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

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)×10^9 y	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

• 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

1. "Radioactive Human Body". Retrieved 2011-05-18.
2. "Universal Nuclide Chart". nucleonica. (registration required (help)).
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.

• 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.