Potassium ( ) is a chemical element. It has the symbol K ( , from ), atomic number 19, and atomic mass 39.0983. The name "potassium" comes from the word "potash", as potassium was first isolated from potash. Potassium is a soft silvery-white metallic alkali metal that occurs naturally bound to other elements in seawater and many minerals. It oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the evolved hydrogen. In many respects, potassium and sodium are chemically similar, although they have very different functions in organisms in general, and in animal cells in particular.

Occurrence

Production

Pure potassium metal can be isolated by electrolysis of its hydroxide in a process that has changed little since Davy. Thermal methods also are employed in potassium production, using potassium chloride Humphry Davy extracted this metal in 1807 along with sodium.Fact: date=February 2008

Isotopes

main: isotopes of potassium There are 24 known isotopes of potassium. Three isotopes occur naturally: ³⁹K (93.3%), ⁴⁰K (0.0117%) and ⁴¹K (6.7%). Naturally occurring ⁴⁰K decays to stable ⁴⁰Ar (11.2%) by electron capture and by positron emission, and decays to stable ⁴⁰Ca (88.8%) by beta decay; ⁴⁰K has a half-life of 1.250×10⁹ years. The decay of ⁴⁰K to ⁴⁰Ar enables a commonly used m argon at the time of formation and that all the subsequent radiogenic argon (i.e., ⁴⁰Ar) was quantitatively retained. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic ⁴⁰Ar that has accumulated. The minerals that are best suited for dating include biotite, muscovite, plutonic/high grade metamorphic hornblende, and volcanic feldspar; whole rock samples from volcanic flows and shallow instrusives can also be dated if they are unaltered.

Outside of dating, potassium isotopes have been used extensively as tracers in studies of weathering. They have also been used for nutrient cycling studies because potassium is a macronutrient required for life.

⁴⁰K 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. In healthy animals and people, ⁴⁰K represents the largest source of radioactivity, greater even than ¹⁴C. In a human body of 70 kg mass, about 4,400 nuclei of ⁴⁰K decay per second.

The activity of natural potassium is 31 Bq/g.