Isotopes of potassium
Encyclopedia
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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