Nihonium is a chemical element with symbol Nh and atomic number 113. It is a synthetic element and is extremely radioactive; its most stable known isotope, nihonium-286, has a half-life of about 8 seconds. Nihonium was first reported to have been created in 2003 by the Joint Institute for Nuclear Research in Dubna, Russia, and in 2004 by a team of Japanese scientists at RIKEN. In December 2015, the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP) recognized the element and assigned the priority of the discovery to RIKEN.^[7] In November 2016, the IUPAC published a declaration defining the name to be nihonium.^[8] The name comes from the common Japanese name for Japan (日本, nihon). On 28 November 2016, the name became official.^[9][10]

In the periodic table, it is a p-block transactinide element. It is a member of the 7th period and is placed in the boron group, although it has not been confirmed to behave as the heavier homologue to thallium in the boron group. Nihonium is calculated to have some similar properties to its lighter homologues, boron, aluminium, gallium, indium, and thallium, and behave as a post-transition metal, although it should also show several major differences from them. Unlike all the other p-block elements, it may be able to involve its d-electrons in bonding, although these predictions are disputed.

History

Early indications

The synthesis of elements 107 to 112 inclusive, bohrium through copernicium, were conducted at the GSI via cold fusion reactions (bombarding closed-shell lead and bismuth targets with 3d transition metal ions, creating fused nuclei with low excitation energies due to the magic shells of the targets). However, the yields from this reaction decreased significantly with increasing atomic number, so that a 1998 GSI attempt on element 113 via cold fusion (repeated in 2003) was unsuccessful. The next element that was synthesised was element 114 (flerovium) instead of 113 (nihonium), with a "hot fusion" reaction of a heavy actinide target with doubly magic calcium-48 ions. Flerovium was first synthesized in December 1998 by a team of scientists at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, led by Yuri Oganessian, who bombarded a target of plutonium-244 with accelerated nuclei of calcium-48:

244
94Pu
+ 48
20Ca
→ 292
114Fl
* → 290
114Fl
+ 2 1
0
n

A single atom of flerovium, decaying by alpha emission with a lifetime of 30.4 seconds, was detected. The decay energy measured was 9.71 MeV, giving an expected half-life of 2–23 s.^[11] This observation was assigned to the isotope flerovium-289 and was published in January 1999.^[11] The experiment was later repeated, but an isotope with these decay properties was never found again and hence the exact identity of this activity is unknown. It is possible that it was due to the metastable isomer ^289mFl,^[12] but because the presence of a whole series of longer-lived isomers in its decay chain would be rather doubtful, the most likely assignment of this chain is to the 2n channel leading to ²⁹⁰Fl and electron capture to ²⁹⁰Nh, which fits well with the systematics and trends across flerovium isotopes. This would then have been the first report of a decay chain from an isotope of nihonium, but it was not recognised as such at the time, and the assignment is still uncertain in the absence of confirmation.^[6]

Dubna–Livermore collaboration

The first report of nihonium was in August 2003, when it was identified as an alpha decay product of element 115, moscovium. These results were published on February 1, 2004, by a team composed of Russian scientists at Dubna (Joint Institute for Nuclear Research), and American scientists at the Lawrence Livermore National Laboratory:^[13][14]

243
95Am
+ 48
20Ca
→ 288
115Mc
+ 3 1
0n
→ 284
113Nh
+
α

243
95Am
+ 48
20Ca
→ 287
115Mc
+ 4 1
0n
→ 283
113Nh
+
α

The Dubna–Livermore collaboration strengthened their claim for the discovery of nihonium by conducting chemical experiments on ²⁶⁸Db, the final decay product of ²⁸⁸Mc. This was valuable as none of the nuclides in this decay chain were previously known, so that their claim was not supported by any previously obtained experimental data, and chemical experimentation would strengthen the case for their claim. In June 2004 and again in December 2005, this dubnium isotope was successfully identified by extracting the final decay products, measuring spontaneous fission (SF) activities and using chemical identification techniques to confirm that they behave like a group 5 element (as dubnium is known to be in group 5 of the periodic table).^[1][15] Both the half-life and decay mode were confirmed for the proposed ²⁶⁸Db which lends support to the assignment of the parent and daughter nuclei to moscovium and nihonium respectively.^[15][16] Further experiments at Dubna in 2005 have fully confirmed the decay data for moscovium and nihonium, but in 2011, the IUPAC/IUPAP Joint Working Party (JWP) did not recognize the two elements as having been discovered because current theory could not distinguish between group 4 and group 5 elements by their chemical properties with sufficient confidence, and the identification of the daughter dubnium isotope was the most important factor in confirming the discovery of moscovium and nihonium.^[17] Furthermore, the decay properties of all the nuclei in the decay chain of moscovium had not been previously characterized before the Dubna experiments, a situation which the JWP generally considers "troublesome, but not necessarily exclusive".^[17]

RIKEN

Starting on 5 September 2003,^[18] a team of Japanese scientists at the RIKEN Nishina Center for Accelerator-Based Science led by Kōsuke Morita, bombarded a target of bismuth-209 with accelerated nuclei of zinc-70, continuing from their previous study from 2001 of cold fusion reactions and confirmation of the discoveries of elements 108, 110, 111, and 112 at the GSI Helmholtz Centre for Heavy Ion Research. The RIKEN team chose to use cold fusion, despite the much lower yield expected than for the JINR's hot fusion technique with calcium-48, as the synthesized isotopes would alpha decay to known daughter nuclides and make the discovery much more certain, and would not necessitate the use of radioactive targets.^[19] They detected a single atom of nihonium-278 on 23 July 2004 and published their results on September 28, 2004:^[20]

209
83Bi
+ 70
30Zn
→ 278
113Nh
+ 1
0n

Previously, in 2000, a team led by P. A. Wilk had identified one atom of the decay product ²⁶⁶Bh as decaying with identical properties to what the Japanese team had observed, thus lending support for their claim. However, they also observed the daughter of ²⁶⁶Bh, ²⁶²Db, undergo alpha decay instead of spontaneous fission: the Japanese team observed the latter decay mode.^[17]

The RIKEN team produced a further atom on 2 April 2005, although the decay data were slightly different from the first chain, perhaps due to either the formation of a metastable state or an alpha particle escaping from the detector before depositing its full energy.^[17] In 2004, the RIKEN team also studied the ²⁰⁵Tl(⁷⁰Zn,n)²⁷⁴Rg reaction, retaining the zinc beam and impinging it on a thallium rather than a bismuth target, in an effort to directly produce ²⁷⁴Rg in a cross-bombardment as the immediate daughter of ²⁷⁸Nh. However, the thallium target was weak compared to the more commonly used lead and bismuth targets, and it deteriorated significantly and became non-uniform in thickness; hence no atoms of ²⁷⁴Rg were observed. This reaction was repeated in 2010 with upgraded equipment, but again without success. The reasons for this weakness are still unknown, given that thallium has a higher melting point than bismuth.^[21]

In 2009, the RIKEN team studied the ²⁴⁸Cm(²³Na,5n)²⁶⁶Bh reaction to synthesize the decay product ²⁶⁶Bh directly and establish its link with ²⁷⁸Nh as a cross-bombardment; they also established the branched decay of ²⁶²Db, which sometimes underwent spontaneous fission and sometimes underwent the previously known alpha decay to ²⁵⁸Lr.^[22] Due to these inconsistencies in the decay data, the small number of nihonium atoms produced, and the lack of unambiguous anchors to known isotopes, the JWP did not accept this as a conclusive discovery of nihonium in 2011.^[17]

After 450 more days of irradiation of bismuth with zinc projectiles, production and identification of another ²⁷⁸Nh atom occurred at RIKEN on 12 August 2012.^[23][24] In this case, a series of six alpha decays was observed, leading down to an isotope of mendelevium:

278
113Nh
→ 274
111Rg
+
α
→ 270
109Mt
+
α
→ 266
107Bh
+
α
→ 262
105Db
+
α
→ 258
103Lr
+
α
→ 254
101Md
+
α

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 scientists on this team calculated the probability of accidental coincidence to be 10⁻²⁸, or totally negligible.^[23] The resulting ²⁵⁴Md atom then underwent beta plus decay to ²⁵⁴Fm, which itself finally underwent the seventh alpha decay in the chain to the long-lived ²⁵⁰Cf, which has a half-life of around thirteen years.^[25]

This experiment practically exhausted cold fusion as a method for making new elements, due to the extremely low cross section of the reaction (probability of fusion): the value of 20 femtobarns obtained for the ²⁰⁹Bi(⁷⁰Zn,n)²⁷⁸Nh is the lowest among all superheavy fusion reactions that have been successful.^[26] Nevertheless, it may still be of use in making new isotopes of already known elements: RIKEN had plans to later investigate light isotopes of the next element 114, flerovium, by fusing a lead-208 target with germanium-76 projectiles in the reaction ²⁰⁸Pb(⁷⁶Ge,n)²⁸³Fl.^[18][27] The cross-section for this reaction of lead and germanium producing flerovium is expected to be 200 fb, greater than the 30 fb for the reaction with bismuth and zinc producing nihonium.^[6][28][29]

Naming

Using Mendeleev's nomenclature for unnamed and undiscovered elements, nihonium should 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),^[30] a systematic element name as a placeholder, until the discovery of the element is confirmed and a name is decided on. Although widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, the recommendations were mostly ignored among scientists in the field, who called it "element 113", with the symbol of E113, (113), or even simply 113.^[1]

Claims to the discovery of nihonium were put forward by both the Dubna and RIKEN teams. The Japanese team suggested various names: japonium, symbol Jp, after their home country;^[31] nishinanium, symbol Nh, after Japanese physicist Yoshio Nishina, the "founding father of modern physics research in Japan";^[32] and rikenium, symbol Rk, after the team itself.^[31] In 2011, the IUPAC evaluated the 2004 RIKEN experiments and 2004 and 2007 Dubna experiments, and concluded that they did not meet the criteria for discovery.^[17]

On August 12, 2012, researchers at the RIKEN Nishina Center for Accelerator-Based Science in Japan, claimed to have successfully repeated their experiment and produced a third atom of ²⁷⁸Nh.^[33] In December 2015, IUPAC recognized the element and assigned the priority of the discovery to RIKEN, noting that while the individual decay energies of each nuclide in the decay chain of ²⁷⁸Nh were inconsistent, their sum was now confirmed to be consistent, strongly suggesting that the initial and final states in ²⁷⁸Nh and its daughter ²⁶²Db were the same for all three events. Furthermore, the decay of ²⁶²Db to ²⁵⁸Lr and ²⁵⁴Md was previously known, firmly anchoring the decay chain of ²⁷⁸Nh to known regions of the chart of nuclides. While the Dubna collaboration had also confirmed its results in 2013, their claim did not meet the discovery criteria as they had not convincingly determined the atomic numbers of their nuclides through cross-bombardments (making the daughters of ²⁸⁸Mc and ²⁸⁴Nh directly through other reactions), since their decay chains were not anchored to previously known nuclides;^[34][35] in any case, this confirmation came the year after RIKEN had confirmed its results.^[21] For the first time in history, a team of Asian physicists named a new element.^[33] Kōsuke Morita recollected:^[36]

Early in the morning, at 5:50 a.m. (JST), on the last day of 2015, I received an e-mail from the president of IUPAC (International Union of Pure and Applied Chemistry) Division II, Professor Jan Reedijk. He wrote, "May I first of all congratulate you and all of your colleagues in the Riken collaboration on the fact that the discovery of the element with Atomic Numbers of 113 has been assigned to work that you and your collaborating team has carried out." It was the moment where it truly hit me that our group had become the very first Asian scientific research group to discover a new element.^[36]

— Kōsuke Morita

On 8 June 2016, IUPAC disclosed the name of element 113 as nihonium. Prior to the formal approval by the IUPAC Council, a five-month public review was set, expiring 8 November 2016; the name was officially approved on 28 November 2016.

The sum argument advanced by the JWP in the approval of the discovery of nihonium was later criticized in a May 2016 study from Lund University and the GSI Helmholtz Centre for Heavy Ion Research, 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 ²⁷⁸Nh decay chain was not small enough to rule out this possibility; and if this is the case, similarity in lifetimes of intermediate daughters become a meaningless argument, as different isomers of the same nuclide can have wildly different half-lives (the ground state of ¹⁸⁰Ta has a half-life of mere hours, but an excited state ^180mTa has never been observed to decay). However, although this study found reason to doubt and criticize the IUPAC approval of the discoveries of moscovium and tennessine, the data from RIKEN for nihonium was found to be congruent, and the data from the Dubna team for moscovium and nihonium to probably be so, thus necessitating no criticism of the IUPAC approval of the discovery of element 113.^[40][41]

Isotopes

Nihonium has no stable or naturally occurring isotopes. Several radioactive isotopes have been synthesized in the laboratory, either by fusing two atoms or by observing the decay of heavier elements. Seven different isotopes of nihonium have been reported with atomic masses 278, 282–286, and 290; they all decay through alpha decay to isotopes of roentgenium,^[42] although nihonium-284 may have an electron capture branch to copernicium-284.^[45]

Stability and half-lives

All nihonium isotopes are extremely unstable and radioactive; however, the heavier nihonium isotopes are more stable than the lighter. The most stable known nihonium isotope, ²⁸⁶Nh, is also the heaviest confirmed nihonium isotope; it has a half-life of 20 seconds. The isotope ²⁸⁵Nh and the unconfirmed ²⁹⁰Nh have been reported to also have half-lives of over a second. The isotopes ²⁸⁴Nh and ²⁸³Nh have half-lives of 0.48 and 0.10 seconds respectively. The remaining two isotopes have half-lives between 0.1 and 100 milliseconds: ²⁸²Nh has a half-life of 70 milliseconds, and ²⁷⁸Nh, the lightest known nihonium isotope, is also the shortest-lived known nihonium isotope, with a half-life of just 0.24 milliseconds. It is predicted that even heavier undiscovered nihonium isotopes could be much more stable: for example, ²⁸⁷Nh is predicted to have a half-life of around 20 minutes,^[46] close to two orders of magnitude more than that of ²⁸⁶Nh.^[42]

Theoretical estimates of alpha decay half-lives of isotopes of nihonium are in good agreement with the experimental data.^[47] The undiscovered isotope ²⁹³Nh has been predicted to be the most stable towards beta decay;^[48] however, no known nihonium isotope has been observed to undergo beta decay.^[42]

The stability of nuclei decreases greatly with the increase in atomic number after plutonium, the heaviest primordial element, so that all isotopes with an atomic number above 101 decay radioactively with a half-life under a day, with the exception of dubnium-268. Nevertheless, for reasons not very well understood yet, there is a slight increased nuclear stability around atomic numbers 110–114, which leads to the appearance of what is known in nuclear physics as the "island of stability". This concept, proposed by University of California professor Glenn Seaborg, explains why superheavy elements last longer than predicted.^[49]

Predicted properties

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. It is predicted to show many differences from its lighter homologues: a largely contributing effect is the spin–orbit (SO) interaction. It is especially strong for the superheavy elements, because their electrons move much faster than in lighter atoms, at velocities comparable to the speed of light.^[51] In relation to nihonium atoms, it lowers the 7s and the 7p electron energy levels (stabilizing the corresponding electrons), but two of the 7p electron energy levels are stabilized more than the other four.^[52] The stabilization of the 7s electrons is called the inert pair effect, and the effect "tearing" the 7p subshell into the more stabilized and the less stabilized parts is called the subshell splitting. Computation 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 stabilized and less stabilized parts of the 7p subshell, respectively.^{[51][note 1]} For many theoretical purposes, the valence electron configuration may be represented to reflect the 7p subshell split as 7s²7p_1/2¹.^[1] These effects stabilize lower oxidation states: the first ionization energy of nihonium is expected to be 7.306 eV, the highest in group 13. Hence, the most stable oxidation state of nihonium is predicted to be the +1 state,^[1] and nihonium is expected to be less reactive than thallium.^[4] Differences for other electron levels also exist. For example, the 6d electron levels (also split in halves, with four being 6d_3/2 and six being 6d_5/2) are both raised, so that they are close in energy to the 7s ones.^[52] Thus, the 6d electron levels, being destabilized, should still be able to participate in chemical reactions in nihonium^[1] (as well as in the next 7p element, flerovium),^[4] thus making it behave in some ways like transition metals and allow higher oxidation states.^[1] Nihonium should hence also be able to show stable +3 and possibly also +5 oxidation states. However, the +3 state should still be less stable than the +1 state, following periodic trends. Nihonium should be the most electronegative among all the group 13 elements:^[1] for example, in the compound NhTs, the negative charge is expected to be on the nihonium atom rather than the tennessine atom, the opposite of what would be expected from simple periodicity.^[50] The electron affinity of nihonium is calculated to be around 0.68 eV; in comparison, that of thallium is 0.4 eV.^[1] The high electron affinity and electronegativity of nihonium are due to it being only one electron short of the closed-shell valence electron configuration of flerovium (7s²7p_1/2²):^[1] this would make the −1 oxidation state of nihonium more stable than that of its lighter congener thallium.^[5] The standard electrode potential for the Nh⁺/Nh couple is predicted to be −0.6 V.^[4]

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. However, 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 exceptional in the 7p series of elements; all other MH (M = Fl, Mc, Lv, Ts, Og) molecules have relativistic expansion of the bond length instead of contraction.^[53] The analogous monofluoride (NhF) should also exist.^[50] Nihonium should also be able to form the trihydride (NhH₃), trifluoride (NhF₃), and trichloride (NhCl₃), with nihonium in the +3 oxidation state. Because the 6d electrons are involved in bonding instead of the 7s ones, these molecules are predicted to be T-shaped and not trigonal planar. Although the polyfluoride anion NhF−
6 should be stable, the corresponding neutral fluoride NhF₅ should be unstable, spontaneously decomposing into the trifluoride and elemental fluorine. 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 an excess of hydrochloric acid or in ammonia while TlCl is not. Additionally, in contrast to the strongly basic TlOH, nihonium(I) should instead form Nh₂O, which would be weakly water-soluble and readily ammonia-soluble.^[4] The adsorption behavior of nihonium on gold surfaces in thermochromatographical experiments is expected to be closer to that of astatine than that of thallium.^[54]

Nihonium is expected to be much denser than thallium, having a predicted density of about 16 to 18 g/cm³, due to the relativistic stabilization and contraction of its 7s and 7p_1/2 orbitals.^[1][50] This is because calculations estimate it to have an atomic radius of about 170 pm, the same as that of thallium, even though periodic trends would predict it to have an atomic radius larger than that of thallium due to it being one period further down in the periodic table.^[1] The melting and boiling points of nihonium are not definitely known, but have been calculated to be 430 °C and 1100 °C respectively, exceeding the values for gallium, indium, and thallium, following periodic trends.^[1][2]

Experimental chemistry

Unambiguous determination of the chemical characteristics of nihonium has yet to have been established.^[55][56] The isotopes ²⁸⁴Nh, ²⁸⁵Nh, and ²⁸⁶Nh have half-lives long enough for chemical investigation. 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.^[56] From 2010 to 2012, some preliminary chemical experiments were performed at the Joint Institute for Nuclear Research to determine the volatility of nihonium. The reaction used was ²⁴³Am(⁴⁸Ca,3n)²⁸⁸Mc; the isotope ²⁸⁸Mc has a short half-life and would quickly decay to the longer-lived ²⁸⁴Nh, which would be chemically investigated. Teflon capillaries at 70 °C connecting the recoil chamber, where the nihonium atoms were synthesized, and the gold-covered detectors: the nihonium atoms would be carried along the capillaries by a carrier gas. While about ten to twenty atoms of ²⁸⁴Nh were produced, none of these atoms were registered by the gold-covered detectors, suggesting either that nihonium was similar in volatility to the noble gases or, more plausibly, that pure nihonium was not very volatile and thus could not efficiently pass through the Teflon capillaries at 70 °C.^[56] Formation of the hydroxide NhOH would ease the transport, as nihonium hydroxide is expected to be more volatile than elemental nihonium, and this reaction could be facilitated by adding more water vapor into the carrier gas. However, it seems likely that this formation is not kinetically favored, so that the longer-lived isotope ²⁸⁶Nh, created as the granddaughter of ²⁹⁴Ts, might be more desirable for future experiments.^[56]

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 Teflon. No nihonium atoms were observed after chemical separation, implying an unexpectedly large retention of nihonium atoms on Teflon surfaces. This experimental result for the interaction limit of nihonium atoms with a Teflon surface (−ΔHTeflon
ads(Nh) > 45 kJ/mol) disagrees significantly with previous theory, which expected a far lower value of 14.00 kJ/mol. This also implies 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 behavior of elemental nihonium.^[57]

See also

• Isotopes of nihonium
• Island of stability