Editing Stable nuclide (section)

== Definition of stability, and naturally occurring nuclides ==

Most naturally occurring [[nuclide]]s are stable (about 251; see list at the end of this article), and about 35 more (total of 286) are known to be radioactive with long enough half-lives (also known) to occur primordially. If the half-life of a [[nuclide]] is comparable to, or greater than, the Earth's age (4.5 billion years), a significant amount will have survived since the [[Formation and evolution of the Solar System|formation of the Solar System]], and then is said to be [[Primordial nuclide|primordial]]. It will then contribute in that way to the natural isotopic composition of a chemical element. Primordial radioisotopes are easily detected with half-lives as short as 700 million years (e.g., [[uranium-235|{{sup|235}}U]]). This is the present limit of detection,{{citation needed|date=February 2021}} as shorter-lived nuclides have not yet been detected undisputedly in nature except when recently produced, such as decay products or cosmic ray spallation.

Many naturally occurring radioisotopes (another 53 or so, for a total of about 339) exhibit still shorter half-lives than 700 million years, but they are made freshly, as daughter products of decay processes of primordial nuclides (for example, radium from uranium), or from ongoing energetic reactions, such as [[cosmogenic nuclide]]s produced by present bombardment of Earth by [[cosmic rays]] (for example, {{sup|14}}C made from nitrogen).

Some isotopes that are classed as stable (i.e. no radioactivity has been observed for them) are predicted to have extremely long half-lives (sometimes 10{{sup|18}} years or more).<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> If the predicted half-life falls into an experimentally accessible range, such isotopes have a chance to move from the list of stable nuclides to the radioactive category, once their activity is observed. For example, {{sup|209}}Bi and {{sup|180}}W were formerly classed as stable, but were found to be [[alpha particle|alpha]]-active in 2003. However, such nuclides do not change their status as primordial when they are found to be radioactive.

Most stable isotopes on Earth are believed to have been formed in processes of [[nucleosynthesis]], either in the [[Big Bang]], or in generations of stars that preceded the [[formation of the Solar System]]. However, some stable isotopes also show abundance variations in the earth as a result of decay from long-lived radioactive nuclides. These decay-products are termed [[radiogenic]] isotopes, in order to distinguish them from the much larger group of 'non-radiogenic' isotopes.