated. In addition, they claimed JINR was able to undoubtedly demonstrate the synthesis of element 105 at least a year after they did. JINR and GSI endorsed the report. The scientists from the former JWP rejected the criticisms from LBL.
In 1994, IUPAC published a recommendation on naming the disputed elements following the previous reports. For element 105, they proposed the name joliotium (Jl), after the French physicist Frédéric Joliot-Curie, a significant 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. (The name nielsbohrium for element 107 was transformed to bohrium to conform with the practice set by all then-named elements.) This recommendation paper was generally met with criticism from the American scientists: their recommendations were scrambled (the names rutherfordium and hahnium, originally suggested by Berkeley for elements 104 and 105, were used for elements 106 and 108, respectively); both elements 104 and 105 were given names suggested by JINR despite earlier recognition of LBL as of an equal co-discoverer; and especially because the name seaborgium for element 106 was rejected for honoring a living person, a rule that had only just been approved. These names were to be accepted on a Council meeting in 1995.
After a new round of negotiations between the parties, IUPAC decided to allow the name seaborgium for element 106 and the name flerovium for element 102 after Georgy Flerov following the recognition by the 1993 report that the element had been first convincingly synthesized in Dubna. This compromise was also 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 finally approved and published in 1997. Element 105 got its final name, dubnium (Db), after the Russian town of Dubna, the location of the Joint Institute for Nuclear Research. This decision was "reluctantly" approved by the American scientists. IUPAC stated the Berkeley laboratory had already been recognized several times in the naming of elements (i.e., berkelium, californium, 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. (The matter of naming element 107 was transferred to the Royal Danish Academy of Sciences and Letters, who recommended the name bohrium.)
Dubnium, having an atomic number of 105, is a superheavy element; like all elements with such high atomic numbers, it is highly unstable. The most stable known isotope of dubnium, 268Db, has a half-life of around a day. Thus, even if the element had once existed on Earth, it would have decayed completely long ago and would only be obtained by artificial production.[b]
The short half-life of dubnium limits the scope of experimentation. This is amplified by the trend of growth of the neutron-to-proton ratio of the most stable isotopes of an element with growth of the atomic number; this trend is expected to continue to the superheavy elements. This complicates synthesis of the most stable isotopes as the isotopes in question will have more neutrons per proton than both the target and beam nuclei that could be employed. (Although a different technique based on rapid neutron capture is being considered as of the 2010s, those based on the collision of a big and small nucleus dominate research in the area nowadays.)
Only a few atoms of 268Db can be produced in each experiment, and thus the measured lifetimes vary significantly during the process. During three experiments, 23 atoms were created in total, with a resulting half-life of 28+11
−4 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.
While it yet remains to be seen if there are principally more stable isotopes, there is no theoretical suggestion this could be the case. In a 2012 calculation by JINR, it was suggested that the half-lives of all dubnium isotopes, synthesized or not, would not significantly exceed a day.[c]
According to the periodic law, dubnium should belong to group 5, under 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,[d] which dramatically change physical properties on both atomic and macroscopic scales. These properties have remained challenging to measure for a number of 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 presence of many unwanted activities apart from those of synthesis of superheavy atoms. So far, because of technological challenges, studies have only been performed on single atoms.
Atomic and physicalEdit
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 (take on themselves) the charge of the nucleus more effectively, leaving even less for the outer d and f electrons, which therefore move on larger orbitals. Dubnium is greatly affected by this: unlike the previous group 5 members, its 7s electrons are more difficult to extract than its 6d electrons, although these energy levels remain close to each other.
Another effect is the spin–orbit interaction, particularly the 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. While all ten energy levels are actually raised, therefore becoming less stable energetically, four of them are lower than the other six. (The three 6d electrons normally occupy the energy levels of lowest energy, 6d3/2.)
While 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 homologues. Despite the changes, dubnium is still expected to have five valence electrons; 7p energy levels have not been shown to influence dubnium and its properties. 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 (see below).
Computational chemistry is simplest in gas-phase chemistry, in which interactions between molecules may be ignored as negligible. Multiple authors have researched on dubnium pentachloride; calculations show it to be consistent with the periodic laws by exhibiting properties of a compound of a group 5 element. For example, the molecular orbital levels indicate dubnium uses three 6d electron levels as expected. Compared to its tantalum analog, dubnium pentachloride is expected to show increased covalence: 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 that of niobium and tantalum and the +3 state 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 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, however, 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 over a decade later, 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 tracers of the group 5 elements niobium and tantalum and the group 4 elements zirconium and hafnium produced on‐line under similar conditions. The group 5 elements are known to sorb on glass surfaces while 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 ketones differed between dubnium, tantalum, and niobium. Dubnium did not extract and its behavior resembled niobium more closely than tantalum, indicating that details of complexing behavior could not be predicted based only on 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; at 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 DbOX−
4 or [Db(OH)
. 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 both protactinium and 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 DbOBr
3, which was predicted to be less volatile than DbBr
5. 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 TaOCl
3. 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, another anion-exchange experiment was conducted in Japan at the JAEA tandem accelerator. Dubnium was processed in nitric and hydrofluoric acid solution, at concentrations where niobium forms NbOF−
4 and tantalum forms TaF−
6. Dubnium's behavior was close to that of niobium but not tantalum; it was thus deduced that dubnium formed DbOF−
4. From the available information, it was concluded that dubnium often behaved like niobium, sometimes like protactinium, but rarely as tantalum.
- This notation marks that the nucleus is a spontaneously fissionable isomer; i.e., an isomer that decays via spontaneous fission.
- While the modern theory of the atomic nucleus does not suggest a long-lived isotope of dubnium, claims were raised in the past that unknown isotopes of superheavy elements actually existed primordially on the Earth: for example, such a claim was raised for 267108 of half-life of 400 to 500 million years in 1963 or 292122 of half-life of over 100 million years in 2009; however, neither claim gained acceptance.
- While this does not conform with the current experimental value of 28+11
−4 hours for 268Db, it must be noted that the statistical law of large numbers, on which relies the regular determination of half-lives, cannot be directly applied due to a very limited number of experiments (decays). The range of uncertainty is an indication of that the half-life period lies within this range with the probability of the standard figure of 95%, which has been arbitrarily chosen to represent the probability of a true event.
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