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Isotopes of potassium (₁₉K)
ε	40Ar
β+	40Ar
Standard atomic weight Ar°(K)
	39.0983±0.0001; 39.098±0.001 (abridged);
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Potassium (
₁₉K) has 25 known isotopes from ³⁴
K to ⁵⁷
K as well as ³¹
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]

List of isotopes

Nuclide ^{[n 1]}	Z	N	Isotopic mass (Da)^[8] ^{[n 2]}^{[n 3]}	Half-life^[9] ^{[n 4]}	Decay mode^[9]	Daughter isotope ^{[n 5]}	Spin and parity^[9] ^{[n 6]}^{[n 4]}	Natural abundance (mole fraction)
Nuclide ^{[n 1]}	Excitation energy^{[n 4]}			Half-life^[9] ^{[n 4]}	Decay mode^[9]	Daughter isotope ^{[n 5]}	Spin and parity^[9] ^{[n 6]}^{[n 4]}	Normal proportion^[9]	Range of variation
³¹ K^[5]^[6]	19	12	31.03678(32)#	<10 ps	3p	²⁸S	3/2+#
³⁴K^[10]	19	15	33.998404(18)		p	³³Ar
³⁵K	19	16	34.98800541(55)	175.2(19) ms	β⁺ (99.63%)	³⁵Ar	3/2+
³⁵K	19	16	34.98800541(55)	175.2(19) ms	β⁺, p (0.37%)	³⁴Cl	3/2+
³⁶K	19	17	35.98130189(35)	341(3) ms	β⁺ (99.95%)	³⁶Ar	2+
					β⁺, p (0.048%)	³⁵Cl
					β⁺, α (0.0034%)	³²S
³⁷K	19	18	36.97337589(10)	1.23651(94) s	β⁺	³⁷Ar	3/2+
³⁸K	19	19	37.96908111(21)	7.651(19) min	β⁺	³⁸Ar	3+
^38m1K	130.15(4) keV			924.35(12) ms	β⁺ (99.97%)	³⁸Ar	0+
^38m1K	130.15(4) keV			924.35(12) ms	IT (0.0330%)	³⁸K	0+
^38m2K	3458.10(17) keV			21.95(11) μs	IT	³⁸K	(7)+
³⁹K	19	20	38.9637064848(49)	Stable			3/2+	0.932581(44)
⁴⁰K^{[n 7]}^{[n 8]}	19	21	39.963998165(60)	1.248(3)×10⁹ y	β⁻ (89.28%)	⁴⁰Ca	4−	1.17(1)×10⁻⁴
					EC (10.72%)	⁴⁰Ar
					β⁺ (0.001%)^[11]	⁴⁰Ar
^40mK	1643.638(11) keV			336(12) ns	IT	⁴⁰K	0+
⁴¹K	19	22	40.9618252561(40)	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	42.96073470(44)	22.3(1) h	β⁻	⁴³Ca	3/2+
^43mK	738.30(6) keV			200(5) ns	IT	⁴³K	7/2−
⁴⁴K	19	25	43.96158698(45)	22.13(19) min	β⁻	⁴⁴Ca	2−
⁴⁵K	19	26	44.96069149(56)	17.8(6) min	β⁻	⁴⁵Ca	3/2+
⁴⁶K	19	27	45.96198158(78)	96.30(8) s	β⁻	⁴⁶Ca	2−
⁴⁷K	19	28	46.9616616(15)	17.38(3) s	β⁻	⁴⁷Ca	1/2+
⁴⁸K	19	29	47.96534118(83)	6.83(14) s	β⁻ (98.86%)	⁴⁸Ca	1−
⁴⁸K	19	29	47.96534118(83)	6.83(14) s	β⁻, n (1.14%)	⁴⁷Ca	1−
⁴⁹K	19	30	48.96821075(86)	1.26(5) s	β⁻, n (86%)	⁴⁸Ca	1/2+
⁴⁹K	19	30	48.96821075(86)	1.26(5) s	β⁻ (14%)	⁴⁹Ca	1/2+
⁵⁰K	19	31	49.9723800(83)	472(4) ms	β⁻ (71.4%)	⁵⁰Ca	0−
					β⁻, n (28.6%)	⁴⁹Ca
					β⁻, 2n?	⁴⁸Ca
^50mK	172.0(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
					β⁻, 2n?	⁴⁹Ca
⁵²K	19	33	51.981602(36)	110(4) ms	β⁻, n (72.2%)	⁵¹Ca	2−#
					β⁻ (25.5%)	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	53.99447(43)#	10(5) ms	β⁻	⁵⁴Ca	2−#
					β⁻, n?	⁵³Ca
					β⁻, 2n?	⁵²Ca
⁵⁵K	19	36	55.00051(54)#	10# ms [>620 ns]	β⁻?	⁵⁵Ca	3/2+#
					β⁻, n?	⁵⁴Ca
					β⁻, 2n?	⁵⁴Ca
⁵⁶K	19	37	56.00857(64)#	5# ms [>620 ns]	β⁻?	⁵⁶Ca	2−#
					β⁻, n?	⁵⁵Ca
					β⁻, 2n?	⁵⁴Ca
⁵⁷K	19	38	57.01517(64)#	2# ms [>400 ns]	β⁻?	⁵⁷Ca	3/2+#
					β⁻, n?	⁵⁶Ca
					β⁻, 2n?	⁵⁵Ca
⁵⁹K^[3]^{[n 10]}	19	40	59.03086(86)#	1# ms [>400 ns]	β⁻?	⁵⁹Ca	3/2+#
					β⁻, n?	⁵⁸Ca
					β⁻, 2n?	⁵⁷Ca
This table header & footer: view

^ ^mK – Excited nuclear isomer.
^ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
^ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
^ ^a ^b ^c # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
^ Bold symbol as daughter – Daughter product is stable.
^ ( ) spin value – Indicates spin with weak assignment arguments.
^ Used in potassium-argon dating
^ Primordial radionuclide
^ Decay product of ⁴²Ar
^ Discovery of this isotope is unconfirmed.

References

^ "Standard Atomic Weights: Potassium". CIAAW. 1979.
^ 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.
^ ^a ^b Neufcourt, Léo; Cao, Yuchen; Nazarewicz, Witold; et al. (14 February 2019). "Neutron Drip Line in the Ca Region from Bayesian Model Averaging". Physical Review Letters. 122 (6): 062502. arXiv:1901.07632. Bibcode:2019PhRvL.122f2502N. doi:10.1103/PhysRevLett.122.062502. PMID 30822058.
^ "Radioactive Human Body". Retrieved 2011-05-18.
^ ^a ^b "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.
^ ^a ^b 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.
^ Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf.
^ ^a ^b ^c ^d Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
^ Dronchi, N.; Charity, R. J.; Sobotka, L. G.; Brown, B. A.; Weisshaar, D.; Gade, A.; Brown, K. W.; Reviol, W.; Bazin, D.; Farris, P. J.; Hill, A. M.; Li, J.; Longfellow, B.; Rhodes, D.; Paneru, S. N.; Gillespie, S. A.; Anthony, A. K.; Rubino, E.; Biswas, S. (2024-09-12). "Evolution of shell gaps in the neutron-poor calcium region from invariant-mass spectroscopy of ^37,38Sc, ³⁵Ca, and ³⁴K". Physical Review C. 110 (3). doi:10.1103/PhysRevC.110.L031302. ISSN 2469-9985.
^ 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.

[8] K – Excited nuclear isomer.

[10] ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.

[TMS-11] # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).

[TNN-13] # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).

[14] Bold symbol as daughter – Daughter product is stable.

[15] ( ) spin value – Indicates spin with weak assignment arguments.

[17] Used in potassium-argon dating

[18] Primordial radionuclide

[c-20] Decay product of ⁴²Ar

[21] Discovery of this isotope is unconfirmed.

[1] "Standard Atomic Weights: Potassium". CIAAW. 1979.

[CIAAW2021-2] 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.

[neufcourt2019-3] Neufcourt, Léo; Cao, Yuchen; Nazarewicz, Witold; et al. (14 February 2019). "Neutron Drip Line in the Ca Region from Bayesian Model Averaging". Physical Review Letters. 122 (6): 062502. arXiv:1901.07632. Bibcode:2019PhRvL.122f2502N. doi:10.1103/PhysRevLett.122.062502. PMID 30822058.

[4] "Radioactive Human Body". Retrieved 2011-05-18.

[31K-1-5] "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.

[31K-2-6] 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.

[AME2020_II-9] Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf.

[NUBASE2020-12] Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.

[16] Dronchi, N.; Charity, R. J.; Sobotka, L. G.; Brown, B. A.; Weisshaar, D.; Gade, A.; Brown, K. W.; Reviol, W.; Bazin, D.; Farris, P. J.; Hill, A. M.; Li, J.; Longfellow, B.; Rhodes, D.; Paneru, S. N.; Gillespie, S. A.; Anthony, A. K.; Rubino, E.; Biswas, S. (2024-09-12). "Evolution of shell gaps in the neutron-poor calcium region from invariant-mass spectroscopy of ^37,38Sc, ³⁵Ca, and ³⁴K". Physical Review C. 110 (3). doi:10.1103/PhysRevC.110.L031302. ISSN 2469-9985.

[19] 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.

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[2]

[3]

[4]

[5]

[6]

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[n 1]

[8]

[n 2]

[n 3]

[9]

[n 4]

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[11]

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Isotopes of potassium

List of isotopes

See also

References

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