Editing Decay chain (section)

== History ==
The chemical elements came into being in two phases. The first commenced shortly after the [[Big Bang]]. From ten seconds to 20 minutes after the beginning of the universe the [[Big Bang nucleosynthesis|earliest condensation of light atoms]] was responsible for the manufacture of the four lightest elements. The vast majority of this primordial production consisted of the three lightest isotopes of [[hydrogen]]—[[Isotopes of hydrogen|protium]], [[deuterium]] and [[tritium]]—and two of the nine known isotopes of [[helium]]—[[helium-3]] and [[helium-4]]. Trace amounts of [[Isotopes of lithium#Lithium-7|lithium-7]] and [[Isotopes of beryllium#Beryllium-7|beryllium-7]] were likely also produced. 

So far as is known, all heavier elements came into being starting around 100 million years later, in a second phase of [[nucleosynthesis]] that commenced with the birth of the [[Primordial star|first stars]].<ref>{{Cite web |last=Bromm |first=Richard B. Larson, Volker |title=The First Stars in the Universe |url=https://www.scientificamerican.com/article/the-first-stars-in-the-un/ |access-date=2024-09-29 |website=Scientific American |language=en}}</ref> The nuclear furnaces that power stellar evolution were necessary to create large quantities of all elements heavier than helium, and the [[R-process|r-]] and [[s-process]]<nowiki/>es of neutron capture that occur in stellar cores are thought to have created all such elements up to [[iron]] and [[nickel]] (atomic numbers 26 and 28). The [[Supernova nucleosynthesis|extreme conditions]] that attend [[Supernova|supernovae]] explosions are capable of creating the elements between [[oxygen]] and [[rubidium]] (i.e., atomic numbers 8 through 37). The creation of heavier elements, including those without stable isotopes—all elements with atomic numbers greater than lead's, 82—appears to rely on r-process nucleosynthesis operating amid the immense concentrations of free neutrons released during [[Neutron star merger|neutron star mergers]].

Most of the isotopes of each chemical element present in the Earth today were formed by such processes no later than the time of [[History of Earth|our planet's condensation]] from the solar [[Protoplanetary disk|protoplanetary disc]], around 4.5 billion years ago. The exceptions to these so-called ''primordial'' elements are those that have resulted from the radioactive disintegration of unstable parent nuclei as they progress down one of several decay chains, each of which terminates with the production of one of the 251 stable isotopes known to exist. Aside from cosmic or stellar nucleosynthesis, and decay chains the only other ways of producing a chemical element rely on [[Nuclear weapon|atomic weapons]], nuclear reactors ([[Natural nuclear fission reactor|natural]] or [[Nuclear reactor|manmade]]) or the laborious atom-by-atom [[Superheavy element|assembly]] of nuclei with [[Particle accelerator|particle accelerators]].

Unstable isotopes decay to their daughter products (which may sometimes be even more unstable) at a given rate; eventually, often after a series of decays, a stable isotope is reached: there are 251 stable isotopes in the universe. In stable isotopes, light elements typically have a lower ratio of neutrons to protons in their nucleus than heavier elements. Light elements such as [[helium-4]] have close to a 1:1 neutron:proton ratio. The heaviest elements such as uranium have close to 1.5 neutrons per proton (e.g. 1.587 in [[uranium-238]]). No nuclide heavier than lead-208 is stable; these heavier elements have to shed mass to achieve stability, mostly by [[alpha decay]]. The other common way for isotopes with a high neutron to proton ratio (n/p) to decay is [[beta decay]], in which the nuclide changes elemental identity while keeping the same mass number and lowering its n/p ratio. For some isotopes with a relatively low n/p ratio, there is an [[positron emission|inverse beta decay]], by which a proton is transformed into a neutron, thus moving towards a stable isotope; however, since fission almost always produces products which are neutron heavy, [[positron emission]] or [[electron capture]] are rare compared to electron emission. There are many relatively short beta decay chains, at least two (a heavy, beta decay and a light, [[positron]] decay) for every discrete weight up to around 207 and some beyond, but for the higher mass elements (isotopes heavier than lead) there are only four pathways which encompass all decay chains.{{Citation needed|date=July 2023}} This is because there are just two main decay methods: [[alpha radiation]], which reduces the mass by 4 [[atomic mass units]] (amu), and beta, which does not change the mass number (just the atomic number and the p/n ratio). The four paths are termed 4n, 4n&nbsp;+&nbsp;1, 4n&nbsp;+&nbsp;2, and 4n&nbsp;+&nbsp;3; the remainder from dividing the atomic mass by four gives the chain the isotope will use to decay. There are other decay modes, but they invariably occur at a lower probability than alpha or beta decay. (It should not be supposed that these chains have no branches: the diagram below shows a few branches of chains, and in reality there are many more, because there are many more isotopes possible than are shown in the diagram.) For example, the third atom of [[nihonium-278]] synthesised underwent six alpha decays down to [[mendelevium-254]],<ref name="six-alpha" /> followed by an [[electron capture]] (a form of beta decay) to [[fermium-254]],<ref name="six-alpha" /> and then a seventh alpha to [[californium-250]],<ref name="six-alpha">{{cite journal|journal=Journal of the Physical Society of Japan|volume=81|pages=103201 |date=2012|title=New Results in the Production and Decay of an Isotope, <sup>278</sup>113, of the 113th Element|author=K. Morita|doi=10.1143/JPSJ.81.103201|last2=Morimoto|first2=Kouji|last3=Kaji|first3=Daiya|last4=Haba|first4=Hiromitsu|last5=Ozeki|first5=Kazutaka|last6=Kudou|first6=Yuki|last7=Sumita|first7=Takayuki|last8=Wakabayashi|first8=Yasuo|last9=Yoneda|first9=Akira|first10=Kengo |last10=Tanaka|first11=Sayaka |last11=Yamaki|first12=Ryutaro |last12=Sakai|first13=Takahiro |last13=Akiyama|first14=Shin-ichi |last14=Goto|first15=Hiroo |last15=Hasebe|first16=Minghui |last16=Huang|first17=Tianheng |last17=Huang|first18=Eiji |last18=Ideguchi|first19=Yoshitaka |last19=Kasamatsu|first20=Kenji |last20=Katori|first21=Yoshiki |last21=Kariya|first22=Hidetoshi |last22=Kikunaga|first23=Hiroyuki |last23=Koura|first24=Hisaaki |last24=Kudo|first25=Akihiro |last25=Mashiko|first26=Keita |last26=Mayama|first27=Shin-ichi |last27=Mitsuoka|first28=Toru |last28=Moriya|first29=Masashi |last29=Murakami|first30=Hirohumi |last30=Murayama|first31=Saori |last31=Namai|first32=Akira |last32=Ozawa|first33=Nozomi |last33=Sato|first34=Keisuke |last34=Sueki|first35=Mirei |last35=Takeyama|first36=Fuyuki |last36=Tokanai|first37=Takayuki |last37=Yamaguchi|first38=Atsushi |last38=Yoshida
|issue=10|display-authors=10|arxiv = 1209.6431 |bibcode = 2012JPSJ...81j3201M |s2cid=119217928 }}</ref> upon which it would have followed the 4n&nbsp;+&nbsp;2 chain (radium series) as given in this article. However, the heaviest [[superheavy element|superheavy]] nuclides synthesised do not reach the four decay chains, because they reach a [[spontaneous fission|spontaneously fissioning]] nuclide after a few alpha decays that terminates the chain: this is what happened to the first two atoms of nihonium-278 synthesised,<ref name="Nh278 04Mo01">{{cite journal|title=Experiment on the Synthesis of Element 113 in the Reaction <sup>209</sup>Bi(<sup>70</sup>Zn, n)<sup>278</sup>113|doi=10.1143/JPSJ.73.2593|year=2004|last1=Morita |first1=Kosuke |journal=Journal of the Physical Society of Japan |volume=73 |pages=2593–2596 |last2=Morimoto |first2=Kouji |last3=Kaji |first3=Daiya |last4=Akiyama |first4=Takahiro |last5=Goto |first5=Sin-Ichi |last6=Haba |first6=Hiromitsu |last7=Ideguchi |first7=Eiji |last8=Kanungo |first8=Rituparna |last9=Katori |first9=Kenji |last10=Koura |first10=Hiroyuki |last11=Kudo |first11=Hisaaki |last12=Ohnishi |first12=Tetsuya |last13=Ozawa |first13=Akira |last14=Suda |first14=Toshimi |last15=Sueki |first15=Keisuke |last16=Xu |first16=Hushan |last17=Yamaguchi |first17=Takayuki |last18=Yoneda |first18=Akira |last19=Yoshida |first19=Atsushi |last20=Zhao |first20=Yuliang |display-authors=8 |issue=10 |bibcode = 2004JPSJ...73.2593M |doi-access= }}</ref><ref name="JWP Nh278">{{cite journal |last1=Barber |first1=Robert C. |last2=Karol |first2=Paul J |last3=Nakahara |first3=Hiromichi |last4=Vardaci |first4=Emanuele |last5=Vogt |first5=Erich W. |title=Discovery of the elements with atomic numbers greater than or equal to 113 (IUPAC Technical Report)|doi=10.1351/PAC-REP-10-05-01 |journal=Pure and Applied Chemistry |year=2011 |volume=83|issue=7 |page=1485|doi-access=free }}</ref> as well as to all heavier nuclides produced.

Three of those chains have a long-lived isotope (or nuclide) near the top; this long-lived nuclide is a bottleneck in the process through which the chain flows very slowly, and keeps the chain below them "alive" with flow. The three long-lived nuclides are uranium-238 (half-life 4.5 billion years), uranium-235 (half-life 700 million years) and thorium-232 (half-life 14 billion years). The fourth chain has no such long-lasting bottleneck nuclide near the top, so almost all of the nuclides in that chain have long since decayed down to just before the end: bismuth-209. This nuclide was long thought to be stable, but in 2003 it was found to be unstable, with a very long half-life of 20.1 billion billion years;<ref>{{Cite journal|author=J.W. Beeman|display-authors=et al|date=2012|title=First Measurement of the Partial Widths of <sup>209</sup>Bi Decay to the Ground and to the First Excited States|journal=Physical Review Letters |volume=108 |issue=6 |pages=062501|doi=10.1103/PhysRevLett.108.062501|pmid=22401058|arxiv=1110.3138|s2cid=118686992 }}</ref> it is the last step in the chain before stable thallium-205. Because this bottleneck is so long-lived, very small quantities of the final decay product have been produced, and for most practical purposes bismuth-209 is the final decay product.

In the distant past, during the first few million years of the history of the Solar System, there were more kinds of unstable high-mass nuclides in existence, and the four chains were longer, as they included nuclides that have since decayed away. Notably, <sup>244</sup>Pu, <sup>237</sup>Np, and <sup>247</sup>Cm have half-lives over a million years and would have then been lesser bottlenecks high in the 4n, 4n+1, and 4n+3 chains respectively.<ref name=Davis>{{cite journal |last1=Davis |first1=Andrew M. |date=2022 |title=Short-Lived Nuclides in the Early Solar System: Abundances, Origins, and Applications |journal=Annual Review of Nuclear and Particle Science |volume=72 |issue= |pages=339–363 |doi=10.1146/annurev-nucl-010722-074615 |doi-access=free |bibcode=2022ARNPS..72..339D }}</ref> (There is no nuclide with a half-life over a million years above <sup>238</sup>U in the 4n+2 chain.) Today some of these formerly extinct isotopes are again in existence as they have been manufactured. Thus they again take their places in the chain: plutonium-239, used in nuclear weapons, is the major example, decaying to uranium-235 via alpha emission with a half-life 24,500 years. There has also been large-scale production of neptunium-237, which has resurrected the hitherto extinct fourth chain.<ref>{{cite book|last1=Koch|first1=Lothar|title=Transuranium Elements, in Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley|date=2000|doi=10.1002/14356007.a27_167}}</ref> The tables below hence start the four decay chains at isotopes of [[californium]] with mass numbers from 249 to 252.

{|class=wikitable
|+Summary of the four decay chain pathways
|'''Name of series'''||Thorium||Neptunium||Uranium||Actinium
|-
|'''Mass numbers'''||4''n''||4''n''+1||4''n''+2||4''n''+3
|-
|'''Long-lived nuclide'''||<sup>232</sup>Th<br>(<sup>244</sup>Pu)||<sup>209</sup>Bi<br>(<sup>237</sup>Np)||<sup>238</sup>U<br>&nbsp;||<sup>235</sup>U<br>(<sup>247</sup>Cm)
|-
|'''Half-life'''<br/>(billions of years)||14<br>(0.08)|| {{val|20100000000}}<br>(0.00214) ||4.5<br>&nbsp;||0.7<br>(0.0156)
|-
|'''End of chain'''||<sup>208</sup>Pb||<sup>205</sup>Tl||<sup>206</sup>Pb||<sup>207</sup>Pb
|}

These four chains are summarised in the chart in the following section.