Editing Extended periodic table (section)

===Nuclear properties===
{{multiple image
| direction =vertical
| width = 540
| image1 = Island of Stability derived from Zagrebaev.svg
| image2 = Superheavy decay modes predicted.png
| footer = Predicted half-lives (top) and decay modes (bottom) of superheavy nuclei. The line of synthesized proton-rich nuclei is expected to be broken soon after ''Z'' = 120, because of half-lives shorter than 1 microsecond from ''Z'' = [[unbiunium|121]], the increasing contribution of spontaneous fission instead of alpha decay from ''Z'' = [[unbibium|122]] onward until it dominates from ''Z'' = 125, and the proton [[nuclear drip line|drip line]] around ''Z'' = 130. The white rings denote the expected location of the island of stability; the two squares outlined in white denote <sup>291</sup>[[copernicium|Cn]] and <sup>293</sup>Cn, predicted to be the longest-lived nuclides on the island with half-lives of centuries or millennia.<ref name=Karpov/> The black square near the bottom of the second picture is [[uranium-238]], the heaviest confirmed [[primordial nuclide]] (a nuclide stable enough to have survived from the Earth's formation to the present day).
}}

====Magic numbers and the island of stability====
The stability of nuclei decreases greatly with the increase in atomic number after [[curium]], element 96, so that all isotopes with an atomic number above [[mendelevium|101]] [[radioactive decay|decay radioactively]] with a [[half-life]] under a day. No elements with [[atomic number]]s above 82 (after [[lead]]) have stable isotopes.<ref>{{cite journal|last = Marcillac|first = Pierre de |author2= Noël Coron|author3= Gérard Dambier|author4= Jacques Leblanc|author5= Jean-Pierre Moalic|date=April 2003|title = Experimental detection of α-particles from the radioactive decay of natural bismuth|journal = Nature|volume = 422|pages = 876–878|pmid=12712201|doi = 10.1038/nature01541|issue = 6934|bibcode = 2003Natur.422..876D|s2cid = 4415582 }}</ref> Nevertheless, because of [[magic number (physics)|reasons]] not very well understood yet, there is a slight increased nuclear stability around atomic numbers [[darmstadtium|110]]–[[flerovium|114]], which leads to the appearance of what is known in nuclear physics as the "[[island of stability]]". This concept, proposed by [[University of California, Berkeley|University of California]] professor [[Glenn Seaborg]], explains why [[superheavy element]]s last longer than predicted.<ref>{{cite book|title=Van Nostrand's scientific encyclopedia|first1=Glenn D. |last1= Considine |first2=Peter H. |last2= Kulik|publisher=Wiley-Interscience |year=2002|edition=9|isbn=978-0-471-33230-5|oclc=223349096}}</ref>

Calculations according to the [[Hartree–Fock method|Hartree–Fock–Bogoliubov method]] using the non-relativistic [[Skyrme interaction]] have proposed ''Z''&nbsp;=&nbsp;126 as a [[nuclear shell model|closed proton shell]]. In this region of the periodic table, ''N''&nbsp;=&nbsp;184, ''N''&nbsp;=&nbsp;196, and ''N''&nbsp;=&nbsp;228 have been suggested as closed neutron shells. Therefore, the isotopes of most interest are <sup>310</sup>126, <sup>322</sup>126, and <sup>354</sup>126, for these might be considerably longer-lived than other isotopes. Element 126, having a [[magic number (physics)|magic number]] of [[proton]]s, is predicted to be more stable than other elements in this region, and may have [[nuclear isomer]]s with very long [[half-life|half-lives]].<ref name="emsley2"/> It is also possible that the [[island of stability]] is instead centered at <sup>306</sup>[[unbibium|122]], which may be spherical and [[doubly magic]].<ref name="Kratz"/> Probably, the island of stability occurs around ''Z''&nbsp;=&nbsp;114–126 and ''N''&nbsp;=&nbsp;184, with lifetimes probably around hours to days. Beyond the shell closure at ''N''&nbsp;=&nbsp;184, spontaneous fission lifetimes should drastically drop below 10<sup>−15</sup> seconds – too short for a nucleus to obtain an electron cloud and participate in any chemistry. That being said, such lifetimes are very model-dependent, and predictions range across many orders of magnitude.<ref name=gamowstates/>

Taking nuclear deformation and relativistic effects into account, an analysis of single-particle levels predicts new magic numbers for superheavy nuclei at ''Z''&nbsp;=&nbsp;126, 138, 154, and 164 and ''N''&nbsp;=&nbsp;228, 308, and 318.<ref name=fossilfission>{{cite web|last1=Maly|first1=J.|last2=Walz|first2=D.R.|title=Search for superheavy elements among fossil fission tracks in zircon|date=1980|url=http://www.slac.stanford.edu/pubs/slacpubs/2500/slac-pub-2554.pdf|access-date=2018-12-07}}</ref><ref name=magickoura>{{cite journal|last1=Koura|first1=H.|last2=Chiba|first2=S.|date=2013|title=Single-Particle Levels of Spherical Nuclei in the Superheavy and Extremely Superheavy Mass Region|journal=Journal of the Physical Society of Japan|volume=82|issue=1|pages=014201|url=https://www.researchgate.net/publication/258799250 |doi=10.7566/JPSJ.82.014201|bibcode=2013JPSJ...82a4201K}}</ref> Therefore, in addition to the island of stability centered at <sup>291</sup>Cn, <sup>293</sup>Cn,<ref name=Zagrebaev/> and <sup>298</sup>Fl, further islands of stability may exist around the doubly magic <sup>354</sup>126 as well as <sup>472</sup>164 or <sup>482</sup>164.<ref name="eurekalert.org"/><ref name="link.springer.com"/> These nuclei are predicted to be [[beta-decay stable isobars|beta-stable]] and decay by alpha emission or spontaneous fission with relatively long half-lives, and confer additional stability on neighboring ''N''&nbsp;=&nbsp;228 [[isotone]]s and elements 152–168, respectively.<ref name=SHlimit>{{cite conference|last=Koura|first=H.|date=2011|title=Decay modes and a limit of existence of nuclei in the superheavy mass region|url=http://tan11.jinr.ru/pdf/10_Sep/S_2/05_Koura.pdf|conference=4th International Conference on the Chemistry and Physics of the Transactinide Elements|access-date=18 November 2018}}</ref> On the other hand, the same analysis suggests that proton shell closures may be relatively weak or even nonexistent in some cases such as <sup>354</sup>126, meaning that such nuclei might not be doubly magic and stability will instead be primarily determined by strong neutron shell closures.<ref name=magickoura/> Additionally, due to the enormously greater forces of [[coulomb repulsion|electromagnetic repulsion]] that must be overcome by the strong force at the second island (''Z''&nbsp;=&nbsp;164),<ref name=greinernuclei>{{cite journal|last=Greiner|first=W.|date=2013|title=Nuclei: superheavy-superneutronic-strange-and of antimatter|url=http://inspirehep.net/record/1221632/files/jpconf13_413_012002.pdf|journal=Journal of Physics: Conference Series|volume=413|issue=1|pages=012002|doi=10.1088/1742-6596/413/1/012002|bibcode=2013JPhCS.413a2002G|doi-access=free}}</ref> it is possible that nuclei around this region only exist as [[resonance (particle physics)|resonances]] and cannot stay together for a meaningful amount of time. It is also possible that some of the superactinides between these series may not actually exist because they are too far from both islands,<ref name=greinernuclei/> in which case the periodic table might end around ''Z''&nbsp;=&nbsp;130.<ref name="BFricke"/> The area of elements 121–156 where periodicity is in abeyance is quite similar to the gap between the two islands.<ref name=primefan/>

Beyond element 164, the [[fissility]] line defining the limit of stability with respect to spontaneous fission may converge with the [[neutron drip line]], posing a limit to the existence of heavier elements.<ref name=SHlimit/> Nevertheless, further magic numbers have been predicted at ''Z''&nbsp;=&nbsp;210, 274, and 354 and ''N''&nbsp;=&nbsp;308, 406, 524, 644, and 772,<ref name=Denisov>{{cite journal|last=Denisov|first=V.|date=2005|title=Magic numbers of ultraheavy nuclei|journal=Physics of Atomic Nuclei|url= https://www.researchgate.net/publication/225734594|volume=68|issue=7|pages=1133–1137|doi=10.1134/1.1992567|bibcode=2005PAN....68.1133D|s2cid=119430168}}</ref> with two beta-stable doubly magic nuclei found at <sup>616</sup>210 and <sup>798</sup>274; the same calculation method reproduced the predictions for <sup>298</sup>Fl and <sup>472</sup>164. (The doubly magic nuclei predicted for ''Z''&nbsp;=&nbsp;354 are beta-unstable, with <sup>998</sup>354 being neutron-deficient and <sup>1126</sup>354 being neutron-rich.) Although additional stability toward alpha decay and fission are predicted for <sup>616</sup>210 and <sup>798</sup>274, with half-lives up to hundreds of microseconds for <sup>616</sup>210,<ref name=Denisov/> there will not exist islands of stability as significant as those predicted at ''Z''&nbsp;=&nbsp;114 and 164. As the existence of superheavy elements is very strongly dependent on stabilizing effects from closed shells, nuclear instability and fission will likely determine the end of the periodic table beyond these islands of stability.<ref name="BFricke"/><ref name=limit146/><ref name=SHlimit/>

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, which is the time it takes for the nucleus to form an electron cloud. However, a [[nuclide]] is generally considered to exist if its lifetime is longer than about 10<sup>−22</sup> seconds, which is the time it takes for [[nuclear structure]] to form. Consequently, it is possible that some ''Z'' values can only be realised in nuclides and that the corresponding elements do not exist.<ref name=colloq/>

It is also possible that no further islands actually exist beyond 126, as the nuclear shell structure gets smeared out (as the electron shell structure already is expected to be around oganesson) and low-energy decay modes become readily available.<ref name=relqed>{{cite journal |last1=Schwerdtfeger |first1=Peter |last2=Pašteka |first2=Lukáš F. |last3=Punnett |first3=Andrew |last4=Bowman |first4=Patrick O. |date=2015 |title=Relativistic and quantum electrodynamic effects in superheavy elements |journal=Nuclear Physics A |volume=944 |issue=December 2015 |pages=551–577 |doi=10.1016/j.nuclphysa.2015.02.005|bibcode=2015NuPhA.944..551S }}</ref>

In some regions of the table of nuclides, there are expected to be additional regions of stability due to non-spherical nuclei that have different magic numbers than spherical nuclei do; the egg-shaped <sup>270</sup>[[Hassium|Hs]] {{nowrap|1=(''Z'' = 108, ''N'' = 162)}} is one such deformed doubly magic nucleus.<ref>{{cite journal|last1=Dvorak|first1=J.|last2=Brüchle|first2=W.|last3=Chelnokov|first3=M.|last4=Dressler|first4=R.|last5=Düllmann|first5=Ch. E.|last6=Eberhardt|first6=K.|last7=Gorshkov|first7=V.|last8=Jäger|first8=E.|last9=Krücken|first9=R.|last10=Kuznetsov|first10=A.|last11=Nagame|first11=Y.|last12=Nebel|first12=F.|last13=Novackova|first13=Z.|last14=Qin|first14=Z.|last15=Schädel|first15=M.|last16=Schausten|first16=B.|last17=Schimpf|first17=E.|last18=Semchenkov|first18=A.|last19=Thörle|first19=P.|last20=Türler|first20=A.|last21=Wegrzecki|first21=M.|last22=Wierczinski|first22=B.|last23=Yakushev|first23=A.|last24=Yeremin|first24=A.|title=Doubly Magic Nucleus <sub>108</sub><sup>270</sup>Hs<sub>162</sub> |journal=Physical Review Letters|volume=97|issue=24|pages=242501|year=2006|doi=10.1103/PhysRevLett.97.242501|pmid=17280272|bibcode=2006PhRvL..97x2501D|url=https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A16351}}</ref> In the superheavy region, the strong Coulomb repulsion of protons may cause some nuclei, including isotopes of oganesson, to assume a bubble shape in the ground state with a reduced central density of protons, unlike the roughly uniform distribution inside most smaller nuclei.<ref>{{cite journal |last1=LaForge |first1=Evan |last2=Price |first2=Will |last3=Rafelski |first3=Johann |title=Superheavy elements and ultradense matter |journal=The European Physical Journal Plus |date=15 September 2023 |volume=138 |issue=9 |page=812 |doi=10.1140/epjp/s13360-023-04454-8|arxiv=2306.11989 |bibcode=2023EPJP..138..812L }}</ref><ref>{{cite news |title=Physicists are pushing the periodic table to its limits {{!}} Science News |url=https://www.sciencenews.org/article/physics-periodic-table-future-superheavy-elements |access-date=25 December 2023 |date=27 February 2019}}</ref> Such a shape would have a very low fission barrier, however.<ref>{{cite journal |last1=Dechargé |first1=J. |last2=Berger |first2=J.-F. |last3=Girod |first3=M. |last4=Dietrich |first4=K. |title=Bubbles and semi-bubbles as a new kind of superheavy nuclei |journal=Nuclear Physics A |date=March 2003 |volume=716 |pages=55–86 |doi=10.1016/S0375-9474(02)01398-2|bibcode=2003NuPhA.716...55D }}</ref> Even heavier nuclei in some regions, such as <sup>342</sup>136 and <sup>466</sup>156, may instead become [[torus|toroidal]] or [[red blood cell]]-like in shape, with their own magic numbers and islands of stability, but they would also fragment easily.<ref>{{cite journal |last1=Agbemava |first1=S. E. |last2=Afanasjev |first2=A. V. |title=Hyperheavy spherical and toroidal nuclei: The role of shell structure |journal=Physical Review C |date=25 March 2021 |volume=103 |issue=3 |pages=034323 |doi=10.1103/PhysRevC.103.034323 |arxiv=2012.13799|bibcode=2021PhRvC.103c4323A }}</ref><ref>{{cite journal |last1=Afanasjev |first1=A.V. |last2=Agbemava |first2=S.E. |last3=Gyawali |first3=A. |title=Hyperheavy nuclei: Existence and stability |journal=Physics Letters B |date=July 2018 |volume=782 |pages=533–540 |doi=10.1016/j.physletb.2018.05.070|doi-access=free |arxiv=1804.06395 |bibcode=2018PhLB..782..533A }}</ref>

====Predicted decay properties of undiscovered elements====
As the main island of stability is thought to lie around <sup>291</sup>Cn and <sup>293</sup>Cn, undiscovered elements beyond [[oganesson]] may be very unstable and undergo [[alpha decay]] or [[spontaneous fission]] in microseconds or less. The exact region in which half-lives exceed one microsecond is unknown, though various models suggest that isotopes of elements heavier than [[unbinilium]] that may be produced in fusion reactions with available targets and projectiles will have half-lives under one microsecond and therefore may not be detected.<ref name=Karpov/> It is consistently predicted that there will exist regions of stability at ''N'' = 184 and ''N'' = 228, and possibly also at ''Z'' ~ 124 and ''N'' ~ 198. These nuclei may have half-lives of a few seconds and undergo predominantly alpha decay and spontaneous fission, though minor [[positron emission|beta-plus decay]] (or [[electron capture]]) branches may also exist.<ref name=Palenzuela>{{cite journal|last1=Palenzuela|first1=Y. M.|last2=Ruiz|first2=L. F.|last3=Karpov|first3=A.|last4=Greiner|first4=W.|year=2012|title=Systematic Study of Decay Properties of Heaviest Elements|journal=Bulletin of the Russian Academy of Sciences: Physics|volume=76|issue=11|pages=1165–1171|issn=1062-8738|url=http://nrv.jinr.ru/karpov/publications/Palenzuela12_BRAS.pdf|doi=10.3103/S1062873812110172|bibcode=2012BRASP..76.1165P|s2cid=120690838}}</ref> Outside these regions of enhanced stability, [[fission barrier]]s are expected to drop significantly due to loss of stabilization effects, resulting in fission half-lives below [[attosecond|10<sup>−18</sup>]] seconds, especially in [[even and odd atomic nuclei|even–even nuclei]] for which hindrance is even lower due to [[nucleon pair breaking in fission|nucleon pairing]].<ref name=SHlimit/> In general, alpha decay half-lives are expected to increase with neutron number, from nanoseconds in the most neutron-deficient isotopes to seconds closer to the [[beta-stability line]].<ref name="sciencedirect1"/> For nuclei with only a few neutrons more than a magic number, [[nuclear binding energy|binding energy]] substantially drops, resulting in a break in the trend and shorter half-lives.<ref name="sciencedirect1"/> The most neutron deficient isotopes of these elements may also be unbound and undergo [[proton emission]]. [[Cluster decay]] (heavy particle emission) has also been proposed as an alternative decay mode for some isotopes,<ref>{{cite journal |last1=Poenaru |first1=Dorin N. |last2=Gherghescu |first2=R. A. |last3=Greiner |first3=W. |date=2012 |title=Cluster decay of superheavy nuclei |url=https://www.researchgate.net/publication/235507943 |journal=Physical Review C |volume=85 |issue=3 |pages=034615 |doi=10.1103/PhysRevC.85.034615 |access-date=2 May 2017|bibcode=2012PhRvC..85c4615P }}</ref> posing yet another hurdle to identification of these elements.