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A period 8 element is any one of 46 hypothetical chemical elements (ununennium through unhexquadium) belonging to an eighth period of the periodic table of the elements. They may be referred to using IUPAC systematic element names. None of these elements have been synthesized, and it is possible that none have isotopes with stable enough nuclei to receive significant attention in the near future. It is also probable that, due to drip instabilities, only the lower period 8 elements are physically possible and the periodic table may end soon after the island of stability at unbihexium with atomic number 126. The names given to these unattested elements are all IUPAC systematic names.
If it were possible to produce sufficient quantities of sufficiently long-lived isotopes of these elements that would allow the study of their chemistry, these elements may well behave very differently from those of previous periods. This is because their electronic configurations may be altered by quantum and relativistic effects, as the energy levels of the 5g, 6f, 7d and 8p1/2 orbitals are so close to each other that they may well exchange electrons with each other. This would result in a large number of elements in the superactinide series that would have extremely similar chemical properties that would be quite unrelated to elements of lower atomic number.
History
There are currently seven periods in the periodic table of chemical elements, culminating with atomic number 118. If further elements with higher atomic numbers than this are discovered, they will be placed in additional periods, laid out (as with the existing periods) to illustrate periodically recurring trends in the properties of the elements concerned. Any additional periods are expected to contain a larger number of elements than the seventh period, as they are calculated to contain elements with filled g-orbitals in their ground state. An eight-period table containing these elements was suggested by Glenn T. Seaborg in 1969. No elements in this region have been synthesized or discovered in nature. While Seaborg's version of the extended period had the heavier elements following the pattern set by lighter elements, as it did not take into account relativistic effects, models that take relativistic effects into account do not. Pekka Pyykkö and B. Fricke used computer modeling to calculate the positions of elements up to Z = 172 (comprising periods 8 and 9), and found that several were displaced from the Madelung rule. Fricke predicted the structure of the extended periodic table up to Z = 172 to be:
Predicted properties
Chemical and physical properties
8s elements
The first two elements of period 8 are expected to be ununennium and unbinilium, elements 119 and 120. Their electron configurations should have the 8s shell being filled. However, the 8s orbital is relativistically stabilized and contracted and thus, elements 119 and 120 should be more like caesium and barium than their immediate neighbours above, francium and radium. Another effect of the relativistic contraction of the 8s orbital is that the atomic radii of these two elements should be about the same of those of francium and radium. They should behave like normal alkali and alkaline earth metals, normally forming +1 and +2 oxidation states respectively, but the relativistic destabilization of the 7p3/2 subshell and the relatively low ionization energies of the 7p3/2 electrons should make higher oxidation states like +3 and +4 (respectively) possible as well.
Superactinides
The superactinide series is expected to contain elements 121 to 155. In the superactinide series, the 7d3/2, 8p1/2, 6f5/2 and 5g7/2 shells should all fill simultaneously. The first superactinide, unbiunium (element 121), should be a congener of lanthanum and actinium and should have similar properties to them. In the first few superactinides, the binding energies of the added electrons are predicted to be small enough that they can lose all their valence electrons; for example, unbihexium (element 126) would usually form a +8 oxidation state, and even higher oxidation states for the next few elements may be possible. The presence of electrons in g-orbitals, which do not exist in the ground state electron configuration of any currently known element, should allow presently unknown hybrid orbitals to form and influence the chemistry of the superactinides in new ways, although the absence of g electrons in known elements makes predicting their chemistry more difficult.
In the later superactinides, the oxidation states should become lower. By element 132, the predominant most stable oxidation state will be only +6; this is further reduced to +3 and +4 by element 144, and at the end of the superactinide series it will be only +2 (and possibly even 0) because the 6f shell, which is being filled at that point, is deep inside the electron cloud and the 8s and 8p1/2 electrons are bound too strongly to be chemically active. The 5g shell should be filled at element 144 and the 6f shell at around element 154, and at this region of the superactinides the 8p1/2 electrons are bound so strongly that they are no longer active chemically, so that only a few electrons can participate in chemical reactions. Calculations by Fricke et al. predict that at element 154, the 6f shell is full and there are no d- or other electron wave functions outside the chemically inactive 8s and 8p1/2 shells. This would cause element 154 to be very unreactive, so that it may exhibit properties similar to those of the noble gases.
Similarly to the lanthanide and actinide contractions, there should be a superactinide contraction in the superactinide series where the ionic radii of the superactinides are smaller than expected. In the lanthanides, the contraction is about 4.4 pm per element; in the actinides, it is about 3 pm per element. The contraction is larger in the lanthanides than in the actinides due to the greater localization of the 4f wave function as compared to the 5f wave function. Comparisons with the wave functions of the outer electrons of the lanthanides, actinides, and superactinides lead to a prediction of a contraction of about 2 pm per element in the superactinides; although this is smaller than the contractions in the lanthanides and actinides, its total effect is larger due to the fact that 32 electrons are filled in the deeply buried 5g and 6f shells, instead of just 14 electrons being filled in the 4f and 5f shells in the lanthanides and actinides respectively.
Transition metals
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Nuclear properties
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Synthesis
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See also
Notes
- The heaviest element that has been synthesized to date is ununoctium with atomic number 118, which is the last period 7 element.
References
- Emsley, John (2011). Nature's Building Blocks: An A-Z Guide to the Elements (New ed.). New York, NY: Oxford University Press. ISBN 978-0-19-960563-7.
- Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1063/1.1672054, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with
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- ^ Fricke, B.; Greiner, W.; Waber, J. T. (1971). "The continuation of the periodic table up to Z = 172. The chemistry of superheavy elements". Theoretica chimica acta. 21 (3). Springer-Verlag: 235–260. doi:10.1007/BF01172015. Retrieved 28 November 2012.
- Seaborg, Glenn (August 26, 1996). "An Early History of LBNL".
- Frazier, K. (1978). "Superheavy Elements". Science News. 113 (15): 236–238. doi:10.2307/3963006. JSTOR 3963006.
- "Extended elements: new periodic table". 2010.
- ^ Haire, Richard G. (2006). "Transactinides and the future elements". In Morss; Edelstein, Norman M.; Fuger, Jean (eds.). The Chemistry of the Actinide and Transactinide Elements (3rd ed.). Dordrecht, The Netherlands: Springer Science+Business Media. ISBN 1-4020-3555-1.
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