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Ununoctium, også kendt som eka-radon eller grundstof 118, er det midlertidige IUPAC navn[1] til det transactinide grundstof med atomnummeret 118 og det midlertidige symbol Uuo. I det periodiske system over grundstofferne, er stoffet et p-blok-element og det sidste i den 7. periode. Ununocitum er i øjeblikket det eneste syntetiske medlem af gruppe 18, og har det højeste atomnummer og den højeste atommasse tildelt et fundet grundstof.

Det radioaktive ununoctium-atom er meget ustabilt, og siden 2002 er kun tre atomer af isotopen 294Uuo blevet opdaget.[2] Da dette kun tillader meget lidt eksperimentel karakterisering af stoffets egenskaber og dets forbindelser, har man foretaget mange teoretiske beregninger, der har givet mange forudsigelser om stoffets egenskaber, blandt andet nogle meget uventede. For eksempel kunne stoffet, der er medlem af ædelgas-gruppen, have en større kemisk reaktivitet end nogle elementer uden for denne gruppe.[3] Desuden er det forudset at stoffet sandsynligvis er fast under standardbetingelser.[3][4]

Historie[redigér wikikode]

Ikke-succesfulde forsøg[redigér wikikode]

I slutningen af 1998 offentliggjorde den polske fysiker Robert Smolanczuk beregninger omkring fusionen af atomkerner mod syntesen af supertunge atom, deriblandt grundstof 118.[5] Hans beregninger viste, at det måske ville ære muligt at fremstille ununoctium ved fusion mellem bly og krypton under særdeles kontrollerede forhold.[5]

I 1999 brugte forskere på Lawrence Berkeley National Laboratory disse forudsigelser og offentliggjorde opdagelsen af grundstofferne 116 og 118 i en afhandling offentliggjort i Physical Review Letters,[6] og snart derefter rapporteredes der om resultaterne i Science.[7] Forskerne påstod at havde udført den kemiske reaktion:

86Krypton + 208Bly → 293Ununoctium + neutron

Året efter trak de dog resultaterne tilbage, da andre forskere ikke kunne genskabe resultaterne.[8] I juni 2002 offentliggjorde direktøren for det laboratorium, der først havde opdaget grundstoffet, at opdagelsen var baseret på data fabrikeret af hovedforfatteren Victor Ninov.[9]

Discovery[redigér wikikode]

9. oktober 2006 offentliggjorde forskere fra Joint Institute for Nuclear Research (JINR) og Lawrence Livermore National Laboratory i Californien, USA, der arbejdede på JINR i Dubna, Rusland i Physical Review C[10], at de indirekte havde sporet tre atomkerner af stoffet ununoctium-294 (en i 2002[11] og to mere i 2005) skab via kollisioner mellem californium-249-atomer og calcium-48-ioner:[12][13][14][15][16]

Californium249 + Calcium48 → Ununoctium294 + 3 neutroner
Vejen for det radioaktive hanfald fra isotopn Uuo-294.[10] Henfaldsenergien og de gennemsnitlige halveringstider er givet for the parent isotope og hvert henfaldsprodukt. Fraktionen af atomer, der undergår spontan fission (SF) er givet i grøn.

Because of the very small fusion reaction probability (the fusion cross section is 0.5 pb = 5×10−41 m²) the experiment took 4 months and involved a beam dose of 4×1019 calcium ions that had to be shot at the californium target to produce the first recorded event believed to be the synthesis of ununoctium.[17] Nevertheless, researchers are highly confident that the results are not a false positive, since the chance that the detections were random events was estimated to be less than one part in 100,000.[18]

In the experiments, the decay of three atoms of ununoctium was observed. A half-life of 0.89 ms was calculated: Skabelon:SimpleNuclide decays into Skabelon:SimpleNuclide by alpha decay. Since there were only three nuclei, the halflife derived from observed lifetimes has a large uncertainty: 0.89Skabelon:± ms.[10]

294
118
Uuo
unknown element Ununhexium . + Skabelon:SimpleNuclide

The identification of the Skabelon:SimpleNuclide nuclei was verified by separately creating the putative daughter nucleus Skabelon:SimpleNuclide by means of a bombardment of Skabelon:SimpleNuclide with Skabelon:SimpleNuclide ions,

245
96
Cm
+ 48
20
Ca
unknown element Ununhexium . + 3 Skabelon:SubatomicParticle

and checking that the Skabelon:SimpleNuclide decay matched the decay chain of the Skabelon:SimpleNuclide nuclei.[10] The daughter nucleus Skabelon:SimpleNuclide is very unstable, decaying with a halflife of 14 milliseconds into Skabelon:SimpleNuclide, which may undergo spontaneous fission or alpha decay into Skabelon:SimpleNuclide, which will undergo spontaneous fission.[10]

In a quantum tunneling model, the alpha decay half-life of 294118 was predicted to be 0.66(+0.23,-0.18)ms[19] with the experimental Q-value published in 2004.[20] Calculation with theoretical Q-values from the macroscopic-microscopic model of Muntian-Hofman-Patyk-Sobiczewski gives somewhat low but comparable results.[21]

Following the success in obtaining ununoctium, the discoverers have started similar experiments in the hope of creating element 120 from Skabelon:SimpleNuclide and Skabelon:SimpleNuclide.[22] Isotopes of the element 120 are predicted to have alpha decay half lives of the order of micro-seconds.[23][24]

Naming[redigér wikikode]

Element 118 is still called eka-radon[kilde mangler], but until the 1960s it was also known as eka-emanation (for the old name for radon).[25] Only in 1979 the IUPAC published recommendations according to which the element started to be called ununoctium.[26] The name ununoctium is a systematic element name, used as a placeholder until it is confirmed by other research groups and the IUPAC decides on a name.

Before the retraction in 2002, the researchers from Berkeley had intended to name the element ghiorsium (Gh), after Albert Ghiorso (a leading member of the research team).[27] Several years later, when the Russian discoverers reported their synthesis in 2006, rumors appeared that they were planning on calling it after the place of the discovery, dubnadium (Dn) (very similar to the name of the 105th element, dubnium (Db)).[28] Nevertheless, during an interview with a Russian newspaper, the head of the Russian institute stated the team were considering two names for the new element, Flyorium in honor of Georgy Flyorov, the founder of the research institute; and moskovium (also spelled moscovium or even moscowium), in recognition of the Moscow Oblast where Dubna lies.[29] He also stated that although the element was discovered as an American collaboration, who provided the californium target, the element should rightly be named in honour of Russia since the Flerov Laboratory of Nuclear Reactions at JINR is the only facility in the world which can achieve this result.[30][31]

Characteristics[redigér wikikode]

Nucleus stability and isotopes[redigér wikikode]

Element 118 comes right at the end of the "island of stability" and thus its nuclei are slightly more stable than predicted.

Skabelon:Seealso

There are no elements with an atomic number above 82 (after lead) that have stable isotopes. The stability of nuclei decreases with the increase in atomic number, such that all isotopes with an atomic number above 101 decay radioactively with a halflife under a day. Nevertheless, due to reasons not very well understood yet, there is a slight increased nuclear stability around elements 110–114, which leads to the appearance of what is known in nuclear physics as the "island of stability". This concept, proposed by UC Berkeley professor Glenn Seaborg, explains why superheavy elements last longer than predicted.[32] Ununoctium is radioactive and has half-life that appears to be less than a millisecond. Nonetheless, this is still longer than some predicted values,[19][33] thus giving further support to the idea of this "island of stability"..[34]

Quantum tunneling model predicts existence of several neutron-rich isotopes of the element 118 with alpha decay half lives close to ms.[23][24]

Theoretical calculations done on the synthetic pathways for, and the halflife of, other isotopes have shown that some could be slightly more stable than the synthesized isotope Skabelon:SimpleNuclide, most likely Skabelon:SimpleNuclide, Skabelon:SimpleNuclide, Skabelon:SimpleNuclide, Skabelon:SimpleNuclide, Skabelon:SimpleNuclide, Skabelon:SimpleNuclide and Skabelon:SimpleNuclide.[19][35] Of these, Skabelon:SimpleNuclide might provide the best chances for obtaining longer-lived nuclei,[19][35] and thus might become the focus of future work with this element. Some isotopes with many more neutrons, such as some located around Skabelon:SimpleNuclide, could also provide longer-lived nuclei.[36]

Properties[redigér wikikode]

Skabelon:Seealso Ununoctium is a member of the inert gases, the zero-valence elements. Consequently, ununoctium is expected to have similar physical and chemical properties to other members of its group, most closely resembling the noble gas above it in the periodic table, radon.[37] The members of this group are inert to most common chemical reactions (such as combustion, for example) because the outer valence shell is completely filled with eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound.[38] It is thought that similarly, ununoctium has a closed outer valence shell in which its valence electrons are arranged in a 7s², 7p6 configuration.[3]

The expected electron shell diagram for ununoctium. Note the eight electrons in the outer shell.

Following the periodic trend, ununoctium is expected to be slightly more reactive than radon; but theoretical calculations have shown that it could be quite reactive for its "noble" labeling.[39] In addition to being far more reactive than radon, ununoctium may be even more reactive than elements 114 and 112.[3] The reason for the apparent enhancement of the chemical activity of element 118 relative to radon is an energetic destabilization and a radial expansion of the last occupied 7p subshell.[3][40] More precisely, considerable spin-orbit interactions between the 7p electrons with the inert 7s² electrons, effectively lead to a second valence shell closing at element 114, and a significant decrease in stabilization of the closed shell of element 118.[3] It has also been calculated that ununoctium, unlike other noble gases, binds an electron with release of energy—or in other words, it exhibits positive electron affinity.[41][42][43]

Ununoctium is expected to have by far the broadest polarizability of all elements before it in the periodic table, and almost twofold of radon.[3] By extrapolating this to the other noble gases, it is expected that ununoctium has a boiling point between 320 and 380 K.[3] This is very different from the previously estimated values of 263 K[44] or 247 K.[45] Even given the large uncertainties of the calculations, it seems highly unlikely that element 118 would be a gas under standard conditions.[3][4] And as the liquid range of the other gases is very narrow, between 2 and 9 kelvins, this element should be solid at standard conditions. If ununoctium forms a gas under standard conditions nevertheless, it would be one of the densest substances gaseous at standard conditions (even if it is monatomic like the other noble gases).

Because of its tremendous polarizability, ununoctium is expected to have an anomalously low ionization potential (similar to that of lead which is 70% of that of radon[46] and significantly smaller than that of element 114[47]) and a standard state condensed phase.[3]

Compounds and uses[redigér wikikode]

XeF4 and RnF4 have a square planar configuration
UuoF4 is predicted to have a tetrahedral configuration

Skabelon:Seealso

No compounds of ununoctium have been synthesized yet, but calculations on theoretical compounds have been performed since 1964.[25] It is expected that if the ionization energy of the element is high enough, it will be difficult to oxidize and therefore, the most common oxidation state will be 0 (as for other noble gases).[48]

Calculations on the dimeric molecule Uuo2 showed a bonding interaction roughly equivalent to that calculated for Hg2, and a dissociation energy of 6 kJ/mol, roughly 4 times of that of Rn2.[3] But most strikingly, it was calculated to have a bond length shorter than in Rn2 by .16 Å, which would be indicative of a significant bonding interaction.[3] On the other hand, the compound UuoH+ exhibits a dissociation energy (in other words proton affinity of Uuo) that is smaller than that of RnH+.[3]

The bonding between ununoctium and hydrogen in UuoH is very weak and can be regarded as a pure van der Waals interaction rather than a true chemical bond.[46] On the other hand, with highly electronegative elements, ununoctium seems to form more stable compounds than for example element 112 or element 114.[46] The stable oxidation states +2 and +4 have been predicted to exist in the fluorinated compounds UuoF2 and UuoF4.[49] This is a result of the same spin-orbit interactions that make ununoctium unusually reactive. For example, it was shown that the reaction of Uuo with F2 to form the compound UuoF2, would release an energy of 106 kcal/mol of which about 46 kcal/mol come from these interactions.[46] For comparison, the spin-orbit interaction for the similar molecule RnF2 is about 10 kcal/mol out of a formation energy of 49 kcal/mol.[46] The same interaction stabilizes the tetrahedral Td configuration for UuoF4, as opposed to the square planar D4h one of XeF4 and RnF4.[49] The Uuo-F bond will most probably be ionic rather than covalent, rendering the UuoFn compounds non-volatile.[39][50] Unlike the other noble gases, ununoctium was predicted to be sufficiently electropositive to form a Uuo-Cl bond with chlorine.[39]

Since only three atoms of ununoctium have ever been produced, it currently has no uses outside of basic scientific research. It would constitute a radiation hazard if enough were ever assembled in one place.[51]

References[redigér wikikode]

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See also[redigér wikikode]

Wikinews Wikinews: Controversy-Plagued Element 118, the Heaviest Atom Yet, Finally Discovered – relateret nyhedssag på engelsk
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