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Nihonium

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Nihonium,  113Nh
General properties
Pronunciation
Mass number 286 (most stable isotope)
Nihonium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Tl

Nh

(Uhs)
coperniciumnihoniumflerovium
Atomic number (Z) 113
Group, period group 13 (boron group), period 7
Block p-block
Element category   unknown chemical properties, but probably a post-transition metal
Electron configuration [Rn] 5f14 6d10 7s2 7p1 (predicted)[1]
Electrons per shell
2, 8, 18, 32, 32, 18, 3 (predicted)
Physical properties
Phase (at STP) solid (predicted)[1][2][3]
Melting point 700 K ​(430 °C, ​810 °F) (predicted)[1]
Boiling point 1430 K ​(1130 °C, ​2070 °F) (predicted)[1][4]
Density (near r.t.) 16 g/cm3 (predicted)[4]
Heat of fusion 7.61 kJ/mol (extrapolated)[3]
Heat of vaporization 130 kJ/mol (predicted)[2][4]
Atomic properties
Oxidation states −1, 1, 3, 5 (predicted)[1][4][5]
Ionization energies
  • 1st: 704.9 kJ/mol (predicted)[1]
  • 2nd: 2238.5 kJ/mol (predicted)[4]
  • 3rd: 3203.3 kJ/mol (predicted)[4]
  • (more)
Atomic radius empirical: 170 pm (predicted)[1]
Covalent radius 172–180 pm (extrapolated)[3]
Miscellanea
CAS Number 54084-70-7
History
Naming After Japan (Nihon in Japanese)
Discovery RIKEN (2004)
Main isotopes of nihonium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
290Nh[6] syn 2 s? α 286Rg
287Nh[7] syn 5.5 s? α 283Rg
286Nh syn 9.5 s α 282Rg
285Nh syn 4.2 s α 281Rg
284Nh syn 0.91 s α 280Rg
EC 284Cn
283Nh syn 75 ms α 279Rg
282Nh syn 73 ms α 278Rg
278Nh syn 1.4 ms α 274Rg
| references | in Wikidata

Nihonium is a synthetic chemical element with symbol Nh and atomic number 113. It 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.[8] In November 2016, the IUPAC published a declaration defining the name to be nihonium.[9] The name comes from the common Japanese name for Japan (日本, nihon). On 28 November 2016, the name became official.[10][11]

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[edit]

Early indications[edit]

The syntheses of elements 107 to 112 inclusive, bohrium through copernicium, were conducted at the GSI from 1981 to 1996 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). This technique had been pioneered by Yuri Oganessian at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia. 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. Furthermore, the resulting nuclei were severely neutron-deficient and did not live very long.[12][13]

Faced with this problem, Oganessian and his team at Dubna turned its attention to the previous "hot fusion" technique, in which heavy actinide targets were reacted 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 inevitable neutron deficiencies of the nuclides produced. Being doubly magic, it would also confer similar benefits in stability to the fused nuclei as the lead and bismuth targets from cold fusion had. In collaboration with the team at the Lawrence Livermore National Laboratory in the United States, they made an attempt on element 114 (which was predicted to be a magic number, closing a proton shell, and hence more stable than element 113). 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:[12]

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

n

This reaction had been attempted before, but without success; for this 1998 attempt, the JINR had upgraded all of its equipment to detect and separate the produced atoms better and bombard the target more intensely.[12] 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.[14] This observation was assigned to the isotope flerovium-289 and was published in January 1999.[14] 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,[15] 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] A similar long-lived activity observed by the Dubna team in March 1999 in the 242Pu+48Ca reaction may similarly be due to the electron-capture daughter of 287Fl, 287Nh, but this assignment is likewise tentative.[7]

Dubna–Livermore collaboration[edit]

The now-confirmed discovery of element 114 was made in June 1999 when the Dubna team repeated the first reaction from 1998. This time, two atoms of flerovium were produced; they alpha decayed with a half-life of 2.6 s, different from the 1998 result.[16] This activity was initially assigned to 288Fl in error, due to the confusion regarding the previous observations that were assumed to come from 289Fl. Further work in December 2002 finally allowed a positive reassignment of the June 1999 atoms to 289Fl.[17] During and after this confirmation of the new element 114, the Dubna–Livermore collaboration sought to bolster its discovery by cross-reactions, producing elements 116 and 118 in 2000 and 2002 respectively to identify known isotopes of element 114 as their daughters and verify the observed decay properties: this was done by retaining the calcium-48 beam and changing the targets to curium-248 and californium-249 respectively. They then turned their attention to the missing odd-numbered elements, as they were expected to possibly be even more stable as the odd protons and possibly neutrons would hinder decay by spontaneous fission.[12][18]

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:[18][19]

243
95
Am
+ 48
20
Ca
288
115
Mc
+ 3 1
0
n
284
113
Nh
+
α
243
95
Am
+ 48
20
Ca
287
115
Mc
+ 4 1
0
n
283
113
Nh
+
α

Later, Mark Stoyer, one of the Livermore scientists, recalled:[12]

I was on sabbatical, ... and I remember thinking to myself "Why are we rushing, why are we doing this so fast? There isn't anybody else doing this experiment." About six months later, the Japanese results came out.[12]

— Mark Stoyer

RIKEN[edit]

While the Dubna–Livermore collaboration had been approaching element 113 from above, a team of Japanese scientists at the RIKEN Nishina Center for Accelerator-Based Science led by Kōsuke Morita had been approaching it from below. In 2001, this team had been studying cold fusion reactions, confirming the GSI's discoveries of elements 108, 110, 111, and 112. They then chose to make a new attempt on element 113. 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 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.[20] Bombardment of a bismuth-209 target with accelerated nuclei of zinc-70 began on 5 September 2003.[21] The team detected a single atom of nihonium-278 on 23 July 2004 and published their results on September 28, 2004:[22]

209
83
Bi
+ 70
30
Zn
278
113
Nh
+ 1
0
n

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.[23]

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.[23] 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.[24]

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.[12] 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.[21][25] 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][26][27]

Road to confirmation[edit]

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][28] 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.[28][29] 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.[23] 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".[23]

In June 2006, the Dubna-Livermore team synthesised a new isotope of nihonium directly by bombarding a neptunium-237 target with accelerated calcium-48 nuclei:

237
93
Np
+ 48
20
Ca
282
113
Nh
+ 1
0
n

Two atoms of 282Nh were detected. The aim of this experiment had been to synthesise the isotopes 281Nh and 282Nh that would fill in the gap between isotopes produced via hot fusion (283Nh to 286Nh) and cold fusion (278Nh): these nuclides could decay to known portions of the chart of nuclides and provide unambiguous confirmation of proton and neutron numbers of the original nuclide synthesized, thus qualifying for IUPAC approval of the discovery. The first decay chain terminated with spontaneous fission after four alpha decays, presumably originating from 266Db or its electron-capture daughter 266Rf. Spontaneous fission was not observed in the second chain even after four alpha decays. A fifth alpha decay in each chain could have been missed, since 266Db can theoretically undergo alpha decay, in which case the first decay chain would have ended at the known 262Lr or 262No and the second might have continued to the known long-lived 258Md, which has a half-life of 51.5 days, longer than the duration of the experiment itself. However, in the absence of direct detection of the long-lived alpha decays, these interpretations remain unconfirmed, and there is still no known link between any superheavy nuclides produced by hot fusion and the well-known main body of the chart of nuclides.[30]

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.[31] 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.[23]

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.[32][33] In this case, a series of six alpha decays was observed, leading down to an isotope of mendelevium:

278
113
Nh
274
111
Rg
+
α
270
109
Mt
+
α
266
107
Bh
+
α
262
105
Db
+
α
258
103
Lr
+
α
254
101
Md
+
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.[32] 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.[34]

In August 2013, a team of researchers at Lund University and at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany announced they had repeated the 2004 experiment, confirming Dubna's findings.[35][36] The same year, the 2004 experiment had been repeated at Dubna, now additionally also creating the isotope 289Mc that could serve as a cross-bombardment for confirming the discovery of the tennessine isotope 293Ts in 2010, as well as its daughter 285Nh as part of its decay chain.[37] Further confirmation was published by the team at Lawrence Berkeley National Laboratory in 2015.[38]

IUPAC approval of discoveries[edit]

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;[39] nishinanium, symbol Nh, after Japanese physicist Yoshio Nishina, the "founding father of modern physics research in Japan";[40] and rikenium, symbol Rk, after the team itself.[39]

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, and confirmation had also come externally from the GSI in 2013 and Berkeley in 2015, 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;[41][37] in any case, this confirmation had come the year after RIKEN had confirmed its results.[24] For the first time in history, a team of Asian physicists named a new element.[42] Kōsuke Morita recollected:[43]

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.[43]

— Kōsuke Morita

IUPAC had simultaneously given recognition to the Dubna–Livermore collaboration for elements 115, 117, and 118; they were joined by the Oak Ridge National Laboratory and Vanderbilt University for the former two, who had procured the rare and highly radioactive berkelium-249 target necessary to synthesise element 117 (completing the calcium-48 campaign) and confirm the synthesis of its daughter, element 115. They had previously been accorded credit in 2011 from IUPAC for the discoveries of elements 114 and 116.[41] Regarding the awarding of element 113 to RIKEN, the JINR responded:[44]

To us, this is somewhat unexpected decision. Moreover, in the IUPAC practice, there are enough precedents of acknowledging “joint” priority (examples can be the cases with elements 103, 104 and 105 with authorship of discovery shared between JIND (Dubna) and Berkeley (USA)). We are glad for our colleagues from RIKEN especially, because the leader of the work, Prof. K.Morita is to a certain extent the trainee of Dubna; here, in JINR he for quite a long time learned the basics of synthesis of new elements. However, the method of synthesizing the superheavy elements, chosen by RIKEN researchers is completely exhausted; moreover, today they plan future experiments using only the method proposed in Dubna.

We respect the decision of IUPAC. However, our position regarding the decision on element 113 will be determined only after the reports of the IUPAC-IUPAP JWP are officially published and studied in detail.[44]

After the publication of the JWP reports in January 2016, Sergey Dimitriev, the lab director of the Flerov lab at JINR where the discoveries were made, remarked:[12]

Of six new elements, Iupac recognises five for our institute. We're quite happy – our Japanese colleagues spent 10 years for the synthesis of three nuclei. Professor Morita is our good friend, he spent many years in our laboratory. He's our pupil, so it's not a problem for us![12]

— Sergey Dimitriev

One further cloud hung over the recognition, which had been announced early and unilaterally by IUPAC because the news of RIKEN being awarded credit for element 113 had been leaked to Japanese newspapers. The former president of IUPAP, Cecilia Jarlskog, had complained at the Nobel Symposium on Superheavy Elements in 2016 about the lack of openness involved in the process of approving the new elements, and stated that she believed that the JWP's work was flawed and should be redone by a new JWP. However, after a survey of many physicists, it was determined that while many physicists felt that some aspects of the JWP report merited concern, the consensus was that the conclusions would hold up if the work was redone, and hence the new president, Bruce McKellar, ruled that the proposed names should be released in a joint IUPAP–IUPAC press release.[45]

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. 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: for example, 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.[46][47]

Naming[edit]

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),[48] 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]

In March 2016, Kōsuke Morita proposed the name "nihonium" to IUPAC, after its place of discovery and referencing Japanese chemist Masataka Ogawa's 1908 discovery of rhenium, which he named "nipponium".[41] IUPAC accepted the proposal[49] and set a term expiring 8 November to collect comments, after which the final name would be formally established at a conference.[50][51]

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.[9]

Isotopes[edit]

List of nihonium isotopes
Isotope
Half-life
[52]
Decay
mode[52]
Discovery
year
Reaction
278Nh 1.4 ms α 2004 209Bi(70Zn,n)[22]
282Nh 70 ms α 2006 237Np(48Ca,3n)[53]
283Nh 0.1 s α 2003 287Mc(—,α)[53]
284Nh 1 s α, EC 2003 288Mc(—,α)[53]
285Nh 4 s α 2009 293Ts(—,2α)[54]
286Nh 8 s α 2009 294Ts(—,2α)[54]
287Nh 5.5 s? α 1999? 287Fl(ee)?
290Nh 2 s? α 1998? 290Fl(ee)?

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. Eight different isotopes of nihonium have been reported with atomic masses 278, 282–287, and 290 (although 287Nh and 290Nh are unconfirmed); they all decay through alpha decay to isotopes of roentgenium,[52] although nihonium-284 may have an electron capture branch to copernicium-284.[55]

Stability and half-lives[edit]

Nihonium (row 113) is expected to be within the "island of stability" (white circle) and thus its nuclei are slightly more stable than would otherwise be predicted.

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 8 seconds. The isotope 285Nh, as well as the unconfirmed 287Nh and 290Nh, have been reported to also have half-lives of over a second. The isotopes 284Nh and 283Nh have half-lives of 1 and 0.1 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 1.4 milliseconds.[56]

Theoretical estimates of alpha decay half-lives of isotopes of nihonium are in good agreement with the experimental data.[57] The undiscovered isotope 293Nh has been predicted to be the most stable towards beta decay;[58] however, no known nihonium isotope has been observed to undergo beta decay.[52]

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 110114, 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.[59]

Predicted properties[edit]

Atomic energy levels of outermost s, p, and d electrons of thallium and nihonium[60]

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.[61] 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.[62] 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.[61][note 1] For many theoretical purposes, the valence electron configuration may be represented to reflect the 7p subshell split as 7s27p1/21.[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 6d3/2 and six being 6d5/2) are both raised, so that they are close in energy to the 7s ones.[62] 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.[60] 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 (7s27p1/22):[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 7p1/2 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 7p1/2 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.[63] The analogous monofluoride (NhF) should also exist.[60] 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.[64]

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 7p1/2 orbitals.[1][60] 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[edit]

Unambiguous determination of the chemical characteristics of nihonium has yet to have been established.[65][66] 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.[66] 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.[66] 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;[66] 285Nh (the daughter of 289Mc and granddaughter of 293Ts) would also be suitable.[67]

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.[68] It has been suggested to use bromine saturated with boron tribromide as a carrier gas for experiments on nihonium chemistry; this acts as a strong brominating agent and oxidises nihonium's lighter congener thallium to thallium(III), providing an avenue to investigate the oxidation states of nihonium, similar to earlier experiments done on the bromides of group 5 elements, including the superheavy dubnium.[69]

See also[edit]

Notes[edit]

  1. ^ The quantum number corresponds to the letter in the electron orbital name: 0 to s, 1 to p, 2 to d, etc. See azimuthal quantum number for more information.

References[edit]

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External links[edit]