Roentgenium

Roentgenium
111Rg
Au ↑ Rg ↓ (Uht) Periodic table darmstadtium ← roentgenium → copernicium
Appearance
silvery (predicted)
General properties
Name, symbol, number	roentgenium, Rg, 111
Pronunciation	/ r ʌ n t ˈ ɡ ɛ n i ə m / runt-GEN-ee-əm or / r ɛ n t ˈ ɡ ɛ n i ə m / rent-GEN-ee-əm
Element category	unknown but probably a transition metal
Group, period, block	11, 7, d
Standard atomic weight	[281]
Electron configuration	[Rn] 5f14 6d9 7s2 (predicted) 2, 8, 18, 32, 32, 17, 2 (predicted)
History
Discovery	Gesellschaft für Schwerionenforschung (1994)
Physical properties
Phase	solid (predicted)
Density (near r.t.)	28.7 (predicted) g·cm−3
Atomic properties
Oxidation states	5, 3, 1, −1 (predicted)
Ionization energies ( more)	1st: 1022.7 (estimated) kJ·mol−1
	2nd: 2074.4 (estimated) kJ·mol−1
	3rd: 3077.9 (estimated) kJ·mol−1
Atomic radius	114 (predicted) pm
Covalent radius	121 (estimated) pm
Miscellanea
CAS registry number	54386-24-2
Most stable isotopes
Main article: Isotopes of roentgenium
iso NA half-life DM DE ( MeV) DP 282Rg syn 0.5 s α 9.00 278Mt 281Rg syn 26 s SF 280Rg syn 3.6 s α 9.75 276Mt 279Rg syn 0.17 s α 10.37 275Mt only isotopes with half-lives over 0.1 seconds are included here

Roentgenium is a chemical element with the symbol Rg and atomic number 111. It is an extremely radioactive synthetic element (an element that can be created in a laboratory but is not found in nature); the most stable known isotope, roentgenium-281, has a half-life of 26 seconds. Roentgenium was first created in 1994 by the GSI Helmholtz Centre for Heavy Ion Research near Darmstadt, Germany. It is named after the physicist Wilhelm Röntgen (also spelled Roentgen).

In the periodic table, it is a d-block transactinide element. It is a member of the 7th period and is placed in the group 11 elements, although no chemical experiments have been carried out to confirm that it behaves as the heavier homologue to gold in group 11. Roentgenium is calculated to have similar properties to its lighter homologues, copper, silver, and gold, although it may show some differences from them.

History

Official discovery

Roentgenium was first synthesized by an international team led by Sigurd Hofmann at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany, on December 8, 1994. The team bombarded a target of bismuth-209 with accelerated nuclei of nickel-64 and detected a single atom of the isotope roentgenium-272:

209
83Bi + 64
28Ni → 272
111Rg + 1
0n

In 2001, the IUPAC/IUPAP Joint Working Party (JWP) concluded that there was insufficient evidence for the discovery at that time. The GSI team repeated their experiment in 2002 and detected three more atoms. In their 2003 report, the JWP decided that the GSI team should be acknowledged for the discovery of this element.

Naming

The name roentgenium (Rg) was suggested by the GSI team in honour of the German physicist Wilhelm Conrad Röntgen, the discoverer of X-rays, in 2004. This name was accepted by IUPAC on November 1, 2004.

Nucleosynthesis

Super-heavy elements such as roentgenium are produced by bombarding lighter elements in particle accelerators that induce fusion reactions. Whereas the lightest isotope of roentgenium, roentgenium-272, can be synthesized directly this way, all the heavier roentgenium isotopes have only been observed as decay products of elements with higher atomic numbers.

Depending on the energies involved, fusion reactions can be categorized as "hot" or "cold". In hot fusion reactions, very light, high-energy projectiles are accelerated toward very heavy targets (actinides), giving rise to compound nuclei at high excitation energy (~40–50 MeV) that may either fission or evaporate several (3 to 5) neutrons. In cold fusion reactions, the produced fused nuclei have a relatively low excitation energy (~10–20 MeV), which decreases the probability that these products will undergo fission reactions. As the fused nuclei cool to the ground state, they require emission of only one or two neutrons, and thus, allows for the generation of more neutron-rich products. The latter is a distinct concept from that of where nuclear fusion claimed to be achieved at room temperature conditions (see cold fusion).

Cold fusion

Before the first successful synthesis of roentgenium in 1994 by the GSI team, a team at the Joint Institute for Nuclear Research in Dubna, Russia, also tried to synthesize roentgenium by bombarding bismuth-209 with nickel-64 in 1986. No roentgenium atoms were identified. After an upgrade of their facilities, the team at GSI successfully detected 3 atoms of ²⁷²Rg in their discovery experiment. A further 3 atoms were synthesized in 2002. The discovery of roentgenium was confirmed in 2003 when a team at RIKEN measured the decays of 14 atoms of ²⁷²Rg.

The same roentgenium isotope was also observed by an American team at the Lawrence Berkeley National Laboratory (LBNL) from the reaction:

208
82Pb + 65
29Cu → 272
111Rg + n

This reaction was conducted as part of their study of projectiles with odd atomic number in cold fusion reactions.

As decay product

All the isotopes of roentgenium except roentgenium-272 have been detected only in the decay chains of elements with a higher atomic number, such as ununtrium. Ununtrium currently has six known isotopes; all of them undergo alpha decays to become roentgenium nuclei, with mass numbers between 274 and 282. Parent ununtrium nuclei can be themselves decay products of ununpentium or ununseptium. To date, no other elements have been known to decay to roentgenium. For example, in January 2010, the Dubna team ( JINR) identified roentgenium-281 as a final product in the decay of ununseptium via an alpha decay sequence:

293
117Uus → 289
115Uup + 4
2He

289
115Uup → 285
113Uut + 4
2He

285
113Uut → 281
111Rg + 4
2He

Isotopes

Roentgenium has no stable or naturally-occurring isotopes. Several radioactive isotopes have been synthesized in the laboratory, either by fusion of the nuclei of lighter elements or as intermediate decay products of heavier elements. Seven different isotopes of roentgenium have been reported with atomic masses 272, 274, and 278–282, two of which, roentgenium-272 and roentgenium-274, have known but unconfirmed metastable states. All of these decay through alpha decay except roentgenium-281, which undergoes spontaneous fission.

Stability and half-lives

All roentgenium isotopes are extremely unstable and radioactive; in general, the heavier isotopes are more stable than the lighter. The most stable known roentgenium isotope, ²⁸¹Rg, is also the second-heaviest known roentgenium isotope; it has a half-life of 26 seconds. The isotope ²⁸⁰Rg has been reported to also have a half-life of over a second. The isotopes ²⁸²Rg and ²⁷⁹Rg have half-lives of 0.5 and 0.17 seconds respectively. The remaining four isotopes have half-lives between 1 and 10 milliseconds. The undiscovered isotope ²⁸⁷Rg has been predicted to be the most stable towards beta decay; however, no known roentgenium isotope has been observed to undergo beta decay. The unknown isotopes ²⁷⁷Rg and ²⁸³Rg are also expected to have long half-lives of 1 second and 10 minutes respectively. Before their discovery, the isotopes ²⁷⁸Rg, ²⁸¹Rg, and ²⁸²Rg were predicted to have long half-lives of 1 second, 1 minute, and 4 minutes respectively; however, they were discovered to have shorter half-lives of 4.2 milliseconds, 26 seconds, and 0.5 seconds respectively.

Nuclear isomerism

²⁷⁴Rg

Two atoms of ²⁷⁴Rg have been observed in the decay chain of ²⁷⁸Uut. The They decay by alpha emission, emitting alpha particles with different energies, and have different lifetimes. In addition, the two entire decay chains appear to be different. This suggests the presence of two nuclear isomers but further research is required.

²⁷²Rg

Four alpha particles emitted from ²⁷²Rg with energies of 11.37, 11.03, 10.82, and 10.40 MeV have been detected. The GSI measured ²⁷²Rg to have a half-life of 1.6 ms whilst recent data from RIKEN have given a half-life of 3.8 ms. The conflicting data may be due to nuclear isomers but the current data are insufficient to come to any firm assignments.

Predicted properties

Chemical

Roentgenium is the ninth member of the 6d series of transition metals. Since copernicium (element 112) has been shown to be a transition metal, it is expected that all the elements from 104 to 112 would form a fourth transition metal series. Calculations on its ionization potentials and atomic and ionic radii are similar to that of its lighter homologue gold, thus implying that roentgenium's basic properties will resemble those of the other group 11 elements, copper, silver, and gold; however, it is also predicted to show several differences from its lighter homologues.

Roentgenium is predicted to be a noble metal. Based on the most stable oxidation states of the lighter group 11 elements, roentgenium is predicted to show stable +5, +3, and −1 oxidation states, with a less stable +1 state. The +3 state is predicted to be the most stable. Roentgenium(III) is expected to be of comparable reactivity to gold(III), but should be more stable and form a larger variety of compounds. Gold also forms a somewhat stable −1 state due to relativistic effects, and roentgenium may do so as well. The 6d orbitals are destabilized by relativistic effects and spin–orbit interactions near the end of the fourth transition metal series, thus making higher oxidation states like roentgenium(V) and copernicium(IV) more stable than their lighter homologues gold(V) and mercury(IV) (each of which are known only in one compound) as the 6d electrons participate in bonding to a greater extent. The spin-orbit interactions stabilize molecular roentgenium compounds with more bonding 6d electrons; for example, RgF−
6 is expected to be more stable than RgF−
4, which is expected to be more stable than RgF−
2. Roentgenium(I) is expected to be difficult to obtain.

The probable chemistry of roentgenium has received more interest than that of the two previous elements, meitnerium and darmstadtium, as the valence s- subshells of the group 11 elements are expected to be relativistically contracted most strongly at roentgenium. Calculations on the molecular compound RgH show that relativistic effects double the strength of the roentgenium–hydrogen bond, even though spin–orbit interactions also weaken it by 0.7  eV. The compounds AuX and RgX, where X = F, Cl, Br, O, Au, or Rg, were also studied.

Physical and atomic

Roentgenium is expected to be a solid under normal conditions. It should be a very heavy metal with a density of around 28.7 g/cm³; in comparison, the densest known element that has had its density measured, osmium, has a density of only 22.61 g/cm³. This results from roentgenium's high atomic weight, the lanthanide and actinide contractions, and relativistic effects, although production of enough roentgenium to measure this quantity would be impractical, and the sample would quickly decay.

The stable group 11 elements, copper, silver, and gold, all have an outer electron configuration nd¹⁰(n+1)s¹. For each of these elements, the first excited state of their atoms 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 the atomic number increases, the excited levels are stabilized by relativistic effects and in gold the energy gap decreases again and it appears gold. For roentgenium, calculations indicate that the 6d⁹7s² level is stabilized to such an extent that it becomes the ground state and the 6d¹⁰7s¹ level becomes the first excited 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. The atomic radius of roentgenium is expected to be around 114 pm.

Experimental chemistry

Unambiguous determination of the chemical characteristics of roentgenium has yet to have been established due to the low yields of reactions that produce roentgenium isotopes. For chemical studies to be carried out on a transactinide, at least four atoms must be produced, the half-life of the isotope used must be at least 1 second, and the rate of production must be at least one atom per week. Even though the half-life of ²⁸¹Rg, the most stable known roentgenium isotope, is 26 seconds, long enough to perform chemical studies, another obstacle is the need to increase the rate of production of roentgenium isotopes and allow experiments to carry on for weeks or months so that statistically significant results can be obtained. Separation and detection must be carried out continuously to separate out the roentgenium isotopes and automated systems can then experiment on the gas-phase and solution chemistry of roentgenium as the yields for heavier elements are predicted to be smaller than those for lighter elements. However, the experimental chemistry of roentgenium has not received as much attention as that of the heavier elements copernicium and flerovium.