Potassium

Potassium

Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

(K) has 25 known isotope

Isotope

Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

s from ³²K to ⁵⁶K. Three isotopes occur naturally: stable ³⁹K (93.3%) and ⁴¹K (6.7%), and the long-lived radioisotope ⁴⁰K

Potassium-40

Potassium-40 is a radioactive isotope of potassium which has a very long half-life of 1.248 years, or about 39.38 seconds.Potassium-40 is a rare example of an isotope which undergoes all three types of beta decay. About 89.28% of the time, it decays to calcium-40 with emission of a beta particle...

 (0.012%). The standard atomic mass is 39.0983(1) u. Naturally occurring ⁴⁰K decays to stable ⁴⁰Ar

Argon

Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

 (11.2% of decays) by electron capture

Electron capture

Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...

 or positron emission

Positron emission

Positron emission or beta plus decay is a type of beta decay in which a proton is converted, via the weak force, to a neutron, releasing a positron and a neutrino....

 (giving it the longest known positron-emitter nuclide half-life). Alternately and most of the time (88.8%) it decays to stable ⁴⁰Ca

Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 by beta decay

Beta decay

In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...

; ⁴⁰K has a half-life

Half-life

Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...

 of 1.248×10⁹ years. All other 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.

The decay of ⁴⁰K to ⁴⁰Ar enables a commonly used method for dating rocks. The conventional K-Ar dating method

Potassium-argon dating

Potassium–argon dating or K–Ar dating is a radiometric dating method used in geochronology and archeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium into argon . Potassium is a common element found in many materials, such as micas, clay minerals,...

 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. Mineral

Mineral

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

s 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

Biotite

Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...

, muscovite

Muscovite

Muscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...

, plutonic/high grade metamorphic hornblende

Hornblende

Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....

, and volcanic feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

; 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 tracer

Radioactive tracer

A radioactive tracer, also called a radioactive label, is a substance containing a radioisotope that is used to measure the speed of chemical processes and to track the movement of a substance through a natural system such as a cell or tissue...

s in studies of weathering

Weathering

Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

. They have also been used for nutrient cycling studies because potassium is a macronutrient required for life

Life

Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

.

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

Carbon-14

Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues , to date archaeological, geological, and hydrogeological...

. In a human body of 70 kg mass, about 4,400 nuclei of ⁴⁰K decay per second.

Table

nuclide symbol	Z(p Proton The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number.... )	N(n Neutron The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of... )	isotopic mass (u)	half-life	decay mode(s)	daughter isotope(s)Bold for stable isotopes, bold italic for nearly-stable isotopes (half-life longer than the age of the universe Age of the universe The age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model... )	nuclear spin	representative isotopic composition (mole fraction)
	excitation energy
32K	19	13	32.02192(54)#	unknown	p Proton emission Proton emission is a type of radioactive decay in which a proton is ejected from a nucleus. Proton emission can occur from high-lying excited states in a nucleus following a beta decay, in which case the process is known as beta-delayed proton emission, or can occur from the ground state of very...	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+ Beta decay In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...  (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 Alpha decay Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...  (.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 Potassium-40 Potassium-40 is a radioactive isotope of potassium which has a very long half-life of 1.248 years, or about 39.38 seconds.Potassium-40 is a rare example of an isotope which undergoes all three types of beta decay. About 89.28% of the time, it decays to calcium-40 with emission of a beta particle... Used in potassium-argon dating Potassium-argon dating Potassium–argon dating or K–Ar dating is a radiometric dating method used in geochronology and archeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium into argon . Potassium is a common element found in many materials, such as micas, clay minerals,... Primordial Primordial nuclide In geochemistry and geonuclear physics, primordial nuclides or primordial isotopes are nuclides found on the earth that have existed in their current form since before Earth was formed. Only 288 such nuclides are known...  radionuclide Radionuclide A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits gamma...	19	21	39.96399848(21)	1.248(3)×109 years	β- (89.28%)	40Ca	4-	1.17(1)×10−4
					β+ (10.72%)	40Ar
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 Neutron emission Neutron emission is a type of radioactive decay of atoms containing excess neutrons, in which a neutron is simply ejected from the nucleus. Two examples of isotopes which emit neutrons are helium-5 and beryllium-13... (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

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