Nihonium

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^[l] 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.^[56][57]

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).^[56]

In 1998, the JINR–LLNL collaboration started their attempt on element 114, bombarding a target of plutonium-244 with ions of calcium-48:^[56]

²⁴⁴
₉₄Pu
+ ⁴⁸
₂₀Ca
→ ²⁹²114* → ²⁹⁰114 + 2
n
+ e⁻ → ²⁹⁰113 + ν_e ?

A single atom was observed which was thought to be the isotope ²⁸⁹114: the results were published in January 1999.^[58] 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.^[59] 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.^[9] A similar long-lived activity observed by the JINR team in March 1999 in the ²⁴²Pu + ⁴⁸Ca reaction may be due to the electron-capture daughter of ²⁸⁷114, ²⁸⁷113; this assignment is also tentative.^[8]

JINR–LLNL collaboration

The now-confirmed discovery of element 114 was made in June 1999 when the JINR team repeated the first ²⁴⁴Pu + ⁴⁸Ca reaction from 1998;^[60][61] 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.^[56][62]

The first report of element 113 was in August 2003, when it was identified as an alpha decay product of element 115. Element 115 had been produced by bombarding a target of americium-243 with calcium-48 projectiles. The JINR–LLNL collaboration published its results in February 2004:^[62]

²⁴³
₉₅Am
+ ⁴⁸
₂₀Ca
→ ²⁹¹115* → ²⁸⁸115 + 3
n
→ ²⁸⁴113 +
α

²⁴³
₉₅Am
+ ⁴⁸
₂₀Ca
→ ²⁹¹115* → ²⁸⁷115 + 4
n
→ ²⁸³113 +
α

Four further alpha decays were observed, ending with the spontaneous fission of isotopes of element 105, dubnium.^[62]

Riken

While the JINR–LLNL collaboration had been studying fusion reactions with ⁴⁸Ca, a team of Japanese scientists at the Riken Nishina Center for Accelerator-Based Science in Wakō, Japan, led by Kōsuke Morita had been studying cold fusion reactions. Morita had previously studied the synthesis of superheavy elements at the JINR before starting his own team at Riken. In 2001, his team confirmed the GSI's discoveries of elements 108, 110, 111, and 112. They then made a new attempt on element 113, using the same ²⁰⁹Bi + ⁷⁰Zn reaction that the GSI had attempted unsuccessfully in 1998. Despite the much lower yield expected than for the JINR's hot fusion technique with calcium-48, the Riken team chose to use cold fusion as the synthesised isotopes would alpha decay to known daughter nuclides and make the discovery much more certain, and would not require the use of radioactive targets.^[63] 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.^[64]

The bombardment of ²⁰⁹Bi with ⁷⁰Zn at Riken began in September 2003.^[65] The team detected a single atom of ²⁷⁸113 in July 2004 and published their results that September:^[66]

²⁰⁹
₈₃Bi
+ ⁷⁰
₃₀Zn
→ ²⁷⁹113* → ²⁷⁸113 +
n

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.^[66] The decay data they observed for the alpha decay of ²⁶⁶Bh matched the 2000 data, lending support for their claim. Spontaneous fission of its daughter ²⁶²Db had not been previously known; the American team had observed only alpha decay from this nuclide.^[64]

Road to confirmation

When the discovery of a new element is claimed, the Joint Working Party (JWP) of the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP) assembles to examine the claims according to their criteria for the discovery of a new element, and decides scientific priority and naming rights for the elements. According to the JWP criteria, a discovery must demonstrate that the element has an atomic number different from all previously observed values. It should also preferably be repeated by other laboratories, although this requirement has been waived where the data is of very high quality. Such a demonstration must establish properties, either physical or chemical, of the new element and establish that they are those of a previously unknown element. The main techniques used to demonstrate atomic number are cross-reactions (creating claimed nuclides as parents or daughters of other nuclides produced by a different reaction) and anchoring decay chains to known daughter nuclides. For the JWP, priority in confirmation takes precedence over the date of the original claim. Both teams set out to confirm their results by these methods.^[67]

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.^[80] A joint 2016 announcement by IUPAC and IUPAP had been scheduled to coincide with the publication of the JWP reports, but IUPAC alone decided on an early release because the news of Riken being awarded credit for element 113 had been leaked to Japanese newspapers.^[81] For the first time in history, a team of Asian physicists would name a new element.^[80] 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.^[82]

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.^[67][70][83]

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.^[56][82]

The sum argument advanced by the JWP in the approval of the discovery of element 113 was later criticised in a May 2016 study from Lund University and the GSI, as it is only valid if no gamma decay or internal conversion takes place along the decay chain, which is not likely for odd nuclei, and the uncertainty of the alpha decay energies measured in the ²⁷⁸113 decay chain was not small enough to rule out this possibility. If this is the case, similarity in lifetimes of intermediate daughters becomes a meaningless argument, as different isomers of the same nuclide can have different half-lives: for example, the ground state of ¹⁸⁰Ta has a half-life of hours, but an excited state ^180mTa has never been observed to decay. This study found reason to doubt and criticise the IUPAC approval of the discoveries of elements 115 and 117, but the data from Riken for element 113 was found to be congruent, and the data from the JINR team for elements 115 and 113 to probably be so, thus endorsing the IUPAC approval of the discovery of element 113.^[84][85] 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.^[86]

Naming

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),^[87] 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.^[1]

Before the JWP recognition of their priority, the Japanese team had unofficially suggested various names: japonium, after their home country;^[88] nishinanium, after Japanese physicist Yoshio Nishina, the "founding father of modern physics research in Japan";^[89] and rikenium, after the institute.^[88] 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 nihon (日本), one of the two Japanese pronunciations for the name of Japan.^[90] The discoverers also intended to reference the support of their research by the Japanese people (Riken being almost entirely government-funded),^[91] recover lost pride and trust in science among those who were affected by the Fukushima Daiichi nuclear disaster,^[92] and honour Japanese chemist Masataka Ogawa's 1908 discovery of rhenium, which he named "nipponium" with symbol Np after the other Japanese pronunciation of Japan's name.^[83] As Ogawa's claim had not been accepted, the name "nipponium" could not be reused for a new element, and its symbol Np had since been used for neptunium.^[m] In March 2016, Morita proposed the name "nihonium" to IUPAC, with the symbol Nh.^[83] The naming realised what had been a national dream in Japanese science ever since Ogawa's claim.^[76]

The former president of IUPAP, Cecilia Jarlskog, complained at the Nobel Symposium on Superheavy Elements in Bäckaskog Castle, Sweden, in June 2016 about the lack of openness involved in the process of approving new elements, and stated that she believed that the JWP's work was flawed and should be redone by a new JWP. A survey of physicists determined that many felt that the Lund–GSI 2016 criticisms of the JWP report were well-founded, but it was also generally thought that the conclusions would hold up if the work was redone. Thus the new president, Bruce McKellar, ruled that the proposed names should be released in a joint IUPAP–IUPAC press release.^[81] IUPAC and IUPAP publicised the proposal of nihonium that June,^[92] and set a five-month term to collect comments, after which the name would be formally established at a conference.^[95][96] The name was officially approved on 28 November 2016.^[97] 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.^[98]

Stability and half-lives

The stability of nuclei quickly decreases with the increase in atomic number after curium, element 96, whose half-life is over ten thousand times longer than that of any subsequent element. All isotopes with an atomic number above 101 undergo radioactive decay with half-lives of less than 30 hours: this is because of the ever-increasing Coulomb repulsion of protons, so that the strong nuclear force cannot hold the nucleus together against spontaneous fission for long. Calculations suggest that in the absence of other stabilising factors, elements with more than 103 protons should not exist. Researchers in the 1960s suggested that the closed nuclear shells around 114 protons and 184 neutrons should counteract this instability, and create an "island of stability" containing nuclides with half-lives reaching thousands or millions of years. The existence of the island is still unproven, but the existence of the superheavy elements (including nihonium) confirms that the stabilising effect is real, and in general the known superheavy nuclides become longer-lived as they approach the predicted location of the island.^[102][103]

All nihonium isotopes are unstable and radioactive; the heavier nihonium isotopes are more stable than the lighter ones, as they are closer to the centre of the island. The most stable known nihonium isotope, ²⁸⁶Nh, is also the heaviest; it has a half-life of 8 seconds. The isotope ²⁸⁵Nh, as well as the unconfirmed ²⁸⁷Nh and ²⁹⁰Nh, have also been reported to have half-lives of over a second. The isotopes ²⁸⁴Nh and ²⁸³Nh have half-lives of 0.90 and 0.12 seconds respectively. The remaining two isotopes have half-lives between 0.1 and 100 milliseconds: ²⁸²Nh has a half-life of 61 milliseconds, and ²⁷⁸Nh, the lightest known nihonium isotope, is also the shortest-lived, with a half-life of 2.0 milliseconds. This rapid increase in the half-lives near the closed neutron shell at N = 184 is seen in roentgenium, copernicium, and nihonium (elements 111 through 113), where each extra neutron so far multiplies the half-life by a factor of 5 to 20.^[103][104]

The unknown isotopes in the gap between ²⁷⁸Nh and ²⁸²Nh are too heavy to be produced by cold fusion and too light to be produced by hot fusion. The missing ²⁸⁰Nh and ²⁸¹Nh may be populated as daughters of ²⁸⁴Mc and ²⁸⁵Mc, producible in the ²⁴¹Am+⁴⁸Ca reaction, but this has not yet been attempted.^[105] Of particular interest is ²⁸¹Nh, as it is the expected great-granddaughter of ²⁹³119, a possible product of the ²⁴³Am+⁵⁴Cr reaction.^[106] Production of ²⁸²Mc and ²⁸³Mc is possible in the ²⁴³Am+⁴⁴Ca reaction (though it has a lower cross-section), and their daughters would be ²⁷⁸Nh (known) and ²⁷⁹Nh.^[105] The heavier isotopes ²⁸⁷Nh through ²⁹⁰Nh might be synthesised using charged-particle evaporation, using the ²⁴²Pu+⁴⁸Ca and ²⁴⁴Pu+⁴⁸Ca reactions where one proton and some neutrons are evaporated.^[107][108]

Physical and atomic

Nihonium is the first member of the 7p series of elements and the heaviest group 13 element on the periodic table, below boron, aluminium, gallium, indium, and thallium. All the group 13 elements except boron are metals, and nihonium is expected to follow suit. Nihonium is predicted to show many differences from its lighter homologues. The major reason for this is the spin–orbit (SO) interaction, which is especially strong for the superheavy elements, because their electrons move much faster than in lighter atoms, at velocities close to the speed of light.^[111]: 63 In relation to nihonium atoms, it lowers the 7s and the 7p electron energy levels (stabilising those electrons), but two of the 7p electron energy levels are stabilised more than the other four.^[112] The stabilisation of the 7s electrons is called the inert pair effect, and the separation of the 7p subshell into the more and less stabilised parts is called subshell splitting. Computational chemists see the split as a change of the second, azimuthal quantum number l, from 1 to 1/2 and 3/2 for the more and less stabilised parts of the 7p subshell, respectively.^[111]: 63 The quantum number corresponds to the letter in the electron orbital name: 0 to s, 1 to p, 2 to d, etc. For theoretical purposes, the valence electron configuration may be represented to reflect the 7p subshell split as 7s² 7p_1/2¹.^[1] The first ionisation energy of nihonium is expected to be 7.306 eV, the highest among the metals of group 13.^[1] Similar subshell splitting should exist for the 6d electron levels, with four being 6d_3/2 and six being 6d_5/2. Both these levels are raised to be close in energy to the 7s ones, high enough to possibly be chemically active. This would allow for the possibility of exotic nihonium compounds without lighter group 13 analogues.^[112]

Periodic trends would predict nihonium to have an atomic radius larger than that of thallium due to it being one period further down the periodic table, but calculations suggest nihonium has an atomic radius of about 170 pm, the same as that of thallium, due to the relativistic stabilisation and contraction of its 7s and 7p_1/2 orbitals. Thus, nihonium is expected to be much denser than thallium, with a predicted density of about 16 to 18 g/cm³ compared to thallium's 11.85 g/cm³, since nihonium atoms are heavier than thallium atoms but have the same volume.^[1][110] Bulk nihonium is expected to have a hexagonal close-packed crystal structure, like thallium.^[5] The melting and boiling points of nihonium have been predicted to be 430 °C and 1100 °C respectively, exceeding the values for indium and thallium, following periodic trends.^[1][2] Nihonium should have a bulk modulus of 20.8 GPa, about half that of thallium (43 GPa).^[6]

Chemical

The chemistry of nihonium is expected to be very different from that of thallium. This difference stems from the spin–orbit splitting of the 7p shell, which results in nihonium being between two relatively inert closed-shell elements (copernicium and flerovium).^[113] Nihonium is expected to be less reactive than thallium, because of the greater stabilisation and resultant chemical inactivity of the 7s subshell in nihonium compared to the 6s subshell in thallium.^[4] The standard electrode potential for the Nh⁺/Nh couple is predicted to be 0.6 V. Nihonium should be a rather noble metal.^[4]

The metallic group 13 elements are typically found in two oxidation states: +1 and +3. The former results from the involvement of only the single p electron in bonding, and the latter results in the involvement of all three valence electrons, two in the s-subshell and one in the p-subshell. Going down the group, bond energies decrease and the +3 state becomes less stable, as the energy released in forming two additional bonds and attaining the +3 state is not always enough to outweigh the energy needed to involve the s-electrons. Hence, for aluminium and gallium +3 is the most stable state, but +1 gains importance for indium and by thallium it becomes more stable than the +3 state. Nihonium is expected to continue this trend and have +1 as its most stable oxidation state.^[1]

The simplest possible nihonium compound is the monohydride, NhH. The bonding is provided by the 7p_1/2 electron of nihonium and the 1s electron of hydrogen. The SO interaction causes the binding energy of nihonium monohydride to be reduced by about 1 eV^[1] and the nihonium–hydrogen bond length to decrease as the bonding 7p_1/2 orbital is relativistically contracted. This is unique among the 7p element monohydrides; all the others have relativistic expansion of the bond length instead of contraction.^[114] Another effect of the SO interaction is that the Nh–H bond is expected to have significant pi bonding character (side-on orbital overlap), unlike the almost pure sigma bonding (head-on orbital overlap) in thallium monohydride (TlH).^[115] The analogous monofluoride (NhF) should also exist.^[110] Nihonium(I) is predicted to be more similar to silver(I) than thallium(I):^[1] the Nh⁺ ion is expected to more willingly bind anions, so that NhCl should be quite soluble in excess hydrochloric acid or ammonia; TlCl is not. In contrast to Tl⁺, which forms the strongly basic hydroxide (TlOH) in solution, the Nh⁺ cation should instead hydrolyse all the way to the amphoteric oxide Nh₂O, which would be soluble in aqueous ammonia and weakly soluble in water.^[4]

The adsorption behaviour of nihonium on gold surfaces in thermochromatographical experiments is expected to be closer to that of astatine than that of thallium. The destabilisation of the 7p_3/2 subshell effectively leads to a valence shell closing at the 7s² 7p² configuration rather than the expected 7s² 7p⁶ configuration with its stable octet. As such, nihonium, like astatine, can be considered to be one p-electron short of a closed valence shell. Hence, even though nihonium is in group 13, it has several properties similar to the group 17 elements. (Tennessine in group 17 has some group-13-like properties, as it has three valence electrons outside the 7s² 7p² closed shell.^[116]) Nihonium is expected to be able to gain an electron to attain this closed-shell configuration, forming the −1 oxidation state like the halogens (fluorine, chlorine, bromine, iodine, and astatine). This state should be more stable than it is for thallium as the SO splitting of the 7p subshell is greater than that for the 6p subshell.^[111]: 63 Nihonium should be the most electronegative of the metallic group 13 elements,^[1] even more electronegative than tennessine, the period 7 congener of the halogens: in the compound NhTs, the negative charge is expected to be on the nihonium atom rather than the tennessine atom.^[110] The −1 oxidation should be more stable for nihonium than for tennessine.^[1][117] The electron affinity of nihonium is calculated to be around 0.68 eV, higher than thallium's at 0.4 eV; tennessine's is expected to be 1.8 eV, the lowest in its group.^[1] It is theoretically predicted that nihonium should have an enthalpy of sublimation around 150 kJ/mol and an enthalpy of adsorption on a gold surface around −159 kJ/mol.^[118]

BCl
₃ has a trigonal structure.

NhCl
₃ is predicted to be T-shaped.

Significant 6d involvement is expected in the Nh–Au bond, although it is expected to be more unstable than the Tl–Au bond and entirely due to magnetic interactions. This raises the possibility of some transition metal character for nihonium.^[113] On the basis of the small energy gap between the 6d and 7s electrons, the higher oxidation states +3 and +5 have been suggested for nihonium.^[1][4] Some simple compounds with nihonium in the +3 oxidation state would be the trihydride (NhH₃), trifluoride (NhF₃), and trichloride (NhCl₃). These molecules are predicted to be T-shaped and not trigonal planar as their boron analogues are:^[p] this is due to the influence of the 6d_5/2 electrons on the bonding.^[115][q] 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.^[115]

The bonding in the lighter NhX₃ molecules can be considered as that of a linear NhX⁺
₂ species (similar to HgF₂ or AuF⁻
₂) with an additional Nh–X bond involving the 7p orbital of nihonium perpendicular to the other two ligands. These compounds are all expected to be highly unstable towards the loss of an X₂ molecule and reduction to nihonium(I):^[115]

NhX₃ → NhX + X₂

Nihonium thus continues the trend down group 13 of reduced stability of the +3 oxidation state, as all five of these compounds have lower reaction energies than the unknown thallium(III) iodide.^[r] The +3 state is stabilised for thallium in anionic complexes such as TlI⁻
₄, and the presence of a possible vacant coordination site on the lighter T-shaped nihonium trihalides is expected to allow a similar stabilisation of NhF⁻
₄ and perhaps NhCl⁻
₄.^[115]

The +5 oxidation state is unknown for all lighter group 13 elements: calculations predict that nihonium pentahydride (NhH₅) and pentafluoride (NhF₅) should have a square pyramidal molecular geometry, but also that both would be highly thermodynamically unstable to loss of an X₂ molecule and reduction to nihonium(III). Again, some stabilisation is expected for anionic complexes, such as NhF⁻
₆. The structures of the nihonium trifluoride and pentafluoride molecules are the same as those for chlorine trifluoride and pentafluoride.^[115]