Isotopes of potassium

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%)

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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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)
	Excitation energy[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+
					β+, p (0.37%)	34Cl
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+
					IT (0.0330%)	38K
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]
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−
					β−, n (1.14%)	47Ca
49K	19	30	48.96821075(86)	1.26(5) s	β−, n (86%)	48Ca	1/2+
					β− (14%)	49Ca
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

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. # – 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

• Banana equivalent dose

References

1. "Standard Atomic Weights: Potassium". CIAAW. 1979.
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.
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.
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.
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.
8. 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.
9. 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.
10. 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.
11. 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.