Nihonium

Nihonium is a synthetic chemical element; it has symbol Nh and atomic number 113. It is extremely radioactive: its most stable known isotope, nihonium-286, has a half-life of about 10 seconds. In the periodic table, nihonium is a transactinide element in the p-block. It is a member of period 7 and group 13.

Nihonium was first reported to have been created in experiments carried out between 14 July and 10 August 2003, by a Russian–American collaboration at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, working in collaboration with the Lawrence Livermore National Laboratory in Livermore, California, and on 23 July 2004, by a team of Japanese scientists at Riken in Wakō, Japan. The confirmation of their claims in the ensuing years involved independent teams of scientists working in the United States, Germany, Sweden, and China, as well as the original claimants in Russia and Japan. In 2015, the IUPAC/IUPAP Joint Working Party recognised the element and assigned the priority of the discovery and naming rights for the element to Riken. The Riken team suggested the name nihonium in 2016, which was approved in the same year. The name comes from the common Japanese name for .

Very little is known about nihonium, as it has been made only in very small amounts that decay within seconds. The anomalously long lives of some superheavy nuclides, including some nihonium isotopes, are explained by the island of stability theory. Experiments to date have supported the theory, with the half-lives of the confirmed nihonium isotopes increasing from milliseconds to seconds as neutrons are added and the island is approached. Nihonium has been calculated to have similar properties to its homologues boron, aluminium, gallium, indium, and thallium. All but boron are post-transition metals, and nihonium is expected to be a post-transition metal as well. It should also show several major differences from them; for example, nihonium should be more stable in the +1 oxidation state than the +3 state, like thallium, but in the +1 state nihonium should behave more like silver and astatine than thallium. Preliminary experiments have shown that elemental nihonium is not very volatile, and that it is less reactive than its lighter homologue thallium.

Introduction

History

Early indications

The syntheses of elements 107 to 112 were conducted at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, from 1981 to 1996. These elements were made by cold fusion reactions, in which targets made of lead and bismuth, which are around the stable configuration of 82 protons, are bombarded with heavy ions of period 4 elements. This creates fused nuclei with low excitation energies due to the stability of the targets' nuclei, significantly increasing the yield of superheavy elements. Cold fusion was pioneered by Yuri Oganessian and his team in 1974 at the Joint Institute for Nuclear Research (JINR) in Dubna, Soviet Union. Yields from cold fusion reactions were found to decrease significantly with increasing atomic number; the resulting nuclei were severely neutron-deficient and short-lived. The GSI team attempted to synthesise element 113 via cold fusion in 1998 and 2003, bombarding bismuth-209 with zinc-70; both attempts were unsuccessful.

Faced with this problem, Oganessian and his team at the JINR turned their renewed attention to the older hot fusion technique, in which heavy actinide targets were bombarded with lighter ions. Calcium-48 was suggested as an ideal projectile, because it is very neutron-rich for a light element (combined with the already neutron-rich actinides) and would minimise the neutron deficiencies of the nuclides produced. Being doubly magic, it would confer benefits in stability to the fused nuclei. In collaboration with the team at the Lawrence Livermore National Laboratory (LLNL) in Livermore, California, United States, they made an attempt on element 114 (which was predicted to be a magic number, closing a proton shell, and more stable than element 113). Despite numerous attempts to repeat this reaction, an isotope with these decay properties has never again been found, and the exact identity of this activity is unknown. A 2016 paper by Sigurd Hofmann et al. considered that the most likely explanation of the 1998 result is that two neutrons were emitted by the produced compound nucleus, leading to ²⁹⁰114 and electron capture to ²⁹⁰113, while more neutrons were emitted in all other produced chains. This would have been the first report of a decay chain from an isotope of element 113, but it was not recognised at the time, and the assignment is still uncertain. following this, the JINR team used the same hot fusion technique to synthesize elements 116 and 118 in 2000 and 2002 respectively via the ²⁴⁸Cm + ⁴⁸Ca and ²⁴⁹Cf + ⁴⁸Ca reactions. They then turned their attention to the missing odd-numbered elements, as the odd protons and possibly neutrons would hinder decay by spontaneous fission and result in longer decay chains.

: + → ²⁹¹115* → ²⁸⁸115 + 3 → ²⁸⁴113 +

: + → ²⁹¹115* → ²⁸⁷115 + 4 → ²⁸³113 +

Four further alpha decays were observed, ending with the spontaneous fission of isotopes of element 105, dubnium. In particular, the isotope ²⁷⁸113 expected to be produced in this reaction would decay to the known ²⁶⁶Bh, which had been synthesised in 2000 by a team at the Lawrence Berkeley National Laboratory (LBNL) in Berkeley. The team detected a single atom of ²⁷⁸113 in July 2004<!--the 23rd--> and published their results that September<!--the 28th-->:

: + → ²⁷⁹113* → ²⁷⁸113 +

The Riken team observed four alpha decays from ²⁷⁸113, creating a decay chain passing through ²⁷⁴Rg, ²⁷⁰Mt, and ²⁶⁶Bh before terminating with the spontaneous fission of ²⁶²Db. Further experiments at the JINR in 2005 confirmed the observed decay data.

2009–2015

The JWP published its report on elements 113–116 and 118 in 2011. It recognised the JINR–LLNL collaboration as having discovered elements 114 and 116, but did not accept either team's claim to element 113 and did not accept the JINR–LLNL claims to elements 115 and 118. The JINR–LLNL claim to elements 115 and 113 had been founded on chemical identification of their daughter dubnium, but the JWP objected that current theory could not distinguish between superheavy group 4 and group 5 elements by their chemical properties with enough confidence to allow this assignment. The decay properties of all the nuclei in the decay chain of element 115 had not been previously characterised before the JINR experiments, a situation which the JWP generally considers "troublesome, but not necessarily exclusive", and with the small number of atoms produced with neither known daughters nor cross-reactions the JWP considered that their criteria had not been fulfilled.

In late 2009, the JINR–LLNL collaboration studied the ²⁴⁹Bk + ⁴⁸Ca reaction in an effort to produce element 117, which would decay to elements 115 and 113 and bolster their claims in a cross-reaction. They were now joined by scientists from Oak Ridge National Laboratory (ORNL) and Vanderbilt University, both in Tennessee, United States,

: + → ²⁹⁷117* → ²⁹⁴117 + 3 → ²⁹⁰115 + α → ²⁸⁶113 + α

: + → ²⁹⁷117* → ²⁹³117 + 4 → ²⁸⁹115 + α → ²⁸⁵113 + α

The new isotopes ²⁸⁵113 and ²⁸⁶113 produced did not overlap with the previously claimed ²⁸²113, ²⁸³113, and ²⁸⁴113, so this reaction could not be used as a cross-bombardment to confirm the 2003 or 2006 claims.

After 450 more days of irradiation of bismuth with zinc projectiles, Riken produced and identified another ²⁷⁸113 atom in August 2012<!--the 12th-->. Although electricity prices had soared since the 2011 Tōhoku earthquake and tsunami, and Riken had ordered the shutdown of the accelerator programs to save money, Morita's team was permitted to continue with one experiment, and they chose their attempt to confirm their synthesis of element 113. In this case, a series of six alpha decays was observed, leading to an isotope of mendelevium:

:²⁷⁸113 → + → + → + → + → + → +

This decay chain differed from the previous observations at Riken mainly in the decay mode of ²⁶²Db, which was previously observed to undergo spontaneous fission, but in this case instead alpha decayed; the alpha decay of ²⁶²Db to ²⁵⁸Lr is well-known. The team calculated the probability of accidental coincidence to be 10⁻²⁸, or totally negligible.

The ²⁴⁹Bk + ⁴⁸Ca experiment was repeated at the JINR in 2012 and 2013 with consistent results, and again at the GSI in 2014. The same year, the 2003 experiment had been repeated at the JINR, now also creating the isotope ²⁸⁹115 that could serve as a cross-bombardment for confirming their discovery of the element 117 isotope ²⁹³117, as well as its daughter ²⁸⁵113 as part of its decay chain.

Approval of discoveries

In December 2015, the conclusions of a new JWP report were published by IUPAC in a press release, in which element 113 was awarded to Riken; elements 115, 117, and 118 were awarded to the collaborations involving the JINR. The JINR considered the awarding of element 113 to Riken unexpected, citing their own 2003 production of elements 115 and 113, and pointing to the precedents of elements 103, 104, and 105 where IUPAC had awarded joint credit to the JINR and LBNL. They stated that they respected IUPAC's decision, but reserved determination of their position for the official publication of the JWP reports.

The full JWP reports were published on 21 January 2016. The JWP recognised the discovery of element 113, assigning priority to Riken. They noted that while the individual decay energies of each nuclide in the decay chain of ²⁷⁸113 were inconsistent, their sum was now confirmed to be consistent, strongly suggesting that the initial and final states in ²⁷⁸113 and its daughter ²⁶²Db were the same for all three events. The decay of ²⁶²Db to ²⁵⁸Lr and ²⁵⁴Md was previously known, firmly anchoring the decay chain of ²⁷⁸113 to known regions of the chart of nuclides. The JWP considered that the JINR–LLNL collaborations of 2004 and 2007, producing element 113 as the daughter of element 115, did not meet the discovery criteria as they had not convincingly determined the atomic numbers of their nuclides through cross-bombardments, which were considered necessary since their decay chains were not anchored to previously known nuclides. They also considered that the previous JWP's concerns over their chemical identification of the dubnium daughter had not been adequately addressed. The JWP recognised the JINR–LLNL–ORNL–Vanderbilt collaboration of 2010 as having discovered elements 117 and 115, and accepted that element 113 had been produced as their daughter, but did not give this work shared credit.

After the publication of the JWP reports, Sergey Dimitriev, the lab director of the Flerov lab at the JINR where the discoveries were made, remarked that he was happy with IUPAC's decision, mentioning the time Riken spent on their experiment and their good relations with Morita, who had learnt the basics of synthesising superheavy elements at the JINR. Two members of the JINR team published a journal article rebutting these criticisms against the congruence of their data on elements 113, 115, and 117 in June 2017.

Naming

thumb|[[Kōsuke Morita and Hiroshi Matsumoto, celebrating the naming on 1 December 2016.|alt=Lead researcher Kosuke Morita and Riken president Hiroshi Matsumoto from Riken showing "Nh" being added to the periodic table]]

Using Mendeleev's nomenclature for unnamed and undiscovered elements, nihonium would be known as eka-thallium. In 1979, IUPAC published recommendations according to which the element was to be called ununtrium (with the corresponding symbol of Uut), a systematic element name as a placeholder, until the discovery of the element is confirmed and a name is decided on. The recommendations were widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, but were mostly ignored among scientists in the field, who called it "element 113", with the symbol of E113, (113), or even simply 113. and rikenium, after the institute. After the recognition, the Riken team gathered in February 2016 to decide on a name. Morita expressed his desire for the name to honour the fact that element 113 had been discovered in Japan. Japonium was considered, making the connection to Japan easy to identify for non-Japanese, but it was rejected as Jap is considered an ethnic slur. The name nihonium was chosen after an hour of deliberation: it comes from , one of the two Japanese pronunciations for the name of Japan. The discoverers also intended to reference the support of their research by the Japanese people (Riken being almost entirely government-funded), recover lost pride and trust in science among those who were affected by the Fukushima Daiichi nuclear disaster, In March 2016, Morita proposed the name "nihonium" to IUPAC, with the symbol Nh. IUPAC and IUPAP publicised the proposal of nihonium that June, and set a five-month term to collect comments, after which the name would be formally established at a conference. The name was officially approved on 28 November 2016. The naming ceremony for the new element was held in Tokyo, Japan, on 14 March 2017, with Naruhito, then the Crown Prince of Japan, in attendance.

Isotopes

The heavier nihonium tribromide (NhBr₃) and triiodide (NhI₃) are trigonal planar due to the increased steric repulsion between the peripheral atoms; accordingly, they do not show significant 6d involvement in their bonding, though the large 7s–7p energy gap means that they show reduced sp² hybridisation compared to their boron analogues. The +3 state is stabilised for thallium in anionic complexes such as , and the presence of a possible vacant coordination site on the lighter T-shaped nihonium trihalides is expected to allow a similar stabilisation of and perhaps . From 2010 to 2012, some preliminary chemical experiments were performed at the JINR to determine the volatility of nihonium. The isotope ²⁸⁴Nh was investigated, made as the daughter of ²⁸⁸Mc produced in the ²⁴³Am+⁴⁸Ca reaction. The nihonium atoms were synthesised in a recoil chamber and then carried along polytetrafluoroethylene (PTFE) capillaries at 70&nbsp;°C by a carrier gas to the gold-covered detectors. About ten to twenty atoms of ²⁸⁴Nh were produced, but none of these atoms were registered by the detectors, suggesting either that nihonium was similar in volatility to the noble gases (and thus diffused away too quickly to be detected) or, more plausibly, that pure nihonium was not very volatile and thus could not efficiently pass through the PTFE capillaries.

A 2017 experiment at the JINR, producing ²⁸⁴Nh and ²⁸⁵Nh via the ²⁴³Am+⁴⁸Ca reaction as the daughters of ²⁸⁸Mc and ²⁸⁹Mc, avoided this problem by removing the quartz surface, using only PTFE. No nihonium atoms were observed after chemical separation, implying an unexpectedly large retention of nihonium atoms on PTFE surfaces. This experimental result for the interaction limit of nihonium atoms with a PTFE surface disagrees significantly with previous theory, which expected a lower value of 14.00&nbsp;kJ/mol. This suggests that the nihonium species involved in the previous experiment was likely not elemental nihonium but rather nihonium hydroxide, and that high-temperature techniques such as vacuum chromatography would be necessary to further probe the behaviour of elemental nihonium.

A 2024 experiment at the GSI, producing ²⁸⁴Nh via the ²⁴³Am+⁴⁸Ca reaction as daughter of ²⁸⁸Mc, studied the adsorption of nihonium and moscovium on SiO₂ and gold surfaces. The adsorption enthalpy of nihonium on SiO₂ was determined experimentally as (68% confidence interval). Nihonium was determined to be less reactive with the SiO₂ surface than its lighter congener thallium, but more reactive than its closed-shell neighbours copernicium and flerovium. This arises because of the relativistic stabilisation of the 7p_1/2 shell.

Notes

References

Bibliography

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External links

• Nihonium at The Periodic Table of Videos (University of Nottingham)
• Uut and Uup Add Their Atomic Mass to Periodic Table
• Discovery of Elements 113 and 115
• Superheavy elements
• WebElements.com: Nihonium