Isotopes of potassium
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Potassium (19K) has 25 known isotopes from 34K to 57K as well as 31K, as well as an unconfirmed report of 59K. Three of those isotopes occur naturally: the two stable forms 39K (93.26%) and 41K (6.72%), and the long-lived radioisotope 40K (0.012%).
Naturally occurring radioactive 40K decays with a half-life of 1.248×109 years. 89% of those decays are to stable 40Ca by beta decay, whilst 11% are to 40Ar 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. 40K has the longest known half-life for any positron-emitting nuclide. The long half-life of this primordial radioisotope is caused by a highly spin-forbidden transition: 40K has a nuclear spin of 4, while both of its decay daughters are even–even isotopes with spins of 0.
40K 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] 40K is the largest source of natural radioactivity in healthy animals and humans, greater even than 14C. In a human body of 70 kg mass, about 4300 nuclei of 40K decay per second.
The decay of 40K to 40Ar is used in potassium-argon dating of rocks. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic 40Ar that has accumulated. 40K 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 unbound 31K was discovered in 2019 and emits three protons; its half-life was measured to be shorter than 10 picoseconds.
Stable potassium isotopes have been used for several nutrient cycling studies since potassium is a macronutrient required for life.
List of isotopes
| Nuclide | Z | N | Isotopic mass (Da) | Discovery year | Half-life | Decay mode | Daughter isotope | Spin and parity | Natural abundance (mole fraction) | |
|---|---|---|---|---|---|---|---|---|---|---|
| Excitation energy | Normal proportion | Range of variation | ||||||||
| 31K | 19 | 12 | 31.03678(32)# | <10 ps | p | 30Ar | 3/2+# | |||
| 34K | 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 | 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%) | ||||||||||
| 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 | 19 | 40 | 59.03086(86)# | 1# ms [>400 ns] | β−? | 59Ca | 3/2+# | |||
| β−, n? | 58Ca | |||||||||
| β−, 2n? | 57Ca | |||||||||
| This table header & footer: view |
See also
Daughter products other than potassium