Ununseptium (117)

The essentials

Ununseptium is the superheavy artificial chemical element with temporary symbol Uus and atomic number 117. The element, also known as eka-astatine or simply element 117 (MUCH easier to remember for the general populous), 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 2010—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. 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".

An article published in Physical Review Letters on 5 April 2010 (submitted 15 March 2010, "Synthesis of a new chemical element with atomic number Z=117", Joint Institute for Nuclear Research, RU-141980 Dubna, Russian Federation, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA, University of Nevada Las Vegas, Las Vegas, Nevada 89154, USA, Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA, Lawrence Livemore National Laboratory, Livermore, California 94551, USA, and Research Institute of Atomic Reactors, RU-433510 Dimitrovgrad, Russian Federation) claims the identification of six atoms of the isotopes ²⁹³Uus (five atoms) and ²⁹⁴Uus (one atom) in fusion reactions between ⁴⁸Ca and ²⁴⁹Bk.

⁴⁸₂₀Ca + ²⁴⁹₉₇Bk → ²⁹⁷₁₁₇Uus* → ²⁹³₁₁₇Uus + 4 n

⁴⁸₂₀Ca + ²⁴⁹₉₇Bk → ²⁹⁷₁₁₇Uus* → ²⁹⁴₁₁₇Uus + 3 n

Decay chains involving eleven nuclei were identified by means of the Dubna Gas Filled Recoil Separator. It is said that the measured decay properties show a rise of stability for heavier isotopes with Z>=111, validating the concept of the "long sought island of enhanced stability for super-heavy nuclei".

The half-life for ²⁹³Uus is 0.014(+0.011-0.004) seconds and that for ²⁹⁴Uus is 0.078(+0.370-0.036) seconds. Each undergoes sequential decay chains down to ²⁸¹Rg and ²⁷⁰Db respectively.

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. 

Name: Ununseptium Symbol: Uus Atomic number: 117 Atomic weight: [ 294 ] Standard state: presumably a solid at 298 K CAS Registry ID: 87658-56-8	Group in periodic table: 17 Group name: Halogen Period in periodic table: 7 Block in periodic table: p-block Color: unknown, but probably metallic and dark in appearance Classification: Unknown

Historical information

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 2013, however, and the synthesized isotopes tend to have fewer neutrons than those expected to be in the island of stability.

Decay chain of the ununseptium isotopes produced. The figures near the arrows describe the decay characteristics: half-life time and decay energy. For each couple of values, the upper one is obtained experimentally (in black) while the lower one is predicted theoretically (in blue).

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:

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 2007–2011. 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

Physical properties

• Melting point: 573-773 K; 300-500 C; 572-932 F
• Boiling point: 823 K; 550 C; 1022 F
• Density of solid: 7.1-7.3 (predicted) g cm^-3

Orbital properties

• Ground state electron configuration:  [Rn].5f¹⁴.6d¹⁰.7s².7p⁵ (a guess based upon that of astatine)
• Shell structure:  2.8.18.32.32.18.7
• Term symbol:   ²P_3/2 (a guess based upon guessed electronic structure)
•  First ionization energy: 742.9 kJ mol^-1 (predicted)

Isolation

An article published in Physical Review Letters on 5 April 2010 ("Synthesis of a new chemical element with atomic number Z=117") claims the identification of six atoms of the isotopes ²⁹³Uus (five atoms) and ²⁹⁴Uus (one atom) in fusion reactions between ⁴⁸Ca and ²⁴⁹Bk.

⁴⁸₂₀Ca + ²⁴⁹₉₇Bk → ²⁹⁷₁₁₇Uus* → ²⁹³₁₁₇Uus + 4 n

⁴⁸₂₀Ca + ²⁴⁹₉₇Bk → ²⁹⁷₁₁₇Uus* → ²⁹⁴₁₁₇Uus + 3 n

Decay chains involving eleven nuclei were identified by means of the Dubna Gas Filled Recoil Separator. It is said that the measured decay properties show a rise of stability for heavier isotopes with Z>=111, validating the concept of the "long sought island of enhanced stability for super-heavy nuclei".

The half-life for ²⁹³Uus is 0.014(+0.011-0.004) seconds and that for ²⁹⁴Uus is 0.078(+0.370-0.036) seconds. Each undergoes sequential decay chains down to ²⁸¹Rg and ²⁷⁰Db respectively.