Open main menu

Isotopes of potassium

Potassium (19K) has 25 known isotopes from 33K to 57K, with unconfirmed detection of 59K.[2] Three isotopes occur naturally: stable 39K (93.3%) and 41K (6.7%), and the long-lived radioisotope 40K (0.012%).

Main isotopes of potassium (19K)
Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
39K 93.258% stable
40K 0.012% 1.248(3)×109 y β 40Ca
ε 40Ar
β+ 40Ar
41K 6.730% stable
Standard atomic weight Ar, standard(K)

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.[citation needed] 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.[3]

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 is 31K, a three-proton emitter discovered in 2019; its half-life was measured to be shorter than 10 picoseconds.[4][5]

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

List of isotopesEdit

Nuclide[6]
[n 1]
Z N Isotopic mass (u)[7]
[n 2][n 3]
Half-life
[n 4][n 5]
Decay
mode

Daughter
isotope

[n 6]
Spin and
parity
[n 7][n 5]
Natural abundance (mole fraction)
Excitation energy[n 5] Normal proportion Range of variation
33K 19 14 33.00756(21)# <25 ns p 32Ar 3/2+#
34K 19 15 33.99869(21)# <40 ns p 33Ar 1+#
35K 19 16 34.9880054(6) 178(8) ms β+ (99.63%) 35Ar 3/2+
β+, p (.37%) 34Cl
36K 19 17 35.9813020(4) 341(3) ms β+ (99.95%) 36Ar 2+
β+, p (.048%) 35Cl
β+, α (.0034%) 32S
37K 19 18 36.97337589(10) 1.2365(9) s β+ 37Ar 3/2+
38K 19 19 37.96908112(21) 7.636(18) min β+ 38Ar 3+
38m1K 130.50(28) keV 924.46(14) ms β+ 38Ar 0+
38m2K 3458.0(2) keV 21.95(11) µs IT 38K (7+)
39K 19 20 38.963706487(5) Stable 3/2+ 0.932581(44)
40K[n 8][n 9] 19 21 39.96399817(6) 1.248(3)×109 y β (89.28%) 40Ca 4− 1.17(1)×10−4
EC (10.72%) 40Ar
β+ (0.001%)[8]
40mK 1643.639(11) keV 336(12) ns IT 40K 0+
41K 19 22 40.961825258(4) Stable 3/2+ 0.067302(44)
42K 19 23 41.96240231(11) 12.355(7) h β 42Ca 2−
43K 19 24 42.9607347(4) 22.3(1) h β 43Ca 3/2+
43mK 738.30(6) keV 200(5) ns IT 43K 7/2−
44K 19 25 43.9615870(5) 22.13(19) min β 44Ca 2−
45K 19 26 44.9606915(6) 17.8(6) min β 45Ca 3/2+
46K 19 27 45.9619816(8) 105(10) s β 46Ca 2−
47K 19 28 46.9616616(15) 17.50(24) s β 47Ca 1/2+
48K 19 29 47.9653412(8) 6.8(2) s β (98.86%) 48Ca 1−
β, n (1.14%) 47Ca
49K 19 30 48.9682108(9) 1.26(5) s β, n (86%) 48Ca (3/2+)
β (14%) 49Ca
50K 19 31 49.972380(8) 472(4) ms β (71%) 50Ca 0−
β, n (29%) 49Ca
50mK 171.4(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
52K 19 33 51.98160(4) 110(4) ms β, n (74%) 51Ca 2−#
β (23.7%) 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 53.99463(64)# 10(5) ms β (>99.9%) 54Ca 2−#
β, n (<.1%) 53Ca
55K 19 36 55.00076(75)# 3# ms β 55Ca 3/2+#
β, n 54Ca
56K 19 37 56.00851(86)# 1# ms β 56Ca 2−#
β, n 55Ca
57K[9][2] 19 38 β 57Ca
59K[2][n 10] 19 40 β 59Ca
  1. ^ mK – Excited nuclear isomer.
  2. ^ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. ^ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. ^ Bold half-life – nearly stable, half-life longer than age of universe.
  5. ^ a b c # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  6. ^ Bold symbol as daughter – Daughter product is stable.
  7. ^ ( ) spin value – Indicates spin with weak assignment arguments.
  8. ^ Used in potassium-argon dating
  9. ^ Primordial radionuclide
  10. ^ Discovery of this isotope is unconfirmed

See alsoEdit

ReferencesEdit

  1. ^ Meija, Juris; et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry. 88 (3): 265–91. doi:10.1515/pac-2015-0305.
  2. ^ a b c Tarasov, O.B. (2017). "Production of very neutron rich isotopes: What should we know?".
  3. ^ "Radioactive Human Body". Retrieved 2011-05-18.
  4. ^ "A peculiar atom shakes up assumptions of nuclear structure". Nature. 573 (7773): 167. 6 September 2019. Bibcode:2019Natur.573T.167.. doi:10.1038/d41586-019-02655-9. PMID 31506620.
  5. ^ Kostyleva, D.; et al. (2019). "Towards the Limits of Existence of Nuclear Structure: Observation and First Spectroscopy of the Isotope 31K by Measuring Its Three-Proton Decay". Physical Review Letters. 123 (9): 092502. arXiv:1905.08154. Bibcode:2019PhRvL.123i2502K. doi:10.1103/PhysRevLett.123.092502. PMID 31524489.
  6. ^ Half-life, decay mode, nuclear spin, and isotopic composition is sourced in:
    Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017). "The NUBASE2016 evaluation of nuclear properties" (PDF). Chinese Physics C. 41 (3): 030001. Bibcode:2017ChPhC..41c0001A. doi:10.1088/1674-1137/41/3/030001.
  7. ^ Wang, M.; Audi, G.; Kondev, F. G.; Huang, W. J.; Naimi, S.; Xu, X. (2017). "The AME2016 atomic mass evaluation (II). Tables, graphs, and references" (PDF). Chinese Physics C. 41 (3): 030003–1—030003–442. doi:10.1088/1674-1137/41/3/030003.
  8. ^ 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.
  9. ^ Neufcourt, L.; Cao, Y.; Nazarewicz, W.; Olsen, E.; Viens, F. (2019). "Neutron drip line in the Ca region from Bayesian model averaging". Physical Review Letters. 122 (6): 062502–1–062502–6. arXiv:1901.07632. Bibcode:2019PhRvL.122f2502N. doi:10.1103/PhysRevLett.122.062502. PMID 30822058.