Roentgenium (German:[ʁœntˈɡeːni̯ʊm]ⓘ) is asynthetic chemical element; it hassymbolRg andatomic number 111. It is extremely radioactive and can only be created in a laboratory. The most stable known isotope, roentgenium-282, has ahalf-life of 130 seconds, although the unconfirmed roentgenium-286 may have a longer half-life of about 10.7 minutes. Roentgenium was first created in December 1994 by theGSI Helmholtz Centre for Heavy Ion Research nearDarmstadt, Germany. It is named after the physicistWilhelm Röntgen (also spelled Roentgen), who discoveredX-rays. Only a few roentgenium atoms have ever been synthesized, and they have no practical application.
In theperiodic table, it is ad-blocktransactinide element. It is a member of the7th period and is placed in thegroup 11 elements, although no chemical experiments have been carried out to confirm that it behaves as the heavierhomologue togold in group 11 as the ninth member of the 6d series oftransition metals. Roentgenium is calculated to have similar properties to its lighter homologues,copper,silver, andgold, although it may show some differences from them.
A graphic depiction of anuclear fusion reaction. Two nuclei fuse into one, emitting aneutron. 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.
A superheavy[a]atomic nucleus is created in a nuclear reaction that combines two other nuclei of unequal size[b] into one; roughly, the more unequal the two nuclei in terms ofmass, the greater the possibility that the two react.[18] The material made of the heavier nuclei is made into a target, which is then bombarded by thebeam of lighter nuclei. Two nuclei can onlyfuse into one if they approach each other closely enough; normally, nuclei (all positively charged) repel each other due toelectrostatic repulsion. Thestrong interaction can overcome this repulsion but only within a very short distance from a nucleus; beam nuclei are thus greatlyaccelerated in order to make such repulsion insignificant compared to the velocity of the beam nucleus.[19] The energy applied to the beam nuclei to accelerate them can cause them to reach speeds as high as one-tenth of thespeed of light. However, if too much energy is applied, the beam nucleus can fall apart.[19]
Coming close enough alone is not enough for two nuclei to fuse: when two nuclei approach each other, they usually remain together for about 10−20 seconds and then part ways (not necessarily in the same composition as before the reaction) rather than form a single nucleus.[19][20] This happens because during the attempted formation of a single nucleus, electrostatic repulsion tears apart the nucleus that is being formed.[19] Each pair of a target and a beam is characterized by itscross section—the probability that fusion will occur if two nuclei approach one another expressed in terms of the transverse area that the incident particle must hit in order for the fusion to occur.[c] This fusion may occur as a result of the quantum effect in which nuclei cantunnel through electrostatic repulsion. If the two nuclei can stay close past that phase, multiple nuclear interactions result in redistribution of energy and an energy equilibrium.[19]
The resulting merger is anexcited state[23]—termed acompound nucleus—and thus it is very unstable.[19] To reach a more stable state, the temporary merger mayfission without formation of a more stable nucleus.[24] Alternatively, the compound nucleus may eject a fewneutrons, which would carry away the excitation energy; if the latter is not sufficient for a neutron expulsion, the merger would produce agamma ray. This happens in about 10−16 seconds after the initial nuclear collision and results in creation of a more stable nucleus.[24] The definition by theIUPAC/IUPAP Joint Working Party (JWP) states that achemical element can only be recognized as discovered if a nucleus of it has notdecayed within 10−14 seconds. This value was chosen as an estimate of how long it takes a nucleus to acquireelectrons and thus display its chemical properties.[25][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.[27] 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 asurface-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.[27] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[30] The nucleus is recorded again once its decay is registered, and the location, theenergy, and the time of the decay are measured.[27]
Stability of a nucleus is provided by the strong interaction. However, its range is very short; as nuclei become larger, its influence on the outermostnucleons (protons and neutrons) weakens. At the same time, the nucleus is torn apart by electrostatic repulsion between protons, and its range is not limited.[31] Totalbinding energy provided by the strong interaction increases linearly with the number of nucleons, whereas electrostatic repulsion increases with the square of the atomic number, i.e. the latter grows faster and becomes increasingly important for heavy and superheavy nuclei.[32][33] Superheavy nuclei are thus theoretically predicted[34] and have so far been observed[35] to predominantly decay via decay modes that are caused by such repulsion:alpha decay andspontaneous fission.[f] Almost all alpha emitters have over 210 nucleons,[37] and the lightest nuclide primarily undergoing spontaneous fission has 238.[38] In both decay modes, nuclei are inhibited from decaying by correspondingenergy barriers for each mode, but they can be tunneled through.[32][33]
Scheme of an apparatus for creation of superheavy elements, based on the Dubna Gas-Filled Recoil Separator set up in theFlerov Laboratory of Nuclear Reactions in JINR. The trajectory within the detector and the beam focusing apparatus changes because of adipole magnet in the former andquadrupole magnets in the latter.[39]
Alpha particles are commonly produced in radioactive decays because the mass of an alpha particle per nucleon is small enough to leave some energy for the alpha particle to be used as kinetic energy to leave the nucleus.[40] Spontaneous fission is caused by electrostatic repulsion tearing the nucleus apart and produces various nuclei in different instances of identical nuclei fissioning.[33] As the atomic number increases, spontaneous fission rapidly becomes more important: spontaneous fission partial half-lives decrease by 23 orders of magnitude fromuranium (element 92) tonobelium (element 102),[41] and by 30 orders of magnitude fromthorium (element 90) tofermium (element 100).[42] The earlierliquid drop model thus suggested that spontaneous fission would occur nearly instantly due to disappearance of thefission barrier for nuclei with about 280 nucleons.[33][43] The laternuclear shell model suggested that nuclei with about 300 nucleons would form anisland of stability in which nuclei will be more resistant to spontaneous fission and will primarily undergo alpha decay with longer half-lives.[33][43] Subsequent discoveries suggested that the predicted island might be further than originally anticipated; they also showed that nuclei intermediate between the long-lived actinides and the predicted island are deformed, and gain additional stability from shell effects.[44] Experiments on lighter superheavy nuclei,[45] as well as those closer to the expected island,[41] have shown greater than previously anticipated stability against spontaneous fission, showing the importance of shell effects on nuclei.[g]
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 easily determined.[h] (That all decays within a decay chain were indeed related to each other is established by the location of these decays, which must be in the same place.)[27] The known nucleus can be recognized by the specific characteristics of decay it undergoes such as decay energy (or more specifically, thekinetic energy of the emitted particle).[i] Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.[j]
The information available to physicists aiming to synthesize a superheavy element 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.[k]
This reaction had previously been conducted at theJoint Institute for Nuclear Research inDubna (then in theSoviet Union) in 1986, but no atoms of272111 had then been observed.[57] In 2001, theIUPAC/IUPAP Joint Working Party (JWP) concluded that there was insufficient evidence for the discovery at that time.[58] The GSI team repeated their experiment in 2002 and detected three more atoms.[59][60] In their 2003 report, the JWP decided that the GSI team should be acknowledged for the discovery of this element.[61]
Backdrop for presentation of the discovery and recognition of roentgenium at GSI Darmstadt
UsingMendeleev's nomenclature for unnamed and undiscovered elements, roentgenium should be known aseka-gold. In 1979, IUPAC published recommendations according to which the element was to be calledunununium (with the corresponding symbol ofUuu),[62] asystematic element name as aplaceholder, 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 itelement 111, with the symbol ofE111,(111) or even simply111.[2]
The nameroentgenium (Rg) was suggested by the GSI team[63] in 2004, to honor the German physicistWilhelm Conrad Röntgen, the discoverer ofX-rays.[63] This name was accepted byIUPAC on November 1, 2004.[63]
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 unconfirmedmetastable states. All of these decay through alpha decay or spontaneous fission,[65] though280Rg may also have anelectron capture branch.[66]
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 unconfirmed286Rg 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 unconfirmed283Rg appears to have a long half-life of about 5.1 minutes. The isotopes280Rg and281Rg have also been reported to have half-lives over a second. The remaining isotopes have half-lives in the millisecond range.[65]
The missing isotopes between274Rg and278Rg are too light to be produced by hot fusion and too heavy to be produced by cold fusion. A possible synthesis method is to populate them from above, as daughters of nihonium or moscovium isotopes that can be produced by hot fusion.[67] The isotopes283Rg and284Rg could be synthesised using charged-particle evaporation, using the238U+48Ca reaction where a proton is evaporated alongside some neutrons.[68][69]
Other than nuclear properties, no properties of roentgenium or its compounds have been measured; this is due to its extremely limited and expensive production[18] 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 oftransition metals.[70] Calculations on itsionization potentials andatomic andionic radii are similar to that of its lighter homologuegold, thus implying that roentgenium's basic properties will resemble those of the othergroup 11 elements,copper,silver, andgold; however, it is also predicted to show several differences from its lighter homologues.[2]
Roentgenium is predicted to be anoble metal. Thestandard 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 ionization energy of 1020 kJ/mol almost matches that of thenoble gasradon at 1037 kJ/mol.[2] Its predicted second ionization energy, 2070 kJ/mol, is almost the same as that ofsilver. 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:[2] nevertheless, theelectron affinity of roentgenium is expected to be around 1.6 eV (150 kJ/mol), significantly lower than gold's value of 2.3 eV (220 kJ/mol), so roentgenides may not be stable or even possible.[71]
The 6d orbitals are destabilized byrelativistic effects andspin–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 ingold 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,RgF− 6 is expected to be more stable thanRgF− 4, which is expected to be more stable thanRgF− 2.[2] The stability ofRgF− 6 is homologous to that ofAuF− 6; the silver analogueAgF− 6 is unknown and is expected to be only marginally stable to decomposition toAgF− 4 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.[72] Roentgenium(I) is expected to be difficult to obtain.[2][73][74] Gold readily forms thecyanidecomplexAu(CN)− 2, which is used in its extraction from ore through the process ofgold cyanidation; roentgenium is expected to follow suit and formRg(CN)− 2.[75]
The probable chemistry of roentgenium has received more interest than that of the two previous elements,meitnerium anddarmstadtium, as the valence s-subshells of the group 11 elements are expected to be relativistically contracted most strongly at roentgenium.[2] 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 (68 kJ/mol). The compoundsAuX and RgX, where X =F,Cl,Br,O, Au, or Rg, were also studied.[2][76] Rg+ is predicted to be thesoftest metal ion, even softer than Au+, although there is disagreement on whether it would behave as anacid or abase.[77][78] In aqueous solution, Rg+ would form theaqua ion [Rg(H2O)2]+, with an Rg–O bond distance of 207.1 pm. It is also expected to form Rg(I) complexes withammonia,phosphine, andhydrogen sulfide.[78]
Roentgenium is expected to be a solid under normal conditions and to crystallize in thebody-centered cubic structure, unlike its lightercongeners which crystallize in theface-centered cubic structure, due to its being expected to have different electron charge densities from them.[3] It should be a very heavy metal with adensity of around 22–24 g/cm3; in comparison, the densest known element that has had its density measured,osmium, has a density of 22.61 g/cm3.[4][5] The atomic radius of roentgenium is expected to be around 114 pm.[7]
Unambiguous determination of the chemical characteristics of roentgenium has yet to have been established[79] due to the low yields of reactions that produce roentgenium isotopes.[2] For chemical studies to be carried out on atransactinide, 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.[70] Even though the half-life of282Rg, 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 fromcopernicium tolivermorium,[2][79][80] despite early interest in theoretical predictions due to relativistic effects on thens subshell in group 11 reaching a maximum at roentgenium.[2] The isotopes280Rg and281Rg are promising for chemical experimentation and may be produced as the granddaughters of themoscovium isotopes288Mc and289Mc respectively;[81] their parents are thenihonium isotopes284Nh and285Nh, which have already received preliminary chemical investigations.[39]
^Innuclear physics, an element is calledheavy 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 than103 (although there are other definitions, such as atomic number greater than100[13] or112;[14] sometimes, the term is presented an equivalent to the term "transactinide", which puts an upper limit before the beginning of the hypotheticalsuperactinide series).[15] 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 the JINR led by Oganessian published results of their attempt to create hassium in a symmetric136Xe + 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.[16] 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.[17]
^The amount of energy applied to the beam particle to accelerate it can also influence the value of cross section. For example, in the28 14Si +1 0n →28 13Al +1 1p reaction, cross section changes smoothly from 370 mb at 12.3 MeV to 160 mb at 18.3 MeV, with a broad peak at 13.5 MeV with the maximum value of 380 mb.[21]
^This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.[26]
^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.[28] Such separation can also be aided by atime-of-flight measurement and a recoil energy measurement; a combination of the two may allow to estimate the mass of a nucleus.[29]
^It was already known by the 1960s that ground states of nuclei differed in energy and shape as well as that certain magic numbers of nucleons corresponded to greater stability of a nucleus. However, it was assumed that there was no nuclear structure in superheavy nuclei as they were too deformed to form one.[41]
^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 superheavy nuclei.[46] The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL.[47] 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).[48]
^If the decay occurred in a vacuum, then since total momentum of an isolated system before and after the decaymust be preserved, the daughter nucleus would also receive a small velocity. The ratio of the two velocities, and accordingly the ratio of the kinetic energies, would thus be inverse to the ratio of the two masses. The decay energy equals the sum of the known kinetic energy of the alpha particle and that of the daughter nucleus (an exact fraction of the former).[37] The calculations hold for an experiment as well, but the difference is that the nucleus does not move after the decay because it is tied to the detector.
^Spontaneous fission was discovered by Soviet physicistGeorgy Flerov,[49] a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility.[50] 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.[26] They thus preferred to link new isotopes to the already known ones by successive alpha decays.[49]
^For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics inStockholm,Stockholm County,Sweden.[51] 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.[52] 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.[52] JINR insisted that they were the first to create the element and suggested a name of their own for the new element,joliotium;[53] the Soviet name was also not accepted (JINR later referred to the naming of the element 102 as "hasty").[54] This name was proposed to IUPAC in a written response to their ruling on priority of discovery claims of elements, signed 29 September 1992.[54] The name "nobelium" remained unchanged on account of its widespread usage.[55]
^Different sources give different values for half-lives; the most recently published values are listed.
^abcdefghijklmnHoffman, Darleane C.; Lee, Diana M.; Pershina, Valeria (2006). "Transactinides and the future elements". In Morss; Edelstein, Norman M.; Fuger, Jean (eds.).The Chemistry of the Actinide and Transactinide Elements (3rd ed.). Dordrecht, The Netherlands:Springer Science+Business Media.ISBN978-1-4020-3555-5.
^abGyanchandani, Jyoti; Sikka, S. K. (May 10, 2011). "Physical properties of the 6 d -series elements from density functional theory: Close similarity to lighter transition metals".Physical Review B.83 (17) 172101.Bibcode:2011PhRvB..83q2101G.doi:10.1103/PhysRevB.83.172101.
^abKratz; Lieser (2013).Nuclear and Radiochemistry: Fundamentals and Applications (3rd ed.). p. 631.
^abHoffman, Darleane C.; Lee, Diana M.; Pershina, Valeria (2010). "Transactinide Elements and Future Elements".The Chemistry of the Actinide and Transactinide Elements. Dordrecht: Springer Netherlands.doi:10.1007/978-94-007-0211-0_14.ISBN978-94-007-0210-3.
^Oganessian, Yuri Ts.; Abdullin, F. Sh.; Alexander, C.; Binder, J.; et al. (May 30, 2013). "Experimental studies of the249Bk + 48Ca reaction including decay properties and excitation function for isotopes of element 117, and discovery of the new isotope277Mt".Physical Review C.87 (054621). American Physical Society.Bibcode:2013PhRvC..87e4621O.doi:10.1103/PhysRevC.87.054621.
^abcdHofmann, 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.Cite error: The named reference "Hofmann2016-EXON-Remarks" was defined multiple times with different content (see thehelp page).
^"Популярная библиотека химических элементов. Сиборгий (экавольфрам)" [Popular library of chemical elements. Seaborgium (eka-tungsten)].n-t.ru (in Russian). RetrievedJanuary 7, 2020. Reprinted from"Экавольфрам" [Eka-tungsten].Популярная библиотека химических элементов. Серебро – Нильсборий и далее [Popular library of chemical elements. Silver through nielsbohrium and beyond] (in Russian).Nauka. 1977.
^Hofmann, S.; Ninov, V.; Heßberger, F.P.; Armbruster, P.; Folger, H.; Münzenberg, G.; Schött, H. J.; Popeko, A. G.; Yeremin, A. V.; Andreyev, A. N.; Saro, S.; Janik, R.; Leino, M. (1995). "The new element 111".Zeitschrift für Physik A.350 (4):281–282.Bibcode:1995ZPhyA.350..281H.doi:10.1007/BF01291182.S2CID18804192.
^abSonzogni, Alejandro."Interactive Chart of Nuclides". National Nuclear Data Center: Brookhaven National Laboratory. Archived fromthe original on July 28, 2018. RetrievedJune 6, 2008.
^Hong, J.; Adamian, G. G.; Antonenko, N. V.; Jachimowicz, P.; Kowal, M. (April 26, 2023).Interesting fusion reactions in superheavy region(PDF). IUPAP Conference "Heaviest nuclei and atoms". Joint Institute for Nuclear Research. RetrievedJuly 30, 2023.
^Conradie, 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.S2CID189944098.
^Seth, M.; Cooke, F.; Schwerdtfeger, P.; Heully, J.-L.; Pelissier, M. (1998). "The chemistry of the superheavy elements. II. The stability of high oxidation states in group 11 elements: Relativistic coupled cluster calculations for the di-, tetra- and hexafluoro metallates of Cu, Ag, Au, and element 111".J. Chem. Phys.109 (10):3935–43.Bibcode:1998JChPh.109.3935S.doi:10.1063/1.476993.hdl:2292/5208.S2CID54803557.
^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. Vol. 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.
^abDüllmann, Christoph E. (2012). "Superheavy elements at GSI: a broad research program with element 114 in the focus of physics and chemistry".Radiochimica Acta.100 (2):67–74.doi:10.1524/ract.2011.1842.S2CID100778491.
^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.ISBN978-3-642-37466-1.
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