Editing Extended periodic table (section)

==History==
Elements beyond the [[actinide]]s were first proposed to exist as early as 1895, when Danish chemist [[Hans Peter Jørgen Julius Thomsen]] predicted that [[thorium]] and [[uranium]] formed part of a 32-element period which would end at a chemically inactive element with atomic weight 292 (not far from the 294 for the only known isotope of [[oganesson]]). In 1913, Swedish physicist [[Johannes Rydberg]] similarly predicted that the next noble gas after [[radon]] would have atomic number 118, and purely formally derived even heavier congeners of radon at ''Z''&nbsp;=&nbsp;168, 218, 290, 362, and 460, exactly where the [[Aufbau principle]] would predict them to be. In 1922, [[Niels Bohr]] predicted the electronic structure of this next [[noble gas]] at ''Z''&nbsp;=&nbsp;118, and suggested that the reason why elements beyond uranium were not seen in nature was because they were too unstable. The German physicist and engineer Richard Swinne published a review paper in 1926 containing predictions on the [[transuranic element]]s (he may have coined the term) in which he anticipated modern predictions of an [[island of stability]]: he first hypothesised in 1914 that half-lives should not decrease strictly with atomic number, but suggested instead that there might be some longer-lived elements at ''Z''&nbsp;=&nbsp;98–102 and ''Z''&nbsp;=&nbsp;108–110, and speculated that such elements might exist in the [[Earth's core]], in [[iron meteorites]], or in the [[Greenland ice sheet|ice caps of Greenland]] where they had been locked up from their supposed cosmic origin.<ref>{{cite book |last=Kragh |first=Helge |date=2018 |title=From Transuranic to Superheavy Elements: A Story of Dispute and Creation |publisher=Springer |pages=6–10 |isbn=9783319758138}}</ref> By 1955, these elements were called ''superheavy'' elements.<ref name=Transuraniumppl/>

The first predictions on properties of undiscovered superheavy elements were made in 1957, when the concept of [[nuclear shell model|nuclear shells]] was first explored and an island of stability was theorized to exist around element&nbsp;126.<ref name=fossilfission/> In 1967, more rigorous calculations were performed, and the island of stability was theorized to be centered at the then-undiscovered [[flerovium]] (element 114); this and other subsequent studies motivated many researchers to search for superheavy elements in nature or attempt to [[synthetic element|synthesize]] them at accelerators.<ref name=Transuraniumppl/> Many searches for superheavy elements were conducted in the 1970s, all with negative results. {{As of|2022|4}}, synthesis has been attempted for every element up to and including unbiseptium (''Z''&nbsp;=&nbsp;127), except unbitrium (''Z''&nbsp;=&nbsp;123),<ref name="emsley"/><ref name=beyonduranium/><ref name=superlourds/> with the heaviest successfully synthesized element being [[oganesson]] in 2002 and the most recent discovery being that of [[tennessine]] in 2010.<ref name="emsley"/>

As some superheavy elements were predicted to lie beyond the seven-period periodic table, an additional eighth period containing these elements was first proposed by [[Glenn T. Seaborg]] in 1969. This model continued the pattern in established elements and introduced a new g-block and superactinide series beginning at element&nbsp;121, raising the number of elements in period 8 compared to known periods.<ref name=LBL/><ref name=SHE78/><ref name=Transuraniumppl/> These early calculations failed to consider relativistic effects that break down periodic trends and render simple extrapolation impossible, however. In 1971, Fricke calculated the periodic table up to ''Z''&nbsp;=&nbsp;172, and discovered that some elements indeed had different properties that break the established pattern,<ref name="Fricke"/> and a 2010 calculation by [[Pekka Pyykkö]] also noted that several elements might behave differently than expected.<ref name="PT172"/> It is unknown how far the periodic table might extend beyond the known 118 elements, as heavier elements are predicted to be increasingly unstable. [[Glenn T. Seaborg]] suggested that practically speaking, the end of the periodic table might come as early as around ''Z''&nbsp;=&nbsp;120 due to nuclear instability.<ref name=EB/>