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 placed as the heaviest member of group 15 (VA) although a sufficiently stable isotope is not known at this time that would allow chemical experiments to confirm its position. It was first observed in 2003 and about 50 atoms of ununpentium have been synthesized to date, with about 25 direct decays of the parent element having been detected. Four consecutive isotopes are currently known, ^287–290Uup, with ²⁸⁹Uup having the longest measured half-life of ~200 ms.^[1]

History

Discovery profile

On February 2, 2004, synthesis of ununpentium was reported in Physical Review C by a team composed of Russian scientists at the Joint Institute for Nuclear Research in Dubna, and American scientists at the Lawrence Livermore National Laboratory.^[2]^[3] The team reported that they bombarded americium-243 with calcium-48 ions to produce four atoms of ununpentium. These atoms, they report, decayed by emission of alpha-particles to ununtrium in approximately 100 milliseconds.

48
20Ca + 243
95Am → 291
115Uup*
→ 288
113Uut

The Dubna-Livermore collaboration has strengthened their claim for the discovery of ununpentium by conducting chemical experiments on the decay daughter ²⁶⁸Db. In experiments in June 2004 and December 2005, the Dubnium isotope was successfully identified by milking the Db fraction and measuring any SF activities.^[4]^[5] Both the half-life and decay mode were confirmed for the proposed ²⁶⁸Db which lends support to the assignment of Z=115 to the parent nuclei.

Sergei Dmitriev from the Flerov Laboratory of Nuclear Reactions (FLNR) in Dubna, Russia, has formally put forward their claim of discovery of ununpentium to the Joint Working Party (JWP) from IUPAC and IUPAP.^[6] In 2011, the IUPAC evaluated the Dubna-Livermore results and concluded that they did not meet the criteria for discovery.^[7]

Recent experiments at Dubna have fully confirmed the data for ununpentium and ununtrium but have yet to be fully published and reviewed by the JWP. This process is likely not to occur for some time.

Naming

Ununpentium is historically known as eka-bismuth. Ununpentium is a temporary IUPAC systematic element name derived from the digits 115, where "un-" represents Latin unum. "Pent-" represents the Greek word for 5, and it was chosen because the Latin word for 5 starts with 'q', which would have caused confusion with ununquadium, element 114. Research scientists usually refer to the element simply as element 115.

Current and future experiments

The team at Dubna are currently running another series of experiments on the ²⁴³Am(⁴⁸Ca,xn) reaction. They are attempting to complete the 4n excitation function and confirm the data for ²⁸⁷115. They are also hoping to identify some decays from the 2n and 5n exit channels. This reaction will run until the Christmas shutdown.

The FLNR also have future plans to study light isotopes of element 115 using the reaction ²⁴¹Am + ⁴⁸Ca.^[8]

Isotopes and nuclear properties

Nucleosynthesis

Target-projectile combinations leading to Z=115 compound nuclei

The table below contains various combinations of targets and projectiles which could be used to form compound nuclei with Z=115.

Target	Projectile	CN	Attempt result
²⁰⁸Pb	⁷⁵As	²⁸³Uup	Reaction yet to be attempted
²³²Th	⁵⁵Mn	²⁸⁷Uup	Reaction yet to be attempted
²³⁸U	⁵¹V	²⁸⁹Uup	Failure to date
²³⁷Np	⁵⁰Ti	²⁸⁷Uup	Reaction yet to be attempted
²⁴⁴Pu	⁴⁵Sc	²⁸⁹Uup	Reaction yet to be attempted
²⁴³Am	⁴⁸Ca	²⁹¹Uup	Successful reaction
²⁴¹Am	⁴⁸Ca	²⁸⁹Uup	Reaction yet to be attempted
²⁴⁸Cm	⁴¹K	²⁸⁹Uup	Reaction yet to be attempted
²⁴⁹Bk	⁴⁰Ar	²⁸⁹Uup	Reaction yet to be attempted
²⁴⁹Cf	³⁷Cl	²⁸⁶Uup	Reaction yet to be attempted

Hot fusion

This section deals with the synthesis of nuclei of ununpentium by so-called "hot" fusion reactions. These are processes which create compound nuclei at high excitation energy (~40–50 MeV, hence "hot"), leading to a reduced probability of survival from fission. The excited nucleus then decays to the ground state via the emission of 3–5 neutrons. Fusion reactions utilizing ⁴⁸Ca nuclei usually produce compound nuclei with intermediate excitation energies (~30–35 MeV) and are sometimes referred to as "warm" fusion reactions. This leads, in part, to relatively high yields from these reactions.

²³⁸U(⁵¹V,xn)^289−xUup

There are strong indications that this reaction was performed in late 2004 as part of a uranium(IV) fluoride target test at the GSI. No reports have been published suggesting that no products atoms were detected, as anticipated by the team.^[9]

²⁴³Am(⁴⁸Ca,xn)^291−xUup (x=2,3,4)

This reaction was first performed by the team in Dubna in July–August 2003. In two separate runs they were able to detect 3 atoms of ²⁸⁸Uup and a single atom of ²⁸⁷Uup. The reaction was studied further in June 2004 in an attempt to isolate the descendant ²⁶⁸Db from the ²⁸⁸Uup decay chain. After chemical separation of a +4/+5 fraction, 15 SF decays were measured with a lifetime consistent with ²⁶⁸Db. In order to prove that the decays were from dubnium-268, the team repeated the reaction in August 2005 and separated the +4 and +5 fractions and further separated the +5 fractions into tantalum-like and niobium-like ones. Five SF activities were observed, all occurring in the +5 fractions and none in the tantalum-like fractions, proving that the product was indeed isotopes of dubnium.

In a series of experiments between October 2010 - February 2011, scientists at the FLNR studied this reaction at a range of excitation energies. They were able to detect 21 atoms of ²⁸⁸115 and one atom of ²⁸⁹115, from the 2n exit channel. This latter result was used to support the synthesis of ununseptium. The 3n excitation function was completed with a maximum at ~8 pb. The data was consistent with that found in the first experiments in 2003.

Chronology of isotope discovery

Isotope	Year discovered	Discovery reaction
²⁸⁷Uup	2003	²⁴³Am(⁴⁸Ca,4n)
²⁸⁸Uup	2003	²⁴³Am(⁴⁸Ca,3n)
²⁸⁹Uup	2009	²⁴⁹Bk(⁴⁸Ca,4n)^[1]
²⁹⁰Uup	2009	²⁴⁹Bk(⁴⁸Ca,3n)^[1]

Yields of isotopes

Hot fusion

The table below provides cross-sections and excitation energies for hot fusion reactions producing ununpentium isotopes directly. Data in bold represent maxima derived from excitation function measurements. + represents an observed exit channel.

Projectile	Target	CN	2n	3n	4n	5n
⁴⁸Ca	²⁴³Am	²⁹¹Uup		3.7 pb, 39.0 MeV	0.9 pb, 44.4 MeV

Theoretical calculations

Decay characteristics

Theoretical calculations using a quantum-tunneling model support the experimental alpha-decay half-lives.^[10]

Evaporation residue cross sections

The table below contains various target-projectile combinations for which calculations have provided estimates for cross section yields from various neutron evaporation channels. The channel with the highest expected yield is given.

MD = multi-dimensional; DNS = Di-nuclear system; σ = cross section

Target	Projectile	CN	Channel (product)	σ_max	Model	Ref
²⁴³Am	⁴⁸Ca	²⁹¹Uup	3n (²⁸⁸Uup)	3 pb	MD	^[11]
²⁴³Am	⁴⁸Ca	²⁹¹Uup	4n (²⁸⁷Uup)	2 pb	MD	^[11]
²⁴³Am	⁴⁸Ca	²⁹¹Uup	3n (²⁸⁸Uup)	1 pb	DNS	^[12]
²⁴²Am	⁴⁸Ca	²⁹⁰Uup	3n (²⁸⁷Uup)	2.5 pb	DNS	^[12]

Chemical properties

Extrapolated chemical properties

Oxidation states

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 +V but with differing stability. For nitrogen, the +V state is very difficult to achieve due to the lack of low-lying d-orbitals and the inability of the small nitrogen atom to accommodate five ligands. The +V state is well represented for phosphorus, arsenic, and antimony. However, for bismuth it is rare due to the reluctance of the 6s² electron to participate in bonding. This effect is known as the "inert pair effect" and is commonly linked to relativistic stabilisation of the 6s-orbitals. It is expected that ununpentium will continue this trend and portray only +III and +I oxidation states. Nitrogen(I) and bismuth(I) are known but rare and Uup(I) is likely to show some unique properties ^[13] because of spin-orbit coupling, Ununquadium may display closed-shell or noble gas-like properties; if this is the case, Uup will likely be monovalent as a result, since the cation Uup⁺ will have the same electron configuration as Uuq.