Unbinilium

Unbinilium, also known as eka-radium or simply element 120, is the hypothetical chemical element in the periodic table with symbol Ubn and atomic number 120. Unbinilium and Ubn are the temporary systematic IUPAC name and symbol, 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 alkaline earth metal, and the second element in the eighth period. It has attracted attention because of some predictions that it may be in the island of stability, although newer calculations expect the island to actually occur at a slightly lower atomic number, closer to copernicium and flerovium.

Unbinilium, ₁₂₀Ubn

Unbinilium
Pronunciation	/ˌuːnbaɪˈnɪliəm/ ​(OON-by-NIL-ee-əm)
Alternative names	element 120, eka-radium
Mass number	[299] (unconfirmed)
Unbinilium 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 Ra ↑ Ubn ↓ — ununennium ← unbinilium → unbiunium
Atomic number (Z)	120
Group	group 2 (alkaline earth metals)
Period	period 8
Block	s-block
Electron configuration	[Og] 8s^2 (predicted)[1]
Electrons per shell	2, 8, 18, 32, 32, 18, 8, 2 (predicted)
Physical properties
Phase at STP	solid (predicted)[1][2]
Melting point	953 K ​(680 °C, ​1256 °F) (predicted)[1]
Boiling point	1973 K ​(1700 °C, ​3092 °F) (predicted)[3]
Density (near r.t.)	7 g/cm^3 (predicted)[1]
Heat of fusion	8.03–8.58 kJ/mol (extrapolated)[2]
Atomic properties
Oxidation states	(+1),[4] (+2), (+4) (predicted)[1]
Electronegativity	Pauling scale: 0.91 (predicted)[5]
Ionization energies	1st: 563.3 kJ/mol (predicted)[6] 2nd: 895–919 kJ/mol (extrapolated)[2]
Atomic radius	empirical: 200 pm (predicted)[1]
Covalent radius	206–210 pm (extrapolated)[2]
Other properties
Crystal structure	​body-centered cubic (bcc) (extrapolated)[7]
CAS Number	54143-58-7
History
Naming	IUPAC systematic element name
Main isotopes of unbinilium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct ^299Ubn[8] syn 3.7 s? α ^295Og

Unbinilium has not yet been synthesized, despite multiple attempts from German and Russian teams. One 2011 attempt from the German team at the GSI Helmholtz Centre for Heavy Ion Research had a suggestive but not conclusive result suggesting the possible production of ²⁹⁹Ubn, but the data were incomplete and did not match theoretical expectations. Experimental evidence from these attempts shows that the period 8 elements would likely be far more difficult to synthesise than the previous known elements, and that unbinilium may even be the last element that can be synthesized with current technology.

Unbinilium's position as the seventh alkaline earth metal suggests that it would have similar properties to its lighter congeners, beryllium, magnesium, calcium, strontium, barium, and radium; however, relativistic effects may cause some of its properties to differ from those expected from a straight application of periodic trends. For example, unbinilium is expected to be less reactive than barium and radium and be closer in behavior to strontium, and while it should show the characteristic +2 oxidation state of the alkaline earth metals, it is also predicted to show the +4 oxidation state, which is unknown in any other alkaline earth metal.

Introduction

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.

External video
Visualization of unsuccessful nuclear fusion, based on calculations by the Australian National University[9]

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.[15] 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.[16] 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.[16][17] 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.[18][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.[21] 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.[21] The transfer takes about 10⁻⁶ seconds; in order to be detected, the nucleus must survive this long.[24] The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.[21]

Stability of a nucleus is provided by the strong interaction. However, its range is very short; as nuclei become larger, its influence on the outermost nucleons (protons and neutrons) weakens. At the same time, the nucleus is torn apart by electrostatic repulsion between protons, as it has unlimited range.[25] Nuclei of the heaviest elements are thus theoretically predicted[26] and have so far been observed[27] 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

Transactinide elements, such as unbinilium, 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.[40] 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, and is currently the only method to produce the superheavy elements from flerovium (element 114) onward.[41]

Ununennium and unbinilium (elements 119 and 120) are the elements with the lowest atomic numbers that have not yet been synthesized: all the preceding elements have been synthesized, culminating in oganesson (element 118), the heaviest-known element, which completes the seventh row of the periodic table. Attempts to synthesize elements 119 and 120 push the limits of current technology, due to the decreasing cross sections of the production reactions and their probably short half-lives,[42] expected to be on the order of microseconds.[1][43] Heavier elements would likely be too short-lived to be detected with current technology: they would decay within a microsecond, before reaching the detectors.[42]

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.[44] (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.[47] Furthermore, using calcium-48 to synthesize unbinilium would require a target of fermium-257, which cannot yet be produced in large enough quantities (only picograms can presently be produced; in comparison, milligrams of berkelium and californium are available), and would in any case have a lower yield than using an einsteinium target with calcium-48 projectiles to produce ununennium.[44][48] More practical production of further superheavy elements would require projectiles heavier than ⁴⁸Ca, but this has the drawback of resulting in more symmetrical fusion reactions that are colder and less likely to succeed.[44]

Synthesis attempts

Following their success in obtaining oganesson by the reaction between ²⁴⁹Cf and ⁴⁸Ca in 2006, the team at the Joint Institute for Nuclear Research (JINR) in Dubna started similar experiments in March–April 2007, in hope of creating unbinilium from nuclei of ⁵⁸Fe and ²⁴⁴Pu.[49][50] Initial analysis revealed that no atoms of unbinilium were produced, providing a limit of 400 fb for the cross section at the energy studied.[51]

²⁴⁴
₉₄Pu
+ ⁵⁸
₂₆Fe
→ ³⁰²
₁₂₀Ubn
* → no atoms

The Russian team planned to upgrade their facilities before attempting the reaction again.[51]

In April 2007, the team at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany attempted to create unbinilium using uranium-238 and nickel-64:[52]

²³⁸
₉₂U
+ ⁶⁴
₂₈Ni
→ ³⁰²
₁₂₀Ubn
* → no atoms

No atoms were detected, providing a limit of 1.6 pb for the cross section at the energy provided. The GSI repeated the experiment with higher sensitivity in three separate runs in April–May 2007, January–March 2008, and September–October 2008, all with negative results, reaching a cross section limit of 90 fb.[52]

In 2011, after upgrading their equipment to allow the use of more radioactive targets, scientists at the GSI attempted the rather asymmetrical fusion reaction:[53]

²⁴⁸
₉₆Cm
+ ⁵⁴
₂₄Cr
→ ³⁰²
₁₂₀Ubn
* → no atoms

It was expected that the change in reaction would quintuple the probability of synthesizing unbinilium,[54] as the yield of such reactions is strongly dependent on their asymmetry.[42] Although this reaction is less asymmetric than the ²⁴⁹Cf+⁵⁰Ti reaction, it also creates more neutron-rich unbinilium isotopes that should receive increased stability from their proximity to the shell closure at N = 184.[55]

Three correlated signals were observed on 18 May 2011 that matched the predicted alpha decay energies of ²⁹⁹Ubn and its daughter ²⁹⁵Og, as well as the experimentally known decay energy of its granddaughter ²⁹¹Lv: the decay chain could thus be interpreted as beginning from ²⁹⁹Ubn and undergoing four successive alpha decays to the spontaneously fissioning ²⁸³Cn, with the final alpha from ²⁸⁷Fl having been missed. The observed lifetimes for ²⁸⁷Fl and ²⁸³Cn were rather longer than those measured and accepted for those isotopes and ²⁷⁹Ds, but agree well with those measured at Dubna in an early 1999 experiment aimed at synthesising ²⁸⁷Fl; both these chains may originate from isomeric states, or the electron capture of ²⁸⁷Fl leading to ²⁸⁷Nh and its spontaneously fissioning daughter ²⁸³Rg.[8] However, the results could not be confirmed due to lack of beam time, even though the probability that the observations were due to accidental coincidence was calculated as 4 × 10⁻⁸.[56][57][55]

Observed decay chains possibly arising from even-Z superheavy nuclides (Z = 114, 116, 118, 120) as of 2016. Dotted nuclides (chain 3 from Darmstadt and chains 5 and 8 from Dubna) are tentatively assigned.[8]

In August–October 2011, a different team at the GSI using the TASCA facility tried a new, even more asymmetrical reaction:[53][58]

²⁴⁹
₉₈Cf
+ ⁵⁰
₂₂Ti
→ ²⁹⁹
₁₂₀Ubn
* → no atoms

Because of its asymmetry,[59] the reaction between ²⁴⁹Cf and ⁵⁰Ti was predicted to be the most favorable practical reaction for synthesizing unbinilium, although it is also somewhat cold, and is further away from the neutron shell closure at N = 184 than any of the other three reactions attempted. No unbinilium atoms were identified, implying a limiting cross section of 200 fb.[58] Jens Volker Kratz predicted the actual maximum cross section for producing unbinilium by any of the four reactions ²³⁸U+⁶⁴Ni, ²⁴⁴Pu+⁵⁸Fe, ²⁴⁸Cm+⁵⁴Cr, or ²⁴⁹Cf+⁵⁰Ti to be around 0.1 fb;[60] in comparison, the world record for the smallest cross section of a successful reaction was 30 fb for the reaction ²⁰⁹Bi(⁷⁰Zn,n)²⁷⁸Nh,[42] and Kratz predicted a maximum cross section of 20 fb for producing ununennium.[60] If these predictions are accurate, then synthesizing ununennium would be at the limits of current technology, and synthesizing unbinilium would require new methods.[60]

This reaction was investigated again in April to September 2012 at the GSI. This experiment used a ²⁴⁹Bk target and a ⁵⁰Ti beam, to produce element 119, but since ²⁴⁹Bk decays to ²⁴⁹Cf with a half-life of about 327 days, both elements 119 and 120 could be searched for simultaneously. No atoms were identified, implying a cross-section limit of 200 fb for the ²⁴⁹Cf+⁵⁰Ti reaction.[61]

The Russian team at the Joint Institute for Nuclear Research in Dubna, Russia planned to conduct an experiment before 2012, and no results were released, strongly implying that either the experiment was not done or no unbinilium atoms were identified.

Naming

Mendeleev's nomenclature for unnamed and undiscovered elements would call unbinilium eka-radium. The 1979 IUPAC recommendations temporarily call it unbinilium (symbol Ubn) until it is discovered, the discovery is confirmed and a permanent name chosen.[62] Although the IUPAC systematic names are widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, scientists who work theoretically or experimentally on superheavy elements typically call it "element 120", with the symbol E120, (120) or 120.[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 elliptical region encloses the predicted location of the island of stability.[42]

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.[60]

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.[63] Nevertheless, because of 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.[64]

Isotopes of unbinilium are predicted to have alpha decay half-lives of the order of microseconds.[65][66] In a quantum tunneling model with mass estimates from a macroscopic-microscopic model, the alpha-decay half-lives of several unbinilium isotopes (^292–304Ubn) have been predicted to be around 1–20 microseconds.[67][68][69][70] Some heavier isotopes may be more stable; Fricke and Waber predicted ³²⁰Ubn to be the most stable unbinilium isotope in 1971.[3] Since unbinilium is expected to decay via a cascade of alpha decays leading to spontaneous fission around copernicium, the total half-lives of unbinilium isotopes are also predicted to be measured in microseconds.[1][43] This has consequences for the synthesis of unbinilium, as isotopes with half-lives below one microsecond would decay before reaching the detector.[1][43] Nevertheless, new theoretical models show that the expected gap in energy between the proton orbitals 2f_7/2 (filled at element 114) and 2f_5/2 (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.[60]

Given that element 120 fills the 2f_5/2 proton orbital, much attention has been given to the compound nucleus ³⁰²Ubn* and its properties. Several experiments have been performed between 2000 and 2008 at the Flerov Laboratory of Nuclear Reactions in Dubna studying the fission characteristics of the compound nucleus ³⁰²Ubn*. Two nuclear reactions have been used, namely ²⁴⁴Pu+⁵⁸Fe and ²³⁸U+⁶⁴Ni. The results have revealed how nuclei such as this fission predominantly by expelling closed shell nuclei such as ¹³²Sn (Z = 50, N = 82). It was also found that the yield for the fusion-fission pathway was similar between ⁴⁸Ca and ⁵⁸Fe projectiles, suggesting a possible future use of ⁵⁸Fe projectiles in superheavy element formation.[71]

In 2008, the team at GANIL, France, described the results from a new technique which attempts to measure the fission half-life of a compound nucleus at high excitation energy, since the yields are significantly higher than from neutron evaporation channels. It is also a useful method for probing the effects of shell closures on the survivability of compound nuclei in the super-heavy region, which can indicate the exact position of the next proton shell (Z=114, 120, 124, or 126). The team studied the nuclear fusion reaction between uranium ions and a target of natural nickel:[72][73]

²³⁸
₉₂U
+ ^nat
₂₈Ni
→ ^{296,298,299,300,302}
₁₂₀Ubn
* → fission

The results indicated that nuclei of unbinilium were produced at high (~70 MeV) excitation energy which underwent fission with measurable half-lives just over 10⁻¹⁸ s.[72][73] Although very short (indeed insufficient for the element to be considered by IUPAC to exist, because a compound nucleus has no internal structure and its nucleons have not been arranged into shells until it has survived for 10⁻¹⁴ s, when it forms an electronic cloud),[74] the ability to measure such a process indicates a strong shell effect at Z=120. At lower excitation energy (see neutron evaporation), the effect of the shell will be enhanced and ground-state nuclei can be expected to have relatively long half-lives. This result could partially explain the relatively long half-life of ²⁹⁴Og measured in experiments at Dubna. Similar experiments have indicated a similar phenomenon at element 124 but not for flerovium, suggesting that the next proton shell does in fact lie beyond element 120.[72][73] In September 2007, the team at RIKEN began a program utilizing ²⁴⁸Cm targets and have indicated future experiments to probe the possibility of 120 being the next proton magic number (and 184 being the next neutron magic number) using the aforementioned nuclear reactions to form ³⁰²Ubn*, as well as ²⁴⁸Cm+⁵⁴Cr. They also planned to further chart the region by investigating the nearby compound nuclei ²⁹⁶Og*, ²⁹⁸Og*, ³⁰⁶Ubb*, and ³⁰⁸Ubb*.[75]

Atomic and physical

Being the second period 8 element, unbinilium is predicted to be an alkaline earth metal, below beryllium, magnesium, calcium, strontium, barium, and radium. Each of these elements has two valence electrons in the outermost s-orbital (valence electron configuration ns²), which is easily lost in chemical reactions to form the +2 oxidation state: thus the alkaline earth metals are rather reactive elements, with the exception of beryllium due to its small size. Unbinilium is predicted to continue the trend and have a valence electron configuration of 8s². It is therefore expected to behave much like its lighter congeners; however, it is also predicted to differ from the lighter alkaline earth metals in some properties.[1]

The main reason for the predicted differences between unbinilium and the other alkaline earth 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 velocities comparable to the speed of light—than those in lighter atoms.[4] In unbinilium 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.[76] 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.[4][lower-alpha 12] Thus, the outer 8s electrons of unbinilium are stabilized and become harder to remove than expected, while the 7p_3/2 electrons are correspondingly destabilized, perhaps allowing them to participate in chemical reactions.[1] This stabilization of the outermost s-orbital (already significant in radium) is the key factor affecting unbinilium's chemistry, and causes all the trends for atomic and molecular properties of alkaline earth metals to reverse direction after barium.[77]

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][78]

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][78]

Due to the stabilization of its outer 8s electrons, unbinilium's first ionization energy—the energy required to remove an electron from a neutral atom—is predicted to be 6.0 eV, comparable to that of calcium.[1] The electron of the hydrogen-like unbinilium atom—oxidized so it has only one electron, Ubn¹¹⁹⁺—is predicted to move so quickly that its mass is 2.05 times that of a non-moving electron, a feature coming from the relativistic effects. For comparison, the figure for hydrogen-like radium is 1.30 and the figure for hydrogen-like barium is 1.095.[4] According to simple extrapolations of relativity laws, that indirectly indicates the contraction of the atomic radius[4] to around 200 pm,[1] very close to that of strontium (215 pm); the ionic radius of the Ubn²⁺ ion is also correspondingly lowered to 160 pm.[1] The trend in electron affinity is also expected to reverse direction similarly at radium and unbinilium.[77]

Unbinilium should be a solid at room temperature, with melting point 680 °C:[79] this continues the downward trend down the group, being lower than the value 700 °C for radium.[80] The boiling point of unbinilium is expected to be around 1700 °C, which is lower than that of all the previous elements in the group (in particular, radium boils at 1737 °C), following the downward periodic trend.[3] The density of unbinilium has been predicted to be 7 g/cm³, continuing the trend of increasing density down the group: the value for radium is 5.5 g/cm³.[3][2]

Chemical

Bond lengths and bond-dissociation energies of alkaline earth metal dimers. Data for Ba₂, Ra₂ and Ubn₂ is predicted.[77]

Compound	Bond length (Å)	Bond-dissociation energy (eV)
Ca2	4.277	0.14
Sr2	4.498	0.13
Ba2	4.831	0.23
Ra2	5.19	0.11
Ubn2	5.65	0.02

The chemistry of unbinilium is predicted to be similar to that of the alkaline earth metals,[1] but it would probably behave more like calcium or strontium[1] than barium or radium. Like strontium, unbinilium should react vigorously with air to form an oxide (UbnO) and with water to form the hydroxide (Ubn(OH)₂), which would be a strong base, and releasing hydrogen gas. It should also react with the halogens to form salts such as UbnCl₂.[81] While these reactions would be expected from periodic trends, their lowered intensity is somewhat unusual, as ignoring relativistic effects, periodic trends would predict unbinilium to be even more reactive than barium or radium. This lowered reactivity is due to the relativistic stabilization of unbinilium's valence electron, increasing unbinilium's first ionization energy and decreasing the metallic and ionic radii;[82] this effect is already seen for radium.[1] the chemistry of unbinilium in the +2 oxidation state should be more similar to the chemistry of strontium than to that of radium. On the other hand, the ionic radius of the Ubn²⁺ ion is predicted to be larger than that of Sr²⁺, because the 7p orbitals are destabilized and are thus larger than the p-orbitals of the lower shells. Unbinilium may also show the +4 oxidation state,[1] which is not seen in any other alkaline earth metal,[83] in addition to the +2 oxidation state that is characteristic of the other alkaline earth metals and is also the main oxidation state of all the known alkaline earth metals: this is because of the destabilization and expansion of the 7p_3/2 spinor, causing its outermost electrons to have a lower ionization energy than what would otherwise be expected.[1][83] The +1 state may also be stable in isolation.[4] Many unbinilium compounds are expected to have a large covalent character, due to the involvement of the 7p_3/2 electrons in the bonding: this effect is also seen to a lesser extent in radium, which shows some 6s and 6p_3/2 contribution to the bonding in radium fluoride (RaF₂) and astatide (RaAt₂), resulting in these compounds having more covalent character.[4] The standard reduction potential of the Ubn²⁺/Ubn couple is predicted to be −2.9 V, which is almost exactly the same as that for the Sr²⁺/Sr couple of strontium (−2.899 V).[79]

Bond lengths and bond-dissociation energies of MAu (M = an alkaline earth metal). All data is predicted, except for CaAu.[77]

Compound	Bond length (Å)	Bond-dissociation energy (kJ/mol)
CaAu	2.67	2.55
SrAu	2.808	2.63
BaAu	2.869	3.01
RaAu	2.995	2.56
UbnAu	3.050	1.90

In the gas phase, the alkaline earth metals do not usually form covalently bonded diatomic molecules like the alkali metals do, since such molecules would have the same number of electrons in the bonding and antibonding orbitals and would have very low dissociation energies.[84] Thus, the M–M bonding in these molecules is predominantly through van der Waals forces.[77] The metal–metal bond lengths in these M₂ molecules increase down the group from Ca₂ to Ubn₂. On the other hand, their metal–metal bond-dissociation energies generally increase from Ca₂ to Ba₂ and then drop to Ubn₂, which should be the most weakly bound of all the group 2 homodiatomic molecules. The cause of this trend is the increasing participation of the p_3/2 and d electrons as well as the relativistically contracted s orbital.[77] From these M₂ dissociation energies, the enthalpy of sublimation (ΔH_sub) of unbinilium is predicted to be 150 kJ/mol.[77]

Bond lengths, harmonic frequency, vibrational anharmonicity and bond-dissociation energies of MH and MAu (M = an alkaline earth metal). Data for UbnH and UbnAu are predicted.[85] Data for BaH is taken from experiment,[86] except bond-dissociation energy.[85] Data for BaAu is taken from experiment,[87] except bond-dissociation energy and bond length.[85]

Compound	Bond length (Å)	Harmonic frequency, cm^−1	Vibrational anharmonicity, cm^−1	Bond-dissociation energy (eV)
UbnH	2.38	1070	20.1	1.00
BaH	2.23	1168	14.5	2.06
UbnAu	3.03	100	0.13	1.80
BaAu	2.91	129	0.18	2.84

The Ubn–Au bond should be the weakest of all bonds between gold and an alkaline earth metal, but should still be stable. This gives extrapolated medium-sized adsorption enthalpies (−ΔH_ads) of 172 kJ/mol on gold (the radium value should be 237 kJ/mol) and 50 kJ/mol on silver, the smallest of all the alkaline earth metals, that demonstrate that it would be feasible to study the chromatographic adsorption of unbinilium onto surfaces made of noble metals.[77] The ΔH_sub and −ΔH_ads values are correlated for the alkaline earth metals.[77]

See also

• Island of stability: flerovium–unbinilium–unbihexium

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[10] or 112;[11] sometimes, the term is presented an equivalent to the term "transactinide", which puts an upper limit before the beginning of the hypothetical superactinide series).[12] 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.[13] 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.[14]
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.[18]
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.[19] This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.[20]
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.[22] 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.[23]
6. Not all decay modes are caused by electrostatic repulsion. For example, beta decay is caused by the weak interaction.[28]
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.[29] The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL.[30] 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).[31]
8. Spontaneous fission was discovered by Soviet physicist Georgy Flerov,[32] a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility.[33] 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.[20] They thus preferred to link new isotopes to the already known ones by successive alpha decays.[32]
9. For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics in Stockholm, Stockholm County, Sweden.[34] 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.[35] 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.[35] JINR insisted that they were the first to create the element and suggested a name of their own for the new element, joliotium;[36] the Soviet name was also not accepted (JINR later referred to the naming of element 102 as "hasty").[37] The name "nobelium" remained unchanged on account of its widespread usage.[38]
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).[39]
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,[45] 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).[45] The trend is expected to continue to the superheavy elements,[46] 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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