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| Standard atomic weightAr°(K) | ||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Potassium (
19K) has 25 knownisotopes from34
K to57
K as well as31
K, as well as an unconfirmed report of59
K.[3] Three of those isotopes occur naturally: the two stable forms39
K (93.26%) and41
K (6.72%), and the long-lived radioisotope40
K (0.012%).
Naturally occurring radioactive40
K decays with ahalf-life of 1.248×109 years. 89% of those decays are to stable40
Ca bybeta decay, whilst 11% are to40
Ar by eitherelectron capture orpositron emission. This latter decay branch has produced an isotopic abundance of argon on Earth which differs greatly from that seen ingas giants and stellar spectra.40
K has the longest known half-life for any positron-emittingnuclide.[4] The long half-life of thisprimordial radioisotope is caused by a highlyspin-forbidden transition:40
K has anuclear spin of 4, while both of its decay daughters areeven–even isotopes with spins of 0.
40
K occurs in natural potassium in sufficient quantity that large bags ofpotassium chloride commercialsalt substitutes can be used as aradioactive source for classroom demonstrations.[citation needed]40
K is the largest source of natural radioactivity in healthy animals and humans, greater even than14
C. In a human body of 70 kg mass, about 4300 nuclei of40
K decay per second.[5]
The decay of40
K to40
Ar is used inpotassium-argon dating of rocks.Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic40
Ar that has accumulated.40
K has also been extensively used as aradioactive tracer in studies ofweathering.[citation needed]
All other potassium isotopes have half-lives under a day, most under a minute. The unbound31
K was discovered in 2019 and emitsthreeprotons; its half-life was measured to be shorter than 10picoseconds.[6][7]
Stable potassium isotopes have been used for severalnutrient cycling studies since potassium is amacronutrient required for life.[8]
| Nuclide [n 1] | Z | N | Isotopic mass(Da)[9] [n 2][n 3] | Half-life[10] [n 4] | Decay mode[10] | Daughter isotope [n 5] | Spin and parity[10] [n 6][n 4] | Natural abundance(mole fraction) | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Excitation energy[n 4] | Normal proportion[10] | Range of variation | |||||||||||||||||
| 31 K[6][7] | 19 | 12 | 31.03678(32)# | <10 ps | 3p | 28S | 3/2+# | ||||||||||||
| 34K[11] | 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%)[12] | |||||||||||||||||||
| 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− | ||||||||||||
| 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? | 53Ca | ||||||||||||||||||
| 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 9] | 19 | 40 | 59.03086(86)# | 1# ms [>400 ns] | β−? | 59Ca | 3/2+# | ||||||||||||
| β−, n? | 58Ca | ||||||||||||||||||
| β−, 2n? | 57Ca | ||||||||||||||||||
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Daughter products other than potassium