Editing Decay chain (section)

== Types of decay ==
[[Image:Radioactive decay chains diagram.svg|thumb|right|350px| This diagram illustrates the four decay chains discussed in the text: thorium (4n, in blue), neptunium (4n+1, in pink), radium (4n+2, in red) and actinium (4n+3, in green).]] The four most common modes of radioactive decay are: alpha decay, beta decay, [[inverse beta decay]] (considered as both positron emission and electron capture), and [[isomeric transition]].  Of these decay processes, only alpha decay (fission of a [[helium-4]] nucleus) changes the [[atomic mass]] number (''A'') of the nucleus, and always decreases it by four. Because of this, almost any decay will result in a nucleus whose atomic mass number has the same [[modular arithmetic|residue]] mod 4. This divides the list of nuclides into four classes. All the members of any possible decay chain must be drawn entirely from one of these classes.

Three main decay chains (or families) are observed in nature. These are commonly called the thorium series, the radium or uranium series, and the [[actinium]] series, representing three of these four classes, and ending in three different, stable isotopes of [[lead]]. The mass number of every isotope in these chains can be represented as ''A''&nbsp;=&nbsp;4''n'', ''A''&nbsp;=&nbsp;4''n''&nbsp;+&nbsp;2, and A&nbsp;=&nbsp;4''n''&nbsp;+&nbsp;3, respectively. The long-lived starting isotopes of these three isotopes, respectively [[thorium-232]], [[uranium-238]], and [[uranium-235]], have existed since the formation of the Earth, ignoring the artificial isotopes and their decays created since the 1940s.

Due to the relatively short [[half-life]] of its starting isotope [[neptunium-237]] (2.14 million years), the fourth chain, the [[neptunium]] series with ''A''&nbsp;=&nbsp;4''n''&nbsp;+&nbsp;1, is already extinct in nature, except for the final rate-limiting step, decay of [[bismuth-209]]. Traces of <sup>237</sup>Np and its decay products do occur in nature, however, as a result of neutron reactions in uranium ore; neutron capture by natural thorium to give <sup>233</sup>U is also possible.<ref name=4n1/> The ending isotope of this chain is now known to be [[thallium-205]]. Some older sources give the final isotope as bismuth-209, but in 2003 it was discovered that it is very slightly radioactive, with a half-life of {{val|2.01|e=19|u=years}}.<ref name=nubase>{{NUBASE2016}}</ref>

There are also non-transuranic decay chains of unstable isotopes of light elements, for example those of [[magnesium-28]] and [[chlorine-39]]. On Earth, most of the starting isotopes of these chains before 1945 were generated by [[cosmic radiation]]. Since 1945, the testing and use of nuclear weapons has also released numerous radioactive [[fission products]]. Almost all such isotopes decay by either β<sup>−</sup> or β<sup>+</sup> decay modes, changing from one element to another without changing atomic mass. These later daughter products, being closer to stability, generally have longer half-lives until they finally reach stability.