Isotopes of potassium
Potassium (
19K) has 26 known isotopes from 31
K to 57
K, with the exception of still-unknown 32
K, as well as an unconfirmed report of 59
K. Three of those isotopes occur naturally: the two stable forms 39
K (93.3%) and 41
K (6.7%), and a very long-lived radioisotope 40
K (0.012%)
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| Standard atomic weight Ar°(K) | ||||||||||||||||||||||||||||||
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Naturally occurring radioactive 40
K decays with a half-life of 1.248×109 years. 89% of those decays are to stable 40
Ca by beta decay, whilst 11% are to 40
Ar by either electron capture or positron emission. This latter decay branch has produced an isotopic abundance of argon on Earth which differs greatly from that seen in gas giants and stellar spectra. 40
K 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: 40
K has a nuclear spin of 4, while both of its decay daughters are even–even isotopes with spins of 0.
40
K 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. 40
K is the largest source of natural radioactivity in healthy animals and humans, greater even than 14
C. In a human body of 70 kg mass, about 4,400 nuclei of 40
K decay per second.
The decay of 40
K to 40
Ar is used in potassium-argon dating of rocks. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic 40
Ar that has accumulated. Typically, the method assumes that the rocks contained no argon at the time of formation and all subsequent radiogenic argon (i.e., 40
Ar) was retained. 40
K 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 31
K, a three-proton emitter discovered in 2019; its half-life was measured to be shorter than 10 picoseconds.
Stable potassium isotopes have been used for several nutrient cycling studies since potassium is a macronutrient required for life.