Isotopes of argon
Encyclopedia
Argon
(Ar) has 24 known isotopes, from 30Ar to 53Ar and 1 isomer
(32mAr), three of which are stable
, 36Ar, 38Ar, and 40Ar. On Earth, 40Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are 39Ar with a half-life of 269 years, 42Ar with a half-life of 32.9 years, and 37Ar with a half-life of 35.04 days. All other isotopes have half-lives less than 2 hours, and most less than a minute. The least stable is 30Ar with a half-life shorter than 20 nanoseconds.
Naturally occurring 40K
with a half-life
of 1.248 (3) years, decays to stable 40Ar (10.72%) by electron capture
and by positron emission
, and also transforms to stable 40Ca (89.28%) via beta decay
. These properties and ratios are used to determine the age of rocks
through potassium-argon dating
.
Despite trapping of 40Ar in many rocks, it can be released by melting, grinding, and diffusion. Almost all of the argon in the Earth's atmosphere is the product of potassium-40 decay, since 99.6% of Earth atmospheric argon is 40Ar, whereas in the Sun and presumably in primordial star-forming clouds, argon consists of < 15% 40Ar and mostly (85%) 36Ar.
In the Earth's atmosphere
, radioactive 39Ar (half-life 269 years) is made by cosmic ray
activity, primarily from 40Ar. In the subsurface environment, it is also produced through neutron capture
by 39K or alpha emission
by calcium
. The content of 39Ar in natural argon is measured to be of (8.0±0.6)×10−16 g/g, or (1.01±0.08) Bq/kg of 36, 38, 40Ar. The content of 42Ar (half-life 33 years) in the Earth's atmosphere is lower than 6×10−21 parts per part of 36, 38, 40Ar.
Radioactive 37Ar is a synthetic radionuclide that is created from the neutron spallation
of 40Ca as a result of subsurface nuclear explosions
. It has a half-life of 35 days.
Standard argon atomic mass: 39.948(1) u.
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...
(Ar) has 24 known isotopes, from 30Ar to 53Ar and 1 isomer
Nuclear isomer
A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...
(32mAr), three of which are stable
Stable isotope
Stable isotopes are chemical isotopes that may or may not be radioactive, but if radioactive, have half-lives too long to be measured.Only 90 nuclides from the first 40 elements are energetically stable to any kind of decay save proton decay, in theory...
, 36Ar, 38Ar, and 40Ar. On Earth, 40Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are 39Ar with a half-life of 269 years, 42Ar with a half-life of 32.9 years, and 37Ar with a half-life of 35.04 days. All other isotopes have half-lives less than 2 hours, and most less than a minute. The least stable is 30Ar with a half-life shorter than 20 nanoseconds.
Naturally occurring 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...
with 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 (3) years, decays to stable 40Ar (10.72%) 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...
and by 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....
, and also transforms to stable 40Ca (89.28%) via 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...
. These properties and ratios are used to determine the age of rocks
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
through 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,...
.
Despite trapping of 40Ar in many rocks, it can be released by melting, grinding, and diffusion. Almost all of the argon in the Earth's atmosphere is the product of potassium-40 decay, since 99.6% of Earth atmospheric argon is 40Ar, whereas in the Sun and presumably in primordial star-forming clouds, argon consists of < 15% 40Ar and mostly (85%) 36Ar.
In the Earth's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
, radioactive 39Ar (half-life 269 years) is made by cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
activity, primarily from 40Ar. In the subsurface environment, it is also produced through neutron capture
Neutron capture
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
by 39K or alpha emission
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...
by calcium
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...
. The content of 39Ar in natural argon is measured to be of (8.0±0.6)×10−16 g/g, or (1.01±0.08) Bq/kg of 36, 38, 40Ar. The content of 42Ar (half-life 33 years) in the Earth's atmosphere is lower than 6×10−21 parts per part of 36, 38, 40Ar.
Radioactive 37Ar is a synthetic radionuclide that is created from the neutron spallation
Spallation
In general, spallation is a process in which fragments of material are ejected from a body due to impact or stress. In the context of impact mechanics it describes ejection or vaporization of material from a target during impact by a projectile...
of 40Ca as a result of subsurface nuclear explosions
Nuclear testing
Nuclear weapons tests are experiments carried out to determine the effectiveness, yield and explosive capability of nuclear weapons. Throughout the twentieth century, most nations that have developed nuclear weapons have tested them...
. It has a half-life of 35 days.
Standard argon atomic mass: 39.948(1) u.
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)Abbreviations: EC: 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... |
daughter isotope(s)Bold for stable isotopes |
nuclear spin |
representative isotopic composition (mole fraction) |
range of natural variation (mole fraction) |
|---|---|---|---|---|---|---|---|---|---|
| excitation energy | |||||||||
| 30Ar | 18 | 12 | 30.02156(32)# | <20 ns | 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... |
29Cl | 0+ | ||
| 31Ar | 18 | 13 | 31.01212(22)# | 14.4(6) 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... , p (55.0%) |
30S | 5/2(+#) | ||
| β+ (40.4%) | 31Cl | ||||||||
| β+, 2p (2.48%) | 29P | ||||||||
| β+, 3p (2.1%) | 28Si | ||||||||
| 32Ar | 18 | 14 | 31.9976380(19) | 98(2) ms | β+ (56.99%) | 32Cl | 0+ | ||
| β+, p (43.01%) | 31S | ||||||||
| 32mAr | 5600(100)# keV | unknown | 5-# | ||||||
| 33Ar | 18 | 15 | 32.9899257(5) | 173.0(20) ms | β+ (61.35%) | 33Cl | 1/2+ | ||
| β+, p (38.65%) | 32S | ||||||||
| 34Ar | 18 | 16 | 33.9802712(4) | 844.5(34) ms | β+ | 34Cl | 0+ | ||
| 35Ar | 18 | 17 | 34.9752576(8) | 1.775(4) s | β+ | 35Cl | 3/2+ | ||
| 36Ar | 18 | 18 | 35.967545106(29) | Observationally Stable | 0+ | 0.003336(4) | |||
| 37Ar | 18 | 19 | 36.96677632(22) | 35.04(4) d | EC Electron capture Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino... |
37Cl | 3/2+ | ||
| 38Ar | 18 | 20 | 37.9627324(4) | Stable | 0+ | 6.29(1)×10−4 | |||
| 39ArUsed in argon-argon dating Argon-argon dating Argon-argon dating is a radiometric dating method invented to supersede potassium-argon dating in accuracy. The older method required two samples for dating while the newer method requires only one... |
18 | 21 | 38.964313(5) | 269(3) a | β− | 39K | 7/2- | TraceCosmogenic nuclide Cosmogenic nuclide See also Environmental radioactivity#NaturalCosmogenic nuclides are rare isotopes created when a high-energy cosmic ray interacts with the nucleus of an in situ solar system atom, causing cosmic ray spallation... |
|
| 40ArUsed in argon-argon dating Argon-argon dating Argon-argon dating is a radiometric dating method invented to supersede potassium-argon dating in accuracy. The older method required two samples for dating while the newer method requires only one... and 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,... |
18 | 22 | 39.9623831225(29) | Stable | 0+ | 0.996035(4) | |||
| 41Ar | 18 | 23 | 40.9645006(4) | 109.61(4) min | β− | 41K | 7/2- | ||
| 42Ar | 18 | 24 | 41.963046(6) | 32.9(11) a | β− | 42K | 0+ | Trace | |
| 43Ar | 18 | 25 | 42.965636(6) | 5.37(6) min | β− | 43K | (5/2-) | ||
| 44Ar | 18 | 26 | 43.9649240(17) | 11.87(5) min | β− | 44K | 0+ | ||
| 45Ar | 18 | 27 | 44.9680400(6) | 21.48(15) s | β− | 45K | (1/2,3/2,5/2)- | ||
| 46Ar | 18 | 28 | 45.96809(4) | 8.4(6) s | β− | 46K | 0+ | ||
| 47Ar | 18 | 29 | 46.97219(11) | 1.23(3) s | β− (99%) | 47K | 3/2-# | ||
| 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%) |
46K | ||||||||
| 48Ar | 18 | 30 | 47.97454(32)# | 0.48(40) s | β− | 48K | 0+ | ||
| 49Ar | 18 | 31 | 48.98052(54)# | 170(50) ms | β− | 49K | 3/2-# | ||
| 50Ar | 18 | 32 | 49.98443(75)# | 85(30) ms | β− | 50K | 0+ | ||
| 51Ar | 18 | 33 | 50.99163(75)# | 60# ms [>200 ns] | β− | 51K | 3/2-# | ||
| 52Ar | 18 | 34 | 51.99678(97)# | 10# ms | β− | 52K | 0+ | ||
| 53Ar | 18 | 35 | 53.00494(107)# | 3# ms | β− | 53K | (5/2-)# | ||
| β−, n | 52K | ||||||||