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=                             Tennessine                             =
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                            Introduction
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Tennessine is a synthetic element; it has symbol Ts and atomic number
117. It has the second-highest atomic number, the joint-highest atomic
mass of all known elements, and is the penultimate element of the 7th
period of the periodic table. It is named after the U.S. state of
Tennessee, where key research institutions involved in its discovery
are located (however, the IUPAC says that the element is named after
the "region of Tennessee").

The discovery of tennessine was officially announced in Dubna, Russia,
by a Russian-American collaboration in April 2010, which makes it the
most recently discovered element. One of its daughter isotopes was
created directly in 2011, partially confirming the experiment's
results. The experiment was successfully repeated by the same
collaboration in 2012 and by a joint German-American team in May 2014.
In December 2015, the Joint Working Party of the International Union
of Pure and Applied Chemistry (IUPAC) and the International Union of
Pure and Applied Physics (IUPAP), which evaluates claims of discovery
of new elements, recognized the element and assigned the priority to
the Russian-American team. In June 2016, the IUPAC published a
declaration stating that the discoverers had suggested the name
'tennessine', a name which was officially adopted in November 2016.

Tennessine may be located in the "island of stability", a concept that
explains why some superheavy elements are more stable despite an
overall trend of decreasing stability for elements beyond bismuth on
the periodic table. The synthesized tennessine atoms have lasted tens
and hundreds of milliseconds. In the periodic table, tennessine is
expected to be a member of group 17, the halogens. Some of its
properties may differ significantly from those of the lighter halogens
due to relativistic effects. As a result, tennessine is expected to be
a volatile metal that neither forms anions nor achieves high oxidation
states. A few key properties, such as its melting and boiling points
and its first ionization energy, are nevertheless expected to follow
the periodic trends of the halogens.


Pre-discovery
===============
In December 2004, the Joint Institute for Nuclear Research (JINR) team
in Dubna, Moscow Oblast, Russia, proposed a joint experiment with the
Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, United
States, to synthesize element 117 -- so called for the 117 protons in
its nucleus. Their proposal involved fusing a berkelium (element 97)
target and a calcium (element 20) beam, conducted via bombardment of
the berkelium target with calcium nuclei: this would complete a set of
experiments done at the JINR on the fusion of actinide targets with a
calcium-48 beam, which had thus far produced the new elements 113-116
and 118. ORNL--then the world's only producer of berkelium--could not
then provide the element, as they had temporarily ceased production,
and re-initiating it would be too costly. Plans to synthesize element
117 were suspended in favor of the confirmation of element 118, which
had been produced earlier in 2002 by bombarding a californium target
with calcium. The required berkelium-249 is a by-product in
californium-252 production, and obtaining the required amount of
berkelium was an even more difficult task than obtaining that of
californium, as well as costly: It would cost around 3.5 million
dollars, and the parties agreed to wait for a commercial order of
californium production, from which berkelium could be extracted.

The JINR team sought to use berkelium because calcium-48, the isotope
of calcium used in the beam, has 20 protons and 28 neutrons, making a
neutron-proton ratio of 1.4; and it is the lightest stable or
near-stable nucleus with such a large neutron excess. Thanks to the
neutron excess, the resulting nuclei were expected to be heavier and
closer to the sought-after island of stability. Of the aimed for 117
protons, calcium has 20, and thus they needed to use berkelium, which
has 97 protons in its nucleus.

In February 2005, the leader of the JINR team -- Yuri Oganessian --
presented a colloquium at ORNL. Also in attendance were
representatives of Lawrence Livermore National Laboratory, who had
previously worked with JINR on the discovery of elements 113-116 and
118, and Joseph Hamilton of Vanderbilt University, a collaborator of
Oganessian.

Hamilton checked if the ORNL high-flux reactor produced californium
for a commercial order: The required berkelium could be obtained as a
by-product. He learned that it did not and there was no expectation
for such an order in the immediate future. Hamilton kept monitoring
the situation, making the checks once in a while. (Later, Oganessian
referred to Hamilton as "the father of 117" for doing this work.)


Discovery
===========
ORNL resumed californium production in spring 2008. Hamilton noted the
restart during the summer and made a deal on subsequent extraction of
berkelium (the price was about $600,000). During a September 2008
symposium at Vanderbilt University in Nashville, Tennessee,
celebrating his 50th year on the Physics faculty, Hamilton introduced
Oganessian to James Roberto (then the deputy director for science and
technology at ORNL). They established a collaboration among JINR,
ORNL, and Vanderbilt. Clarice Phelps was part of ORNL's team that
collaborated with JINR; this is particularly notable as because of it
the IUPAC recognizes her as the first African-American woman to be
involved with the discovery of a chemical element. The eventual
collaborating institutions also included  The University of Tennessee
(Knoxville), Lawrence Livermore National Laboratory, The Research
Institute for Advanced Reactors (Russia), and The University of Nevada
(Las Vegas).


In November 2008, the U.S. Department of Energy, which had oversight
over the reactor in Oak Ridge, allowed the scientific use of the
extracted berkelium.

The production lasted 250 days and ended in late December 2008,
resulting in 22 milligrams of berkelium, enough to perform the
experiment. In January 2009, the berkelium was removed from ORNL's
High Flux Isotope Reactor; it was subsequently cooled for 90 days and
then processed at ORNL's Radiochemical Engineering and Development
Center to separate and purify the berkelium material, which took
another 90 days. Its half-life is only 330 days: this means, after
that time, half the berkelium produced would have decayed. Because of
this, the berkelium target had to be quickly transported to Russia;
for the experiment to be viable, it had to be completed within six
months of its departure from the United States. The target was packed
into five lead containers to be flown from New York to Moscow.
Russian customs officials twice refused to let the target enter the
country because of missing or incomplete paperwork. Over the span of a
few days, the target traveled over the Atlantic Ocean five times. On
its arrival in Russia in June 2009, the berkelium was immediately
transferred to Research Institute of Atomic Reactors (RIAR) in
Dimitrovgrad, Ulyanovsk Oblast, where it was deposited as a
300-nanometer-thin layer on a titanium film. In July 2009, it was
transported to Dubna, where it was installed in the particle
accelerator at the JINR. The calcium-48 beam was generated by
chemically extracting the small quantities of calcium-48 present in
naturally occurring calcium, enriching it 500 times. This work was
done in the closed town of Lesnoy, Sverdlovsk Oblast, Russia.

The experiment began in late July 2009. In January 2010, scientists at
the Flerov Laboratory of Nuclear Reactions announced internally that
they had detected the decay of a new element with atomic number 117
via two decay chains: one of an odd-odd isotope undergoing 6 alpha
decays before spontaneous fission, and one of an odd-even isotope
undergoing 3 alpha decays before fission. The obtained data from the
experiment was sent to the LLNL for further analysis. On 9 April 2010,
an official report was released in the journal 'Physical Review
Letters' identifying the isotopes as 294117 and 293117, which were
shown to have half-lives on the order of tens or hundreds of
milliseconds. The work was signed by all parties involved in the
experiment to some extent: JINR, ORNL, LLNL, RIAR, Vanderbilt, the
University of Tennessee (Knoxville, Tennessee, U.S.), and the
University of Nevada (Las Vegas, Nevada, U.S.), which provided data
analysis support. The isotopes were formed as follows:

: +  → 297117* → 294117 + 3  (1 event)

: +  → 297117* → 293117 + 4  (5 events)


Confirmation
==============
All daughter isotopes (decay products) of element 117 were previously
unknown; therefore, their properties could not be used to confirm the
claim of discovery. In 2011, when one of the decay products ((289)115)
was synthesized directly, its properties matched those measured in the
claimed indirect synthesis from the decay of element 117. The
discoverers did not submit a claim for their findings in 2007-2011
when the Joint Working Party was reviewing claims of discoveries of
new elements.

The Dubna team repeated the experiment in 2012, creating seven atoms
of element 117 and confirming their earlier synthesis of element 118
(produced after some time when a significant quantity of the
berkelium-249 target had beta decayed to californium-249). The results
of the experiment matched the previous outcome; the scientists then
filed an application to register the element. In May 2014, a joint
German-American collaboration of scientists from the ORNL and the GSI
Helmholtz Center for Heavy Ion Research in Darmstadt, Hessen, Germany,
claimed to have confirmed discovery of the element. The team repeated
the Dubna experiment using the Darmstadt accelerator, creating two
atoms of element 117.

In December 2015, the JWP officially recognized the discovery of
293117 on account of the confirmation of the properties of its
daughter (289)115, and thus the listed discoverers -- JINR, LLNL, and
ORNL -- were given the right to suggest an official name for the
element. (Vanderbilt was left off the initial list of discoverers in
an error that was later corrected.)

In May 2016, Lund University (Lund, Scania, Sweden) and GSI cast some
doubt on the syntheses of elements 115 and 117. The decay chains
assigned to (289)115, the isotope instrumental in the confirmation of
the syntheses of elements 115 and 117, were found based on a new
statistical method to be too different to belong to the same nuclide
with a reasonably high probability. The reported 293117 decay chains
approved as such by the JWP were found to require splitting into
individual data sets assigned to different isotopes of element 117. It
was also found that the claimed link between the decay chains reported
as from (293)117 and (289)115 probably did not exist. (On the other
hand, the chains from the non-approved isotope (294)117 were found to
be congruent.) The multiplicity of states found when nuclides that are
not even-even undergo alpha decay is not unexpected and contributes to
the lack of clarity in the cross-reactions. This study criticized the
JWP report for overlooking subtleties associated with this issue, and
considered it "problematic" that the only argument for the acceptance
of the discoveries of elements 115 and 117 was a link they considered
to be doubtful.

On 8 June 2017, two members of the Dubna team published a journal
article answering these criticisms, analysing their data on the
nuclides (293)117 and (289)115 with widely accepted statistical
methods, noted that the 2016 studies indicating non-congruence
produced problematic results when applied to radioactive decay: they
excluded from the 90% confidence interval both average and extreme
decay times, and the decay chains that would be excluded from the 90%
confidence interval they chose were more probable to be observed than
those that would be included. The 2017 reanalysis concluded that the
observed decay chains of (293)117 and (289)115 were consistent with
the assumption that only one nuclide was present at each step of the
chain, although it would be desirable to be able to directly measure
the mass number of the originating nucleus of each chain as well as
the excitation function of the  reaction.


Naming
========
Using Mendeleev's nomenclature for unnamed and undiscovered elements,
element 117 should be known as 'eka-astatine'. Using the 1979
recommendations by the International Union of Pure and Applied
Chemistry (IUPAC), the element was temporarily called 'ununseptium'
(symbol 'Uus'), formed from Latin roots "one", "one", and "seven", a
reference to the element's atomic number 117. Many scientists in the
field called it "element 117", with the symbol 'E117', '(117)', or
'117'. According to guidelines of IUPAC valid at the moment of the
discovery approval, the permanent names of new elements should have
ended in "-ium"; this included element 117, even if the element was a
halogen, which traditionally have names ending in "-ine"; however, the
new recommendations published in 2016 recommended using the "-ine"
ending for all new group 17 elements.

After the original synthesis in 2010, Dawn Shaughnessy of LLNL and
Oganessian declared that naming was a sensitive question, and it was
avoided as far as possible. However, Hamilton, who teaches at
Vanderbilt University in Nashville, Tennessee, declared that year, "I
was crucial in getting the group together and in getting the 249Bk
target essential for the discovery. As a result of that, I'm going to
get to name the element. I can't tell you the name, but it will bring
distinction to the region." In a 2015 interview, Oganessian, after
telling the story of the experiment, said, "and the Americans named
this a tour de force, they had demonstrated they could do [this] with
no margin for error. Well, soon they will name the 117th element."

In March 2016, the discovery team agreed on a conference call
involving representatives from the parties involved on the name
"tennessine" for element 117. In June 2016, IUPAC published a
declaration stating the discoverers had submitted their suggestions
for naming the new elements 115, 117, and 118 to the IUPAC; the
suggestion for the element 117 was 'tennessine', with a symbol of
'Ts', after "the region of Tennessee". The suggested names were
recommended for acceptance by the IUPAC Inorganic Chemistry Division;
formal acceptance was set to occur after a five-month term following
publishing of the declaration expires. In November 2016, the names,
including tennessine, were formally accepted. Concerns that the
proposed symbol 'Ts' may clash with a notation for the tosyl group
used in organic chemistry were rejected, following existing symbols
bearing such dual meanings: Ac (actinium and acetyl) and Pr
(praseodymium and propyl). The naming ceremony for moscovium,
tennessine, and oganesson was held on 2 March 2017 at the Russian
Academy of Sciences in Moscow; a separate ceremony for tennessine
alone had been held at ORNL in January 2017.


                        Predicted properties
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Other than nuclear properties, no properties of tennessine or its
compounds have been measured; this is due to its extremely limited and
expensive production and the fact that it decays very quickly.
Properties of tennessine remain unknown and only predictions are
available.


Nuclear stability and isotopes
================================
The stability of nuclei quickly decreases with the increase in atomic
number after curium, element 96, whose half-life is four orders of
magnitude 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. No elements with atomic numbers
above 82 (after lead) have stable isotopes. 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
stabilizing factors, elements with more than 104 protons should not
exist. However, researchers in the 1960s suggested that the closed
nuclear shells around 114 protons and 184 neutrons should counteract
this instability, creating an "island of stability" where nuclides
could have half-lives reaching thousands or millions of years. While
scientists have still not reached the island, the mere existence of
the superheavy elements (including tennessine) confirms that this
stabilizing effect is real, and in general the known superheavy
nuclides become exponentially longer-lived as they approach the
predicted location of the island. Tennessine is the second-heaviest
element created so far, and all its known isotopes have half-lives of
less than one second. Nevertheless, this is longer than the values
predicted prior to their discovery: the predicted lifetimes for 293Ts
and 294Ts used in the discovery paper were 10 ms and 45 ms
respectively, while the observed lifetimes were 21 ms and 112 ms
respectively. The Dubna team believes that the synthesis of the
element is direct experimental proof of the existence of the island of
stability.


It has been calculated that the isotope 295Ts would have a half-life
of about 18 milliseconds, and it may be possible to produce this
isotope via the same berkelium-calcium reaction used in the
discoveries of the known isotopes, 293Ts and 294Ts. The chance of this
reaction producing 295Ts is estimated to be, at most, one-seventh the
chance of producing 294Ts. This isotope could also be produced in a
pxn channel of the 249Cf+48Ca reaction that successfully produced
oganesson, evaporating a proton alongside some neutrons; the heavier
tennessine isotopes 296Ts and 297Ts could similarly be produced in the
251Cf+48Ca reaction. Calculations using a quantum tunneling model
predict the existence of several isotopes of tennessine up to 303Ts.
The most stable of these is expected to be 296Ts with an alpha-decay
half-life of 40 milliseconds. A liquid drop model study on the
element's isotopes shows similar results; it suggests a general trend
of increasing stability for isotopes heavier than 301Ts, with partial
half-lives exceeding the age of the universe for the heaviest isotopes
like 335Ts when beta decay is not considered. Lighter isotopes of
tennessine may be produced in the 243Am+50Ti reaction, which was
considered as a contingency plan by the Dubna team in 2008 if 249Bk
proved unavailable; the isotopes 289Ts through 292Ts could also be
produced as daughters of element 119 isotopes that can be produced in
the 243Am+54Cr and 249Bk+50Ti reactions.


Atomic and physical
=====================
Tennessine is expected to be a member of group 17 in the periodic
table, below the five halogens; fluorine, chlorine, bromine, iodine,
and astatine, each of which has seven valence electrons with a
configuration of . For tennessine, being in the seventh period (row)
of the periodic table, continuing the trend would predict a valence
electron configuration of , and it would therefore be expected to
behave similarly to the halogens in many respects that relate to this
electronic state. However, going down group 17, the metallicity of the
elements increases; for example, iodine already exhibits a metallic
luster in the solid state, and astatine is expected to be a metal. As
such, an extrapolation based on periodic trends would predict
tennessine to be a rather volatile metal.


Calculations have confirmed the accuracy of this simple extrapolation,
although experimental verification of this is currently impossible as
the half-lives of the known tennessine isotopes are too short.
Significant differences between tennessine and the previous halogens
are likely to arise, largely due to spin-orbit interaction--the mutual
interaction between the motion and spin of electrons. The spin-orbit
interaction is especially strong for the superheavy elements because
their electrons move faster--at velocities comparable to the speed of
light--than those in lighter atoms. In tennessine atoms, this lowers
the 7s and the 7p electron energy levels, stabilizing the
corresponding electrons, although two of the 7p electron energy levels
are more stabilized than the other four. The stabilization of the 7s
electrons is called the inert pair effect; the effect that separates
the 7p subshell into the more-stabilized and the less-stabilized parts
is called subshell splitting. Computational chemists understand 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. For many theoretical purposes, the
valence electron configuration may be represented to reflect the 7p
subshell split as .

Differences for other electron levels also exist. For example, the 6d
electron levels (also split in two, with four being 6d3/2 and six
being 6d5/2) are both raised, so they are close in energy to the 7s
ones, although no 6d electron chemistry has ever been predicted for
tennessine. The difference between the 7p1/2 and 7p3/2 levels is
abnormally high; 9.8 eV. Astatine's 6p subshell split is only 3.8 eV,
and its 6p1/2 chemistry has already been called "limited". These
effects cause tennessine's chemistry to differ from those of its upper
neighbors (see below).

Tennessine's first ionization energy--the energy required to remove an
electron from a neutral atom--is predicted to be 7.7 eV, lower than
those of the halogens, again following the trend. Like its neighbors
in the periodic table, tennessine is expected to have the lowest
electron affinity--energy released when an electron is added to the
atom--in its group; 2.6 or 1.8 eV. The electron of the hypothetical
hydrogen-like tennessine atom--oxidized so it has only one electron,
Ts116+--is predicted to move so quickly that its mass is 1.90 times
that of a non-moving electron, a feature attributable to relativistic
effects. For comparison, the figure for hydrogen-like astatine is 1.27
and the figure for hydrogen-like iodine is 1.08. Simple extrapolations
of relativity laws indicate a contraction of atomic radius. Advanced
calculations show that the radius of a tennessine atom that has formed
one covalent bond would be 165 pm, while that of astatine would be 147
pm. With the seven outermost electrons removed, tennessine is finally
smaller; 57 pm for tennessine and 61 pm for astatine.

The melting and boiling points of tennessine are not known; earlier
papers predicted about 350-500 °C and 550 °C, respectively, or 350-550
°C and 610 °C, respectively. These values exceed those of astatine and
the lighter halogens, following periodic trends. A later paper
predicts the boiling point of tennessine to be 345 °C (that of
astatine is estimated as 309 °C, 337 °C, or 370 °C, although
experimental values of 230 °C and 411 °C have been reported). The
density of tennessine is expected to be between 7.1 and 7.3 g/cm3.


Chemical
==========
The known isotopes of tennessine, 293Ts and 294Ts, are too short-lived
to allow for chemical experimentation at present. Nevertheless, many
chemical properties of tennessine have been calculated. Unlike the
lighter group 17 elements, tennessine may not exhibit the chemical
behavior common to the halogens. For example, fluorine, chlorine,
bromine, and iodine routinely accept an electron to achieve the more
stable electronic configuration of a noble gas, obtaining eight
electrons (octet) in their valence shells instead of seven. This
ability weakens as atomic weight increases going down the group;
tennessine would be the least willing group 17 element to accept an
electron. Of the oxidation states it is predicted to form, −1 is
expected to be the least common. The standard reduction potential of
the Ts/Ts− couple is predicted to be −0.25 V; this value is negative,
unlike for all the lighter halogens.

There is another opportunity for tennessine to complete its octet--by
forming a covalent bond. Like the halogens, when two tennessine atoms
meet they are expected to form a Ts-Ts bond to give a diatomic
molecule. Such molecules are commonly bound via single sigma bonds
between the atoms; these are different from pi bonds, which are
divided into two parts, each shifted in a direction perpendicular to
the line between the atoms, and opposite one another rather than being
located directly between the atoms they bind. Sigma bonding has been
calculated to show a great antibonding character in the At2 molecule
and is not as favorable energetically. Tennessine is predicted to
continue the trend; a strong pi character should be seen in the
bonding of Ts2. The molecule tennessine chloride (TsCl) is predicted
to go further, being bonded with a single pi bond.

Aside from the unstable −1 state, three more oxidation states are
predicted; +5, +3, and +1. The +1 state should be especially stable
because of the destabilization of the three outermost 7p3/2 electrons,
forming a stable, half-filled subshell configuration; astatine shows
similar effects. The +3 state should be important, again due to the
destabilized 7p3/2 electrons. The +5 state is predicted to be uncommon
because the 7p1/2 electrons are oppositely stabilized. The +7 state
has not been shown--even computationally--to be achievable. Because
the 7s electrons are greatly stabilized, it has been hypothesized that
tennessine effectively has only five valence electrons.

The simplest possible tennessine compound would be the monohydride,
TsH. The bonding is expected to be provided by a 7p3/2 electron of
tennessine and the 1s electron of hydrogen. The non-bonding nature of
the 7p1/2 spinor is because tennessine is expected not to form purely
sigma or pi bonds. Therefore, the destabilized (thus expanded) 7p3/2
spinor is responsible for bonding. This effect lengthens the TsH
molecule by 17 picometers compared with the overall length of 195 pm.
Since the tennessine p electron bonds are two-thirds sigma, the bond
is only two-thirds as strong as it would be if tennessine featured no
spin-orbit interactions. The molecule thus follows the trend for
halogen hydrides, showing an increase in bond length and a decrease in
dissociation energy compared to AtH. The molecules TlTs and NhTs may
be viewed analogously, taking into account an opposite effect shown by
the fact that the element's p1/2 electrons are stabilized. These two
characteristics result in a relatively small dipole moment (product of
difference between electric charges of atoms and displacement of the
atoms) for TlTs; only 1.67 D, the positive value implying that the
negative charge is on the tennessine atom. For NhTs, the strength of
the effects are predicted to cause a transfer of the electron from the
tennessine atom to the nihonium atom, with the dipole moment value
being −1.80 D. The spin-orbit interaction increases the dissociation
energy of the TsF molecule because it lowers the electronegativity of
tennessine, causing the bond with the extremely electronegative
fluorine atom to have a more ionic character. Tennessine monofluoride
should feature the strongest bonding of all group 17 monofluorides.

VSEPR theory predicts a bent-T-shaped molecular geometry for the group
17 trifluorides. All known halogen trifluorides have this molecular
geometry and have a structure of AX3E2--a central atom, denoted A,
surrounded by three ligands, X, and two unshared electron pairs, E. If
relativistic effects are ignored, TsF3 should follow its lighter
congeners in having a bent-T-shaped molecular geometry. More
sophisticated predictions show that this molecular geometry would not
be energetically favored for TsF3, predicting instead a trigonal
planar molecular geometry (AX3E0). This shows that VSEPR theory may
not be consistent for the superheavy elements. The TsF3 molecule is
predicted to be significantly stabilized by spin-orbit interactions; a
possible rationale may be the large difference in electronegativity
between tennessine and fluorine, giving the bond a partially ionic
character.


                            Bibliography
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