A graphic depiction of a nuclear fusion reaction. Two nuclei fuse into one, emitting a neutron. Reactions that created new elements to this moment were similar, with the only possible difference that several singular neutrons sometimes were released, or none at all.
The heaviest[a]atomic nuclei are created in nuclear reactions that combine two other nuclei of unequal size[b] into one; roughly, the more unequal the two nuclei in terms of mass, the greater the possibility that the two react. The material made of the heavier nuclei is made into a target, which is then bombarded by the beam of lighter nuclei. Two nuclei can only fuse into one if they approach each other closely enough; normally, nuclei (all positively charged) repel each other due to electrostatic repulsion. The strong interaction can overcome this repulsion but only within a very short distance from a nucleus; beam nuclei are thus greatly accelerated in order to make such repulsion insignificant compared to the velocity of the beam nucleus. Coming close alone is not enough for two nuclei to fuse: when two nuclei approach each other, they usually remain together for approximately 10-20 seconds and then part ways (not necessarily in the same composition as before the reaction) rather than form a single nucleus. If fusion does occur, the temporary merger--termed a compound nucleus--is an excited state. To lose its excitation energy and reach a more stable state, a compound nucleus either fissions or ejects one or several neutrons,[c] which carry away the energy. This occurs in approximately 10-16 seconds after the initial collision.[d]
The beam passes through the target and reaches the next chamber, the separator; if a new nucleus is produced, it is carried with this beam. In the separator, the newly produced nucleus is separated from other nuclides (that of the original beam and any other reaction products)[e] and transferred to a surface-barrier detector, which stops the nucleus. The exact location of the upcoming impact on the detector is marked; also marked are its energy and the time of the arrival. The transfer takes about 10-6 seconds; in order to be detected, the nucleus must survive this long. The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.
Stability of a nucleus is provided by the strong interaction. However, its range is very short; as nuclei become larger, its influence on the outermost nucleons (protons and neutrons) weakens. At the same time, the nucleus is torn apart by electrostatic repulsion between protons, as it has unlimited range. Nuclei of the heaviest elements are thus theoretically predicted and have so far been observed to primarily decay via decay modes that are caused by such repulsion: alpha decay and spontaneous fission;[f] these modes are predominant for nuclei of superheavy elements. Alpha decays are registered by the emitted alpha particles, and the decay products are easy to determine before the actual decay; if such a decay or a series of consecutive decays produces a known nucleus, the original product of a reaction can be determined arithmetically.[g] Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.[h]
The information available to physicists aiming to synthesize one of the heaviest elements is thus the information collected at the detectors: location, energy, and time of arrival of a particle to the detector, and those of its decay. The physicists analyze this data and seek to conclude that it was indeed caused by a new element and could not have been caused by a different nuclide than the one claimed. Often, provided data is insufficient for a conclusion that a new element was definitely created and there is no other explanation for the observed effects; errors in interpreting data have been made.[i]
This reaction had previously been conducted at the Joint Institute for Nuclear Research in Dubna (then in the Soviet Union) in 1986, but no atoms of 272Rg had then been observed. 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.
Using Mendeleev's nomenclature for unnamed and undiscovered elements, roentgenium should be known as eka-gold. In 1979, IUPAC published recommendations according to which the element was to be called unununium (with the corresponding symbol of Uuu), a systematic element name as a placeholder, until the element was discovered (and the discovery then confirmed) and a permanent name was decided on. Although widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, the recommendations were mostly ignored among scientists in the field, who called it element 111, with the symbol of E111, (111) or even simply 111.
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. Nine different isotopes of roentgenium have been reported with atomic masses 272, 274, 278-283, and 286 (283 and 286 unconfirmed), two of which, roentgenium-272 and roentgenium-274, have known but unconfirmed metastable states. All of these decay through alpha decay or spontaneous fission, though 280Rg may also have an electron capture branch.
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, 282Rg, is also the heaviest known roentgenium isotope; it has a half-life of 100 seconds. The unconfirmed 286Rg is even heavier and appears to have an even longer half-life of about 10.7 minutes, which would make it one of the longest-lived superheavy nuclides known; likewise, the unconfirmed 283Rg appears to have a long half-life of about 5.1 minutes. The isotopes 280Rg and 281Rg have also been reported to have half-lives over a second. The remaining isotopes have half-lives in the millisecond range.
No properties of roentgenium or its compounds have been measured; this is due to its extremely limited and expensive production and the fact that roentgenium (and its parents) decays very quickly. Properties of roentgenium metal remain unknown and only predictions are available.
Roentgenium is the ninth member of the 6d series of transition metals. Since copernicium (element 112) has been shown to be a group 12 metal, it is expected that all the elements from 104 to 111 would continue 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. The standard electrode potential of 1.9 V for the Rg3+/Rg couple is greater than that of 1.5 V for the Au3+/Au couple. Roentgenium's predicted first ionisation energy of 1020 kJ/mol almost matches that of the noble gasradon at 1037 kJ/mol. Based on the most stable oxidation states of the lighter group 11 elements, roentgenium is predicted to show stable +5 and +3 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 it has been suggested roentgenium may do so as well: nevertheless, the electron affinity of roentgenium is expected to be around 1.6 eV (37 kcal/mol), significantly lower than gold's value of 2.3 eV (53 kcal/mol), so roentgenides may not be stable or even possible. The 6d orbitals are destabilized by relativistic effects and spin-orbit interactions near the end of the fourth transition metal series, thus making the high oxidation state roentgenium(V) more stable than its lighter homologue gold(V) (known only in gold pentafluoride, Au2F10) 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, is expected to be more stable than , which is expected to be more stable than . The stability of is homologous to that of ; the silver analogue is unknown and is expected to be only marginally stable to decomposition to and F2. Moreover, Rg2F10 is expected to be stable to decomposition, exactly analogous to the Au2F10, whereas Ag2F10 should be unstable to decomposition to Ag2F6 and F2. Gold heptafluoride, AuF7, is known as a gold(V) difluorine complex AuF5·F2, which is lower in energy than a true gold(VII) heptafluoride would be; RgF7 is instead calculated to be more stable as a true roentgenium(VII) heptafluoride, although it would be somewhat unstable, its decomposition to Rg2F10 and F2 releasing a small amount of energy at room temperature. Roentgenium(I) is expected to be difficult to obtain. Gold readily forms the cyanidecomplex , which is used in its extraction from ore through the process of gold cyanidation; roentgenium is expected to follow suit and form .
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 (16 kcal/mol). The compounds AuX and RgX, where X = F, Cl, Br, O, Au, or Rg, were also studied. Rg+ is predicted to be the softest metal ion, even softer than Au+, although there is disagreement on whether it would behave as an acid or a base. In aqueous solution, Rg+ would form the aqua ion [Rg(H2O)2]+, with an Rg-O bond distance of 207.1 pm. It is also expected to form Rg(I) complexes with ammonia, phosphine, and hydrogen sulfide.
Physical and atomic
Roentgenium is expected to be a solid under normal conditions and to crystallize in the body-centered cubic structure, unlike its lighter congeners which crystallize in the face-centered cubic structure, due to its being expected to have different electron charge densities from them. 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 138 pm.
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 282Rg, the most stable confirmed roentgenium isotope, is 100 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 allow automated systems to 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 from copernicium to livermorium, despite early interest in theoretical predictions due to relativistic effects on the ns subshell in group 11 reaching a maximum at roentgenium. The isotopes 280Rg and 281Rg are promising for chemical experimentation and may be produced as the granddaughters of the moscovium isotopes 288Mc and 289Mc respectively; their parents are the nihonium isotopes 284Nh and 285Nh, which have already received preliminary chemical investigations.
^In nuclear physics, an element is called heavy if its atomic number is high; lead (element 82) is one example of such a heavy element. The term "superheavy elements" typically refers to elements with atomic number greater than 103 (although there are other definitions, such as atomic number greater than 100 or 112; sometimes, the term is presented an equivalent to the term "transactinide", which puts an upper limit before the beginning of the hypothetical superactinide series). Terms "heavy isotopes" (of a given element) and "heavy nuclei" mean what could be understood in the common language--isotopes of high mass (for the given element) and nuclei of high mass, respectively.
^In 2009, a team at JINR led by Oganessian published results of their attempt to create hassium in a symmetric 136Xe + 136Xe reaction. They failed to observe a single atom in such a reaction, putting the upper limit on the cross section, the measure of probability of a nuclear reaction, as 2.5 pb. In comparison, the reaction that resulted in hassium discovery, 208Pb + 58Fe, had a cross section of ~20 pb (more specifically, 19+19 -11 pb), as estimated by the discoverers.
^The greater the excitation energy, the more neutrons are ejected. If the excitation energy is lower than energy binding each neutron to the rest of the nucleus, neutrons are not emitted; instead, the compound nucleus de-excites by emitting a gamma ray.
^The definition by the IUPAC/IUPAP Joint Working Party states that a chemical element can only be recognized as discovered if a nucleus of it has not decayed within 10-14 seconds. This value was chosen as an estimate of how long it takes a nucleus to acquire its outer electrons and thus display its chemical properties. This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.
^This separation is based on that the resulting nuclei move past the target more slowly then the unreacted beam nuclei. The separator contains electric and magnetic fields whose effects on a moving particle cancel out for a specific velocity of a particle. Such separation can also be aided by a time-of-flight measurement and a recoil energy measurement; a combination of the two may allow to estimate the mass of a nucleus.
^Since mass of a nucleus is not measured directly but is rather calculated from that of another nucleus, such measurement is called indirect. Direct measurements are also possible, but for the most part they have remained unavailable for heaviest nuclei. The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL. Mass was determined from the location of a nucleus after the transfer (the location helps determine its trajectory, which is linked to the mass-to-charge ratio of the nucleus, since the transfer was done in presence of a magnet).
^Spontaneous fission was discovered by Soviet physicist Georgy Flerov, a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility. In contrast, the LBL scientists believed fission information was not sufficient for a claim of synthesis of an element. They believed spontaneous fission had not been studied enough to use it for identification of a new element, since there was a difficulty of establishing that a compound nucleus had only ejected neutrons and not charged particles like protons or alpha particles. They thus preferred to link new isotopes to the already known ones by successive alpha decays.
^For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics in Stockholm, Stockholm County, Sweden. There were no earlier definitive claims of creation of this element, and the element was assigned a name by its Swedish, American, and British discoverers, nobelium. It was later shown that the identification was incorrect. The following year, RL was unable to reproduce the Swedish results and announced instead their synthesis of the element; that claim was also disproved later. JINR insisted that they were the first to create the element and suggested a name of their own for the new element, joliotium; the Soviet name was also not accepted (JINR later referred to the naming of element 102 as "hasty"). The name "nobelium" remained unchanged on account of its widespread usage.
^Different sources give different values for half-lives; the most recently published values are listed.
^ abConradie, Jeanet; Ghosh, Abhik (June 15, 2019). "Theoretical Search for the Highest Valence States of the Coinage Metals: Roentgenium Heptafluoride May Exist". Inorganic Chemistry. 2019 (58): 8735-8738. doi:10.1021/acs.inorgchem.9b01139. PMID31203606.
^Oganessian, Yuri Ts.; Abdullin, F. Sh.; Alexander, C.; et al. (May 30, 2013). "Experimental studies of the 249Bk + 48Ca reaction including decay properties and excitation function for isotopes of element 117, and discovery of the new isotope 277Mt". Physical Review C. American Physical Society. 87 (054621). Bibcode:2013PhRvC..87e4621O. doi:10.1103/PhysRevC.87.054621.
^ abHofmann, S.; Heinz, S.; Mann, R.; et al. (2016). "Remarks on the Fission Barriers of SHN and Search for Element 120". In Peninozhkevich, Yu. E.; Sobolev, Yu. G. (eds.). Exotic Nuclei: EXON-2016 Proceedings of the International Symposium on Exotic Nuclei. Exotic Nuclei. pp. 155-164. doi:10.1142/9789813226548_0024. ISBN9789813226555.
^"? ? ? . ()" [Popular library of chemical elements. Seaborgium (eka-tungsten)]. n-t.ru (in Russian). Retrieved 2020. Reprinted from "" [Eka-tungsten]. ? ? ? . ? -- ? ? [Popular library of chemical elements. Silver through nielsbohrium and beyond] (in Russian). Nauka. 1977.
^Barber, R. C.; Greenwood, N. N.; Hrynkiewicz, A. Z.; Jeannin, Y. P.; Lefort, M.; Sakai, M.; Ulehla, I.; Wapstra, A. P.; Wilkinson, D. H. (1993). "Discovery of the transfermium elements. Part II: Introduction to discovery profiles. Part III: Discovery profiles of the transfermium elements". Pure and Applied Chemistry. 65 (8): 1757. doi:10.1351/pac199365081757. (Note: for Part I see Pure Appl. Chem., Vol. 63, No. 6, pp. 879-886, 1991)
^Liu, W.; van Wüllen, C. (1999). "Spectroscopic constants of gold and eka-gold (element 111) diatomic compounds: The importance of spin-orbit coupling". J. Chem. Phys. 110 (8): 3730-5. Bibcode:1999JChPh.110.3730L. doi:10.1063/1.478237.
^Thayer, John S. (2010). Relativistic Effects and the Chemistry of the Heavier Main Group Elements. Relativistic Methods for Chemists. Challenges and Advances in Computational Chemistry and Physics. 10. p. 82. doi:10.1007/978-1-4020-9975-5_2. ISBN978-1-4020-9974-8.
^ abHancock, Robert D.; Bartolotti, Libero J.; Kaltsoyannis, Nikolas (November 24, 2006). "Density Functional Theory-Based Prediction of Some Aqueous-Phase Chemistry of Superheavy Element 111. Roentgenium(I) Is the 'Softest' Metal Ion". Inorg. Chem. 45 (26): 10780-5. doi:10.1021/ic061282s. PMID17173436.
^Moody, Ken (November 30, 2013). "Synthesis of Superheavy Elements". In Schädel, Matthias; Shaughnessy, Dawn (eds.). The Chemistry of Superheavy Elements (2nd ed.). Springer Science & Business Media. pp. 24-8. ISBN9783642374661.
^Aksenov, Nikolay V.; Steinegger, Patrick; Abdullin, Farid Sh.; Albin, Yury V.; Bozhikov, Gospodin A.; Chepigin, Viktor I.; Eichler, Robert; Lebedev, Vyacheslav Ya.; Mamudarov, Alexander Sh.; Malyshev, Oleg N.; Petrushkin, Oleg V.; Polyakov, Alexander N.; Popov, Yury A.; Sabel'nikov, Alexey V.; Sagaidak, Roman N.; Shirokovsky, Igor V.; Shumeiko, Maksim V.; Starodub, Gennadii Ya.; Tsyganov, Yuri S.; Utyonkov, Vladimir K.; Voinov, Alexey A.; Vostokin, Grigory K.; Yeremin, Alexander; Dmitriev, Sergey N. (July 2017). "On the volatility of nihonium (Nh, Z = 113)". The European Physical Journal A. 53 (158): 158. Bibcode:2017EPJA...53..158A. doi:10.1140/epja/i2017-12348-8.