# Isotopes of potassium

Potassium ( $_{19}$K) has 25 known isotopes from $^{34}$ K to $^{57}$ K as well as $^{31}$ K, as well as an unconfirmed report of $^{59}$ K. 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 1.248×10$^9$ 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. $^{40}$ 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.
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.
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) | Half-life | Decay mode | Daughter isotope | Spin and parity | Natural abundance (mole fraction) | Natural abundance (mole fraction) |
| Nuclide | Excitation energy | Excitation energy | Excitation energy | Half-life | Decay mode | Daughter isotope | Spin and parity | Normal proportion | Range of variation |
| $^{31}$ K | 19 | 12 | 31.03678(32)# | <10 ps | 3p | $^{28}$S | 3/2+# | | |
| $^{34}$K | 19 | 15 | 33.998404(18) | | p | $^{33}$Ar | | | |
| $^{35}$K | 19 | 16 | 34.98800541(55) | 175.2(19) ms | β$^+$ (99.63%) | $^{35}$Ar | 3/2+ | | |
| $^{35}$K | 19 | 16 | 34.98800541(55) | 175.2(19) ms | β$^+$, p (0.37%) | $^{34}$Cl | 3/2+ | | |
| $^{36}$K | 19 | 17 | 35.98130189(35) | 341(3) ms | β$^+$ (99.95%) | $^{36}$Ar | 2+ | | |
| $^{36}$K | 19 | 17 | 35.98130189(35) | 341(3) ms | β$^+$, p (0.048%) | $^{35}$Cl | 2+ | | |
| $^{36}$K | 19 | 17 | 35.98130189(35) | 341(3) ms | β$^+$, α (0.0034%) | $^{32}$S | 2+ | | |
| $^{37}$K | 19 | 18 | 36.97337589(10) | 1.23651(94) s | β$^+$ | $^{37}$Ar | 3/2+ | | |
| $^{38}$K | 19 | 19 | 37.96908111(21) | 7.651(19) min | β$^+$ | $^{38}$Ar | 3+ | | |
| $^{38m1}$K | 130.15(4) keV | 130.15(4) keV | 130.15(4) keV | 924.35(12) ms | β$^+$ (99.97%) | $^{38}$Ar | 0+ | | |
| $^{38m1}$K | 130.15(4) keV | 130.15(4) keV | 130.15(4) keV | 924.35(12) ms | IT (0.0330%) | $^{38}$K | 0+ | | |
| $^{38m2}$K | 3458.10(17) keV | 3458.10(17) keV | 3458.10(17) keV | 21.95(11) μs | IT | $^{38}$K | (7)+ | | |
| $^{39}$K | 19 | 20 | 38.9637064848(49) | Stable | Stable | Stable | 3/2+ | 0.932581(44) | |
| $^{40}$K | 19 | 21 | 39.963998165(60) | 1.248(3)×10$^9$ y | β$^−$ (89.28%) | $^{40}$Ca | 4− | 1.17(1)×10$^{−4}$ | |
| $^{40}$K | 19 | 21 | 39.963998165(60) | 1.248(3)×10$^9$ y | EC (10.72%) | $^{40}$Ar | 4− | 1.17(1)×10$^{−4}$ | |
| $^{40}$K | 19 | 21 | 39.963998165(60) | 1.248(3)×10$^9$ y | β$^+$ (0.001%) | $^{40}$Ar | 4− | 1.17(1)×10$^{−4}$ | |
| $^{40m}$K | 1643.638(11) keV | 1643.638(11) keV | 1643.638(11) keV | 336(12) ns | IT | $^{40}$K | 0+ | | |
| $^{41}$K | 19 | 22 | 40.9618252561(40) | Stable | Stable | Stable | 3/2+ | 0.067302(44) | |
| $^{42}$K | 19 | 23 | 41.96240231(11) | 12.355(7) h | β$^−$ | $^{42}$Ca | 2− | Trace | |
| $^{43}$K | 19 | 24 | 42.96073470(44) | 22.3(1) h | β$^−$ | $^{43}$Ca | 3/2+ | | |
| $^{43m}$K | 738.30(6) keV | 738.30(6) keV | 738.30(6) keV | 200(5) ns | IT | $^{43}$K | 7/2− | | |
| $^{44}$K | 19 | 25 | 43.96158698(45) | 22.13(19) min | β$^−$ | $^{44}$Ca | 2− | | |
| $^{45}$K | 19 | 26 | 44.96069149(56) | 17.8(6) min | β$^−$ | $^{45}$Ca | 3/2+ | | |
| $^{46}$K | 19 | 27 | 45.96198158(78) | 96.30(8) s | β$^−$ | $^{46}$Ca | 2− | | |
| $^{47}$K | 19 | 28 | 46.9616616(15) | 17.38(3) s | β$^−$ | $^{47}$Ca | 1/2+ | | |
| $^{48}$K | 19 | 29 | 47.96534118(83) | 6.83(14) s | β$^−$ (98.86%) | $^{48}$Ca | 1− | | |
| $^{48}$K | 19 | 29 | 47.96534118(83) | 6.83(14) s | β$^−$, n (1.14%) | $^{47}$Ca | 1− | | |
| $^{49}$K | 19 | 30 | 48.96821075(86) | 1.26(5) s | β$^−$, n (86%) | $^{48}$Ca | 1/2+ | | |
| $^{49}$K | 19 | 30 | 48.96821075(86) | 1.26(5) s | β$^−$ (14%) | $^{49}$Ca | 1/2+ | | |
| $^{50}$K | 19 | 31 | 49.9723800(83) | 472(4) ms | β$^−$ (71.4%) | $^{50}$Ca | 0− | | |
| $^{50}$K | 19 | 31 | 49.9723800(83) | 472(4) ms | β$^−$, n (28.6%) | $^{49}$Ca | 0− | | |
| $^{50}$K | 19 | 31 | 49.9723800(83) | 472(4) ms | β$^−$, 2n? | $^{48}$Ca | 0− | | |
| $^{50m}$K | 172.0(4) keV | 172.0(4) keV | 172.0(4) keV | 125(40) ns | IT | $^{50}$K | (2−) | | |
| $^{51}$K | 19 | 32 | 50.975828(14) | 365(5) ms | β$^−$, n (65%) | $^{50}$Ca | 3/2+ | | |
| $^{51}$K | 19 | 32 | 50.975828(14) | 365(5) ms | β$^−$ (35%) | $^{51}$Ca | 3/2+ | | |
| $^{51}$K | 19 | 32 | 50.975828(14) | 365(5) ms | β$^−$, 2n? | $^{49}$Ca | 3/2+ | | |
| $^{52}$K | 19 | 33 | 51.981602(36) | 110(4) ms | β$^−$, n (72.2%) | $^{51}$Ca | 2−# | | |
| $^{52}$K | 19 | 33 | 51.981602(36) | 110(4) ms | β$^−$ (25.5%) | $^{52}$Ca | 2−# | | |
| $^{52}$K | 19 | 33 | 51.981602(36) | 110(4) ms | β$^−$, 2n (2.3%) | $^{50}$Ca | 2−# | | |
| $^{53}$K | 19 | 34 | 52.98680(12) | 30(5) ms | β$^−$, n (64%) | $^{52}$Ca | 3/2+ | | |
| $^{53}$K | 19 | 34 | 52.98680(12) | 30(5) ms | β$^−$ (26%) | $^{53}$Ca | 3/2+ | | |
| $^{53}$K | 19 | 34 | 52.98680(12) | 30(5) ms | β$^−$, 2n (10%) | $^{51}$Ca | 3/2+ | | |
| $^{54}$K | 19 | 35 | 53.99447(43)# | 10(5) ms | β$^−$ | $^{54}$Ca | 2−# | | |
| $^{54}$K | 19 | 35 | 53.99447(43)# | 10(5) ms | β$^−$, n? | $^{53}$Ca | 2−# | | |
| $^{54}$K | 19 | 35 | 53.99447(43)# | 10(5) ms | β$^−$, 2n? | $^{52}$Ca | 2−# | | |
| $^{55}$K | 19 | 36 | 55.00051(54)# | 10# ms [>620 ns] | β$^−$? | $^{55}$Ca | 3/2+# | | |
| $^{55}$K | 19 | 36 | 55.00051(54)# | 10# ms [>620 ns] | β$^−$, n? | $^{54}$Ca | 3/2+# | | |
| $^{55}$K | 19 | 36 | 55.00051(54)# | 10# ms [>620 ns] | β$^−$, 2n? | $^{53}$Ca | 3/2+# | | |
| $^{56}$K | 19 | 37 | 56.00857(64)# | 5# ms [>620 ns] | β$^−$? | $^{56}$Ca | 2−# | | |
| $^{56}$K | 19 | 37 | 56.00857(64)# | 5# ms [>620 ns] | β$^−$, n? | $^{55}$Ca | 2−# | | |
| $^{56}$K | 19 | 37 | 56.00857(64)# | 5# ms [>620 ns] | β$^−$, 2n? | $^{54}$Ca | 2−# | | |
| $^{57}$K | 19 | 38 | 57.01517(64)# | 2# ms [>400 ns] | β$^−$? | $^{57}$Ca | 3/2+# | | |
| $^{57}$K | 19 | 38 | 57.01517(64)# | 2# ms [>400 ns] | β$^−$, n? | $^{56}$Ca | 3/2+# | | |
| $^{57}$K | 19 | 38 | 57.01517(64)# | 2# ms [>400 ns] | β$^−$, 2n? | $^{55}$Ca | 3/2+# | | |
| $^{59}$K | 19 | 40 | 59.03086(86)# | 1# ms [>400 ns] | β$^−$? | $^{59}$Ca | 3/2+# | | |
| $^{59}$K | 19 | 40 | 59.03086(86)# | 1# ms [>400 ns] | β$^−$, n? | $^{58}$Ca | 3/2+# | | |
| $^{59}$K | 19 | 40 | 59.03086(86)# | 1# ms [>400 ns] | β$^−$, 2n? | $^{57}$Ca | 3/2+# | | |
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* ↑ $^m$K – 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).
* 1 2 3 # – 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 $^{42}$Ar
* ↑ Discovery of this isotope is unconfirmed.