Ununennium

Ununennium and unbinilium are the first elements that have not yet been synthesized, and attempts to synthesize them would push the limits of current technology, due to the decreasing cross sections of the production reactions and their probably short half-lives.^[9] 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 neutron-rich isotope calcium-48 which could thus be used as a projectile to produce more neutron-rich isotopes of superheavy elements. However, using calcium-48 to synthesize ununennium would require a target of einsteinium-253 or -254, which while not impossible is very difficult. To more practically produce further superheavy elements, projectiles heavier than ⁴⁸Ca are needed.^[10]

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

${\displaystyle \,_{99}^{254}\mathrm {Es} +\,_{20}^{48}\mathrm {Ca} \to \,_{119}^{302}\mathrm {Uue} ^{*}}$ 

No atoms were identified, leading to a limiting yield of 300 nb.^[11] Later calculations suggest that the yield of the 3n reaction (which would result in ²⁹⁹Uue and three neutrons as products) would be 0.5 pb.^[12]

As of May 2012, plans are under way to attempt to synthesize the isotopes ²⁹⁵Uue and ²⁹⁶Uue by bombarding a target of berkelium with titanium at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany:^[13][14]

${\displaystyle \,_{97}^{249}\mathrm {Bk} +\,_{22}^{50}\mathrm {Ti} \to \,_{119}^{296}\mathrm {Uue} \,+3\,_{0}^{1}\mathrm {n} }$ 

${\displaystyle \,_{97}^{249}\mathrm {Bk} +\,_{22}^{50}\mathrm {Ti} \to \,_{119}^{295}\mathrm {Uue} \,+4\,_{0}^{1}\mathrm {n} }$ 

No atoms were identified, leading to a yielding limit of 70 fb.^[15]

The alpha-decay half-lives of 1700 nuclei with 100 ≤ Z ≤ 130 have been calculated in a quantum tunneling model with alpha-decay Q-values from different mass estimates.^[16][17][18] The alpha-decay half-lives predicted for ^291–307Uue are of the order of micro-seconds. The highest value of the alpha-decay half-life predicted in the quantum tunneling model with the mass estimates from a macroscopic-microscopic model is ~485 microseconds for the isotope ²⁹⁴Uue. For ³⁰²Uue it is ~163 microseconds.

Being the first period 8 element, ununennium is predicted to be the next alkali metal after francium and behave much like their lighter congeners; however, it is also predicted to differ from the lighter alkali metals in some properties.^[1] Its chemistry is predicted to be closer to that of potassium^[19] or rubidium^[1] instead of caesium or francium. This is unusual as periodic trends, ignoring relativistic effects would predict ununennium to be even more reactive than caesium and francium. This lowered reactivity is due to the relativistic stabilisation of ununennium's valence electron, increasing ununennium's first ionisation energy and decreasing the metallic and ionic radii;^[19] this effect is already seen for francium.^[1] The relativistic stabilisation of the 8s orbital also increases ununennium's electron affinity far beyond that of caesium and francium; indeed, ununennium is expected to have an electron affinity higher than all the alkali metals lighter than it. Relativistic effects also cause a very large drop in the polarisability of ununennium.^[1] On the other hand, ununennium is predicted to continue the trend of melting points decreasing going down the group, being expected to have a melting point between 0 °C and 30 °C.^[6]

The stabilisation of ununennium's valence electron and thus the contraction of the 8s orbital cause its atomic radius to be lowered to 240 pm,^[1] very close to that of rubidium (247 pm), so that 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 destabilised 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,^[20] 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 destabilisation and expansion of the 7p_3/2 spinor, causing its outermost electrons to have a lower ionisation energy than what would otherwise be expected.^[20][1] Indeed, many ununennium 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 francium.^[21]

• Extended periodic table

ഫലകം:Extended periodic table (by Fricke, 32 columns, compact)