Roentgenium

iso	NA	half-life	DM	DE (MeV)	DP
²⁸²Rg	syn	~0.5 s	α	9.00	²⁷⁸Mt
²⁸¹Rg	syn	22.8 s	SF
²⁸⁰Rg	syn	3.6 s	α	9.75	²⁷⁶Mt
²⁷⁹Rg	syn	170 ms	α	10.37	²⁷⁵Mt
²⁷⁸Rg	syn	4.2 ms	α	10.69	²⁷⁴Mt
²⁷⁴Rg	syn	15 ms	α	11.23	²⁷⁰Mt
²⁷²Rg	syn	1.6 ms	α	11.02,10.82	²⁶⁸Mt

Roentgenium (pronounced /rɛntˈɡɛniəm/ rent-GEN-ee-əm or /rʌntˈɡɛniəm/ ( listen) runt-GEN-ee-əm) is a synthetic radioactive chemical element with the symbol Rg and atomic number 111. It is placed as the heaviest member of the group 11 (IB) elements, although a sufficiently stable isotope is not known at this time that would allow its position as a heavier homologue of gold to be confirmed.

Roentgenium was called unununium before it was formally discovered.

Roentgenium was first observed in 1994 and several isotopes have been synthesized since its first discovery. The most stable known isotope is ²⁸¹Rg with a half-life of ~20 seconds, which decays by spontaneous fission, like many other N=170 isotones.

History

Official discovery

Roentgenium was officially discovered by Peter Armbruster, Gottfried Münzenberg, and their team working at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany on December 8, 1994.^[1] Only three atoms of it were observed (all ²⁷²Rg), by the cold fusion between nickel ions and a bismuth target in a linear accelerator:

20983Bi + 6428Ni → 272111Rg + 10n

In 2001, the IUPAC/IUPAP Joint Working Party (JWP) concluded that there was insufficient evidence for the discovery at that time.^[2] The GSI team repeated their experiment in 2002 and detected a further 3 atoms.^[3]^[4] In their 2003 report, the JWP decided that the GSI team should be acknowledged as the discoverers.^[5]

Naming

The name roentgenium (Rg) was proposed by the GSI team^[6] in honor of the German physicist Wilhelm Conrad Röntgen, and was accepted as a permanent name on November 1, 2004.^[7] Previously the element was known under the temporary IUPAC systematic element name unununium, Uuu.

Isotopes and nuclear properties

Nucleosynthesis

Target-projectile combinations leading to Z=111 compound nuclei

The below table contains various combinations of targets and projectiles (both at max no. of neutrons) which could be used to form compound nuclei with Z=111.

Target	Projectile	CN	Attempt result
²⁰⁸Pb	⁶⁵Cu	²⁷³Rg	Successful reaction
²⁰⁹Bi	⁶⁴Ni	²⁷³Rg	Successful reaction
²³²Th	⁴⁵Sc	²⁷⁷Rg	Reaction yet to be attempted
²³¹Pa	⁴⁸Ca	²⁷⁹Rg	Reaction yet to be attempted
²³⁸U	⁴¹K	²⁸⁰Rg	Reaction yet to be attempted
²³⁷Np	⁴⁰Ar	²⁷⁷Rg	Reaction yet to be attempted
²⁴⁴Pu	³⁷Cl	²⁸¹Rg	Reaction yet to be attempted
²⁴³Am	³⁶S	²⁷⁹Rg	Reaction yet to be attempted
²⁴⁸Cm	³¹P	²⁷⁹Rg	Reaction yet to be attempted
²⁴⁹Bk	³⁰Si	²⁷⁹Rg	Reaction yet to be attempted
²⁴⁹Cf	²⁷Al	²⁷⁶Rg	Reaction yet to be attempted

Cold fusion

This section deals with the synthesis of nuclei of roentgenium by so-called "cold" fusion reactions. These are processes which create compound nuclei at low excitation energy (~10-20 MeV, hence "cold"), leading to a higher probability of survival from fission. The excited nucleus then decays to the ground state via the emission of one or two neutrons only.

²⁰⁹Bi(⁶⁴Ni,xn)^273−xRg (x=1)

First experiments to synthesize element 111 were performed by the Dubna team in 1986 using this cold fusion reaction. No atoms were identified that could be assigned to atoms of element 111 and a production cross-section limit of 4 pb was determined. After an upgrade of their facilities, the team at GSI successfully detected 3 atoms of ²⁷²Rg in their discovery experiment.^[1] A further 3 atoms were synthesized in 2000.^[3] The discovery of roentgenium was confirmed in 2003 when a team at RIKEN measured the decays of 14 atoms of ²⁷²Rg during the measurement of the 1n excitation function.^[8]

²⁰⁸Pb(⁶⁵Cu,xn)^273−xRg (x=1)

In 2004, as part of their study of odd-Z projectiles in cold fusion reactions, the team at LBNL detected a single atom of ²⁷²Rg in this new reaction.^[9]^[10]

As a decay product

Isotopes of roentgenium have also been observed in the decay of heavier elements. Observations to date are outlined in the table below:

Evaporation residue	Observed Rg isotope
²⁸⁸Uup	²⁸⁰Rg ^[11]
²⁸⁷Uup	²⁷⁹Rg ^[11]
²⁸²Uut	²⁷⁸Rg ^[12]
²⁷⁸Uut	²⁷⁴Rg ^[12]

Chronology of isotope discovery

Isotope	Year discovered	Discovery reaction
²⁷²Rg	1994	²⁰⁹Bi(⁶⁴Ni,n)
²⁷³Rg	unknown
²⁷⁴Rg	2004	²⁰⁹Bi(⁷⁰Zn,n) ^[12]
²⁷⁵Rg	unknown
²⁷⁶Rg	unknown
²⁷⁷Rg	unknown
²⁷⁸Rg	2006	²³⁷Np(⁴⁸Ca,3n) ^[12]
²⁷⁹Rg	2003	²⁴³Am(⁴⁸Ca,4n) ^[11]
²⁸⁰Rg	2003	²⁴³Am(⁴⁸Ca,3n) ^[11]
²⁸¹Rg	2009	²⁴⁹Bk(⁴⁸Ca,4n)
²⁸²Rg	2009	²⁴⁹Bk(⁴⁸Ca,3n)

Nuclear isomerism

²⁷⁴Rg

Two atoms of ²⁷⁴Rg have been observed in the decay chains starting with ²⁷⁸Uut. The two events occur with different energies and with different lifetimes. In addition, the two entire decay chains appear to be different. This suggests the presence of two isomeric levels but further research is required.

²⁷²Rg

The direct production of ²⁷²Rg has provided four alpha lines at 11.37, 11.03, 10.82, and 10.40 MeV. The GSI measured a half-life of 1.6 ms whilst recent data from RIKEN hav given a half-life of 3.8 ms. The conflicting data may be due to isomeric levels but the current data are insufficient to come to any firm assignments.

Chemical yields of isotopes

Cold fusion

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

Projectile	Target	CN	1n	2n	3n
⁶⁴Ni	²⁰⁹Bi	²⁷³Rg	3.5 pb, 12.5 MeV
⁶⁵Cu	²⁰⁸Pb	²⁷³Rg	1.7 pb, 13.2 MeV

Chemical properties

Electronic structure (relativistic)

The stable group 11 elements, copper, silver, and gold all have an outer electron configuration nd¹⁰(n+1)s¹. For each of these elements, their first excited state has a configuration nd⁹(n+1)s². Due to spin-orbit coupling between the d electrons, this state is split into a pair of energy levels. For copper, the difference in energy between the ground state and lowest excited state causes the metal to appear reddish. For silver, the energy gap widens and it becomes silvery. However, as Z increases, the excited levels are stabilised by relativistic effects and in gold the energy gap decreases again and it appears gold. For roentgenium, calculations indicate that the 6d⁹7s² level is stabilised to such an extent that it becomes the ground state. The resulting energy difference between the new ground state and the first excited state is similar to that of silver and roentgenium is expected to be silvery in appearance.^[13]

Extrapolated chemical properties

Oxidation states

Element 111 is projected to be the ninth member of the 6d series of transition metals and the heaviest member of group 11 (IB) in the Periodic Table, below copper, silver, and gold. Each of the members of this group show different stable states. Copper forms a stable +2 state, while silver is predominantly found as silver(I) and gold as gold(III). Copper(I) and silver(II) are also relatively well-known. Roentgenium is therefore expected to predominantly form a stable +3 state.

Chemistry

The heavier members of this group are well known for their lack of reactivity or noble character. Silver and gold are both inert to oxygen, but are attacked by the halogens. In addition, silver is attacked by sulfur and hydrogen sulfide, highlighting its higher reactivity compared to gold. Roentgenium is expected to be even more noble than gold and can be expected to be inert to oxygen and halogens. The most-likely reaction is with fluorine to form a trifluoride, RgF₃.