Isotopes of potassium

Potassium (K) has 24 known isotopes. Three isotopes occur naturally: ³⁹K (93.3%), ⁴⁰K (0.012%) and ⁴¹K (6.7%). The standard atomic mass is 39.0983(1) u. Naturally occurring ⁴⁰K decays to stable ⁴⁰Ar (11.2%) by electron capture and by positron emission, and decays to stable ⁴⁰Ca (88.8%) by beta decay; ⁴⁰K has a half-life of 1.250×10⁹ years. 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. The minerals that are best suited for dating include biotite, muscovite, plutonic/high grade metamorphic hornblende, and volcanic feldspar; whole rock samples from volcanic flows and shallow instrusives can also be dated if they are unaltered.

Outside of dating, potassium isotopes have been used extensively as tracers in studies of weathering. They have also been used for nutrient cycling studies because potassium is a macronutrient required for life.

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]

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	nuclear spin	representative isotopic composition (mole fraction)
nuclide symbol	excitation energy			half-life	nuclear spin	representative isotopic composition (mole fraction)
³²K	19	13	32.02192(54)#		1+#
^32mK	950(100)# keV			?	4+#
³³K	19	14	33.00726(21)#	<25 ns	(3/2+)#
³⁴K	19	15	33.99841(32)#	<40 ns	1+#
³⁵K	19	16	34.988010(21)	178(8) ms	3/2+
³⁶K	19	17	35.981292(8)	342(2) ms	2+
³⁷K	19	18	36.97337589(10)	1.226(7) s	3/2+
³⁸K	19	19	37.9690812(5)	7.636(18) min	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	19	21	39.96399848(21)	1.248(3)E+9 a	4-	0.000117(1)
^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	2-
⁴³K	19	24	42.960716(10)	22.3(1) h	3/2+
⁴⁴K	19	25	43.96156(4)	22.13(19) min	2-
⁴⁵K	19	26	44.960699(11)	17.3(6) min	3/2+
⁴⁶K	19	27	45.961977(17)	105(10) s	2(-)
⁴⁷K	19	28	46.961678(9)	17.50(24) s	1/2+
⁴⁸K	19	29	47.965514(26)	6.8(2) s	(2-)
⁴⁹K	19	30	48.96745(8)	1.26(5) s	(3/2+)
⁵⁰K	19	31	49.97278(30)	472(4) ms	(0-,1,2-)
⁵¹K	19	32	50.97638(54)#	365(5) ms	3/2+#
⁵²K	19	33	51.98261(75)#	105(5) ms	(2-)#
⁵³K	19	34	52.98712(75)#	30(5) ms	(3/2+)#
⁵⁴K	19	35	53.99420(97)#	10(5) ms	2-#
⁵⁵K	19	36	54.99971(107)#	3# ms	3/2+#

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.

References

Isotope masses from Ame2003 Atomic Mass Evaluation by G. Audi, A.H. Wapstra, C. Thibault, J. Blachot and O. Bersillon in Nuclear Physics A729 (2003).
Isotopic compositions and standard atomic masses from Atomic weights of the elements. Review 2000 (IUPAC Technical Report). Pure Appl. Chem. Vol. 75, No. 6, pp. 683-800, (2003) and Atomic Weights Revised (2005).
Half-life, spin, and isomer data selected from these sources. Editing notes on this article's talk page.
- Audi, Bersillon, Blachot, Wapstra. The Nubase2003 evaluation of nuclear and decay properties, Nuc. Phys. A 729, pp. 3-128 (2003).
- National Nuclear Data Center, Brookhaven National Laboratory. Information extracted from the NuDat 2.1 database (retrieved Sept. 2005).
- David R. Lide (ed.), Norman E. Holden in CRC Handbook of Chemistry and Physics, 85th Edition, online version. CRC Press. Boca Raton, Florida (2005). Section 11, Table of the Isotopes.