Editing Island of stability (section)

===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/>