Editing Potassium (section)

===Isotopes===
<!--This section is linked from [[Uranium]]-->
{{main|Isotopes of potassium}}

There are 25 known [[isotope]]s of potassium, three of which occur naturally: {{chem|39|K}} (93.3%), {{chem|40|K}} (0.0117%), and {{chem|41|K}} (6.7%) (by mole fraction). Naturally occurring [[potassium-40|{{chem|40|K}}]] has a [[half-life]] of {{val|1.250e9}} years. It decays to stable [[Argon|{{chem|40|Ar}}]] by [[electron capture]] or [[positron emission]] (11.2%) or to stable [[Calcium|{{chem|40|Ca}}]] by [[beta decay]] (88.8%).<ref name="NUBASE">{{NUBASE 2003}}</ref> The decay of {{chem|40|K}} to {{chem|40|Ar}} is the basis of a common method for dating rocks. The conventional [[Potassium-argon dating|K-Ar dating method]] depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent radiogenic argon ({{chem|40|Ar}}) was quantitatively retained. [[Mineral]]s are dated by measurement of the concentration of potassium and the amount of radiogenic {{chem|40|Ar}} that has accumulated. The minerals best suited for dating include [[biotite]], [[muscovite]], [[metamorphic]] [[hornblende]], and volcanic [[feldspar]]; [[Petrography|whole rock]] samples from volcanic flows and shallow [[Igneous rock|instrusives]] can also be dated if they are unaltered.<ref name="NUBASE" /><ref>{{cite book|chapter-url = https://books.google.com/books?id=k90iAnFereYC&pg=PA207|pages =203–8|chapter= Theory and Assumptions in Potassium–Argon Dating|title = Isotopes in the Earth Sciences|isbn = 978-0-412-53710-3|last1 = Bowen|first1 = Robert|last2 = Attendorn|first2 = H. G.|date = 1988|publisher=Springer}}</ref> Apart from dating, potassium isotopes have been used as [[radioactive tracer|tracers]] in studies of [[weathering]] and for [[nutrient cycling]] studies because potassium is a [[macronutrient (ecology)|macronutrient]] required for [[life]]<ref>{{cite book|author=Anaç, D.|author2=Martin-Prével, P.|title=Improved crop quality by nutrient management|url=https://books.google.com/books?id=9Hr4w6QhPGsC&pg=PA290|date=1999|publisher=Springer|isbn=978-0-7923-5850-3|pages=290–}}</ref> on Earth.

{{chem|40|K}} occurs in natural potassium (and thus in some commercial salt substitutes) in sufficient quantity that large bags of those substitutes can be used as a radioactive source for classroom demonstrations. {{chem|40|K}} is the radioisotope with the largest abundance [[Composition of the human body|in the human body]]. In healthy animals and people, {{chem|40|K}} represents the largest source of radioactivity, greater even than [[Carbon-14|{{chem|14|C}}]]. In a human body of 70 kg, about 4,400 nuclei of {{chem|40|K}} decay per second.<ref>{{cite web|url=http://sciencedemonstrations.fas.harvard.edu/presentations/radioactive-human-body|title=Radiation and Radioactive Decay. Radioactive Human Body|access-date=July 2, 2016|publisher=Harvard Natural Sciences Lecture Demonstrations|archive-date=May 28, 2023|archive-url=https://web.archive.org/web/20230528120501/https://sciencedemonstrations.fas.harvard.edu/presentations/radioactive-human-body|url-status=live}}</ref> The activity of natural potassium is 31 [[Becquerel|Bq]]/g.<ref>{{cite book|url = https://books.google.com/books?id=KRVXMiQWi0cC&pg=PA32|page =32|title = Radioactive fallout in soils, crops and food: a background review|isbn = 978-92-5-102877-3|author1 = Winteringham, F. P. W|author2 = Effects, F.A.O. Standing Committee on Radiation, Land And Water Development Division, Food and Agriculture Organization of the United Nations|date = 1989|publisher=Food & Agriculture Org.}}</ref>