Ununennium

Ununennium, also known as eka-francium or element 119, is the hypothetical chemical element with symbol Uue and atomic number 119. Ununennium and Uue are the temporary systematic IUPAC name and symbol respectively, which are used until the element is discovered, confirmed, and a permanent name is decided upon. In the periodic table of the elements, it is expected to be an s-block element, an alkali metal, and the first element in the eighth period. It is the lightest element that has not yet been synthesized.

Ununennium, 119Uue
Ununennium
Pronunciation/ˌuːn.uːnˈɛniəm/ (listen) ​(OON-oon-EN-ee-əm)
Alternative nameselement 119, eka-francium
Ununennium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Ununennium Unbinilium
Unquadtrium Unquadquadium Unquadpentium Unquadhexium Unquadseptium Unquadoctium Unquadennium Unpentnilium Unpentunium Unpentbium Unpenttrium Unpentquadium Unpentpentium Unpenthexium Unpentseptium Unpentoctium Unpentennium Unhexnilium Unhexunium Unhexbium Unhextrium Unhexquadium Unhexpentium Unhexhexium Unhexseptium Unhexoctium Unhexennium Unseptnilium Unseptunium Unseptbium
 Unbiunium Unbibium Unbitrium Unbiquadium Unbipentium Unbihexium Unbiseptium Unbioctium Unbiennium Untrinilium Untriunium Untribium Untritrium Untriquadium Untripentium Untrihexium Untriseptium Untrioctium Untriennium Unquadnilium Unquadunium Unquadbium
 Fr
↑
Uue
↓
(Ust)
oganesson ← ununennium → unbinilium
Atomic number (Z)119
Groupgroup 1: hydrogen and alkali metals
Periodperiod 8
Block  s-block
Electron configuration[Og] 8s1 (predicted)[1]
Electrons per shell2, 8, 18, 32, 32, 18, 8, 1 (predicted)
Physical properties
Phase at STPunknown (could be solid or liquid)[1]
Melting point273–303 K ​(0–30 °C, ​32–86 °F) (predicted)[1]
Boiling point903 K ​(630 °C, ​1166 °F) (predicted)[2]
Density (near r.t.)3 g/cm3 (predicted)[1]
Heat of fusion2.01–2.05 kJ/mol (extrapolated)[3]
Atomic properties
Oxidation states(+1), (+3) (predicted)[1]
ElectronegativityPauling scale: 0.86 (predicted)[4]
Ionization energies
  • 1st: 463.1 kJ/mol
  • 2nd: 1698.1 kJ/mol
  • (predicted)[5]
Atomic radiusempirical: 240 pm (predicted)[1]
Covalent radius263–281 pm (extrapolated)[3]
Other properties
Crystal structure ​body-centered cubic (bcc)

(extrapolated)[6]
CAS Number54846-86-5
History
NamingIUPAC systematic element name

An attempt to synthesize the element has been ongoing since 2018 in RIKEN in Japan. The Joint Institute for Nuclear Research in Dubna, Russia, plans to make an attempt at some point in the future, but a precise date has not been released to the public. Theoretical and experimental evidence has shown that the synthesis of ununennium will likely be far more difficult than that of the previous elements, and it may even be the penultimate element that can be synthesized with current technology.

Ununennium's position as the seventh alkali metal suggests that it would have similar properties to its lighter congeners. However, relativistic effects may cause some of its properties to differ from those expected from a straight application of periodic trends. For example, ununennium is expected to be less reactive than caesium and francium and closer in behavior to potassium or rubidium, and while it should show the characteristic +1 oxidation state of the alkali metals, it is also predicted to show the +3 oxidation state, which is unknown in any other alkali metal.

Introduction
A graphic depiction of a nuclear fusion reaction. Two nuclei fuse into one, emitting a neutron. Thus far, reactions that created new elements were similar, with the only possible difference that several singular neutrons sometimes were released, or none at all.
External video
Visualization of unsuccessful nuclear fusion, based on calculations by the Australian National University[7]

The heaviest[lower-alpha 1] atomic nuclei are created in nuclear reactions that combine two other nuclei of unequal size[lower-alpha 2] into one; roughly, the more unequal the two nuclei in terms of mass, the greater the possibility that the two react.[13] 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.[14] 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.[14][15] 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,[lower-alpha 3] which carry away the energy. This occurs in approximately 10−16 seconds after the initial collision.[16][lower-alpha 4]

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.[19] In the separator, the newly produced nucleus is separated from other nuclides (that of the original beam and any other reaction products)[lower-alpha 5] 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.[19] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[22] The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.[19]

Stability of a nucleus is provided by the strong interaction. However, its range is very short; as nuclei become larger, their 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.[23] Nuclei of the heaviest elements are thus theoretically predicted[24] and have so far been observed[25] to primarily decay via decay modes that are caused by such repulsion: alpha decay and spontaneous fission;[lower-alpha 6] 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.[lower-alpha 7] Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.[lower-alpha 8]

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.[lower-alpha 9]

History

Superheavy elements are produced by nuclear fusion. These fusion reactions can be divided into "hot" and "cold" fusion,[lower-alpha 10] depending on the excitation energy of the compound nucleus produced. 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 fission, or alternatively evaporate several (3 to 5) neutrons.[38] In cold fusion reactions (which use heavier projectiles, typically from the fourth period, and lighter targets, usually lead and bismuth), the fused nuclei produced 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. However, hot fusion reactions tend to produce more neutron-rich products because the actinides have the highest neutron-to-proton ratios of any elements that can presently be made in macroscopic quantities.[39]

Ununennium and unbinilium (elements 119 and 120) are the elements with the lowest atomic numbers that have not yet been synthesized. Attempts to synthesize them would push the limits of current technology, due to the decreasing cross sections of the production reactions and their probably short half-lives,[40] expected to be on the order of microseconds.[1][41] Elements beyond unbiunium (element 121) would likely be too short-lived to be detected with current technology: they would decay within a microsecond, before reaching the detectors. The possibility of detection of elements 121 through 124 depends greatly on the theoretical model being used, as their half-lives are predicted to be very close to the one-microsecond border.[40] Previously, important help (characterized as "silver bullets") in the synthesis of superheavy elements came from the deformed nuclear shells around hassium-270 which increased the stability of surrounding nuclei, and the existence of the quasi-stable neutron-rich isotope calcium-48 which could be used as a projectile to produce more neutron-rich isotopes of superheavy elements.[42] The more neutron-rich a superheavy nuclide is, the closer it is expected to be to the sought-after island of stability.[lower-alpha 11] Even so, the synthesized isotopes still have fewer neutrons than those expected to be in the island of stability.[45] But using calcium-48 to synthesize ununennium would require a target of einsteinium-253 or -254, which are very difficult to produce in sufficiently large quantities (only micrograms are presently available; in comparison, milligrams of berkelium and californium are available). More practical production of further superheavy elements would require projectiles heavier than 48Ca.[42]

Past

The synthesis of ununennium was first attempted in 1985 by bombarding a target of einsteinium-254 with calcium-48 ions at the superHILAC accelerator at Berkeley, California:

254
99Es
 + 48
20Ca
 → 302
119Uue
* → no atoms

No atoms were identified, leading to a limiting cross section of 300 nb.[46] Later calculations suggest that the cross section of the 3n reaction (which would result in 299Uue and three neutrons as products) would actually be six hundred thousand times lower than this upper bound, at 0.5 pb.[47]

Following the claimed synthesis of 293Og in 1999 at the Lawrence Berkeley National Laboratory from 208Pb and 86Kr, the analogous reactions 209Bi + 86Kr and 208Pb + 87Rb were proposed for the synthesis of element 119 and its then-unknown alpha decay daughters, elements 117, 115, and 113.[48] The retraction of these results in 2001[49] and more recent calculations on the cross sections for "cold" fusion reactions cast doubt on this possibility; for example, a maximum yield of 2 fb is predicted for the production of 294Uue in the former reaction.[50] Radioactive ion beams may provide an alternative method utilizing a lead or bismuth target, and may enable the production of more neutron-rich isotopes should they become available at required intensities.[50]

From April to September 2012, an attempt to synthesize the isotopes 295Uue and 296Uue was made by bombarding a target of berkelium-249 with titanium-50 at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany.[51][52] This reaction between 249Bk and 50Ti was predicted to be the most favorable practical reaction for formation of ununennium,[52] as it is rather asymmetrical,[40] though also somewhat cold.[53] (The reaction between 254Es and 48Ca would be superior, but preparing milligram quantities of 254Es for a target is difficult.)[40] Moreover, as berkelium-249 decays to californium-249 (the next element) with a short half-life of 327 days, this allowed elements 119 and 120 to be searched for simultaneously.[54] Nevertheless, the necessary change from the "silver bullet" 48Ca to 50Ti divides the expected yield of ununennium by about twenty, as the yield is strongly dependent on the asymmetry of the fusion reaction.[40] Due to the predicted short half-lives, the GSI team used new "fast" electronics capable of registering decay events within microseconds.[52][40]

249
97Bk
 + 50
22Ti
 → 299
119Uue
* → no atoms
249
98Cf
 + 50
22Ti
 → 299
120Ubn
* → no atoms

Neither element 119 nor element 120 was observed. This implied a limiting cross-section of 65 fb for producing element 119 in these reactions, and 200 fb for element 120.[53][54] The predicted actual cross section for producing element 119 in this reaction is around 40 fb, which is at the limits of current technology.[40] (The record lowest cross section of an experimentally successful reaction is 30 fb for the reaction between 209Bi and 70Zn producing nihonium.)[40] The experiment was originally planned to continue to November 2012,[55] but was stopped early to make use of the 249Bk target to confirm the synthesis of tennessine (thus changing the projectiles to 48Ca).[53]

Present

In 2017, Hideto En'yo, director of the RIKEN Nishina Centre, predicted that elements 119 and 120 would probably be discovered in five years,[56] but this did not happen. The team at RIKEN in Wakō, Japan began bombarding curium-248 targets with a vanadium-51 beam in June 2018[57] to search for element 119. Curium was chosen as a target, rather than heavier berkelium or californium, as these heavier targets are difficult to prepare.[58] The reduced asymmetry of the reaction is expected to approximately halve the cross section, requiring a sensitivity "on the order of at least 30 fb".[54] The 248Cm targets were provided by Oak Ridge National Laboratory, which had provided the necessary 249Bk target for the synthesis of tennessine (element 117) at Dubna. The experiment began at a cyclotron while RIKEN upgraded its linear accelerators; the upgrade was completed in 2020.[59] Bombardment may be continued with both machines until the first event is observed; the experiment is currently running intermittently for at least 100 days per year.[57][58] The RIKEN team's efforts are being financed by the Emperor of Japan.[60]

248
96Cm
 + 51
23V
 → 299
119Uue
* → no atoms yet

The produced isotopes of ununennium are expected to undergo two alpha decays to known isotopes of moscovium (288Mc and 287Mc respectively), which would anchor them to a known sequence of five further alpha decays and corroborate their production. The predicted cross section for these reactions is about 10 fb.[58] In 2022, the optimal reaction energy for synthesis of ununennium in this reaction was experimentally estimated as 234.8±1.8 MeV at RIKEN.[61]

Planned

The team at the Joint Institute for Nuclear Research in Dubna, Russia, planned to begin new experiments on the synthesis of ununennium in 2019, using the 249Bk + 50Ti reaction and a new experimental complex.[62][63][64][65][66][67] However, this did not occur. The JINR still plans to attempt synthesis of elements 119, 120, and perhaps 121 in future, but a precise timeframe has not been publicly released.[68]

The laboratories at RIKEN in Japan and at the JINR in Russia are best suited to these experiments as they are the only ones in the world where long beam times are accessible for reactions with low predicted cross sections.[69]

Naming

Using Mendeleev's nomenclature for unnamed and undiscovered elements, ununennium should be known as eka-francium. Using the 1979 IUPAC recommendations, the element should be temporarily called ununennium (symbol Uue) until it is discovered, the discovery is confirmed, and a permanent name chosen.[70] Although widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, the recommendations are mostly ignored among scientists who work theoretically or experimentally on superheavy elements, who call it "element 119", with the symbol E119, (119) or 119.[1]

Predicted properties

Nuclear stability and isotopes
A chart of nuclide stability as used by the Dubna team in 2010. Characterized isotopes are shown with borders. Beyond element 118 (oganesson, the last known element), the line of known nuclides is expected to rapidly enter a region of instability, with no half-lives over one microsecond after element 121. The white ring encloses the predicted location of the island of stability.[40]
Orbitals with high azimuthal quantum number are raised in energy, eliminating what would otherwise be a gap in orbital energy corresponding to a closed proton shell at element 114, as shown in the left diagram which does not take this effect into account. This raises the next proton shell to the region around element 120, as shown in the right diagram, potentially increasing the half-lives of element 119 and 120 isotopes.[71]

The stability of nuclei decreases greatly with the increase in atomic number after curium, element 96, whose half-life is four orders of magnitude longer than that of any currently known higher-numbered element. All isotopes with an atomic number above 101 undergo radioactive decay with half-lives of less than 30 hours. No elements with atomic numbers above 82 (after lead) have stable isotopes.[72] Nevertheless, for reasons not yet well understood, there is a slight increase of nuclear stability around atomic numbers 110–114, which leads to the appearance of what is known in nuclear physics as the "island of stability". This concept, proposed by University of California professor Glenn Seaborg, explains why superheavy elements last longer than predicted.[73]

The alpha-decay half-lives predicted for 291–307Uue are on the order of microseconds. The longest alpha-decay half-life predicted is ~485 microseconds for the isotope 294Uue.[74][75][76] When factoring in all decay modes, the predicted half-lives drop further to only tens of microseconds.[1][41] Some heavier isotopes may be more stable; Fricke and Waber predicted 315Uue to be the most stable ununennium isotope in 1971.[2] This has consequences for the synthesis of ununennium, as isotopes with half-lives below one microsecond would decay before reaching the detector, and the heavier isotopes cannot be synthesised by the collision of any known usable target and projectile nuclei.[1][41] Nevertheless, new theoretical models show that the expected gap in energy between the proton orbitals 2f7/2 (filled at element 114) and 2f5/2 (filled at element 120) is smaller than expected, so that element 114 no longer appears to be a stable spherical closed nuclear shell, and this energy gap may increase the stability of elements 119 and 120. The next doubly magic nucleus is now expected to be around the spherical 306Ubb (element 122), but the expected low half-life and low production cross section of this nuclide makes its synthesis challenging.[71]

Atomic and physical

Being the first period 8 element, ununennium is predicted to be an alkali metal, taking its place in the periodic table below lithium, sodium, potassium, rubidium, caesium, and francium. Each of these elements has one valence electron in the outermost s-orbital (valence electron configuration ns1), which is easily lost in chemical reactions to form the +1 oxidation state: thus the alkali metals are very reactive elements. Ununennium is predicted to continue the trend and have a valence electron configuration of 8s1. It is therefore expected to behave much like its lighter congeners; however, it is also predicted to differ from the lighter alkali metals in some properties.[1]

The main reason for the predicted differences between ununennium and the other alkali metals is the spin–orbit (SO) interaction—the mutual interaction between the electrons' motion and spin. The SO interaction is especially strong for the superheavy elements because their electrons move faster—at speeds comparable to the speed of light—than those in lighter atoms.[77] In ununennium atoms, it lowers the 7p and 8s electron energy levels, stabilizing the corresponding electrons, but two of the 7p electron energy levels are more stabilized than the other four.[78] The effect is called subshell splitting, as it splits the 7p subshell into more-stabilized and the less-stabilized parts. Computational chemists understand the split as a change of the second (azimuthal) quantum number ℓ from 1 to 1⁄2 and 3⁄2 for the more-stabilized and less-stabilized parts of the 7p subshell, respectively.[77][lower-alpha 12] Thus, the outer 8s electron of ununennium is stabilized and becomes harder to remove than expected, while the 7p3/2 electrons are correspondingly destabilized, perhaps allowing them to participate in chemical reactions.[1] This stabilization of the outermost s-orbital (already significant in francium) is the key factor affecting ununennium's chemistry, and causes all the trends for atomic and molecular properties of alkali metals to reverse direction after caesium.[4]

Empirical (Na–Cs, Mg–Ra) and predicted (Fr–Uhp, Ubn–Uhh) atomic radii of the alkali and alkaline earth metals from the third to the ninth period, measured in angstroms[1][79]
Empirical (Na–Cs), semi-empirical (Fr), and predicted (Uue) electron affinities of the alkali metals from the third to the eighth period, measured in electron volts.[1][79] They decrease from Li to Cs, but the Fr value, 492±10 meV, is 20 meV higher than that of Cs, and that of Uue is much higher still at 662 meV.[80]
Empirical (Na–Fr, Mg–Ra) and predicted (Uue–Uhp, Ubn–Uhh) ionization energy of the alkali and alkaline earth metals from the third to the ninth period, measured in electron volts[1][79]

Due to the stabilization of its outer 8s electron, ununennium's first ionization energy—the energy required to remove an electron from a neutral atom—is predicted to be 4.53 eV, higher than those of the known alkali metals from potassium onward. This effect is so large that unbiunium (element 121) is predicted to have a lower ionization energy of 4.45 eV, so that the alkali metal in period 8 would not have the lowest ionization energy in the period, as is true for all previous periods.[1] Ununennium's electron affinity is expected to be far greater than that of caesium and francium; indeed, ununennium is expected to have an electron affinity higher than all the alkali metals lighter than it at about 0.662 eV, close to that of cobalt (0.662 eV) and chromium (0.676 eV).[80] Relativistic effects also cause a very large drop in the polarizability of ununennium[1] to 169.7 a.u.[81] Indeed, the static dipole polarisability (αD) of ununennium, a quantity for which the impacts of relativity are proportional to the square of the element's atomic number, has been calculated to be small and similar to that of sodium.[82]

The electron of the hydrogen-like ununennium atom—oxidized so it has only one electron, Uue118+—is predicted to move so quickly that its mass is 1.99 times that of a non-moving electron, a feature coming from the relativistic effects. For comparison, the figure for hydrogen-like francium is 1.29 and the figure for hydrogen-like caesium is 1.091.[77] According to simple extrapolations of relativity laws, that indirectly indicates the contraction of the atomic radius[77] to around 240 pm,[1] very close to that of rubidium (247 pm); the metallic radius is also correspondingly lowered to 260 pm.[1] The ionic radius of Uue+ is expected to be 180 pm.[1]

Ununennium is predicted to have a melting point between 0 °C and 30 °C: thus it may be a liquid at room temperature.[5] It is not known whether this continues the trend of decreasing melting points down the group, as caesium's melting point is 28.5 °C and francium's is estimated to be around 8.0 °C.[83] The boiling point of ununennium is expected to be around 630 °C, similar to that of francium, estimated to be around 620 °C; this is lower than caesium's boiling point of 671 °C.[2][83] The density of ununennium has been variously predicted to be between 3 and 4 g/cm3, continuing the trend of increasing density down the group: the density of francium is estimated at 2.48 g/cm3, and that of caesium is known to be 1.93 g/cm3.[2][3][83]

Chemical
Bond lengths and bond-dissociation energies of alkali metal dimers. Data for Fr2 and Uue2 are predicted.[84]
Dimer Bond length
(Å)		 Bond-dissociation
energy (kJ/mol)
Li2 2.673 101.9
Na2 3.079 72.04
K2 3.924 53.25
Rb2 4.210 47.77
Cs2 4.648 43.66
Fr2 ~ 4.61 ~ 42.1
Uue2 ~ 4.27 ~ 53.4

The chemistry of ununennium is predicted to be similar to that of the alkali metals,[1] but it would probably behave more like potassium[85] or rubidium[1] than caesium or francium. This is due to relativistic effects, as in their absence periodic trends would predict ununennium to be even more reactive than caesium and francium. This lowered reactivity is due to the relativistic stabilization of ununennium's valence electron, increasing ununennium's first ionization energy and decreasing the metallic and ionic radii;[85] this effect is already seen for francium.[1]

The chemistry of ununennium in the +1 oxidation state should be more similar to the chemistry of rubidium than to that of francium. On the other hand, the ionic radius of the Uue+ ion is predicted to be larger than that of Rb+, because the 7p orbitals are destabilized and are thus larger than the p-orbitals of the lower shells. Ununennium may also show the +3 oxidation state,[1] which is not seen in any other alkali metal,[86] in addition to the +1 oxidation state that is characteristic of the other alkali metals and is also the main oxidation state of all the known alkali metals: this is because of the destabilization and expansion of the 7p3/2 spinor, causing its outermost electrons to have a lower ionization energy than what would otherwise be expected.[1][86] Many ununennium compounds are expected to have a large covalent character, due to the involvement of the 7p3/2 electrons in the bonding: this effect is also seen to a lesser extent in francium, which shows some 6p3/2 contribution to the bonding in francium superoxide (FrO2).[77] Thus, instead of ununennium being the most electropositive element, as a simple extrapolation would seem to indicate, caesium instead retains this position, with ununennium's electronegativity most likely being close to sodium's (0.93 on the Pauling scale).[4] The standard reduction potential of the Uue+/Uue couple is predicted to be −2.9 V, the same as that of the Fr+/Fr couple and just over that of the K+/K couple at −2.931 V.[5]

Bond lengths and bond-dissociation energies of MAu (M = an alkali metal). All data are predicted, except for the bond-dissociation energies of KAu, RbAu, and CsAu.[4]
Compound Bond length
(Å)		 Bond-dissociation
energy (kJ/mol)
KAu 2.856 2.75
RbAu 2.967 2.48
CsAu 3.050 2.53
FrAu 3.097 2.75
UueAu 3.074 2.44

In the gas phase, and at very low temperatures in the condensed phase, the alkali metals form covalently bonded diatomic molecules. The metal–metal bond lengths in these M2 molecules increase down the group from Li2 to Cs2, but then decrease after that to Uue2, due to the aforementioned relativistic effects that stabilize the 8s orbital. The opposite trend is shown for the metal–metal bond-dissociation energies. The Uue–Uue bond should be slightly stronger than the K–K bond.[4][84] From these M2 dissociation energies, the enthalpy of sublimation (ΔHsub) of ununennium is predicted to be 94 kJ/mol (the value for francium should be around 77 kJ/mol).[4]

The UueF molecule is expected to have a significant covalent character owing to the high electron affinity of ununennium. The bonding in UueF is predominantly between a 7p orbital on ununennium and a 2p orbital on fluorine, with lesser contributions from the 2s orbital of fluorine and the 8s, 6dz2, and the two other 7p orbitals of ununennium. This is very different from the behaviour of s-block elements, as well as gold and mercury, in which the s-orbitals (sometimes mixed with d-orbitals) are the ones participating in the bonding. The Uue–F bond is relativistically expanded due to the splitting of the 7p orbital into 7p1/2 and 7p3/2 spinors, forcing the bonding electrons into the largest orbital measured by radial extent: a similar expansion in bond length is found in the hydrides AtH and TsH.[87] The Uue–Au bond should be the weakest of all bonds between gold and an alkali metal, but should still be stable. This gives extrapolated medium-sized adsorption enthalpies (−ΔHads) of 106 kJ/mol on gold (the francium value should be 136 kJ/mol), 76 kJ/mol on platinum, and 63 kJ/mol on silver, the smallest of all the alkali metals, that demonstrate that it would be feasible to study the chromatographic adsorption of ununennium onto surfaces made of noble metals.[4] The enthalpy of adsorption of ununennium on a Teflon surface is predicted to be 17.6 kJ/mol, which would be the lowest among the alkali metals: this information would be very useful for future chemistry experiments on ununennium.[81] The ΔHsub and −ΔHads values for the alkali metals change in opposite directions as atomic number increases.[4]

See also

Notes
  1. 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[8] or 112;[9] sometimes, the term is presented an equivalent to the term "transactinide", which puts an upper limit before the beginning of the hypothetical superactinide series).[10] 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.
  2. 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.[11] 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.[12]
  3. 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.[16]
  4. 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.[17] This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.[18]
  5. 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.[20] 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.[21]
  6. Not all decay modes are caused by electrostatic repulsion. For example, beta decay is caused by the weak interaction.[26]
  7. 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.[27] The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL.[28] 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).[29]
  8. Spontaneous fission was discovered by Soviet physicist Georgy Flerov,[30] a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility.[31] 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.[18] They thus preferred to link new isotopes to the already known ones by successive alpha decays.[30]
  9. For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics in Stockholm, Stockholm County, Sweden.[32] 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.[33] The following year, LBNL was unable to reproduce the Swedish results and announced instead their synthesis of the element; that claim was also disproved later.[33] JINR insisted that they were the first to create the element and suggested a name of their own for the new element, joliotium;[34] the Soviet name was also not accepted (JINR later referred to the naming of element 102 as "hasty").[35] The name "nobelium" remained unchanged on account of its widespread usage.[36]
  10. Despite the name, "cold fusion" in the context of superheavy element synthesis is a distinct concept from the idea that nuclear fusion can be achieved in room temperature conditions (see cold fusion).[37]
  11. Stable isotopes of the lightest elements usually have a neutron–proton ratio close or equal to one (for example, the only stable isotope of aluminium has 13 protons and 14 neutrons,[43] making a neutron–proton ratio of 1.077). However, isotopes of heavier elements have higher neutron–proton ratios, increasing with the number of protons (iodine's only stable isotope has 53 protons and 74 neutrons, neutron–proton ratio of 1.396; gold's only stable isotope has 79 protons and 118 neutrons, neutron–proton ratio of 1.494; plutonium's most stable isotope has 94 protons and 150 neutrons, neutron–proton ratio of 1.596).[43] The trend is expected to continue to the superheavy elements,[44] making it difficult to synthesize their most stable isotopes, because the neutron–proton ratios of the elements they are synthesized from are lower than the expected ratios of the most stable isotopes of the superheavy elements.
  12. The quantum number corresponds to the letter in the electron orbital name: 0 to s, 1 to p, 2 to d, etc. See azimuthal quantum number for more information.

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Bibliography
  • The dictionary definition of ununennium at Wiktionary

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