Editing Island of stability (section)

==Introduction==
===Nuclide stability===
{{see also|Valley of stability}}
[[File:Isotopes and half-life.svg|thumb|left|upright=1.2|alt=Complete chart of nuclide half-lives plotted against atomic number Z and neutron number N axes.|Chart of half-lives of known nuclides]]

The composition of a [[nuclide]] ([[atomic nucleus]]) is defined by the [[number of protons]] ''Z'' and the [[number of neutrons]] ''N'', which sum to [[mass number]] ''A''. Proton number ''Z'', also named the atomic number, determines the position of an [[chemical element|element]] in the [[periodic table]]. The approximately 3300 known nuclides<ref name=thoennesweb>{{cite web |last=Thoennessen |first=M. |title=Discovery of Nuclides Project |url=https://people.nscl.msu.edu/~thoennes/isotopes/index.html |date=2018 |access-date=13 September 2019}}</ref> are commonly represented in a [[table of nuclides|chart]] with ''Z'' and ''N'' for its axes and the [[half-life]] for [[radioactive decay]] indicated for each unstable nuclide (see figure).<ref>{{harvnb|Podgorsak|2016|p=512}}</ref> {{As of|2019}}, 251 nuclides are observed to be [[stable nuclide|stable]] (having never been observed to decay);<ref>{{cite web |date=2017 |title=Atomic structure |url=https://www.arpansa.gov.au/understanding-radiation/what-is-radiation/ionising-radiation/atomic-structure |website=Australian Radiation Protection and Nuclear Safety Agency |publisher=Commonwealth of Australia |access-date=16 February 2019}}</ref> generally, as the number of protons increases, stable nuclei have a higher [[neutron–proton ratio]] (more neutrons per proton). The last element in the periodic table that has a stable [[isotope]] is [[lead]] (''Z''&nbsp;=&nbsp;82),{{efn|The heaviest stable element was believed to be bismuth (atomic number 83) until 2003, when its only stable isotope, [[bismuth-209|<sup>209</sup>Bi]], was observed to undergo alpha decay.<ref>{{cite journal|last1 = Marcillac|first1 = P.|last2=Coron |first2=N. |last3=Dambier |first3=G. |last4=Leblanc |first4=J. |last5=Moalic |first5=J.-P. |date=2003 |display-authors=3 |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>}}{{efn|It is theoretically possible for other [[observationally stable]] nuclides to decay, though their predicted half-lives are so long that this process has never been observed.<ref name=bellidecay>{{cite journal |last1=Belli |first1=P. |last2=Bernabei |first2=R. |last3=Danevich |first3=F. A. |last4=Incicchitti |first4=A. |last5=Tretyak |first5=V. I. |display-authors=3 |title=Experimental searches for rare alpha and beta decays |journal=European Physical Journal A |date=2019 |volume=55 |issue=8 |pages=140-1–140-7 |doi=10.1140/epja/i2019-12823-2 |issn=1434-601X |arxiv=1908.11458|bibcode=2019EPJA...55..140B |s2cid=201664098 }}</ref>}} with stability (i.e., half-lives of the longest-lived isotopes) generally decreasing in heavier elements,{{efn|1=A region of increased stability encompasses [[thorium]] (''Z''&nbsp;=&nbsp;90) and [[uranium]] (''Z''&nbsp;=&nbsp;92) whose half-lives are comparable to the [[age of the Earth]]. Elements intermediate between bismuth and thorium have shorter half-lives, and heavier nuclei beyond uranium become more unstable with increasing atomic number.<ref name=greinerVP/>}}<ref name=greinerVP>{{cite journal |last=Greiner |first=W. |date=2012 |title=Heavy into Stability |journal=Physics |volume=5 |pages=115-1–115-3 <!-- Deny Citation Bot-->|doi=10.1103/Physics.5.115|bibcode=2012PhyOJ...5..115G |doi-access=free }}</ref> especially beyond curium (''Z''&nbsp;=&nbsp;96).<ref name=GSI2022>{{cite journal |last1=Terranova |first1=M. L. |last2=Tavares |first2=O. A. P. |date=2022 |title=The periodic table of the elements: the search for transactinides and beyond |journal=Rendiconti Lincei. Scienze Fisiche e Naturali |volume=33 |issue=1 |pages=1–16 |doi=10.1007/s12210-022-01057-w|bibcode=2022RLSFN..33....1T |s2cid=247111430 |doi-access=free }}</ref> The half-lives of nuclei also decrease when there is a lopsided neutron–proton ratio, such that the resulting nuclei have too few or too many neutrons to be stable.<ref name=CN14>{{cite web|url=https://wwwndc.jaea.go.jp/CN14/ |title=Chart of the Nuclides |last1=Koura |first1=H. |last2=Katakura |first2=J. |last3=Tachibana |first3=T. |last4=Minato |first4=F. |date=2015 |publisher=Japan Atomic Energy Agency |access-date=12 April 2019}}</ref>

The stability of a nucleus is determined by its [[nuclear binding energy|binding energy]], higher binding energy conferring greater stability. The binding energy per nucleon increases with atomic number to a broad plateau around ''A''&nbsp;=&nbsp;60, then declines.<ref>{{harvnb|Podgorsak|2016|p=33}}</ref> If a nucleus can be split into two parts that have a lower total energy (a consequence of the [[mass defect]] resulting from greater binding energy), it is unstable. The nucleus can hold together for a finite time because there is a [[potential barrier]] opposing the split, but this barrier can be crossed by [[quantum tunneling]]. The lower the barrier and the masses of the [[fission product|fragments]], the greater the probability per unit time of a split.<ref>{{cite book |last1=Blatt |first1=J. M. |last2=Weisskopf |first2=V. F. |title=Theoretical nuclear physics |date=2012 |publisher=Dover Publications |isbn=978-0-486-13950-0 |pages=7–9}}</ref>

Protons in a nucleus are bound together by the [[strong force]], which counterbalances the [[Coulomb repulsion]] between positively [[electric charge|charged]] protons. In heavier nuclei, larger numbers of uncharged neutrons are needed to reduce repulsion and confer additional stability. Even so, as physicists started to [[synthetic element|synthesize]] elements that are not found in nature, they found the stability decreased as the nuclei became heavier.<ref name=Sacks>{{cite news |last1=Sacks |first1=O. |title=Greetings From the Island of Stability |url=https://www.nytimes.com/2004/02/08/opinion/greetings-from-the-island-of-stability.html |access-date=16 February 2019 |work=The New York Times |date=2004 |url-status=dead |archive-url=https://web.archive.org/web/20180704182825/https://www.nytimes.com/2004/02/08/opinion/greetings-from-the-island-of-stability.html |archive-date=4 July 2018}}</ref> Thus, they speculated that the periodic table might come to an end. The discoverers of [[plutonium]] (element 94) considered naming it "ultimium", thinking it was the last.<ref>{{harvnb|Hoffman|2000|p=34}}</ref> Following the discoveries of heavier elements, of which some decayed in microseconds, it then seemed that instability with respect to [[spontaneous fission]] would limit the existence of heavier elements. In 1939, an upper limit of potential element synthesis was estimated around [[rutherfordium|element 104]],<ref name=liquiddrop /> and following the first discoveries of [[transactinide element]]s in the early 1960s, this upper limit prediction was extended to [[hassium|element 108]].<ref name=Sacks/>

===Magic numbers===
[[File:Next proton shell.svg|thumb|right|upright=1.2|alt=Diagram showing energy levels of known and predicted proton shells, with gaps at atomic number 82, 114, 120, and 126.|Diagram showing energy levels of known and predicted proton shells (left and right show two different models).<ref name="Kratz" /> The gaps at ''Z''&nbsp;=&nbsp;82,&nbsp;114,&nbsp;120, and&nbsp;126 correspond to shell closures,<ref name="Kratz" /> which have particularly stable configurations and thus result in more stable nuclei.<ref name="magickoura" />]]As early as 1914, the possible existence of [[superheavy element]]s with atomic numbers well beyond that of uranium—then the heaviest known element—was suggested, when German physicist Richard Swinne proposed that superheavy elements around ''Z''&nbsp;=&nbsp;108 were a source of radiation in [[cosmic rays]]. Although he did not make any definitive observations, he hypothesized in 1931 that [[transuranium element]]s around ''Z''&nbsp;=&nbsp;100 or ''Z''&nbsp;=&nbsp;108 may be relatively long-lived and possibly exist in nature.<ref name="swinne">{{harvnb|Kragh|2018|pages=9–10}}</ref> In 1955, American physicist [[John Archibald Wheeler]] also proposed the existence of these elements;<ref name="ghiorso1">{{harvnb|Hoffman|2000|p=400}}</ref> he is credited with the first usage of the term "superheavy element" in a 1958 paper published with Frederick Werner.<ref name="LBL665">{{cite report |last1=Thompson |first1=S. G. |last2=Tsang |first2=C. F. |title=Superheavy elements |date=1972 |publisher=[[Lawrence Berkeley National Laboratory]] |id=LBL-665 |page=28 |url=https://escholarship.org/content/qt4qh151mc/qt4qh151mc.pdf}}</ref> This idea did not attract wide interest until a decade later, after improvements in the [[nuclear shell model]]. In this model, the atomic nucleus is built up in "shells", analogous to [[electron shell]]s in atoms. Independently of each other, neutrons and protons have [[energy level]]s that are normally close together, but after a given shell is filled, it takes substantially more energy to start filling the next. Thus, the binding energy per nucleon reaches a local maximum and nuclei with filled shells are more stable than those without.<ref>{{cite web |last=Nave |first=R. |title=Shell Model of Nucleus |work=[[HyperPhysics]] |publisher=Department of Physics and Astronomy, [[Georgia State University]] |url=http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/shell.html |access-date=22 January 2007 }}</ref> This theory of a nuclear shell model originates in the 1930s, but it was not until 1949 that German physicists [[Maria Goeppert Mayer]] and [[Johannes Hans Daniel Jensen]] et al. independently devised the correct formulation.<ref name="shellview">{{cite journal |last1=Caurier |first1=E. |last2=Martínez-Pinedo |first2=G. |last3=Nowacki |first3=F. |last4=Poves |first4=A. |last5=Zuker |first5=A. P. |display-authors=3 |date=2005 |title=The shell model as a unified view of nuclear structure |journal=Reviews of Modern Physics |volume=77 |issue=2 |page=428 |doi=10.1103/RevModPhys.77.427 |arxiv=nucl-th/0402046|bibcode=2005RvMP...77..427C |s2cid=119447053 }}</ref>

The numbers of nucleons for which shells are filled are called [[Magic number (physics)|magic numbers]]. Magic numbers of 2, 8, 20, 28, 50, 82 and 126 have been observed for neutrons, and the next number is predicted to be 184.<ref name=beachhead /><ref>{{cite book |last1=Satake |first1=M. |title=Introduction to nuclear chemistry. |date=2010 |publisher=Discovery Publishing House |isbn=978-81-7141-277-8 |page=36}}</ref> Protons share the first six of these magic numbers,<ref>{{cite book |last1=Ebbing |first1=D. |last2=Gammon |first2=S. D. |title=General chemistry |date=2007 |publisher=Houghton Mifflin |isbn=978-0-618-73879-3 |page=858 |edition=8th}}</ref> and 126 has been predicted as a magic proton number since the 1940s.<ref name=Kragh>{{harvnb|Kragh|2018|p=22}}</ref> Nuclides with a magic number of each—such as [[oxygen-16|<sup>16</sup>O]] (''Z''&nbsp;=&nbsp;8, ''N''&nbsp;=&nbsp;8), <sup>132</sup>Sn (''Z''&nbsp;=&nbsp;50, ''N''&nbsp;=&nbsp;82), and <sup>208</sup>Pb (''Z''&nbsp;=&nbsp;82, ''N''&nbsp;=&nbsp;126)—are referred to as "doubly magic" and are more stable than nearby nuclides as a result of greater binding energies.<ref>{{cite news |last1=Dumé |first1=B. |title="Magic" numbers remain magic |url=https://physicsworld.com/a/magic-numbers-remain-magic/ |access-date=17 February 2019 |work=Physics World |publisher=IOP Publishing |date=2005}}</ref><ref name=DoublyMagic>{{cite journal |last1=Blank |first1=B. |last2=Regan |first2=P. H. |title=Magic and Doubly-Magic Nuclei |date=2000 |journal=Nuclear Physics News |volume=10 |issue=4 |pages=20–27 |url=https://www.researchgate.net/publication/232899048 |doi=10.1080/10506890109411553|s2cid=121966707 }}</ref>

In the late 1960s, more sophisticated shell models were formulated by American physicist William Myers<!--don't link, this William Myers does not have an article--> and Polish physicist [[Władysław Świątecki (physicist)|Władysław Świątecki]], and independently by German physicist Heiner Meldner (1939–2019<ref>{{cite web |url=https://www.llnl.gov/community/retiree-and-employee-resources/in-memoriam/heiner-walter-meldner |title=Heiner Walter Meldner |publisher=Lawrence Livermore National Laboratory |date=2019}}</ref><ref>{{cite web |url=https://www.legacy.com/obituaries/sandiegouniontribune/obituary.aspx?n=heiner-walter-meldner&pid=193040302 |title=Heiner Meldner Obituary |date=2019 |publisher=[[The San Diego Union-Tribune]] |website=Legacy.com}}</ref>). With these models, taking into account Coulomb repulsion, Meldner predicted that the next proton magic number may be 114 instead of 126.<ref name=quest>{{cite journal |last1=Bemis |first1=C. E. |last2=Nix |first2=J. R. |date=1977 |title=Superheavy elements – the quest in perspective |journal=Comments on Nuclear and Particle Physics |volume=7 |issue=3 |pages=65–78 |url=http://inspirehep.net/record/1382449/files/v7-n3-p65.pdf |issn=0010-2709}}</ref> Myers and Świątecki appear to have coined the term "island of stability", and American chemist [[Glenn Seaborg]], later a discoverer of many of the superheavy elements, quickly adopted the term and promoted it.<ref name=Kragh/><ref name=Kragh17>{{cite arXiv |title=The Search for Superheavy Elements: Historical and Philosophical Perspectives |pages=8–9 |eprint=1708.04064 |date=2017|last1=Kragh |first1=H.|class=physics.hist-ph }}</ref> Myers and Świątecki also proposed that some superheavy nuclei would be longer-lived as a consequence of higher [[fission barrier]]s. Further improvements in the nuclear shell model by Soviet physicist [[Vilen Strutinsky]] led to the emergence of the macroscopic–microscopic method, a nuclear mass model that takes into consideration both smooth trends characteristic of the [[liquid drop model]] and local fluctuations such as shell effects. This approach enabled Swedish physicist [[Sven Gösta Nilsson|Sven Nilsson]] et al., as well as other groups, to make the first detailed calculations of the stability of nuclei within the island.<ref name=quest/> With the emergence of this model, Strutinsky, Nilsson, and other groups argued for the existence of the doubly magic nuclide <sup>298</sup>Fl (''Z''&nbsp;=&nbsp;114, ''N''&nbsp;=&nbsp;184), rather than <sup>310</sup>[[unbihexium|Ubh]] (''Z''&nbsp;=&nbsp;126, ''N''&nbsp;=&nbsp;184) which was predicted to be doubly magic as early as 1957.<ref name=quest /> Subsequently, estimates of the proton magic number have ranged from 114 to 126, and there is still no consensus.<ref name=beachhead /><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=014201-1–014201-5<!-- Deny Citation Bot-->|url=https://www.researchgate.net/publication/258799250 |doi=10.7566/JPSJ.82.014201|bibcode=2013JPSJ...82a4201K}}</ref><ref name=newsci10/><ref name=not114>{{cite magazine |url=https://www.science.org/content/article/hopes-evaporate-superheavy-element-flerovium-having-long-life |last=Clery |first=D. |title=Hopes evaporate for the superheavy element flerovium having a long life |journal=[[Science (journal)|Science]] |date=2021 |doi=10.1126/science.abh0581}}</ref>