Editing Extended periodic table (section)

{{Short description|Periodic table of the elements with eight or more periods}}
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{{Periodic table (micro)|number=119|mark=Uue|title=Extended periodic table|caption= Element 119 ([[Ununennium|Uue]], marked here) in period 8 (row 8) marks the start of theorisations.}}
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An '''extended periodic table''' theorizes about [[chemical elements]] beyond those currently known and proven. The element with the highest [[atomic number]] known is [[oganesson]] (''Z''&nbsp;=&nbsp;118), which completes the seventh [[period (periodic table)|period]] (row) in the [[periodic table]]. All elements in the eighth period and beyond thus remain purely hypothetical.

Elements beyond 118 will be placed in additional periods when discovered, laid out (as with the existing periods) to illustrate periodically recurring trends in the properties of the elements. Any additional periods are expected to contain more 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-[[atomic orbital|orbitals]] in each period. An ''eight-period table'' containing this block was suggested by [[Glenn T. Seaborg]] in 1969.<ref name=LBL>{{cite web|url=http://www.lbl.gov/LBL-PID/Nobelists/Seaborg/65th-anniv/29.html|title=An Early History of LBNL|first=Glenn T.|last=Seaborg|author-link=Glenn T. Seaborg|date=August 26, 1996|access-date=2011-02-25|archive-date=2010-11-15|archive-url=https://web.archive.org/web/20101115025829/http://www.lbl.gov/LBL-PID/Nobelists/Seaborg/65th-anniv/29.html|url-status=dead}}</ref><ref name=SHE78>{{cite journal | doi = 10.2307/3963006 | last1 = Frazier | first1 = K. | title = Superheavy Elements | journal = Science News | volume = 113 | issue = 15 | pages = 236–238 | year = 1978 | jstor = 3963006}}</ref> The first element of the g-block may have atomic number 121, and thus would have the [[Systematic element name|systematic name]] [[unbiunium]]. Despite many searches, no elements in this region have been [[Synthetic element|synthesized]] or discovered in nature.<ref>[[Unbibium|Element 122]] was claimed to exist naturally in April 2008, but this claim was widely believed to be erroneous. {{cite web|url=http://www.rsc.org/chemistryworld/News/2008/May/02050802.asp|title=Heaviest element claim criticised|publisher=Rsc.org|date=2008-05-02|access-date=2010-03-16}}</ref>

According to the orbital approximation in [[quantum mechanics|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. 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 quantum chemistry|relativistic effects]]. Models that take relativistic effects into account predict that the pattern will be broken. [[Pekka Pyykkö]] and [[Burkhard Fricke]] used computer modeling to calculate the positions of elements up to ''Z'' = 172, and found that several were displaced from the [[Aufbau principle#Madelung energy ordering rule|Madelung rule]].<ref name="Fricke">{{cite journal |last1=Fricke |first1=B. |last2=Greiner |first2=W. |last3=Waber |first3=J. T. |year=1971 |title=The continuation of the periodic table up to Z = 172. The chemistry of superheavy elements |journal=Theoretica Chimica Acta |volume=21 |issue=3 |pages=235–260 |doi=10.1007/BF01172015|s2cid=117157377 }}</ref> As a result of uncertainty and variability in predictions of chemical and physical properties of elements beyond 120, there is currently no consensus on their placement in the extended periodic table.

Elements in this region are likely to be highly unstable with respect to [[radioactive decay]] and undergo [[alpha decay]] or [[spontaneous fission]] with extremely short [[half-life|half-lives]], though [[unbihexium|element 126]] is hypothesized to be within an [[island of stability]] that is resistant to fission but not to alpha decay. Other islands of stability beyond the known elements may also be possible, including one theorised around element 164, though the extent of stabilizing effects from closed [[nuclear shell model|nuclear shells]] is uncertain. It is not clear how many elements beyond the expected island of stability are physically possible, whether period 8 is complete, or if there is a period 9. The [[International Union of Pure and Applied Chemistry]] (IUPAC) defines an element to exist if its lifetime is longer than 10<sup>−14</sup> seconds (0.01 picoseconds, or 10 femtoseconds), which is the time it takes for the nucleus to form an [[electron cloud]].<ref>{{Cite web |title=Kernchemie |url=http://www.kernchemie.de/Transactinides/Transactinide-2/transactinide-2.html |access-date=2014-11-09 |website=kernchemie.de}}</ref>

As early as 1940, it was noted that a simplistic interpretation of the [[Theory of relativity|relativistic]] [[Dirac equation]] runs into problems with electron orbitals at ''Z''&nbsp;>&nbsp;1/α&nbsp;≈&nbsp;137.036 (the reciprocal of the [[fine-structure constant]]), suggesting that neutral atoms cannot exist beyond element 137, and that a periodic table of elements based on electron orbitals therefore breaks down at this point.<ref>{{cite journal|last1=Schiff |first1=L. I. |last2=Snyder |first2=H. |last3=Weinberg |first3=J. |title=On the Existence of Stationary States of the Mesotron Field |journal=[[Physical Review]] |volume=57 |issue=4 |pages=315–318 |year=1940 |doi=10.1103/PhysRev.57.315|bibcode=1940PhRv...57..315S }}</ref> On the other hand, a more rigorous analysis calculates the analogous limit to be ''Z''&nbsp;≈&nbsp;168–172 where the 1s subshell dives into the [[Dirac sea]], and that it is instead not neutral atoms that cannot exist beyond this point, but bare nuclei, thus posing no obstacle to the further extension of the periodic system. Atoms beyond this critical atomic number are called ''supercritical'' atoms.