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.

Table[edit]

nuclide
symbol
Z(p) N(n)  
isotopic mass (u)
 
half-life decay
mode(s)[2]
daughter
isotope(s)[n 1]
nuclear
spin
representative
isotopic
composition
(mole fraction)
excitation energy
32K 19 13 32.02192(54)# unknown p 31Ar 1+#
32mK 950(100)# keV unknown 4+#
33K 19 14 33.00726(21)# <25 ns p 32Ar (3/2+)#
34K 19 15 33.99841(32)# <25 ns p 33Ar 1+#
35K 19 16 34.988010(21) 178(8) ms β+ (99.63%) 35Ar 3/2+
β+, p (.37%) 34Cl
36K 19 17 35.981292(8) 342(2) ms β+ (99.94%) 36Ar 2+
β+, p (.048%) 35Cl
β+, α (.012%) 32S
37K 19 18 36.97337589(10) 1.226(7) s β+ 37Ar 3/2+
38K 19 19 37.9690812(5) 7.636(18) min β+ 38Ar 3+
38m1K 130.50(28) keV 924.2(3) ms 0+
38m2K 3458.0(2) keV 21.98(11) µs (7+),(5+)
39K 19 20 38.96370668(20) Stable 3/2+ 0.932581(44)
40K[n 2][n 3] 19 21 39.96399848(21) 1.248(3)×109 a β- (89.28%) 40Ca 4- 1.17(1)×10−4
EC (10.72%) 40Ar
β+ (0.001%)[3]
40mK 1643.639(11) keV 336(12) ns 0+
41K 19 22 40.96182576(21) Stable 3/2+ 0.067302(44)
42K 19 23 41.96240281(24) 12.360(12) h β- 42Ca 2-
43K 19 24 42.960716(10) 22.3(1) h β- 43Ca 3/2+
44K 19 25 43.96156(4) 22.13(19) min β- 44Ca 2-
45K 19 26 44.960699(11) 17.3(6) min β- 45Ca 3/2+
46K 19 27 45.961977(17) 105(10) s β- 46Ca 2(-)
47K 19 28 46.961678(9) 17.50(24) s β- 47Ca 1/2+
48K 19 29 47.965514(26) 6.8(2) s β- (98.86%) 48Ca (2-)
β-, n (1.14%) 47Ca
49K 19 30 48.96745(8) 1.26(5) s β-, n (86%) 48Ca (3/2+)
β- (14%) 49Ca
50K 19 31 49.97278(30) 472(4) ms β- (71%) 50Ca (0-,1,2-)
β-, n (29%) 49Ca
51K 19 32 50.97638(54)# 365(5) ms β- (53%) 51Ca 3/2+#
β-, n (47%) 50Ca
52K 19 33 51.98261(75)# 105(5) ms β-, n (64%) 51Ca (2-)#
β-, 2n (21%) 50Ca
β- (15%) 52Ca
53K 19 34 52.98712(75)# 30(5) ms β-, n (67%) 52Ca (3/2+)#
β-, 2n (17%) 51Ca
β- (16%) 53Ca
54K 19 35 53.99420(97)# 10(5) ms β- (>99.9%) 54Ca 2-#
β-, n (<.1%) 53Ca
55K 19 36 54.99971(107)# 3# ms β- 55Ca 3/2+#
β-, n 54Ca
  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

Notes[edit]

  • Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
  • Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.
  • Nuclide masses are given by IUPAP Commission on Symbols, Units, Nomenclature, Atomic Masses and Fundamental Constants (SUNAMCO).
  • Isotope abundances are given by IUPAC Commission on Isotopic Abundances and Atomic Weights.

References[edit]

  1. ^ "Radioactive Human Body". Retrieved 2011-05-18. 
  2. ^ http://www.nucleonica.net/unc.aspx
  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 argon Isotopes of potassium Isotopes of calcium
Table of nuclides