Editing Radiometric dating (section)

===Radioactive decay===
[[File:Thorium decay chain from lead-212 to lead-208.svg|thumb|right|upright=1.35|Example of a radioactive [[decay chain]] from lead-212 (<sup>212</sup>Pb) to lead-208 (<sup>208</sup>Pb) . Each parent nuclide spontaneously decays into a daughter nuclide (the [[decay product]]) via an [[alpha decay|α decay]] or a [[beta decay|β<sup>−</sup> decay]]. The final decay product, lead-208 (<sup>208</sup>Pb), is stable and can no longer undergo spontaneous radioactive decay.]]
All ordinary [[matter]] is made up of combinations of [[chemical element]]s, each with its own [[atomic number]], indicating the number of [[proton]]s in the [[atomic nucleus]]. Additionally, elements may exist in different [[isotope]]s, with each isotope of an element differing in the number of [[neutron]]s in the nucleus. A particular isotope of a particular element is called a [[nuclide]]. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo [[radioactive decay]] and spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including [[alpha decay]] (emission of [[alpha particle]]s) and [[beta decay]] ([[electron]] emission, [[positron]] emission, or [[electron capture]]). Another possibility is [[spontaneous fission]] into two or more nuclides.<ref>{{cite web |title=Spontaneous fission |url=https://www.arpansa.gov.au/understanding-radiation/what-is-radiation/ionising-radiation/radiation-decay |website=Australian Radiation Protection and Nuclear Safety Agency |access-date=29 January 2025}}</ref>

While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays [[exponential decay|exponentially]] at a rate described by a parameter known as the [[half-life]], usually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or [[decay product]]. In many cases, the daughter nuclide itself is radioactive, resulting in a [[decay chain]], eventually ending with the formation of a stable (nonradioactive) daughter nuclide; each step in such a chain is characterized by a distinct half-life. In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter. Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years (e.g., [[tritium]]) to over 100 billion years (e.g., [[samarium-147]]).<ref name="Bernard-Griffiths1989">{{cite book|last1=Bernard-Griffiths|first1=J.|last2=Groan|first2=G.|editor1-last=Roth|editor1-first=Etienne|editor2-last=Poty|editor2-first=Bernard|title=Nuclear Methods of Dating|date=1989|publisher=Springer Netherlands|isbn=978-0-7923-0188-2|pages=53–72|chapter=The samarium–neodymium method}}</ref>

For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially constant.<ref>{{cite journal|last1=Pommé|first1=S.|last2=Stroh|first2=H.|last3=Altzitzoglou|first3=T.|last4=Paepen|first4=J.|last5=Van Ammel|first5=R.|last6=Kossert|first6=K.|last7=Nähle|first7=O.|last8=Keightley|first8=J. D.|last9=Ferreira|first9=K. M.|last10=Verheyen|first10=L.|last11=Bruggeman|first11=M.|date=2018-04-01|title=Is decay constant?|journal=Applied Radiation and Isotopes|series=ICRM 2017 Proceedings of the 21st International Conference on Radionuclide Metrology and its Applications|volume=134|pages=6–12|doi=10.1016/j.apradiso.2017.09.002|pmid=28947247|doi-access=free|bibcode=2018AppRI.134....6P }}</ref> This is known because decay constants measured by different techniques give consistent values within analytical errors and the ages of the same materials are consistent from one method to another. It is not affected by external factors such as [[temperature]], [[pressure]], chemical environment, or presence of a [[magnetic field|magnetic]] or [[electric field]].<ref>{{cite journal|doi=10.1146/annurev.ns.22.120172.001121|doi-access=free |title=Perturbation of Nuclear Decay Rates |year=1972 |author=Emery, G T |journal=[[Annual Review of Nuclear Science]] |volume=22|issue=1 |pages=165–202|bibcode = 1972ARNPS..22..165E }}</ref><ref>{{cite journal|doi=10.1038/264340a0 |title=Direct test of the constancy of fundamental nuclear constants |year=1976 |author=Shlyakhter, A. I. |journal=Nature |volume=264|issue=5584 |pages=340 |bibcode=1976Natur.264..340S|s2cid=4252035 |doi-access=free }}</ref><ref>Johnson, B. (1993). ''How to Change Nuclear Decay Rates'' [http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/decay_rates.html Usenet Physics FAQ]</ref> The only exceptions are nuclides that decay by the process of electron capture, such as [[beryllium-7]], [[strontium-85]], and [[zirconium-89]], whose decay rate may be affected by local electron density. For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time.{{Citation needed|date=October 2022}} This predictability allows the relative abundances of related nuclides to be used as a [[clock]] to measure the time from the incorporation of the original nuclides into a material to the present.