Isotopes of potassium


has 25 known isotopes from 33K to 57K, plus an unconfirmed report of 59K. Three of those isotopes occur naturally: the two stable forms 39K and 41K, and a very long-lived radioisotope 40K.
Naturally occurring radioactive 40K decays with a half-life of 1.248×109 years. 89% of those decays are to stable 40Ca by beta decay, whilst 11% are to 40Ar by either electron capture or positron emission. 40K has the longest known half life for any positron-emitter nuclide. The long half-life of this primordial radioisotope is caused by a highly spin-forbidden transition: 40K has a nuclear spin of 4, while both of its decay daughters are even–even isotopes with spins of 0.
40K occurs in natural potassium in sufficient quantity that large bags of potassium chloride commercial salt substitutes can be used as a radioactive source for classroom demonstrations. 40K is the largest source of natural radioactivity in healthy animals and humans, greater even than 14C. In a human body of 70 kg mass, about 4,400 nuclei of 40K decay per second.
The decay of 40K to 40Ar is used in potassium-argon dating of rocks. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic 40Ar that has accumulated. Typically, the method assumes that the rocks contained no argon at the time of formation and all subsequent radiogenic argon was retained. 40K has also been extensively used as a radioactive tracer in studies of weathering.
All other potassium isotopes have half-lives under a day, most under a minute. The least stable is 31K, a three-proton emitter discovered in 2019; its half-life was measured to be shorter than 10 picoseconds.
Various potassium isotopes have been used for nutrient cycling studies because potassium is a macronutrient required for life.

List of isotopes