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An extended periodic table theorizes about elements beyond element 118 (beyond period 7, or row 7). Currently seven periods in the periodic table of chemical elements are known and proven, 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 have an additional so-called g-block, containing at least 18 elements with partially filled g-orbitals in each period. An eight-period table containing this block was suggested by Glenn T. Seaborg in 1969. IUPAC defines an element to exist if its lifetime is longer than 10−14 seconds, which is the time it takes for the nucleus to form an electronic cloud.〔(Kernchemie )〕 No elements in this region have been synthesized or discovered in nature.〔Element 122 was claimed to exist naturally in April 2008, but this claim was widely believed to be erroneous. 〕 The first element of the g-block may have atomic number 121, and thus would have the systematic name unbiunium. Elements in this region are likely to be highly unstable with respect to radioactive decay, and have extremely short half lives, although element 126 is hypothesized to be within an island of stability that is resistant to fission but not to alpha decay. It is not clear how many elements beyond the expected island of stability are physically possible, if period 8 is complete, or if there is a period 9. According to the orbital approximation in quantum mechanical descriptions of atomic structure, the g-block would correspond to elements with partially filled g-orbitals, but spin-orbit coupling effects reduce the validity of the orbital approximation substantially for elements of high atomic number. 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'' = 184 (comprising periods 8, 9, and the beginning of 10), and found that several were displaced from the Madelung rule. ==History== It is unknown how far the periodic table might extend beyond the known 118 elements. Glenn T. Seaborg suggested that the highest possible element may be under ''Z'' = 130,〔 while Walter Greiner predicted that there may not be a highest possible element.〔 All of these hypothetical undiscovered elements are named by the International Union of Pure and Applied Chemistry (IUPAC) systematic element name standard which creates a generic name for use until the element has been discovered, confirmed, and an official name approved. These names are typically not used in the literature, and are referred to by their atomic numbers; hence, element 164 would usually not be called "unhexquadium" (the IUPAC systematic name), but rather "element 164" with symbol "164", "(164)", or "E164". , synthesis has been attempted for only ununennium, unbinilium, unbibium, unbiquadium, unbihexium, and unbiseptium. (Z = 119, 120, 122, 124, 126, and 127) At element 118, the orbitals 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d, 5f, 6s, 6p, 6d, 7s and 7p are assumed to be filled, with the remaining orbitals unfilled. A simple extrapolation from the Aufbau principle would predict the eighth row to fill orbitals in the order 8s, 5g, 6f, 7d, 8p; but after element 120, the proximity of the electron shells makes placement in a simple table problematic. Although a simple extrapolation of the periodic table, following Seaborg's original concept, would put the elements after 120 as follows: 121-138 form the g-block superactinoids; 139-152 form the f-block superactinoids, 153-162 would be transition metals; 163-166 p-block metals; 167=halogen; 168=noble gas; 169=alkali metal; 170=alkaline earth metal, Dirac-Fock calculations predict that it will most likely go: 121-140 form the g-block superactinoids; 141-154 form the f-block superactinoids; 155-164 form the transition metals; 165=alkali metal; 166=alkaline earth metal; 167-170 p-block metals; 171=halogen; 172=noble gas. Not all models show the higher elements following the pattern established by lighter elements. Pekka Pyykkö, for example, used computer modeling to calculate the positions of elements up to Z=172, and found that several were displaced from the Madelung energy-ordering rule.〔 He predicts that the orbital shells will fill up in this order: *8s, *5g, *the first two spaces of 8p, *6f, *7d, *9s, *the first two spaces of 9p, *the rest of 8p. He also suggests that period 8 be split into three parts: *8a, containing 8s, *8b, containing the first two elements of 8p, *8c, containing 7d and the rest of 8p. Fricke ''et al.'' also predicted the extended periodic table up to 184.〔 This model has been more widely used among scientists and is shown above as the main form of the extended periodic table. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「extended periodic table」の詳細全文を読む スポンサード リンク
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