Editing Potassium (section)

==Properties==
===Physical===
[[File:FlammenfärbungK.png|thumb|upright=0.5|The [[flame test]] of potassium.]]
Potassium is the second least dense metal after [[lithium]]. It is a soft solid with a low [[melting point]], and can be easily cut with a knife. Potassium is silvery in appearance, but it begins to tarnish toward gray immediately on exposure to air.<ref name="g76">[[#Greenwood|Greenwood]], p. 76</ref> In a [[flame test]], potassium and its compounds emit a [[Lilac (color)|lilac color]] with a peak emission wavelength of 766.5 nanometers.<ref>[[#Greenwood|Greenwood]], p. 75</ref><!-- Potassium concentration in solution is commonly determined using [[photoelectric flame photometer|flame photometry]], [[atomic absorption spectrophotometry]], [[inductively coupled plasma]], or [[ion selective electrode]]s. -->

Neutral potassium atoms have 19 electrons, one more than the configuration of the [[noble gas]] [[argon]]. Because of its low first [[ionization energy]] of 418.8{{nbsp}}kJ/mol, the potassium atom is much more likely to lose the last electron and acquire a positive charge, although negatively charged [[alkalide]] {{chem2|K−}} ions are not impossible.<ref name="K-">{{cite journal|journal = [[Angewandte Chemie International Edition]]|year = 1979|last = Dye|first=J. L. |title = Compounds of Alkali Metal Anions|volume = 18|issue = 8|pages = 587–598|doi = 10.1002/anie.197905871}}</ref> In contrast, the second ionization energy is very high (3052{{nbsp}}kJ/mol).

===Chemical===
Potassium reacts with oxygen, water, and carbon dioxide components in air. With oxygen it forms [[potassium peroxide]]. With water potassium forms [[potassium hydroxide]] (KOH). The reaction of potassium with water can be violently [[exothermic]], especially since the coproduced [[hydrogen]] gas can ignite. Because of this, potassium and the liquid sodium-potassium ([[NaK]]) alloy are potent [[desiccant]]s, although they are no longer used as such.<ref>{{cite journal|doi=10.1021/jo101589h|pmid=20945830|title=Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants|journal=The Journal of Organic Chemistry|volume=75|issue=24|pages=8351–8354|year=2010|last1=Williams|first1=D. Bradley G.|last2=Lawton|first2=Michelle|s2cid=17801540}}</ref>

===Compounds===
[[Image:potassium-superoxide-unit-cell-3D-ionic.png|thumb|left|upright|Structure of solid potassium superoxide ({{chem2|KO2}}).]]

Four oxides of potassium are well studied: [[potassium oxide]] ({{chem2|K2O}}), potassium peroxide ({{chem2|K2O2}}), [[potassium superoxide]] ({{chem2|KO2}})<ref>{{cite book|last = Lide|first = David R.|date = 1998|title = Handbook of Chemistry and Physics|edition = 87th|location = Boca Raton, Florida, United States|publisher = CRC Press|isbn = 978-0-8493-0594-8|pages = 477; 520}}</ref> and [[potassium ozonide]] ({{chem2|KO3}}). The binary potassium-oxygen compounds react with water forming KOH.

KOH is a [[strong base]]. Illustrating its [[hydrophilic]] character, as much as 1.21 [[kilogram|kg]] of KOH can dissolve in a single liter of water.<ref>{{RubberBible86th|page=4–80}}</ref><ref>[[#Schultz|Schultz]], p. 94</ref> Anhydrous KOH is rarely encountered. KOH reacts readily with [[carbon dioxide]] ({{chem2|CO2}}) to produce [[potassium carbonate]] ({{chem2|K2CO3}}), and in principle could be used to remove traces of the gas from air. Like the closely related [[sodium hydroxide]], KOH reacts with [[fat]]s to produce [[soap]]s.

In general, potassium compounds are ionic and, owing to the high hydration energy of the {{chem2|K+}} ion, have excellent water solubility. The main species in water solution are the [[Metal aquo complex|aquo complexes]] {{chem2|[K(H2O)_{''n''}]+}} where ''n'' = 6 and 7.<ref name="Lincoln">Lincoln, S. F.; Richens, D. T. and Sykes, A. G. "Metal Aqua Ions" in J. A. McCleverty and T. J. Meyer (eds.) [https://www.sciencedirect.com/referencework/9780080437484/comprehensive-coordination-chemistry-ii ''Comprehensive Coordination Chemistry II''] {{Webarchive|url=https://web.archive.org/web/20190419212906/https://www.sciencedirect.com/referencework/9780080437484/comprehensive-coordination-chemistry-ii |date=2019-04-19 }}, Vol. 1, pp. 515–555, {{ISBN|978-0-08-043748-4}}.</ref>

[[Potassium heptafluorotantalate]] ({{chem2|K2[TaF7]}}) is an intermediate in the purification of [[tantalum]] from the otherwise persistent contaminant of [[niobium]].<ref name="Agulyanski">{{cite book|author=Anthony Agulyanski|editor=Anatoly Agulyanski|chapter=Fluorine chemistry in the processing of tantalum and niobium|title=Chemistry of Tantalum and Niobium Fluoride Compounds |year=2004 |publisher=Elsevier |location=Burlington |isbn=978-0-08-052902-8|edition=1st}}</ref>

[[Organopotassium compound]]s illustrate nonionic compounds of potassium. They feature highly [[Chemical polarity|polar]] [[covalent]] K–C bonds. Examples include [[benzyl potassium]] {{chem2|KCH2C6H5}}. Potassium [[Intercalation (chemistry)|intercalate]]s into [[graphite]] to give a variety of [[graphite intercalation compounds]], including {{chem2|KC8}}.

===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>