|Name, symbol||ununseptium, Uus|
|Alternative names||element 117, eka-astatine|
|Ununseptium in the periodic table|
|Standard atomic weight (Ar)|||
|Element category||unknown, but probably a metalloid|
|Group, block||group 17, p-block|
|Electron configuration||[Rn] 5f14 6d10 7s2 7p5 (predicted)|
|2, 8, 18, 32, 32, 18, 7 (predicted)|
|Melting point||573–773 K (300–500 °C, 572–932 °F) (predicted)|
|Boiling point||823 K (550 °C, 1022 °F) (predicted)|
|Density near r.t.||7.1–7.3 g/cm3 (extrapolated)|
|Oxidation states||−1, +1, +3, +5 (predicted)|
|Ionization energies||1st: 742.9 kJ/mol (prediction)
2nd: 1785.0–1920.1 kJ/mol (extrapolated)
|Atomic radius||empirical: 138 pm (predicted)|
|Covalent radius||156–157 pm (extrapolated)|
|CAS Registry Number||54101-14-3|
|Naming||IUPAC systematic element name|
|Discovery||Joint Institute for Nuclear Research and Lawrence Livermore National Laboratory (2010)|
|Most stable isotopes|
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 second-to-last element of the 7th period of the periodic table. As of 2015[update], 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 2015[update]. 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. 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), which is in charge of examining claims of discovery of superheavy elements, has made no comment on whether the element can be recognized as discovered. Once it is recognized, the discoverers will be invited to give the element an official name. "Ununseptium" is a temporary systematic element name that 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.[a] 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.
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. 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; and it is the lightest stable or near-stable nucleus with such a large neutron excess. The second-lightest such nucleus, zinc-68 (30 protons, 38 neutrons), is much heavier. Since ununseptium has 117 protons in its nucleus and calcium has 20, they needed to use berkelium, which has 97 protons in its nucleus.
In 2008, the American team resumed production of berkelium, producing 22 milligrams of the element, enough to perform the experiment. The berkelium was subsequently cooled in 90 days and, in another 90 days, was chemically purified. The berkelium target had to be quickly transported to Russia because the half-life of berkelium-249, the only isotope of berkelium that can be produced in weighable quantities, is only 330 days: after that time, half the berkelium produced would have decayed away. 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 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 JINR particle accelerator.
The experiment began in June 2009. The calcium-48 beam was generated by chemically extracting the small quantities of calcium-48 present in naturally occurring calcium. Thanks to the neutron excess, the resulting nuclei were expected to be heavier and closer to the sought-after island of stability. [b]
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. 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 isotopes were formed as follows:[c]
97Bk + 48
20Ca → 297
117Uus* → 294
117Uus + 3 1
0n (1 event)
97Bk + 48
20Ca → 297
117Uus* → 293
117Uus + 4 1
0n (5 events)
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.
As of 2015[update], no official name has been suggested for ununseptium. The discoverers will be given the right to suggest a name as soon as it is recognized by the JWP; a Dubna authority said in June 2012 that it "could take up to a year" before this happens. According to the 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. Many scientists in the field call it "element 117", with the symbol (117) or 117.
Nuclear stability and isotopes
The stability of nuclei decreases greatly 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 well understood yet, 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.
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 with alpha decay half-lives up to 40 milliseconds, culminating at 296Uus (the research ends at 303Uus). 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.[d] 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 (SO) interaction—the mutual interaction between the motion and spin of electrons. The SO 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.[e] For many theoretical purposes, the valence electron configuration may be represented to reflect the 7p subshell split as 7s2
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 been predicted for ununseptium. 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 ununseptium's chemistry to differ from those of its upper neighbors (see below).
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 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 the astatine (and all preceding) values, 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.
Unlike the previous group 17 elements, ununseptium may not exhibit chemical behavior common for the halogens. For example, the extant members of the group commonly accept another electron to achieve a stable electronic configuration of a noble gas, having eight electrons (octet) in its valence shell, which is the minimum-energy configuration in which the outer electrons are tightly bound. This ability weakens as the atomic weight grows within the group 17 elements; ununseptium would be the least-willing group member to accept an electron—astatine already forms a positive oxidation state in its hydride. Of the oxidation states the element is predicted to form, −1 is predicted to be the least common.
There is another opportunity for ununseptium to complete the octet—to form a covalent bond. Accordingly, 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 that are located 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, opposite each other 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 7p3/2 electrons, forming a stable, half-filled subshell configuration; astatine already shows similar effects. The +3 state should also be important because of the 7p3/2 electrons destabilization. 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 by chemical reactions. Because the 7s electrons are greatly stabilized, it has been hypothesized that ununseptium has only five valence electrons.
The simplest possible ununseptium compound is the monohydride, UusH. The bonding is provided by a 7p3/2 electron of ununseptium and the 1s electron of hydrogen. The non-bonding nature of the 7p1/2 spinor is because ununseptium 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 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 SO interactions. The molecule thus follows the trend for halogen hydrides, showing an increase in bond length and a decrease in dissociation energy in comparison with AtH. The molecules TlUus and UutUus may be viewed analogously, taking in 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 TlUus; only 1.67 D,[f] the positive value implying that the negative charge is on the ununseptium atom. For UutUus, the strength of the effects caused a transfer of the electron from the ununseptium atom to the ununtrium atom, with the dipole moment value being −1.80 D. The SO interaction increases dissociation energy for the UusF molecule because it lowers the electronegativity of ununseptium, causing the bond with the extremely electronegative fluorine to have a more ionic character. Ununseptium monofluoride should feature the strongest bonding of all group 17 element monofluorides.
The 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 also 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. This molecule is significantly stabilized by the SO interactions; a possible rationale may be the large difference in electronegativity between ununseptium and fluorine, giving the bond a partially ionic character.
- The term "group 17" refers to a column in the periodic table starting with fluorine and is distinct from "halogen", which relates to a common pattern of chemical and physical properties shared only by fluorine, chlorine, bromine, iodine, and astatine, all of which precede ununseptium in group 17. Unlike the other group 17 members, ununseptium may not be a halogen.
- Although stable isotopes of the lightest elements usually have a neutron–proton ratio close or equal to one (for example, the only stable isotope of aluminium has 13 protons and 14 neutrons, making a neutron–proton ratio of 1.077), stable isotopes of heavier elements have higher neutron–proton ratios, increasing with the number of protons (iodine's only stable isotope has 53 protons and 74 neutrons, neutron–proton ratio of 1.396, gold's only stable isotope has 79 protons and 118 neutrons, neutron–proton ratio of 1.494, plutonium's most stable isotope has 94 protons and 150 neutrons, neutron–proton ratio of 1.596), and the trend is expected to continue to the superheavy elements, making it difficult to synthesize the most stable isotopes of these elements, because the neutron–proton ratios of the elements they are synthesized from are lower than the expected ratios of the most stable isotopes of the superheavy elements.
- A nuclide is commonly denoted by the chemical element's symbol immediately preceded by the mass number as a superscript and the atomic number as a subscript. Neutrons are represented as nuclides with atomic mass 1, atomic number 0, and symbol n. Outside the context of nuclear equations, the atomic number is sometimes omitted. An asterisk denotes an extremely short-lived (or even non-existent) intermediate stage of the reaction.
- The letter n stands for the number of the period (horizontal row in the periodic table) the element belongs to. The letters "s" and "p" denote the s and p atomic orbitals, and the subsequent superscript numbers denote the numbers of electrons in each. Hence the notation ns2np5 means that the valence shells of previous group 17 elements are composed of two s electrons and five p electrons, all located in the outermost electron energy level.
- The quantum number corresponds to the letter in the electron orbital name: 0 to s, 1 to p, 2 to d, etc. See azimuthal quantum number for more information.
- For comparison, the values for the ClF, HCl, SO, HF, and HI molecules are 0.89 D, 1.11 D, 1.55 D, 1.83 D, and 1.95 D, correspondingly. Compare to the values for molecules which do not form at standard conditions, GeSe, SnS, TlF, BaO, and NaCl, which are 1.65 D, ~3.2 D, 4.23 D, 7.95 D, and 9.00 D.
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