Ununseptium

"Uus" redirects here. For other uses, see UUS.

Ununseptium,  ₁₁₇Uus

General properties
Name, symbol	ununseptium, Uus
Appearance	semimetallic (predicted)
Pronunciation	i/uːnuːnˈsɛptiəm/ oon-oon-SEP-tee-əm
Alternative names	element 117, eka-astatine
Ununseptium in the periodic table
At ↑ Uus ↓ (Usu) livermorium ← ununseptium → ununoctium
Atomic number (Z)	117
Group, block	group 17, p-block
Period	period 7
Element category	unknown, but probably a metalloid
Standard atomic weight (Ar)	[294]
Electron configuration	[Rn] 5f14 6d10 7s2 7p5 (predicted)
per shell	2, 8, 18, 32, 32, 18, 7 (predicted)
Physical properties
Phase	solid (predicted)
Melting point	623–823 K ​(350–550 °C, ​662–1022 °F) (predicted)
Boiling point	883 K ​(610 °C, ​1130 °F) (predicted)
Density near r.t.	7.1–7.3 g/cm3 (extrapolated)
Atomic properties
Oxidation states	−1, +1, +3, +5 ​(predicted)
Ionization energies	1st: 742.9 kJ/mol (predicted) 2nd: 1785.0–1920.1 kJ/mol (extrapolated)
Atomic radius	empirical: 138 pm (predicted)
Covalent radius	156–157 pm (extrapolated)
Miscellanea
CAS Number	54101-14-3
History
Naming	IUPAC systematic element name
Discovery	Joint Institute for Nuclear Research and Lawrence Livermore National Laboratory (2010)
Most stable isotopes of ununseptium
iso NA half-life DM DE (MeV) DP 294Uus syn 51+41 −16 ms α 10.81 290Uup 293Uus syn 22+8 −4 ms α 11.11, 11.00, 10.91 289Uup

Ununseptium is a superheavy artificial chemical element with an atomic number of 117 and a temporary symbol of Uus. Also known as eka-astatine or element 117, it is the second-heaviest known element and penultimate element of the 7th period of the periodic table. As of 2016, fifteen ununseptium atoms have been observed: six when it was first synthesized in 2010, seven in 2012, and two in 2014.

The discovery of ununseptium was announced in Dubna, Russia, by a Russian–American collaboration in 2010, which makes it the most recently discovered element as of 2016. One of its daughter isotopes was created directly in 2011, partially confirming the results of the experiment. The experiment itself was repeated successfully by the same collaboration in 2012 and by a joint German–American team in 2014. In 2015, the Joint Working Party of the International Union of Pure and Applied Chemistry and the International Union of Pure and Applied Physics, which evaluates claims of discovery of new elements, has recognized the element and assigned the priority to the Russian–American team. Soon after that the discoverers will be invited to give the element an official name. "Ununseptium" is a temporary systematic element name formed from Latin roots "one", "one", and "seven"; it is intended to be used until a permanent one is assigned. The working name "element 117" is commonly used by researchers and in the literature.

Ununseptium may be located in the "island of stability", a concept that explains why some superheavy elements are more stable compared to an overall trend of decreasing stability for elements beyond lead on the periodic table. The synthesized ununseptium atoms have lasted tens and hundreds of microseconds. In the periodic table, ununseptium is expected to be a member of group 17, all other members of which are halogens. Some of its properties are likely to be significantly different from those of the halogens due to relativistic effects. Unlike the halogens, ununseptium is likely to neither commonly form anions nor achieve 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.

History

Pre-discovery

In 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 American team—the world's only producer of berkelium—could not then provide the element, as they had temporarily ceased production. Plans to synthesize element 117 were suspended in favor of the synthesis of element 118, which was produced by bombarding a californium target with calcium.

The Russian 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. Since ununseptium has 117 protons in its nucleus and calcium has 20, they needed to use berkelium, which has 97 protons in its nucleus.

Discovery

The berkelium target used for the synthesis (in solution)

In 2008, the ORNL resumed production of berkelium, producing 22 milligrams of the element, enough to perform the experiment. The berkelium was subsequently cooled for 90 days and then chemically purified for another 90 days. The only isotope of berkelium that can be produced in weighable quantities is berkelium-249. Its half-life is only 330 days: after that time, half the berkelium produced would have decayed away. Because of this, the berkelium target had to be quickly transported to Russia; for the experiment to be viable, it had to begin within six months of its departure from the United States. In mid-2009, 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, 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. It was then transported to Dubna, where it was installed in the particle accelerator at JINR. The calcium-48 beam was generated by the Russian team by chemically extracting the small quantities of calcium-48 present in naturally occurring calcium.

The experiment began in June 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 Lawrence Livermore National Laboratory (LLNL) in Livermore, California, United States, for further analysis. On April 9, 2010, an official report was released in the journal Physical Review Letters identifying the isotopes as ²⁹⁴Uus and ²⁹³Uus, which were shown to have half-lives on the order of tens or hundreds of microseconds. The work was signed by all parties involved in the experiment to some extent: JINR, ORNL, LLNL, RIAR, the Vanderbilt University (Nashville, Tennessee, U.S.), and the University of Nevada (Las Vegas, Nevada, U.S.). The isotopes were formed as follows:

249
97Bk + 48
20Ca → 297
117Uus* → 294
117Uus + 3 ¹
₀n (1 event)

249
97Bk + 48
20Ca → 297
117Uus* → 293
117Uus + 4 ¹
₀n (5 events)

Recognition

Decay chain of the ununseptium isotopes produced in the original experiment. The figures near the arrows describe experimental (black) and theoretical (blue) values for the half-life and energy of each decay.

All of ununseptium's daughter isotopes (decay products) were previously unknown; therefore, their properties could not be used to confirm the claim of discovery. In 2011, when one of the decay products (ununpentium-289) was synthesized directly, its properties matched those measured in the claimed indirect synthesis from the decay of ununseptium. 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 ununseptium. The results of the experiment matched the previous outcome; the scientists have since 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 ununseptium-294.

In December 2015, the JWP officially recognized the discovery, and thus the listed discoverers—JINR, LLNL, and ORNL—will be given the right to suggest an official name for the element. According to existing guidelines of the International Union of Pure and Applied Chemistry (IUPAC), the names of new elements should end in "-ium"; this includes ununseptium, even if the element is a halogen, which traditionally have names ending in "-ine". Using Mendeleev's nomenclature for unnamed and undiscovered elements, ununseptium should be known as eka-astatine. Using the 1979 IUPAC recommendations, the element should be temporarily called ununseptium (symbol Uus) until the discovery is confirmed and a permanent name chosen; the temporary name is formed from Latin roots "one", "one", and "seven", a reference to the element's atomic number of 117. Many scientists in the field call it "element 117", with the symbol (117) or 117.

After the original synthesis in 2010, scientists declared naming was a sensitive question, and it was avoided as far as possible.

Predicted properties

Nuclear stability and isotopes

Main article: Isotopes of ununseptium

See also: Island of stability

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. Ununseptium 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.

A chart of nuclide stability as used by the Dubna team in 2010. Characterized isotopes are shown with borders. According to the discoverers, the ununseptium synthesis serves as definite proof of the existence of the "island of stability" (circled).

It has been calculated that the isotope ²⁹⁵Uus 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, ²⁹³Uus and ²⁹⁴Uus. The chance of this reaction producing ²⁹⁵Uus is estimated to be, at most, one-seventh the chance of producing ²⁹⁴Uus. Calculations using a quantum tunneling model predict the existence of several isotopes of ununseptium up to ³⁰³Uus. The most stable of these is expected to be ²⁹⁶Uus 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 ³⁰¹Uus, with partial half-lives exceeding the age of the universe for the heaviest isotopes like ³³⁵Uus when beta decay is not considered.

Atomic and physical

Ununseptium 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 ununseptium, 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.

Significant differences 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 ununseptium 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 ununseptium. 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 ununseptium's chemistry to differ from those of its upper neighbors (see below).

Atomic energy levels of outermost s, p, and d electrons of astatine and ununseptium

Ununseptium'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, ununseptium 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 ununseptium atom—oxidized so it has only one electron, Uus¹¹⁶⁺—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 ununseptium 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, ununseptium is finally smaller; 57 pm for ununseptium and 61 pm for astatine.

The melting and boiling points of ununseptium 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 ununseptium 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 ununseptium 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

IF
₃ has a T-shape configuration.

UusF
₃ is predicted to have a trigonal one.

Unlike the previous group 17 elements, ununseptium may not exhibit chemical behavior common to the halogens. For example, 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; ununseptium 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.

There is another opportunity for ununseptium to complete its octet—by forming a covalent bond. Like the halogens, when two ununseptium atoms meet they are expected to form a Uus–Uus 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. Ununseptium is predicted to continue the trend; a strong pi character should be seen in the bonding of Uus₂. The molecule UusCl 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 ununseptium effectively has only five valence electrons.

The simplest possible ununseptium compound would be the monohydride, UusH. The bonding is expected to be provided by a 7p_3/2 electron of ununseptium and the 1s electron of hydrogen. The non-bonding nature of the 7p_1/2 spinor is because ununseptium 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 UusH molecule by 17 picometers compared with the overall length of 195 pm. Since the ununseptium p electron bonds are two-thirds sigma, the bond is only two-thirds as strong as it would be if ununseptium 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 TlUus and UutUus 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 TlUus; only 1.67 D, the positive value implying that the negative charge is on the ununseptium atom. For UutUus, the strength of the effects are predicted to cause a transfer of the electron from the ununseptium atom to the ununtrium atom, with the dipole moment value being −1.80 D. The spin–orbit interaction increases the dissociation energy of the UusF molecule because it lowers the electronegativity of ununseptium, causing the bond with the extremely electronegative fluorine atom to have a more ionic character. Ununseptium 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, UusF₃ 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 UusF₃, predicting instead a trigonal planar molecular geometry (AX₃E₀). This shows that VSEPR theory may not be consistent for the superheavy elements. The UusF₃ molecule is predicted to be significantly stabilized by spin–orbit interactions; a possible rationale may be the large difference in electronegativity between ununseptium and fluorine, giving the bond a partially ionic character.

References

Bibliography