Nuclides with atomic number of 19 but with different mass numbers
Isotopes of potassium (19K)
Main isotopes
Decay
abundance
half-life (t1/2)
mode
product
39K
93.3%
stable
40K
0.0120%
1.248×109 y
β−
40Ca
ε
40Ar
β+
40Ar
41K
6.73%
stable
Standard atomic weight Ar°(K)
39.0983±0.0001[1]
39.098±0.001 (abridged)[2]
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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.[3] 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%)
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.[citation needed]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.[4]
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.[citation needed]40 K has also been extensively used as a radioactive tracer in studies of weathering.[citation needed]
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.[5][6]
Stable potassium isotopes have been used for several nutrient cycling studies since potassium is a macronutrient required for life.[7]
^Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; Böhlke, John K.; Chesson, Lesley A.; Coplen, Tyler B.; Ding, Tiping; Dunn, Philip J. H.; Gröning, Manfred; Holden, Norman E.; Meijer, Harro A. J. (2022-05-04). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.
^Cite error: The named reference Tarasov(K) was invoked but never defined (see the help page).
^"Radioactive Human Body". Retrieved 2011-05-18.
^"A peculiar atom shakes up assumptions of nuclear structure". Nature. 573 (7773): 167. 6 September 2019. Bibcode:2019Natur.573T.167.. doi:10.1038/d41586-019-02655-9. PMID 31506620.
^Kostyleva, D.; et al. (2019). "Towards the Limits of Existence of Nuclear Structure: Observation and First Spectroscopy of the Isotope 31K by Measuring Its Three-Proton Decay". Physical Review Letters. 123 (9): 092502. arXiv:1905.08154. Bibcode:2019PhRvL.123i2502K. doi:10.1103/PhysRevLett.123.092502. PMID 31524489. S2CID 159041565.
^"Soil potassium isotope composition during four million years of ecosystem development in Hawai'i". par.nsf.gov. June 2022.
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