Isotopes of potassium

Potassium (K) has 24 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 relative atomic mass is 39.0983(1).

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

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

• 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

• Isotope masses from:
  • G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001. 
• Isotopic compositions and standard atomic masses from:
  • J. R. de Laeter; J. K. Böhlke; P. De Bièvre; H. Hidaka; H. S. Peiser; K. J. R. Rosman; P. D. P. Taylor (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry 75 (6): 683–800. doi:10.1351/pac200375060683. 
  • M. E. Wieser (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry 78 (11): 2051–2066. doi:10.1351/pac200678112051. Lay summary. 
• Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.
  • G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001. 
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005. 
  • N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide. CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0-8493-0485-9. 

Isotopes of the chemical elements
1 H																	2 He
3 Li	4 Be											5 B	6 C	7 N	8 O	9 F	10 Ne
11 Na	12 Mg											13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
19 K	20 Ca	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Ge	33 As	34 Se	35 Br	36 Kr
37 Rb	38 Sr	39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
55 Cs	56 Ba		72 Hf	73 Ta	74 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At	86 Rn
87 Fr	88 Ra		104 Rf	105 Db	106 Sg	107 Bh	108 Hs	109 Mt	110 Ds	111 Rg	112 Cn	113 Uut	114 Fl	115 Uup	116 Lv	117 Uus	118 Uuo
		57 La	58 Ce	59 Pr	60 Nd	61 Pm	62 Sm	63 Eu	64 Gd	65 Tb	66 Dy	67 Ho	68 Er	69 Tm	70 Yb	71 Lu
		89 Ac	90 Th	91 Pa	92 U	93 Np	94 Pu	95 Am	96 Cm	97 Bk	98 Cf	99 Es	100 Fm	101 Md	102 No	103 Lr
Table of nuclides Categories: Isotopes Tables of nuclides Metastable isotopes Isotopes by element