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=                              Nihonium                              =
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                            Introduction
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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.


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).

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

: +  → 292114* → 290114 + 2  + e− → 290113 + νe ?

A single atom was observed which was thought to be the isotope 289114:
the results were published in January 1999. 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. 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.


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;
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.

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:

: +  → 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.


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

: +  → 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. 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.


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.


2004–2008
===========
In June 2004 and again in December 2005, the JINR-LLNL collaboration
strengthened their claim for the discovery of element 113 by
conducting chemical experiments on 268Db, the final decay product of
288115. This was valuable as none of the nuclides in this decay chain
were previously known, so that their claim was not supported by any
previous experimental data, and chemical experimentation would
strengthen the case for their claim, since the chemistry of dubnium is
known. 268Db 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 (dubnium is known to be in group 5). 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
elements 115 and 113 respectively. Further experiments at the JINR in
2005 confirmed the observed decay data.

In November and December 2004, the Riken team studied the 205Tl + 70Zn
reaction, aiming the zinc beam onto a thallium rather than a bismuth
target, in an effort to directly produce 274Rg in a cross-bombardment
as it is the immediate daughter of 278113. The reaction was
unsuccessful, as the thallium target was physically weak compared to
the more commonly used lead and bismuth targets, and it deteriorated
significantly and became non-uniform in thickness. The reasons for
this weakness are unknown, given that thallium has a higher melting
point than bismuth. The Riken team then repeated the original 209Bi +
70Zn reaction and produced a second atom of 278113 in April 2005, with
a decay chain that again terminated with the spontaneous fission of
262Db. The decay data were slightly different from those of the first
chain: this could have been because an alpha particle escaped from the
detector without depositing its full energy, or because some of the
intermediate decay products were formed in metastable isomeric states.

In 2006, a team at the Heavy Ion Research Facility in Lanzhou, China,
investigated the 243Am + 26Mg reaction, producing four atoms of 266Bh.
All four chains started with an alpha decay to 262Db; three chains
ended there with spontaneous fission, as in the 278113 chains observed
at Riken, while the remaining one continued via another alpha decay to
258Lr, as in the 266Bh chains observed at LBNL.

In June 2006, the JINR-LLNL collaboration claimed to have synthesised
a new isotope of element 113 directly by bombarding a neptunium-237
target with accelerated calcium-48 nuclei:

: +  → 285113* → 282113 + 3

Two atoms of 282113 were detected. The aim of this experiment had been
to synthesise the isotopes 281113 and 282113 that would fill in the
gap between isotopes produced via hot fusion (283113 and 284113) and
cold fusion (278113). After five alpha decays, these nuclides would
reach known isotopes of lawrencium, assuming that the decay chains
were not terminated prematurely by spontaneous fission. The first
decay chain ended in 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: this would explain the lack of a spontaneous fission
event in this chain. 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.


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. The JWP did not accept the Riken team's claim either due to
inconsistencies in the decay data, the small number of atoms of
element 113 produced, and the lack of unambiguous anchors to known
isotopes.

In early 2009, the Riken team synthesised the decay product 266Bh
directly in the 248Cm + 23Na reaction to establish its link with
278113 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.

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,
who helped procure the rare and highly radioactive berkelium target
necessary to complete the JINR's calcium-48 campaign to synthesise the
heaviest elements on the periodic table. Two isotopes of element 117
were synthesised, decaying to element 115 and then element 113:

: +  → 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.

In March 2010, the Riken team again attempted to synthesise 274Rg
directly through the 205Tl + 70Zn reaction with upgraded equipment;
they failed again and abandoned this cross-bombardment route.

After 450 more days of irradiation of bismuth with zinc projectiles,
Riken produced and identified another 278113 atom in August 2012.
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 resulting 254Md atom then underwent electron capture
to 254Fm, which underwent the seventh alpha decay in the chain to the
long-lived 250Cf, which has a half-life of around thirteen years.

The 249Bk + 48Ca experiment was repeated at the JINR in 2012 and 2013
with consistent results, and again at the GSI in 2014. In August 2013,
a team of researchers at Lund University in Lund, Sweden, and at the
GSI announced that they had repeated the 2003 243Am + 48Ca experiment,
confirming the findings of the JINR-LLNL collaboration. 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. Confirmation of
288115 and its daughters was published by the team at the LBNL in
August 2015.


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. 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. For the first time in history, a team of Asian physicists
would name a new element. 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.

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. 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
========
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'.

Before the JWP recognition of their priority, the Japanese team had
unofficially suggested various names: 'japonium', after their home
country; 'nishinanium', after Japanese physicist Yoshio Nishina, the
"founding father of modern physics research in Japan"; 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, 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. 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. In March 2016,
Morita proposed the name "nihonium" to IUPAC, with the symbol Nh. The
naming realised what had been a national dream in Japanese science
ever since Ogawa's claim.

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. 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
======================================================================
{{Isotopes summary
|element=nihonium
|reaction ref=
|isotopes=







}}

Nihonium has no stable or naturally occurring isotopes. Several
radioactive isotopes have been synthesised 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 (287Nh and 290Nh are unconfirmed); they
all decay through alpha decay to isotopes of roentgenium. There have
been indications that nihonium-284 can also decay by electron capture
to copernicium-284, though estimates of the partial half-life for this
branch vary strongly by model. A spontaneous fission branch of
nihonium-285 has also been reported.


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.

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.

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. Of particular interest is 281Nh, as it is the
expected great-granddaughter of 293119, a possible product of the
243Am+54Cr reaction. 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. 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.


                        Predicted properties
======================================================================
Very few properties of nihonium or its compounds have been measured;
this is due to its extremely limited and expensive production and the
fact it decays very quickly. Properties of nihonium mostly remain
unknown and only predictions are available.


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. 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. 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. 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 7p1/21. The first ionisation energy of nihonium is expected to be
7.306 eV, the highest among the metals of group 13. Similar subshell
splitting should exist for the 6d electron levels, with four being
6d3/2 and six being 6d5/2. 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.

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 7p1/2 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. Bulk nihonium is expected to have a hexagonal close-packed
crystal structure, like thallium. 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. Nihonium should have a bulk modulus of 20.8 GPa, about half
that of thallium (43 GPa).


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). 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. The standard
electrode potential for the Nh+/Nh couple is predicted to be 0.6 V.
Nihonium should be a rather noble metal.

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.

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. The SO interaction causes the binding energy of
nihonium monohydride to be reduced by about 1 eV and the
nihonium-hydrogen bond length to decrease as the bonding 7p1/2 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. 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). The
analogous monofluoride (NhF) should also exist. Nihonium(I) is
predicted to be more similar to silver(I) than thallium(I): 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.

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 7p3/2
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.) 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.
Nihonium should be the most electronegative of the metallic group 13
elements, 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.
The −1 oxidation should be more stable for nihonium than for
tennessine. 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. 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.



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. 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. 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: this is due to the influence of the 6d5/2
electrons on the bonding. 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 bonding in the lighter NhX3 molecules can be considered as that of
a linear  species (similar to HgF2 or ) 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):
: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. 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 .

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 .
The structures of the nihonium trifluoride and pentafluoride molecules
are the same as those for chlorine trifluoride and pentafluoride.


                       Experimental chemistry
======================================================================
The isotopes 284Nh, 285Nh, and 286Nh have half-lives long enough for
chemical investigation. 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 °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. Formation of the
hydroxide NhOH should 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 vapour into the
carrier gas. It seems likely that this formation is not kinetically
favoured, so the longer-lived isotopes 285Nh and 286Nh were considered
more desirable for future experiments.

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 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.
Bromine saturated with boron tribromide has been suggested as a
carrier gas for experiments on nihonium chemistry; this 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.

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 7p1/2 shell.


                           External links
======================================================================
* [http://www.periodicvideos.com/videos/113.htm Nihonium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [http://www.radiochemistry.org/periodictable/elements/115.html Uut
and Uup Add Their Atomic Mass to Periodic Table]
*
[https://web.archive.org/web/20050623012629/http://www-cms.llnl.gov/e113_115/images.html
Discovery of Elements 113 and 115]
* [https://physicsworld.com/a/superheavy-elements/ Superheavy
elements]
* [http://www.webelements.com/nihonium/ WebElements.com: Nihonium]


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