Isotopes of potassium

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Potassium (K) has 25 known isotopes from 32K to 56K. Three isotopes occur naturally: stable 39K (93.3%) and 41K (6.7%), and the long-lived radioisotope 40K (0.012%).

The standard atomic mass is 39.0983(1) u.

Naturally occurring radioactive 40K decays to stable 40Ar (10.72% of decays) by electron capture or positron emission (giving it the longest known positron-emitter nuclide half-life). Alternately, and most of the time (89.28%), it decays to stable 40Ca by beta decay. 40K has a half-life of 1.248×109 years. 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 (and thus in some commercial salt substitutes) in sufficient quantity that large bags of those substitutes can be used as a radioactive source for classroom demonstrations. In healthy animals and people, 40K represents the largest source of radioactivity, greater even than 14C. In a human body of 70 kg mass, about 4,400 nuclei of 40K decay per second.[1]

The decay of 40K to 40Ar enables a commonly used method for dating rocks. The conventional K-Ar dating method depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent radiogenic argon (i.e., 40Ar) was quantitatively retained. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic 40Ar that has accumulated.

All other potassium isotopes have half-lives under a day, most under a minute. The least stable are 33K and 34K, both with half-lives shorter than 25 nanoseconds. The half-life of 32K is unknown.

Outside of its use in dating 40K has been used extensively as tracers in studies of weathering. Various potassium isotopes also been used for nutrient cycling studies because potassium is a macronutrient required for life.


isotopic mass (u)
isotope(s)[n 1]
(mole fraction)
excitation energy
32mK950(100)# keVunknown4+#
33K191433.00726(21)#<25 nsp32Ar(3/2+)#
34K191533.99841(32)#<25 nsp33Ar1+#
35K191634.988010(21)178(8) msβ+ (99.63%)35Ar3/2+
β+, p (.37%)34Cl
36K191735.981292(8)342(2) msβ+ (99.94%)36Ar2+
β+, p (.048%)35Cl
β+, α (.012%)32S
37K191836.97337589(10)1.226(7) sβ+37Ar3/2+
38K191937.9690812(5)7.636(18) minβ+38Ar3+
38m1K130.50(28) keV924.2(3) ms0+
38m2K3458.0(2) keV21.98(11) µs(7+),(5+)
40K[n 2][n 3]192139.96399848(21)1.248(3)×109 aβ- (89.28%)40Ca4-1.17(1)×10−4
EC (10.72%)40Ar
β+ (0.001%)[3]
40mK1643.639(11) keV336(12) ns0+
42K192341.96240281(24)12.360(12) hβ-42Ca2-
43K192442.960716(10)22.3(1) hβ-43Ca3/2+
44K192543.96156(4)22.13(19) minβ-44Ca2-
45K192644.960699(11)17.3(6) minβ-45Ca3/2+
46K192745.961977(17)105(10) sβ-46Ca2(-)
47K192846.961678(9)17.50(24) sβ-47Ca1/2+
48K192947.965514(26)6.8(2) sβ- (98.86%)48Ca(2-)
β-, n (1.14%)47Ca
49K193048.96745(8)1.26(5) sβ-, n (86%)48Ca(3/2+)
β- (14%)49Ca
50K193149.97278(30)472(4) msβ- (71%)50Ca(0-,1,2-)
β-, n (29%)49Ca
51K193250.97638(54)#365(5) msβ- (53%)51Ca3/2+#
β-, n (47%)50Ca
52K193351.98261(75)#105(5) msβ-, n (64%)51Ca(2-)#
β-, 2n (21%)50Ca
β- (15%)52Ca
53K193452.98712(75)#30(5) msβ-, n (67%)52Ca(3/2+)#
β-, 2n (17%)51Ca
β- (16%)53Ca
54K193553.99420(97)#10(5) msβ- (>99.9%)54Ca2-#
β-, n (<.1%)53Ca
55K193654.99971(107)#3# msβ-55Ca3/2+#
β-, n54Ca
  1. ^ Bold for stable isotopes, bold italic for nearly-stable isotopes (half-life longer than the age of the universe)
  2. ^ Used in potassium-argon dating
  3. ^ Primordial radionuclide



  1. ^ "Radioactive Human Body". Retrieved 2011-05-18. 
  2. ^
  3. ^ 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. 

Isotopes of argonIsotopes of potassiumIsotopes of calcium
Table of nuclides