Tennessine

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. The 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 synthesis of element 118, which was produced earlier 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. The second-lightest such nucleus, palladium-110 (46 protons, 64 neutrons, neutron–proton ratio of 1.391), is much heavier. 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 114, 116, and 118, and Joseph Hamilton, a Vanderbilt professor and 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 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.)

ORNL resumed californium production in spring 2008. Hamilton noted the restart, and during a September 2008 symposium at Vanderbilt University (Nashville, Tennessee, U.S.) celebrating his 50th year on the Physics faculty, he introduced Oganessian to James Roberto (then the deputy director for science and technology at ORNL). They established a collaboration among JINR, ORNL, and Vanderbilt; the team at the Lawrence Livermore National Laboratory (LLNL) in Livermore, California, U.S., was soon invited to join.

Discovery

In the spring of 2008, the ORNL started production of californium for a commercial order by an oil company; Hamilton learned that in the summer, making a deal on subsequent extraction of berkelium. In September 2008, Oganessian, Hamilton, and Roberto assembled to coordinate planning on the experiment. In November 2008, the U.S. Department of Energy, the state department 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: 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 sent on a commercial flight 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 early June 2009, the berkelium was 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 then transported to Dubna, where it was installed in the particle accelerator at 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 in the closed town of Lesnoy, Sverdlovsk Oblast, Russia.

The experiment began later in 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 April 9, 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, and the University of Nevada (Las Vegas, Nevada, U.S.), which provided data analysis support. The isotopes were formed as follows:

249
97Bk
+ 48
20Ca
→ ²⁹⁷117* → ²⁹⁴117 + 3 1
0
n
(1 event)249
97Bk
+ 48
20Ca
→ ²⁹⁷117* → ²⁹³117 + 4 1
0
n
(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 (²⁸⁹115) 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 2014, a joint German–American collaboration of scientists from the ORNL and the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, 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 ²⁸⁹115, and thus the listed discoverers—JINR, LLNL, and ORNL—were given the right to suggest an official name for the element.

A 2016 study from Lund University and the GSI nevertheless cast some doubt on the synthesis of elements 115 and 117 after the IUPAC/IUPAP Joint Working Party recognized these elements as having been discovered in 2009–2010. It found that the decay chains assigned to the isotopes ²⁸⁷115 and ²⁸⁸115 were probably internally consistent, with the uncertainty due to the probable insensitivity of the measurements to very short and very long nuclide lifetimes, incorrect assignments of other decay chains from the ²⁴³Am+⁴⁸Ca reaction to different moscovium isotopes, or uncertainty in the identification of some of the daughters of these isotopes of element 115. On the other hand, the decay chains assigned to ²⁸⁹115, the isotope instrumental in the confirmation of the synthesis of elements 115 and 117, were found not to be internally consistent. Some subsets of these chains were found to be consistent, suggesting however that their true assignment was to ²⁸⁸115, and that their shortness indicated instead new spontaneous fission branches in its daughters ²⁸⁴113 and ²⁸⁰Rg – or, more likely, undetected electron capture branches in these daughters leading to the even–even nuclides ²⁸⁴Cn and ²⁸⁰Ds, which have a very low barrier to spontaneous fission. While the ²⁹⁴117 decay chains were found to be congruent, the ²⁹³117 decay chains approved by the JWP were found to probably not be so and require splitting into individual data sets assigned to different isotopes of element 117. It was also found that the set of chains from ²⁹³117 and ²⁸⁹115 were not congruent, and thus that the claimed link between the decay chains assigned to the two nuclides probably did not exist. 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 criticised the IUPAC/IUPAP JWP report for overlooking subtleties associated with this issue, and noted that the fact that the only argument for the acceptance of the discoveries of elements 115 and 117 was an almost certainly non-existent link was "problematic".

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) until its discovery was confirmed and a permanent name chosen; the temporary name was formed from Latin roots "one", "one", and "seven", a reference to the element's atomic number of 117. Many scientists in the field called it "element 117", with the symbol (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. The IUPAC guidelines specify that the discovery team has naming rights for the element.

After the original synthesis in 2010, Dawn Shaughnessy of LLNL and Oganessian declared naming was a sensitive question, and it was avoided as far as possible. However, Hamilton 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". (Hamilton's institute of Vanderbilt is located in Nashville, Tennessee, U.S.)

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-months 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).

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. Nevertheless, because of reasons not yet well understood, there is a slight increase of nuclear stability around atomic numbers 110–114, which leads to the appearance of what is known in nuclear physics as the "island of stability". This concept, proposed by University of California professor Glenn Seaborg, explains why superheavy elements last longer than predicted. Tennessine is the second-heaviest element created so far, and has a half-life of less than one second; this is longer than the predicted value used in the discovery report. 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 18 ± 7 milliseconds and that 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. 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.

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 ns²np⁵. For tennessine, being in the seventh period (row) of the periodic table, continuing the trend would predict a valence electron configuration of 7s²7p⁵, 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 often classified as a metalloid due to its properties being quite far from those of the four previous halogens. As such, an extrapolation based on periodic trends would predict tennessine to be a rather volatile post-transition 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 7s2
7p2
1/27p3
3/2.

Differences for other electron levels also exist. For example, the 6d electron levels (also split in two, with four being 6d_3/2 and six being 6d_5/2) are both raised, so they are close in energy to the 7s ones, although no 6d electron chemistry has been predicted for tennessine. The difference between the 7p_1/2 and 7p_3/2 levels is abnormally high; 9.8 eV. Astatine's 6p subshell split is only 3.8 eV, and its 6p_1/2 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 1.9 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 an 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·cm⁻³, continuing the trend of increasing density among the halogens; that of astatine is estimated to be between 6.2 and 6.5 g·cm⁻³.

Chemical

Unlike the previous group 17 elements, tennessine may not exhibit the chemical behavior common to the halogens. For example, the extant members of the group routinely accept an electron to achieve the more stable electronic configuration of a noble gas, one having eight electrons (octet) in its valence shell. This ability weakens as atomic weight increases going down the group; tennessine would be the least willing 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 and thus tennessine should not be reduced to the −1 oxidation state under standard conditions, unlike all the previous 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 7p_3/2 electrons, forming a stable, half-filled subshell configuration; astatine shows similar effects. The +3 state should be important, again due to the destabilized 7p_3/2 electrons. The +5 state is predicted to be uncommon because the 7p_1/2 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 7p_3/2 electron of tennessine and the 1s electron of hydrogen. The non-bonding nature of the 7p_1/2 spinor is because tennessine is expected not to form purely sigma or pi bonds. Therefore, the destabilized (thus expanded) 7p_3/2 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 p_1/2 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 element 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.