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
  • 39.098±0.001 (abridged)[1][2]

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. 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.[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. 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.[5][6]

Stable potassium isotopes have been used for several nutrient cycling studies since potassium is a macronutrient required for life.[7]

List of isotopes

Nuclide[8]
[n 1]		 Z N Isotopic mass (Da)[9]
[n 2][n 3]		 Half-life
[n 4]		 Decay
mode
 Daughter
isotope
[n 5]		 Spin and
parity
[n 6][n 4]		 Natural abundance (mole fraction)
Excitation energy[n 4] Normal proportion Range of variation
31
K[5][6]		 19 12 <10 ps 3p 28S
33K 19 14 33.00756(21)# <25 ns p 32Ar 3/2+#
34K 19 15 33.99869(21)# <40 ns p 33Ar 1+#
35K 19 16 34.9880054(6) 178(8) ms β+ (99.63%) 35Ar 3/2+
β+, p (.37%) 34Cl
36K 19 17 35.9813020(4) 341(3) ms β+ (99.95%) 36Ar 2+
β+, p (.048%) 35Cl
β+, α (.0034%) 32S
37K 19 18 36.97337589(10) 1.2365(9) s β+ 37Ar 3/2+
38K 19 19 37.96908112(21) 7.636(18) min β+ 38Ar 3+
38m1K 130.50(28) keV 924.46(14) ms β+ 38Ar 0+
38m2K 3458.0(2) keV 21.95(11) μs IT 38K (7+)
39K 19 20 38.963706487(5) Stable 3/2+ 0.932581(44)
40K[n 7][n 8] 19 21 39.96399817(6) 1.248(3)×109 y β− (89.28%) 40Ca 4− 1.17(1)×10−4
EC (10.72%) 40Ar
β+ (0.001%)[10]
40mK 1643.639(11) keV 336(12) ns IT 40K 0+
41K 19 22 40.961825258(4) Stable 3/2+ 0.067302(44)
42K 19 23 41.96240231(11) 12.355(7) h β− 42Ca 2− Trace[n 9]
43K 19 24 42.9607347(4) 22.3(1) h β− 43Ca 3/2+
43mK 738.30(6) keV 200(5) ns IT 43K 7/2−
44K 19 25 43.9615870(5) 22.13(19) min β− 44Ca 2−
45K 19 26 44.9606915(6) 17.8(6) min β− 45Ca 3/2+
46K 19 27 45.9619816(8) 105(10) s β− 46Ca 2−
47K 19 28 46.9616616(15) 17.50(24) s β− 47Ca 1/2+
48K 19 29 47.9653412(8) 6.8(2) s β− (98.86%) 48Ca 1−
β−, n (1.14%) 47Ca
49K 19 30 48.9682108(9) 1.26(5) s β−, n (86%) 48Ca (3/2+)
β− (14%) 49Ca
50K 19 31 49.972380(8) 472(4) ms β− (71%) 50Ca 0−
β−, n (29%) 49Ca
50mK 171.4(4) keV 125(40) ns IT 50K (2−)
51K 19 32 50.975828(14) 365(5) ms β−, n (65%) 50Ca 3/2+
β− (35%) 51Ca
52K 19 33 51.98160(4) 110(4) ms β−, n (74%) 51Ca 2−#
β− (23.7%) 52Ca
β−, 2n (2.3%) 50Ca
53K 19 34 52.98680(12) 30(5) ms β−, n (64%) 52Ca (3/2+)
β− (26%) 53Ca
β−, 2n (10%) 51Ca
54K 19 35 53.99463(64)# 10(5) ms β− (>99.9%) 54Ca 2−#
β−, n (<.1%) 53Ca
55K 19 36 55.00076(75)# 3# ms β− 55Ca 3/2+#
β−, n 54Ca
56K 19 37 56.00851(86)# 1# ms β− 56Ca 2−#
β−, n 55Ca
57K[11][3] 19 38 β− 57Ca
59K[3][n 10] 19 40 β− 59Ca
This table header & footer:
  1. ↑ mK – Excited nuclear isomer.
  2. ↑ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. ↑ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. 1 2 3 # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  5. ↑ Bold symbol as daughter – Daughter product is stable.
  6. ↑ ( ) spin value – Indicates spin with weak assignment arguments.
  7. ↑ Used in potassium-argon dating
  8. ↑ Primordial radionuclide
  9. ↑ Decay product of 42Ar
  10. ↑ Discovery of this isotope is unconfirmed

See also

References

  1. ↑ "Standard Atomic Weights: Potassium". CIAAW. 1979.
  2. ↑ Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; et al. (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.
  3. 1 2 3 Tarasov, O.B. (2017). "Production of very neutron rich isotopes: What should we know?".
  4. ↑ "Radioactive Human Body". Retrieved 2011-05-18.
  5. 1 2 "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.
  6. 1 2 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.
  7. ↑ "Soil potassium isotope composition during four million years of ecosystem development in Hawai'i". par.nsf.gov. June 2022.
  8. ↑ Half-life, decay mode, nuclear spin, and isotopic composition is sourced in:
    Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017). "The NUBASE2016 evaluation of nuclear properties" (PDF). Chinese Physics C. 41 (3): 030001. Bibcode:2017ChPhC..41c0001A. doi:10.1088/1674-1137/41/3/030001.
  9. ↑ Wang, M.; Audi, G.; Kondev, F. G.; Huang, W. J.; Naimi, S.; Xu, X. (2017). "The AME2016 atomic mass evaluation (II). Tables, graphs, and references" (PDF). Chinese Physics C. 41 (3): 030003-1–030003-442. doi:10.1088/1674-1137/41/3/030003.
  10. ↑ Engelkemeir, D. W.; Flynn, K. F.; Glendenin, L. E. (1962). "Positron Emission in the Decay of K40". Physical Review. 126 (5): 1818. Bibcode:1962PhRv..126.1818E. doi:10.1103/PhysRev.126.1818.
  11. ↑ Neufcourt, L.; Cao, Y.; Nazarewicz, W.; Olsen, E.; Viens, F. (2019). "Neutron drip line in the Ca region from Bayesian model averaging". Physical Review Letters. 122 (6): 062502–1–062502–6. arXiv:1901.07632. Bibcode:2019PhRvL.122f2502N. doi:10.1103/PhysRevLett.122.062502. PMID 30822058. S2CID 73508148.
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