Editing Stable nuclide (section)

== Still-unobserved decay ==
{{further|List of nuclides}}

It is expected that improvement of experimental sensitivity will allow discovery of very mild radioactivity of some isotopes now considered stable. For example, in 2003 it was reported that [[bismuth-209]] (the only primordial isotope of bismuth) is very mildly radioactive, with half-life (1.9 ± 0.2) × 10{{sup|19}} yr,<ref>{{Cite web|url=http://nucleardata.nuclear.lu.se/nucleardata/toi/listnuc.asp?sql=&HlifeMin=1e30&tMinStr=1e30+s&HlifeMax=1e40&tMaxStr=1e+40+s|title=WWW Table of Radioactive Isotopes}} {{dead link|date=May 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{cite journal |last = Marcillac |first = Pierre de |author2 = Noël Coron |author3 = Gérard Dambier |author4 = Jacques Leblanc |author5 = Jean-Pierre Moalic |name-list-style = amp |date=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> confirming earlier theoretical predictions<ref>{{cite journal |author= de Carvalho H. G., de Araújo Penna M.|title = Alpha-activity of {{sup|209}}Bi  |journal = Lett. Nuovo Cimento |date=1972 |volume = 3 |issue = 18 |pages = 720–722 |doi = 10.1007/BF02824346 |url=https://link.springer.com/article/10.1007/BF02824346|url-access = subscription }}</ref> from [[nuclear physics]] that bismuth-209 would very slowly [[alpha decay]].

Isotopes that are theoretically believed to be unstable but have not been observed to decay are termed '''observationally stable'''. Currently there are 105 "stable" isotopes which are theoretically unstable, 40 of which have been observed in detail with no sign of decay, the lightest in any case being {{sup|36}}Ar. Many "stable" nuclides are "[[metastable]]" in that they would release energy if they were to decay,<ref>{{cite web|url=http://www.nndc.bnl.gov/masses/|title=NNDC – Atomic Masses|website=www.nndc.bnl.gov|access-date=2009-01-17|archive-date=2019-01-11|archive-url=https://web.archive.org/web/20190111232533/http://www.nndc.bnl.gov/masses/|url-status=dead}}</ref> and are expected to undergo very rare kinds of [[radioactive decay]], including [[double beta decay]].

146 nuclides from 62 elements with [[atomic number]]s from 1 ([[hydrogen]]) through 66 ([[dysprosium]]) except 43 ([[technetium]]), 61 ([[promethium]]), 62 ([[samarium]]), and 63 ([[europium]]) are theoretically stable to any kind of nuclear decay — except for the theoretical possibility of [[proton decay]], which has never been observed despite extensive searches for it; and [[spontaneous fission]] (SF), which is theoretically possible for the nuclides with [[atomic mass number]]s ≥ 93.<ref name=nucleonica/>

Besides SF, other theoretical decay routes for heavier elements include:<ref name=nucleonica>{{Cite web |url=http://www.nucleonica.net/unc.aspx |title=Nucleonica website |access-date=2014-06-14 |archive-date=2017-02-19 |archive-url=https://web.archive.org/web/20170219043412/http://www.nucleonica.net/unc.aspx |url-status=dead }}</ref>
* [[alpha decay]] – 70 heavy [[nuclide]]s (the lightest two are [[cerium]]-142 and [[neodymium]]-143)
* [[double beta decay]] – 55 nuclides
* [[beta decay]] – [[tantalum]]-180m
* [[electron capture]] – [[tellurium]]-123, tantalum-180m
* [[double electron capture]]
* [[isomeric transition]] – tantalum-180m

These include all nuclides of mass 165 and greater. [[Argon-36]] is the lightest known "stable" nuclide which is theoretically unstable.<ref name=nucleonica/>

The positivity of energy release in these processes means they are allowed kinematically (they do not violate conservation of energy) and, thus, in principle, can occur.<ref name=nucleonica/> They are not observed due to strong but not absolute suppression, by spin-parity selection rules (for beta decays and isomeric transitions) or by the thickness of the potential barrier (for alpha and cluster decays and spontaneous fission).