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Ununennium, 119Uue
Theoretical element
Ununennium
Pronunciation/ˌuːn.uːnˈɛniəm/ ⓘ ​(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
↓
—
oganesson ← ununennium → unbinilium
Atomic number (Z)119
Groupgroup 1: hydrogen and alkali metals
Periodperiod 8 (theoretical, extended table)
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 phase (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 statescommon: (none)
(+1), (+3), (+5)[1][4]
ElectronegativityPauling scale: 0.86 (predicted)[5]
Ionization energies
  • 1st: 463.1 kJ/mol
  • 2nd: 1698.1 kJ/mol
  • (predicted)[6]
Atomic radiusempirical: 240 pm (predicted)[1]
Covalent radius263–281 pm (extrapolated)[3]
Other properties
Crystal structure ​body-centered cubic (bcc)
Body-centered cubic crystal structure for ununennium

(extrapolated)[7]
CAS Number54846-86-5
History
NamingIUPAC systematic element name
Isotopes of ununennium
Experiments and theoretical calculations
| references

Ununennium, also known[8] as eka-francium or element 119, is a hypothetical chemical element; it has symbol Uue and atomic number 119. Ununennium and Uue are the temporary systematic IUPAC name and symbol respectively, which are used until the element has been 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.

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. The Heavy Ion Research Facility in Lanzhou, China (HIRFL) also plans to make an attempt. Theoretical and experimental evidence has shown that the synthesis of ununennium will likely be far more difficult than that of the previous elements.

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 and +5 oxidation states, which are unknown in any other alkali metal.

Introduction

[edit]
This section is an excerpt from Superheavy element § Introduction.[edit]

Synthesis of superheavy nuclei

[edit]
A graphic depiction of a nuclear fusion reaction
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.

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 of mass, the greater the possibility that the two react.[14] 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.[15] The energy applied to the beam nuclei to accelerate them can cause them to reach speeds as high as one-tenth of the speed of light. However, if too much energy is applied, the beam nucleus can fall apart.[15]

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.[15][16] This happens because during the attempted formation of a single nucleus, electrostatic repulsion tears apart the nucleus that is being formed.[15] Each pair of a target and a beam is characterized by its cross 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 can tunnel 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.[15]

External videos
video icon Visualization of unsuccessful nuclear fusion, based on calculations from the Australian National University[18]

The resulting merger is an excited state[19]—termed a compound nucleus—and thus it is very unstable.[15] To reach a more stable state, the temporary merger may fission without formation of a more stable nucleus.[20] Alternatively, the compound nucleus may eject a few neutrons, which would carry away the excitation energy; if the latter is not sufficient for a neutron expulsion, the merger would produce a gamma ray. This happens in about 10−16 seconds after the initial nuclear collision and results in creation of a more stable nucleus.[20] The definition by the IUPAC/IUPAP Joint Working Party (JWP) 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 electrons and thus display its chemical properties.[21][d]

Decay and detection

[edit]

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.[23] 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.[23] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[26] The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.[23]

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, and its range is not limited.[27] Total binding 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.[28][29] Superheavy nuclei are thus theoretically predicted[30] and have so far been observed[31] to predominantly decay via decay modes that are caused by such repulsion: alpha decay and spontaneous fission.[f] Almost all alpha emitters have over 210 nucleons,[33] and the lightest nuclide primarily undergoing spontaneous fission has 238.[34] In both decay modes, nuclei are inhibited from decaying by corresponding energy barriers for each mode, but they can be tunneled through.[28][29]

Apparatus for creation of superheavy elements
Scheme of an apparatus for creation of superheavy elements, based on the Dubna Gas-Filled Recoil Separator set up in the Flerov Laboratory of Nuclear Reactions in JINR. The trajectory within the detector and the beam focusing apparatus changes because of a dipole magnet in the former and quadrupole magnets in the latter.[35]

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.[36] Spontaneous fission is caused by electrostatic repulsion tearing the nucleus apart and produces various nuclei in different instances of identical nuclei fissioning.[29] As the atomic number increases, spontaneous fission rapidly becomes more important: spontaneous fission partial half-lives decrease by 23 orders of magnitude from uranium (element 92) to nobelium (element 102),[37] and by 30 orders of magnitude from thorium (element 90) to fermium (element 100).[38] The earlier liquid drop model thus suggested that spontaneous fission would occur nearly instantly due to disappearance of the fission barrier for nuclei with about 280 nucleons.[29][39] The later nuclear shell model suggested that nuclei with about 300 nucleons would form an island of stability in which nuclei will be more resistant to spontaneous fission and will primarily undergo alpha decay with longer half-lives.[29][39] 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.[40] Experiments on lighter superheavy nuclei,[41] as well as those closer to the expected island,[37] 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.)[23] The known nucleus can be recognized by the specific characteristics of decay it undergoes such as decay energy (or more specifically, the kinetic 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]

History

[edit]

Synthesis attempts

[edit]
Main article: Isotopes of ununennium

Elements 114 to 118 (flerovium through oganesson) were discovered in "hot fusion" reactions at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia. This involved bombarding the actinides plutonium through californium with calcium-48, a quasi-stable neutron-rich isotope which could be used as a projectile to produce more neutron-rich isotopes of superheavy elements.[52] (The term "hot" refers to the high excitation energy of the resulting compound nucleus.) This cannot easily be continued to element 119, because it would require a target of the next actinide einsteinium. Tens of milligrams of einsteinium would be needed for a reasonable chance of success, but only micrograms have so far been produced.[53] An attempt to make element 119 from calcium-48 and less than a microgram of einsteinium was made in 1985 at the superHILAC accelerator at Berkeley, California, but did not succeed.[54]

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

More practical production of further superheavy elements requires projectiles heavier than 48Ca,[52] but this makes the reaction more symmetric[55] and gives it a smaller chance of success.[53] Attempts to synthesize element 119 push the limits of current technology, due to the decreasing cross sections of the production reactions and the probably short half-lives of produced isotopes,[56] expected to be on the order of microseconds.[1][57]

From April to September 2012, an attempt to synthesize 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.[58][59] This reaction between 249Bk and 50Ti was predicted to be the most favorable practical reaction for formation of ununennium,[59] as it is the most asymmetric reaction available.[56] 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.[55] Due to the predicted short half-lives, the GSI team used new "fast" electronics capable of registering decay events within microseconds.[59][56]

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

Neither element 119 nor element 120 was observed.[60][55] The experiment was originally planned to continue to November 2012,[61] but was stopped early to make use of the 249Bk target to confirm the synthesis of tennessine (thus changing the projectile to 48Ca).[60]

The curium oxide targets used by RIKEN for the search for element 119[62]

The team at RIKEN in Wakō, Japan began bombarding curium-248 targets with a vanadium-51 beam in January 2018[63] 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.[64] The 248Cm targets were provided by Oak Ridge National Laboratory. RIKEN developed a high-intensity vanadium beam.[53] The experiment began at a cyclotron while RIKEN upgraded its linear accelerators; the upgrade was completed in 2020.[65] Bombardment may be continued with both machines until the first event is observed.[66][64] The RIKEN team's efforts are being financed by the Emperor of Japan.[67]

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, 287Mc and 288Mc. This would anchor them to a known sequence of five or six further alpha decays, respectively, and corroborate their production.[63][68]

As of September 2023, the team at RIKEN had run the 248Cm+51V reaction for 462 days. A report by the RIKEN Nishina Center Advisory Committee noted that this reaction was chosen because of the availability of the target and projectile materials, despite predictions favoring the 249Bk+50Ti reaction, because the 50Ti projectile is closer to doubly magic 48Ca and has an even atomic number (22); reactions with even-Z projectiles have generally been shown to have greater cross-sections.[69] (With that said, 249Bk has the disadvantage of a short half-life.)[62] The report recommended that if the 5 fb cross-section limit is reached without any events observed, then the team should "evaluate and eventually reconsider the experimental strategy before taking additional beam time."[69] As of August 2024, the team at RIKEN was still running this reaction "24/7".[62]

The team at the JINR plans to attempt synthesis of element 119 in the future, but a precise timeframe has not been publicly released.[70] In late 2023, the JINR reported the first successful synthesis of a superheavy element with a projectile heavier than 48Ca: 238U was bombarded with 54Cr to make a new isotope of livermorium (element 116), 288Lv. Successful synthesis of a superheavy nuclide in this experiment was an unexpectedly good result; the aim was to experimentally determine the cross-section of a reaction with 54Cr projectiles and prepare for the synthesis of element 120.[71] The JINR has also alluded to a future attempt to synthesize element 119 with the same projectile, bombarding 243Am with 54Cr.[72] The team at the Heavy Ion Research Facility in Lanzhou (HIRFL), which is operated by the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences, also plans to try the 243Am+54Cr reaction.[73][74]

Naming

[edit]

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.[75] 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

[edit]

Nuclear stability and isotopes

[edit]
A 2D graph with rectangular cells colored in black-and-white colors, spanning from the llc to the urc, with cells mostly becoming lighter closer to the latter
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.[56]
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.[76]

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.[77] 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.[78]

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.[79][80][81] When factoring in all decay modes, the predicted half-lives drop further to only tens of microseconds.[1][57] 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][57] 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.[76]

The most likely isotopes of ununennium to be synthesised in the near future are 293Uue through 296Uue, because they are populated in the 3n and 4n channels of the 243Am+48Cr and 249Bk+50Ti reactions.[82]

Atomic and physical

[edit]

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.[83] 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.[84] 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.[83][l] 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.[5]

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][85]
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][85] 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.[8]
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][85]

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).[8] Relativistic effects also cause a very large drop in the polarizability of ununennium[1] to 169.7 a.u.[86] 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.[87]

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 consequence of relativistic effects. For comparison, the figure for hydrogen-like francium is 1.29 and the figure for hydrogen-like caesium is 1.091.[83] According to simple extrapolations of relativity laws, that indirectly indicates the contraction of the atomic radius[83] 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.[6] 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.[88] 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][88] 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][88]

Chemical

[edit]
Bond lengths and bond-dissociation energies of alkali metal dimers. Data for Fr2 and Uue2 are predicted.[89]
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[90] 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;[90] 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,[91] 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][91] The 7p3/2 spinor's chemical activity has been suggested to make the +5 oxidation state possible in [UueF6]−, analogous to [SbF6]− or [BrF6]−. The analogous francium(V) compound, [FrF6]−, might also be achievable, but is not experimentally known.[4]

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 (FrO<2).[83] Thus, instead of ununennium being the most electropositive element, as a simple extrapolation would seem to indicate, caesium retains this position, with ununennium's electronegativity most likely being close to sodium's (0.93 on the Pauling scale).[5] 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.[6]

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.[5]
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.[5][89] 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).[5]

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 7p<3/2 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.[92] 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.[5] 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.[86] The ΔHsub and −ΔHads values for the alkali metals change in opposite directions as atomic number increases.[5]

See also

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Ununennium

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Ununennium (Uue) is the provisional systematic name assigned by the International Union of Pure and Applied Chemistry (IUPAC) to the hypothetical with 119, positioned as the lightest member of the group in the periodic table. This undiscovered element, expected to initiate the eighth period, remains unsynthesized despite intensive global efforts, as its production requires overcoming extreme nuclear instability through high-energy fusion reactions in specialized accelerators. Theoretical models predict ununennium to exhibit a ground-state of [Og] 8s¹, where Og denotes (element 118), placing a single in the 8s orbital and suggesting +1 as its primary , akin to other alkali metals like and sodium. However, due to its high , relativistic effects are anticipated to contract the 8s orbital significantly, potentially reducing its reactivity compared to (element 87) and aligning its chemical behavior more closely with lighter elements such as or , while possibly enabling unusual oxidation states like +3. All isotopes of ununennium are forecasted to be highly radioactive with exceedingly short half-lives—estimated in microseconds or less for the most stable candidates—rendering it incapable of occurrence and limiting opportunities for detailed study of its physical properties, such as a predicted around 3–3.7 g/cm³ and an unknown . Current synthesis attempts, led by institutions like Japan's , involve "hot fusion" techniques, such as accelerating vanadium-51 ions toward curium-248 targets to form compound nuclei that may decay into ununennium isotopes, though cross-sections for such reactions are predicted to be vanishingly small, on the order of picobarns, necessitating years of beam time. Alternative approaches, including titanium-50 on berkelium-249 or even multi-step reactions involving oganesson-294, are under theoretical and experimental exploration to enhance production yields, with potential breakthroughs anticipated in the coming years that could extend the periodic table and probe the "" for nuclei.

Introduction

Overview and significance

Ununennium is the systematic temporary name designated by the International Union of Pure and Applied Chemistry (IUPAC) for the hypothetical with 119 and Uue. This follows IUPAC's protocol for undiscovered elements beyond 103, derived from Latin roots indicating the digits of the (un-un-enn-ium for 1-1-9). Predicted to be the lightest member of a new series of elements, ununennium is expected to behave as an in group 1, with its in the 8s orbital, potentially marking the start of period 8 in the periodic table. Superheavy elements are chemical elements with atomic numbers of 104 or higher and are exclusively produced through nuclear reactions in particle accelerators and do not occur naturally. Ununennium would extend this sequence beyond (element 118), the heaviest confirmed element, representing a critical step in mapping the uncharted territory of the periodic table's eighth row. The significance of ununennium lies in its potential to test fundamental theories of , including the boundaries of nuclear stability and the long-hypothesized —a region around atomic numbers 114 to 126 where isotopes with specific "magic" proton and neutron numbers may exhibit unusually long half-lives due to closed nuclear shells. Furthermore, as one of the heaviest conceivable elements, its synthesis would enable detailed study of relativistic quantum effects, which intensify with increasing and profoundly alter orbitals, ionization potentials, and chemical bonding compared to lighter homologues like . As of November 2025, ununennium remains unsynthesized amid active international research efforts at laboratories including in and the GSI Helmholtz Centre in , underscoring the technical challenges in achieving viable fusion reactions for such extreme nuclei.

Position in the periodic table

Ununennium, denoted by the 119, occupies the position of the first element in period 8 and of the periodic table, thereby belonging to the alkali metals series. Its predicted ground-state is [Og] 8s¹, where Og signifies the closed-shell core of (element 118), consistent with the filling of the 8s orbital at the start of the eighth row. As the heaviest alkali metal, ununennium is anticipated to align with the general group trends observed in its lighter homologues—lithium, sodium, potassium, rubidium, caesium, and francium—where metallic character and chemical reactivity progressively increase descending the group due to larger atomic radii and lower ionization energies. However, at this extreme end of the series, these properties are modified by pronounced relativistic effects stemming from the high nuclear charge, which contract the 8s orbitals and elevate the first ionization potential relative to francium, potentially making ununennium less reactive than expected from non-relativistic extrapolations. Ununennium is firmly classified within the s-block of the periodic table, reflecting its single in the 8s subshell, in contrast to the closure of the p-block at , which completes the 7p⁶ configuration of period 7. The ensuing elements from 120 to 126 are projected to fill toward completion of the 8p subshell, marking the onset of a novel transition series influenced by competing orbital energies in the superheavy regime.

History

Theoretical predictions and early concepts

In the early 20th century, theoretical extensions of the periodic table beyond were pioneered by physicists incorporating emerging quantum models of atomic structure. , in his 1922 formulation of the periodic system based on configurations, predicted the existence of elements up to atomic number =118, envisioning a analog to with the configuration 2,8,18,32,32,18,8 and suggesting potential instability for Z>92 due to nuclear limitations, though he speculated on transuranic possibilities. This approach, building on Sommerfeld's refinements, imposed an upper limit near Z=137 from relativistic considerations of electron-nucleus interactions, laying foundational concepts for superheavy extensions without direct nuclear predictions. The and marked a shift toward nuclear shell models predicting enhanced stability for superheavy elements. introduced the "" concept in 1969, forecasting longer-lived isotopes around Z=114–126 due to closed proton and neutron shells near N=184, contrasting the fission-prone actinides and inspiring searches for relatively stable transuranic nuclei. This idea, rooted in semi-empirical mass formulas and shell corrections, evolved from earlier work and suggested that elements in this region could exhibit half-lives extending to seconds or longer, fueling optimism for their accessibility. Advancements in the and incorporated to refine property predictions, revealing significant challenges for high- elements. Relativistic Dirac-Fock methods, as applied in calculations up to Z=120, demonstrated strong spin-orbit coupling destabilizing outer shells, particularly for Z=119, where the 8s orbital experiences extreme relativistic contraction and the 8p_{1/2} level dives below, altering expected alkali-metal behavior and complicating chemical analogies to . These computations highlighted how relativity amplifies electronic distortions in superheavies, shifting focus from stability alone to integrated nuclear-electronic effects. Key milestones included the 1971 Mendeleev centennial predictions by Fricke, Greiner, and Waber, which used Dirac-Slater methods to propose periodic table extensions to Z=172 with detailed radii and configurations for superheavies, and the 1997 IUPAC adoption of systematic , designating element 119 as ununennium (Uue) or eka-francium in provisional terms to standardize references for undiscovered species. By the , theoretical models evolved from initial optimism to emphasize synthesis barriers, integrating fusion-evaporation cross-section estimates with relativistic structure calculations. Predictions indicated cross-sections for Z=119 below 1 picobarn, orders of magnitude smaller than for prior elements, due to barriers and neutron deficiencies, underscoring the need for advanced accelerators despite persistent island-of-stability hopes. This recognition tempered expectations, prioritizing multi-disciplinary simulations over empirical stability assumptions.

Experimental synthesis attempts

In the 2000s, collaborations at the (JINR) in , , primarily focused on synthesizing elements 113 through 118 using beams on targets, with preparatory work laying the groundwork for heavier elements like 119, though no specific attempts for Z=119 were reported during this period. During the 2010s, GSI conducted dedicated searches for ununennium using the TASCA separator, bombarding berkelium-249 targets with titanium-50 projectiles in experiments running from 2011 to 2012; no decay events attributable to element 119 were observed, establishing an upper limit on the production cross section of approximately 70 fb. Concurrently, JINR initiated efforts toward superheavy elements beyond , including test runs for Z=119 reactions such as titanium-50 on berkelium-249, but these early campaigns at the DC280 produced no confirmed syntheses due to insufficient statistics. In 2018, the Nishina Center in launched a major campaign to produce ununennium by fusing vanadium-51 beams with curium-248 targets at the GARIS separator, utilizing high-intensity beams up to 10^13 particles per second; despite several irradiation periods through 2024, no confirmed events have been detected, with challenges including extremely low expected cross sections below 1 picobarn. JINR's Superheavy Element Factory, operational since 2019, has supported further attempts with titanium-50 on berkelium-249 and similar combinations, but as of late 2023, these experiments have not yielded any identifiable ununennium isotopes. As of November 2025, ongoing campaigns at continue with upgraded accelerator capabilities aiming for beam fluxes exceeding 6 × 10^{12} ions per second over extended periods, yet no synthesis of ununennium has been confirmed, highlighting persistent obstacles such as minuscule fusion probabilities and limited availability of target materials. These efforts underscore the technical demands of production, where beam times and detection efficiencies constrain the accumulation of .

Synthesis and detection

Proposed synthesis methods

Ununennium, the provisional name for element 119, has not yet been synthesized, but several hot fusion reactions have been proposed and pursued experimentally to create it by fusing lighter nuclei. These methods involve accelerating beams of medium-mass ions, such as or , onto actinide targets like or , forming a compound nucleus that undergoes neutron evaporation to yield an isotope of ununennium. The cross sections for these reactions are predicted to be extremely low, on the order of picobarns to femtobarns, necessitating high-intensity beams and long irradiation times. The primary ongoing experimental attempt is at RIKEN in Japan, utilizing the reaction 51V+248Cm→299119∗→294Uue+5n^{51}\mathrm{V} + ^{248}\mathrm{Cm} \to ^{299}119^* \to ^{294}\mathrm{Uue} + 5n, where the compound nucleus evaporates multiple neutrons to form the evaporation residue. This hot fusion approach was initiated in 2018 following upgrades to the RIKEN gas-filled recoil separator and cyclotron, with beam energies optimized around 10.5 MeV per nucleon to maximize fusion probability. As of 2025, no ununennium atoms have been detected despite extensive irradiation, but the effort continues with curium targets supplied by Oak Ridge National Laboratory, highlighting the logistical challenges of producing and handling milligram quantities of heavy actinides. Theoretical calculations suggest a maximum cross section of approximately 50 fb for the 4n or 5n channel, underscoring the need for beam intensities exceeding 101310^{13} particles per second. The Joint Institute for Nuclear Research (JINR) in Dubna, Russia, plans future attempts, potentially using similar reactions. Other proposed reactions include 50Ti+249Bk→299119∗→294Uue+5n^{50}\mathrm{Ti} + ^{249}\mathrm{Bk} \to ^{299}119^* \to ^{294}\mathrm{Uue} + 5n, which was tested at GSI Helmholtz Centre in but yielded no detections, with predicted cross sections around 10–20 fb. Theoretical studies also favor 54Cr+243Am→297119∗→292Uue+5n^{54}\mathrm{Cr} + ^{243}\mathrm{Am} \to ^{297}119^* \to ^{292}\mathrm{Uue} + 5n due to its relatively higher survival probability against fission, estimated at 3–27 fb, potentially suitable for facilities like the (JINR) in , though no specific experimental campaign has been confirmed for this combination as of late 2025. Earlier concepts, such as beams on targets (48Ca+251Es^{48}\mathrm{Ca} + ^{251}\mathrm{Es}), were deemed impractical due to the scarcity of . A novel theoretical strategy, proposed in 2024, involves a dual-target setup to exploit freshly produced . In this approach, a 48Ca^{48}\mathrm{Ca} beam first fuses with 249Cf^{249}\mathrm{Cf} to form 294Og^{294}\mathrm{Og}, which then interacts with a downstream or target in a secondary reaction, such as 294Og+2H→295119+n+γ^{294}\mathrm{Og} + ^{2}\mathrm{H} \to ^{295}119 + n + \gamma, potentially increasing production rates by leveraging the short-lived intermediate. While innovative, this method requires validation of the secondary fusion efficiency and has not yet been implemented experimentally.

Detection challenges and techniques

Detecting ununennium atoms after synthesis presents formidable challenges due to their predicted extremely low production cross-sections, on the order of 0.01 to 0.1 picobarns, resulting in yields of potentially one atom per month or less in high-intensity accelerator experiments. These minuscule rates are compounded by short half-lives estimated at less than 1 second for candidate isotopes, necessitating rapid and precise separation and detection to observe decay signatures before the atom disintegrates. Additionally, high from of transfermium products, of beam particles, and induced reactions in the target material complicates signal isolation, demanding advanced to discriminate genuine evaporation residues from contaminants. To address these issues, gas-filled separators are employed to isolate heavy residues from the intense beam and light reaction products. Facilities such as the GARIS (Gas-filled Ion Separator) at in and the DGFRS-2 at the Flerov Laboratory of Nuclear Reactions in , , utilize a low-pressure or gas medium to enhance the charge state of recoiling ions, enabling magnetic and electric deflection based on their and rigidity. Similarly, the SHANS (Spectrometer for Heavy Atoms and Nuclear Structure) at the Heavy Ion Research Facility in , , features a quadrupole-dipole-quadrupole configuration optimized for transmitting residues with efficiencies up to 40-50% while suppressing beam ions by factors exceeding 10^12. These separators transport the residues over distances of several meters to a detection station, where time-of-flight measurements using secondary electron foils provide information, and energy-loss detectors (such as gas or silicon-based) further characterize the ions' and mass. Upon implantation into a position-sensitive silicon detector array, the residues are genetically identified through correlated alpha decay chains that terminate in established isotopes of lighter superheavy elements. For proposed ununennium isotopes like ^{294}Uue or ^{295}Uue, theoretical models predict sequential alpha emissions leading to daughters such as ^{290}Ubh, ^{286}Uut, and eventually linking to known nuclei around moscovium (element 115) or nihonium (element 113), with characteristic energies in the 10-12 MeV range for initial alphas and half-lives spanning milliseconds to seconds. Detection setups record implantation events followed by prompt alpha particles or electron capture (EC) decays, using multi-wire proportional counters or silicon strips to achieve spatial resolutions of ~100 μm and energy resolutions better than 20 keV, enabling the reconstruction of full decay sequences from single-atom events. Recent advancements have mitigated some detection hurdles through enhanced (DSP) techniques, which improve timing resolution to sub-microsecond levels and reduce pile-up effects in high-rate environments by real-time waveform analysis. Cryogenic stopping cells, such as those integrated with the Super-FRS at , employ gas at low temperatures to gently thermalize and buffer ions, facilitating delayed decay studies. These innovations collectively boost sensitivity for . Confirmation of ununennium's discovery adheres to IUPAC guidelines, which emphasize independent replication and robust statistical evidence; historically, recognition has required at least three corroborated decay events from separate laboratories exhibiting consistent chain characteristics, cross-sections, and excitation functions to rule out artifacts.

Predicted properties

Nuclear stability and isotopes

Ununennium (Z = 119) is situated near predicted deformed shell closures in the (SHE) region, particularly at neutron number N = 184, where microscopic shell corrections enhance nuclear binding and potentially increase stability against fission and . However, neutron-deficient isotopes of ununennium are expected to be highly unstable due to low fission barriers, with and competing as dominant modes. Theoretical models, such as the finite-range droplet macroscopic model (FRDM) combined with folded-Yukawa single-particle potentials, indicate that the shell effects at Z = 119 are weaker than at Z = 114 or Z = 120, leading to reduced stability for most accessible isotopes. Relativistic effects in the nuclear potential further lower binding energies, exacerbating instability in this region. Predicted half-lives for ununennium isotopes are generally short, on the order of microseconds to milliseconds, with alpha decay being the primary mode for lighter isotopes. For example, the isotope ^{294}Uue (N = 175) is predicted to undergo alpha decay with a Q_α value of approximately 11–12 MeV and a half-life of about 10^{-3} s. Neutron-rich isotopes closer to the N = 184 shell closure, such as ^{303}Uue, may exhibit slightly longer half-lives due to enhanced binding from shell effects, potentially reaching seconds if synthesized, though spontaneous fission barriers remain low, limiting overall stability. These predictions are derived from systematic calculations using the effective liquid drop model (ELDM) and density-dependent cluster models, which account for deformation and pairing effects in SHEs. Heavier isotopes beyond N = 184 are expected to favor spontaneous fission, with half-lives dropping to microseconds or less due to increased fission probabilities. Theoretical models cover approximately 15–20 hypothetical isotopes of ununennium, spanning mass numbers A = 282 to A = 310. The lightest isotopes (A ≈ 282–290) are highly neutron-deficient and unstable, with half-lives dominated by chains leading to known nuclei. Intermediate isotopes like ^{294}Uue and ^{295}Uue have half-lives around 0.5–1 ms, primarily via . Near the presumed at N ≈ 184 (A ≈ 303), isotopes such as ^{302}Uue and ^{303}Uue show marginal improvements in half-life (up to ~1–10 s in optimistic models), but relativistic reductions in binding and low fission barriers prevent significant stability. For heavier isotopes (A ≈ 305–310), becomes the dominant decay mode, with half-lives < 10^{-6} s, as predicted by macroscopic-microscopic approaches incorporating shell corrections. Ununennium thus serves as a bridge to potentially more stable elements at Z = 120–126, but its isotopes remain challenging for experimental observation due to rapid decay. Nuclear binding energies for SHEs like ununennium are modeled using adaptations of the (SEMF), which includes , surface, , , and terms, augmented by shell corrections to account for quantum effects near closures: B(A,Z)≈avA−asA2/3−acZ(Z−1)A1/3−aa(A−2Z)2A+δ+ΔshellB(A,Z) \approx a_v A - a_s A^{2/3} - a_c \frac{Z(Z-1)}{A^{1/3}} - a_a \frac{(A-2Z)^2}{A} + \delta + \Delta_{\text{shell}} Here, typical coefficients are a_v ≈ 15.5 MeV, a_s ≈ 16.8 MeV, a_c ≈ 0.717 MeV, a_a ≈ 23.285 MeV, and δ is the pairing term; the shell correction Δ_shell is crucial for SHEs, reaching several MeV near N = 184 in deformed configurations. This formula, refined in models like FRDM(2012), allows estimation of Q_α = B(A-4,Z-2) + B(A,Z) - B(A,Z) and fission barriers, highlighting the role of deformation in ununennium's marginal stability.
Representative IsotopeNPredicted Dominant Decay ModeApproximate Half-LifeNotes
^{294}Uue175α decay (Q_α ≈ 11.5 MeV)~10^{-3} sNeutron-deficient, short-lived chain
^{303}Uue184α decay or SF~1–10 s (optimistic)Near shell closure, enhanced binding
^{310}Uue191<10^{-6} sHeavy, fission-dominated
These values are illustrative, based on averaged model predictions; actual synthesis would target neutron-deficient isotopes, while neutron-rich ones remain hypothetical.

Atomic and electronic structure

Ununennium, with 119, is predicted to have a ground-state electronic configuration of [Og]8s1[ \mathrm{Og} ] 8s^1, where the core of (element 118) consists of the closed shells [Rn]5f146d107s27p6[\mathrm{Rn}] 5f^{14} 6d^{10} 7s^2 7p^6. This configuration places ununennium in group 1 of the periodic table, consistent with its expected behavior as an analog, though modified by relativistic influences. Calculations using relativistic methods confirm this filling order, with the occupying the 8s orbital following the closure of the 7p shell in oganesson. Relativistic effects play a dominant role in shaping ununennium's electronic structure due to its high nuclear charge. The strong spin-orbit interaction causes significant splitting of the 7p subshell into 7p1/27p_{1/2} and 7p3/27p_{3/2} components, with the 7p1/27p_{1/2} orbital stabilized by approximately 10 eV relative to the non-relativistic case. This splitting stabilizes the 8s orbital through contraction and penetration effects, while destabilizing the 8p1/28p_{1/2} orbital, potentially inverting the conventional Aufbau filling sequence for superheavy elements. Such effects arise from the high velocities of inner electrons approaching the , leading to a overall contraction of s and p_{1/2} orbitals. The first ionization potential of ununennium is calculated to be approximately 4.78 eV, slightly higher than that of (4.07 eV), owing to the relativistic stabilization and contraction of the 8s valence orbital. This value was obtained using relativistic high-order coupled-cluster theory, incorporating Dirac-Hartree-Fock as the reference and including quantum electrodynamic corrections. The is predicted to be around 240 pm, notably smaller than caesium's 265 pm, as a result of the extended and additional relativistic contraction in the 8s orbital. Theoretical predictions rely on advanced quantum mechanical methods, such as the Dirac-Coulomb-Breit Hamiltonian for multi-electron atoms, which accounts for relativistic , magnetic interactions, and retardation effects to compute orbital energies EnljE_{nlj}. These calculations, often performed via Dirac-Hartree-Fock approaches with correlation corrections, provide the basis for understanding ununennium's atomic structure and highlight the limitations of non-relativistic models for systems.

Physical and chemical properties

Ununennium is predicted to be a solid at , with a estimated at 3–4 g/cm³, reflecting the increasing trend down but moderated by its large atomic size. Its is expected to be low, in the range of 0–30 °C, potentially making it the first alkali metal to be liquid near , while the is forecasted around 630 °C, indicating high volatility consistent with group trends. These physical characteristics arise from relativistic stabilization of the 8s valence orbital, which contracts the atom more than in lighter homologs, though the overall structure remains metallic and soft. Chemically, ununennium is anticipated to exhibit the +1 typical of metals, forming compounds such as ununennium (Uue₂O), (UueH), and (UueF), but with reduced reactivity compared to due to relativistic effects that increase the first potential to approximately 4.8 eV and hinder s-electron promotion to the 8p orbital. This analogue of the may destabilize certain compounds and lower overall reactivity, with the element potentially showing behavior more akin to than its immediate predecessor. A +3 could emerge in specific environments, though it would be less stable. The is predicted to be low at 169.7 , comparable to sodium, further limiting interactions with other species. Spectroscopically, ununennium is expected to display strong absorption in the region due to the 8s to 8p transition at wavelengths around 200–300 nm, shifted by relativistic influences on the energy levels. Due to its extreme radioactivity and short half-lives (on the order of seconds for known isotopes), experimental study would be confined to trace amounts in gas-phase or matrix isolation techniques, preventing bulk environmental assessments but allowing theoretical validation through single-atom chemistry.

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