Ununseptium

Ununseptium is a superheavy artificial chemical element with temporary symbol Uus and atomic number 117. The element, also known as eka-astatine or simply element 117, is the second-heaviest of all the elements that have been created so far and is the second-to-last element of the 7th period of the periodic table. Its discovery was first announced in 2010â€"synthesis was claimed in Dubna, Russia, by a joint Russian-American collaboration, thus making it the most recently discovered element. Another experiment in 2011 created one of its daughter isotopes directly, partially confirming the results of the discovery experiment, and the original experiment was repeated successfully in 2012. In 2014, the GSI Helmholtz Centre for Heavy Ion Research in Germany also claimed to have successfully repeated the original experiment. However, the IUPAC/IUPAP Joint Working Party (JWP), which is in charge of examining claims of discovery of superheavy elements, has made no comment yet on whether the element can be recognized as discovered. Once it is so recognized, it may receive a permanent name which will be suggested for the element by its discoverers; "ununseptium" is a temporary systematic element name that is intended to be used before a permanent one is established. It is commonly called "element 117" by researchers and in the literature instead of "ununseptium".

In the periodic table, ununseptium is located in group 17, all previous members of which are halogens. However, ununseptium is likely to have significantly different properties from the halogens, although a few key properties such as the melting and boiling points, as well as the first ionization energy are expected to follow the periodic trends.

§History

§Pre-discovery

In 2004 the Joint Institute for Nuclear Research (JINR) team in Dubna, Moscow Oblast, Russia proposed an experiment to synthesize element 117 (so-called for the 117 protons in its nucleus) that required fusing a berkelium (element 97) target and a calcium (element 20) beam. However, the team at the Oak Ridge National Laboratory in the United States, the world's only producer of berkelium, could not then provide any, citing a lack of production of the exotic material. Plans to synthesize element 117 were shelved temporarily in favor of the synthesis of element 118, which was produced by bombarding a californium target with calcium.

The Russian team desired to use berkelium, an element they could not access, because the isotope of calcium used in the beam, calcium-48, has 20 protons and 28 neutrons; it is the lightest stable or near-stable nucleus with such a neutron excess. The second-lightest such nucleus, zinc-68, is much heavier. Since ununseptium has 117 protons in its nucleus and calcium has 20, they thus needed to use berkelium, which has 97 protons in its nucleus. The beam is made in Russia by chemically extracting the small quantities of calcium-48 present in Earth's natural calcium. Thus the resulting nuclei become heavier and closer to the sought-after island of stability, a concept wherein some super-heavy atoms can be relatively stable. Sufficiently heavy nuclei have not been created as of 2014, however, and the synthesized isotopes tend to have fewer neutrons than those expected to be in the island of stability.

§Discovery

In 2008, the American team re-launched a program of berkelium production, and the Russian team was contacted. The production resulted in 22 milligrams of berkelium, enough to perform the experiment. The berkelium was subsequently cooled in 90 days and chemically purified in another 90 days. The berkelium target had to be brought to Russia quickly: the half-life of the isotope of berkelium used (berkelium-249) is only 330 days, which means that after this period, half of it would no longer be berkelium. In fact, if the experiment had not begun in six months after the target's departure, it would have had to be canceled due to insufficient quantities of the quickly decaying berkelium. In summer 2009, the target was packed into five lead containers to be sent via a commercial flight from New York to Moscow.

The teams had to deal in advance with the bureaucratic barrier between the two countries to allow the target's timely journey to Russia. This, however, did not prevent such problems: Russian customs twice refused to let the target enter the country because of missing or incomplete paperwork. Even though it traveled over the Atlantic Ocean five times, the journey only took a few days in total. The berkelium was then transferred to Dimitrovgrad, Ulyanovsk Oblast to be fixed on a thin titanium film, and then to Dubna where it was installed in the JINR particle accelerator, the world's most powerful for the synthesis of superheavy elements.

The experiment began in June 2009 and, in January 2010, scientists at the Flerov Laboratory of Nuclear Reactions announced internally that they had succeeded in detecting the decay of a new element with atomic number 117 via two decay chains of an odd-odd isotope (undergoing 6 alpha decays before undergoing spontaneous fission) and of an odd-even one (3 alpha decays before fission). On April 9, 2010 an official report was released in the journal Physical Review Letters. It revealed that the isotopes mentioned in the previous chains referred to ²⁹⁴Uus and ²⁹³Uus, formed as follows:

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

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

None of ununseptium's daughter isotopes (decay products) were known before the actual synthesis of ununseptium; thus, there was no basis for a JWP discovery claim, much less for its recognition. Ununpentium-289, one of ununseptium's daughters, was created directly in 2011, instead of being created indirectly from the decay of ununseptium, yet it matched the claimed decay properties measured from the discovery of ununseptium. The discoverers did not, however, submit a claim for the discovery of ununseptium when JWP was reviewing claims of discoveries of trans-copernicium elements (elements beyond copernicium) in 2007â€"2011. The Dubna team repeated the experiment in 2012 successfully, and its results matched the results of previous experiments. The scientists have since filed a new element registration paper, and a new JWP staffer is working on assigning priority of the claim. On May 2, 2014, scientists from the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany claimed to have confirmed the element's discovery. This also led to the discovery of the new isotope lawrencium-266, obtained from the alpha decay of dubnium-270 (which underwent spontaneous fission in the Dubna experiments).

§Naming

Using Mendeleev's nomenclature for unnamed and undiscovered elements, ununseptium should be known as eka-astatine or dvi-iodine. In 1979, IUPAC published recommendations according to which the element was to be called ununseptium (with the corresponding symbol of Uus), a systematic element name as a placeholder, until the discovery of the element is confirmed and a permanent name is decided on. The recommendations are mostly ignored among scientists, who call it "element 117", with the symbol of (117) or even simply 117. According to current guidelines from IUPAC, the ultimate name for all new elements should end in "-ium", which means the name for ununseptium will almost certainly end in "-ium", not "-ine", even if ununseptium turns out to be a halogen, whose elements traditionally have names ending in "-ine".

No official name has yet been suggested for the element. However, the discoverers will get 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.

§Predicted properties

§Nuclear stability and isotopes

The stability of nuclei decreases greatly with the increase in atomic number after plutonium, the heaviest primordial element, so that all isotopes with an atomic number above 101 decay radioactively with a half-life under a day, with the exception of dubnium-268. No elements with atomic numbers above 82 (after lead) have stable isotopes. Nevertheless, because of reasons not well understood yet, there is a slightly increased nuclear stability around atomic numbers 110â€"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 of all created so far and is radioactive, having a half-life that appears to be less than one second. Nonetheless, this is still longer than the predicted values 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.

It is calculated that the isotope ²⁹⁵Uus would have a half-life of (18 ± 7) milliseconds, and that it may be possible to produce this ununseptium isotope using the same berkeliumâ€"calcium reaction used in the discoveries of the known isotopes, ²⁹³Uus and ²⁹⁴Uus, although 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 on ³⁰³Uus). A liquid drop model study on the element's isotopes shows similar results; additionally, it reveals a general increasing stability trend for isotopes heavier than ³⁰¹Uus, with half-lives exceeding the age of the universe for the heaviest isotopes like ³³⁵Uus when one does not consider beta decay.

§Atomic and physical

Ununseptium is a member of group 17 in the periodic table, below the five halogens (fluorine, chlorine, bromine, iodine, and astatine). Every previous group 17 element has seven electrons in its valence shell, forming a valence electron configuration of ns²np⁵. In ununseptium's case, the trend will be continued and the valence electron configuration is predicted to be 7s²7p⁵; therefore, ununseptium will behave similarly to the halogens in many respects. However, notable differences are likely to arise; a largely contributing effect is the spinâ€"orbit (SO) interactionâ€"the mutual interaction between the electrons' motion and spin. It is especially strong for the superheavy elements, because their electrons move much faster than in lighter atoms, at velocities comparable to the speed of light. In relation to ununseptium atoms, it lowers the 7s and the 7p electron energy levels (stabilizing the corresponding electrons), but two of the 7p electron energy levels are stabilized more than the other four. The stabilization of the 7s electrons is called the inert pair effect, and the effect "tearing" the 7p subshell into the more stabilized and the less stabilized parts is called subshell splitting. Computational chemists see the split as a change of the second (azimuthal) quantum number l from 1 to 1/2 and 3/2 for the more 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 that they are close in energy to the 7s ones, although no 6d electron chemistry has been predicted for the element. The difference between the 7p_1/2 and 7p_3/2 levels is abnormally high, being 9.8 eV. Astatine's 6p subshell split is only 3.8 eV, and its 6p_1/2 chemistry has already been called "limited". All these effects cause ununseptium's chemistry to differ from those of its upper neighbors (see below).

The first ionization energy (the energy required to remove an electron from an atom) is predicted to be 7.7 eV, lower than those of the halogens, again following the trend. Ununseptium is also in line with its neighbors in the periodic table in being expected to have the lowest electron affinity (energy released when an electron is added to the atom) in its group, 2.6 or even 1.8 eV. The electron of the hydrogen-like ununseptium atom (oxidized so that it has only one electron, Uus¹¹⁶⁺) is predicted to move so fast that it has a mass of 1.9 compared to a non-moving electron, a feature coming from the relativistic effects. For comparison, the figure is 1.27 for hydrogen-like astatine and 1.08 for hydrogen-like iodine. According to simple extrapolations of relativity laws, that indirectly indicates the contraction of the atomic radius; however, 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 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 definitely known. Earlier papers predicted about 350â€"500 °C and 550 °C respectively, or even 350â€"550 °C and 610 °C respectively. These values exceed the astatine (and all preceding) values, following periodic trends. A later paper, however, predicts ununseptium's boiling point to be 345 °C (that of astatine is estimated to be 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^âˆ'3, continuing the trend of increasing density from the halogens; that of astatine is estimated to be between 6.2 and 6.5 g·cm^âˆ'3.

§Chemical

The previous members of the group commonly accept another electron to achieve a stable noble-gas electronic configuration, 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 going down the group: as mentioned, ununseptium would be the least willing to accept an electron among the group 17 elements (astatine already forms a positive oxidation state in its hydride). In fact, out of the oxidation states the element is predicted to form, âˆ'1 is predicted to be the least common.

There is another opportunity for ununseptium to complete the octetâ€"to form a covalent bond. Accordingly, when two ununseptium atoms meet, they are expected to form a Uusâ€"Uus bond to give diatomic molecules, just like the halogens. The sigma bonding is calculated to show a great antibonding character in the At₂ molecule; ununseptium is predicted to continue the trend and 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 mentioned unstable âˆ'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 already shows somewhat similar effects. The +3 state should also be important because of the 7p_3/2 electrons destabilization. The +5 state is predicted to be uncommon, as the 7p_1/2 electrons are (oppositely) stabilized. The +7 state has not been shown (even computationally) to exist; moreover, since the 7s electrons are very stabilized, it has been hypothesized that the valence core of the element is only five electrons.

The simplest possible ununseptium compound is the monohydride, UusH. The bonding is 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 to not 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 (compare to 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 as compared to AtH. The molecules TlUus and UutUus may be viewed analogously, taken in account an opposite effect shown by that the metal's p_1/2 electrons are stabilized. These two result in a relatively small dipole moment for TlUus, only 1.67 D (the positive value implying the negative charge is on the ununseptium atom); and for UutUus, the effects' strength caused a transfer of the electron from the ununseptium atom to the ununtrium atom, with the dipole moment value being âˆ'1.80 D. In light of the UusH data, it is interesting that the SO interaction increases dissociation energy for the UusF molecule. This is because it lowers the electronegativity of ununseptium, causing the bond with the extremely electronegative fluorine to have a more ionic character. In fact, 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, having a structure of AX
3E
2 (a central atom, denoted A, surrounded by three ligands, X, and two unshared electron pairs, E). If relativistic effects are not taken into account, UusF
3 should also follow its lighter congeners in having bent-T-shaped molecular geometry. However, since the 7s shell of ununseptium should not participate at all in chemical reactions, VSEPR theory instead predicts a trigonal pyramidal molecular geometry (AX
3E
1). More sophisticated predictions show that this molecular geometry would not be energetically favored for UusF
3, predicting instead a trigonal planar molecular geometry (AX
3E
0) up. This shows that VSEPR theory may not hold consistently for the superheavy elements. Also, this molecule is stabilized very significantly by the SO interactions; a possible rationale may be again the large difference in electronegativity between ununseptium and fluorine, giving the bond a partially ionic character.

§Notes

§References

§Bibliography

§External links

• Ununseptium at The Periodic Table of Videos (University of Nottingham)
• Flerov Lab press release announcing synthesis

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