Ununpentium

Ununpentium is the temporary name of a synthetic superheavy element in the periodic table that has the temporary symbol Uup and has the atomic number 115. It is an extremely radioactive element; its most stable known isotope, ununpentium-289, has a half-life of only 220 milliseconds. It is also known as eka-bismuth or simply element 115. Ununpentium was first created in 2003 by the Joint Institute for Nuclear Research in Dubna, Russia, although this discovery is still awaiting confirmation by IUPAC. About 50 atoms of ununpentium have been observed to date, all of which have been shown to have mass numbers from 287 to 290.

In the periodic table, it is a p-block transactinide element. It is a member of the 7th period and is placed in group 15 as the heaviest pnictogen, although it has not been confirmed to behave as the heavier homologue to the pnictogen bismuth. Ununpentium is calculated to have some similar properties to its lighter homologues, nitrogen, phosphorus, arsenic, antimony, and bismuth, although it should also show several major differences from them.

§History

§Discovery

The first report of the synthesis of ununpentium came in August 2003 and was reported on February 2, 2004, in Physical Review C by a team composed of Russian scientists at the Joint Institute for Nuclear Research (JINR) in Dubna, and American scientists at the Lawrence Livermore National Laboratory. The team reported that they bombarded americium-243 with calcium-48 ions to produce four atoms of ununpentium. These atoms decayed by emission of alpha-particles to ununtrium in approximately 100 milliseconds.

243
95Am + 48
20Ca â†' 288
115Uup + 3 1
0n â†' 284
113Uut + α

243
95Am + 48
20Ca â†' 287
115Uup + 4 1
0n â†' 283
113Uut + α

The Dubnaâ€"Livermore collaboration strengthened their claim for the discoveries of ununpentium and ununtrium by conducting chemical experiments on the final decay product ²⁶⁸Db. None of the nuclides in this decay chain were previously known, so existing experimental data was not available to support their claim. In June 2004, and December 2005, the presence of a dubnium isotope was confirmed by extracting the final decay products, measuring spontaneous fission (SF) activities and using chemical identification techniques to confirm that they behave like a group 5 element (as dubnium is known to be in group 5 of the periodic table). Both the half-life and decay mode were confirmed for the proposed ²⁶⁸Db, lending support to the assignment of the parent nucleus to ununpentium. However, in 2011, the IUPAC/IUPAP Joint Working Party (JWP) did not recognize the two elements as having been discovered, because current theory could not distinguish the chemical properties of group 4 and group 5 elements with sufficient confidence. Furthermore, the decay properties of all the nuclei in the decay chain of ununpentium had not been previously characterized before the Dubna experiments, a situation which the JWP generally considers "troublesome, but not necessarily exclusive".

§Naming

Ununpentium is the second-lightest element that has not yet received an official name. Using Mendeleev's nomenclature for unnamed and undiscovered elements, ununpentium is sometimes known as eka-bismuth. In 1979 IUPAC recommended that the placeholder systematic element name ununpentium (with the corresponding symbol of Uup) be used until the discovery of the element is confirmed and a name is decided on. These recommendations are largely ignored among scientists, who call it "element 115", with the symbol of (115) or even simply 115.

§Road to confirmation

Two heavier isotopes of ununpentium, ²⁸⁹Uup and ²⁹⁰Uup, were discovered in 2009â€"2010 as daughters of the ununseptium isotopes ²⁹³Uus and ²⁹⁴Uus. The JINR also has future plans to study lighter isotopes of element 115 by replacing the americium-243 target with the lighter isotope americium-241.

The IUPAC/IUPAP Joint Working Party (JWP) will decide which team officially discovered ununpentium and give that team the right to suggest a permanent name for it. In 2011, it evaluated the 2004 and 2007 Dubna experiments, and concluded that they did not meet the criteria for discovery. Another evaluation of more recent experiments will take place within the next few years, and a claim to the discovery of ununpentium has been again put forward by Dubna. In August 2013, a team of researchers at Lund University announced they had repeated the 2004 experiment, confirming Dubna's findings. Researchers at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany reported another successful confirmation just two weeks later, on September 10, 2013. These confirmations considerably enhances the chances for IUPAC to officially recognize Dubna's claim to have discovered ununpentium.

§Predicted properties

§Nuclear stability and isotopes

Ununpentium is expected to be in the middle of an island of stability centered around copernicium (element 112) and flerovium (element 114): the reasons for the presence of this island are however still not well understood. Due to the expected high fission barriers, any nucleus within this island of stability exclusively decays by alpha decay and perhaps some electron capture and beta decay. Although the known isotopes of ununpentium do not actually have enough neutrons to be on the island of stability, they can be seen to approach the island as in general, the heavier isotopes are the longer-lived ones.

The isotope ²⁹¹Uup is especially interesting as it has only one neutron more than the heaviest known ununpentium isotope, ²⁹⁰Uup. It could plausibly be synthesized as the daughter of ununseptium-295, which in turn could be made from the reaction ²⁴⁹Bk(⁴⁸Ca,2n)²⁹⁵Uus. Calculations show that it may have a significant electron capture or positron emission decay mode in addition to alpha decaying and also have a relatively long half-life of several seconds. This would produce ²⁹¹Fl, ²⁹¹Uut, and finally ²⁹¹Cn which is expected to be in the middle of the island of stability and have a half-life of about 1200 years, affording the most likely hope of reaching the middle of the island using current technology. Possible drawbacks are that the cross section of the production reaction of ²⁹⁵Uus is expected to be low and the decay properties of superheavy nuclei this close to the line of beta stability are largely unexplored.

Other possibilities to synthesize nuclei on the island of stability include quasifission (partial fusion followed by fission) of a massive nucleus. Such nuclei tend to fission, expelling doubly-magic or nearly doubly-magic fragments such as calcium-40, tin-132, lead-208, or bismuth-209. Recently it has been shown that the multi-nucleon transfer reactions in collisions of actinide nuclei (such as uranium and curium) might be used to synthesize the neutron-rich superheavy nuclei located at the island of stability, although formation of the lighter elements nobelium or seaborgium is more favored. One last possibility to synthesize isotopes near the island is to use controlled nuclear explosions to create a neutron flux high enough to bypass the gaps of instability at ^258â€"260Fm and at mass number 275 (atomic numbers 104 to 108), mimicking the r-process in which the actinides were first produced in nature and the gap of instability around radon bypassed. Some such isotopes (especially ²⁹¹Cn and ²⁹³Cn) may even have been synthesized in nature, but would have decayed away far too quickly (with half-lives of only thousands of years) and be produced in far too small quantities (about 10^âˆ'12 the abundance of lead) to be detectable as primordial nuclides today outside cosmic rays.

§Physical and atomic

In the periodic table, ununpentium is a member of group 15, the pnictogens, below nitrogen, phosphorus, arsenic, antimony, and bismuth. Every previous pnictogen has five electrons in its valence shell, forming a valence electron configuration of ns²np³. In ununpentium's case, the trend should be continued and the valence electron configuration is predicted to be 7s²7p³; therefore, ununpentium will behave similarly to its lighter congeners 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 ununpentium 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. Computation chemists see the split as a change of the second (azimuthal) quantum number l from 1 to ¹â„₂ and ³â„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
1/27p1
3/2. These effects cause ununpentium's chemistry to be somewhat different from that of its lighter congeners.

The valence electrons of ununpentium fall into three subshells: 7s (two electrons), 7p_1/2 (two electrons), and 7p_3/2 (one electron). The first two of these are relativistically stabilized and hence behave as inert pairs, while the last is relativistically destabilized and can easily participate in chemistry. (The 6d electrons are not destabilized enough to participate chemically, although this may still be possible in the two previous elements ununtrium and flerovium.) Thus, the +1 oxidation state should be favored, like Tl⁺, and consistent with this the first ionization potential of ununpentium should be around 5.58 eV, continuing the trend towards lower ionization potentials down the pnictogens. Due to ununpentium and ununtrium both having one electron outside a quasi-closed shell configuration that can be delocalized in the metallic state, they should have similar melting and boiling points (both melting around 400 °C and boiling around 1100 °C) due to the strength of their metallic bonds being similar. Additionally, the predicted ionization potential, ionic radius (1.5 Å for Uup⁺; 1.0 Å for Uup³⁺), and polarizability of Uup⁺ are expected to be more similar to Tl⁺ than its true congener Bi³⁺. Ununpentium should be a dense metal due to its high atomic weight, with a density around 13.5 g/cm³. The electron of the hydrogen-like ununpentium atom (oxidized so that it only has one electron, Uup¹¹⁴⁺) is expected to move so fast that it has a mass 1.82 times that of a stationary electron, due to relativistic effects. For comparison, the figures for hydrogen-like bismuth and antimony are expected to be 1.25 and 1.077 respectively.

§Chemical

Ununpentium is projected to be the third member of the 7p series of chemical elements and the heaviest member of group 15 (VA) in the Periodic Table, below bismuth. In this group, each member is known to portray the group oxidation state of +5 but with differing stability. For nitrogen, the +5 state is mostly a formal explanation of molecules like N₂O₅: it is very difficult to have five covalent bonds to nitrogen due to the lack of low-lying d-orbitals and the inability of the small nitrogen atom to accommodate five ligands. The +5 state is well represented for the essentially non-relativistic typical pnictogens phosphorus, arsenic, and antimony. However, for bismuth it becomes rare due to the relativistic stabilization of the 6s orbitals known as the inert pair effect, so that the 6s electrons are reluctant to bond chemically. It is expected that ununpentium will have an inert pair effect for both the 7s and the 7p_1/2 electrons, as the binding energy of the lone 7p_3/2 electron is noticeably lower than that of the 7p_1/2 electrons. Nitrogen(I) and bismuth(I) are known but rare and ununpentium(I) is likely to show some unique properties, probably behaving more like thallium(I) than bismuth(I). Because of spin-orbit coupling, flerovium may display closed-shell or noble gas-like properties; if this is the case, ununpentium will likely be typically monovalent as a result, since the cation Uup⁺ will have the same electron configuration as flerovium, perhaps giving ununpentium some alkali metal character. However, the Uup³⁺ cation would behave like its true lighter homolog Bi³⁺. The 7s electrons are too stabilized to be able to contribute chemically and hence the +5 state should be impossible and ununpentium may be considered to have only three valence electrons. Ununpentium would be quite a reactive metal, with an oxidation potential of about 1.5 V.

The chemistry of ununpentium in aqueous solution should essentially be that of the Uup⁺ and Uup³⁺ ions. The former should be easily hydrolyzed and not be easily complexed with halides, cyanide, and ammonia. Ununpentium(I) hydroxide (UupOH), carbonate (Uup₂CO₃), oxalate (Uup₂C₂O₄), and fluoride (UupF) should be soluble in water; the sulfide (Uup₂S) should be insoluble; and the chloride (UupCl), bromide (UupBr), iodide (UupI), and thiocyanate (UupSCN) should be only slightly soluble, so that adding excess hydrochloric acid would not noticeably affect the solubility of ununpentium(I) chloride. Uup³⁺ should be about as stable as Tl³⁺ and hence should also be an important part of ununpentium chemistry, although its closest homolog among the elements should be its lighter congener Bi³⁺. Ununpentium(III) fluoride (UupF₃) and thiozonide (UupS₃) should be insoluble in water, similar to the corresponding bismuth compounds, while ununpentium(III) chloride (UupCl₃), bromide (UupBr₃), and iodide (UupI₃) should be readily soluble and easily hydrolyzed to form oxyhalides such as UupOCl and UupOBr, again analogous to bismuth. Both ununpentium(I) and ununpentium(III) should be common oxidation states and their relative stability should depend greatly on what they are complexed with and the likelihood of hydrolysis.

§Experimental chemistry

Unambiguous determination of the chemical characteristics of ununpentium has yet to have been established. In 2011, experiments were conducted to create ununtrium, flerovium, and ununpentium isotopes in the reactions between calcium-48 projectiles and targets of americium-243 and plutonium-244. However, the targets included lead and bismuth impurities and hence some isotopes of bismuth and polonium were generated in nucleon transfer reactions. This, while an unforeseen complication, could give information that would help in the future chemical investigation of the heavier homologs of bismuth and polonium, which are respectively ununpentium and livermorium. The produced nuclides bismuth-213 and polonium-212m were transported as the hydrides ²¹³BiH₃ and ^212mPoH₂ at 850 °C through a quartz wool filter unit held with tantalum, showing that these hydrides were surprisingly thermally stable, although their heavier congeners UupH₃ and LvH₂ would be expected to be less thermally stable from simple extrapolation of periodic trends in the p-block. Further calculations on the stability and electronic structure of BiH₃, UupH₃, PoH₂, and LvH₂ are needed before chemical investigations take place. However, ununpentium and livermorium are expected to be volatile enough as pure elements for them to be chemically investigated in the near future. The chief barrier to their chemical investigation at present is the lack of known isotopes of these elements which are long-lived enough.

§Notes

§References

§External links

• Uut and Uup Add Their Atomic Mass to Periodic Table
• Superheavy elements
• History and etymology
• Ununpentium at The Periodic Table of Videos (University of Nottingham)

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