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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 IUPACsystematic 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 8p 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.
Predicted properties
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Synthesis
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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. ISBN978-0-19-960563-7.
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