Argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice a ...

(₁₈Ar) has 26 known

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...

s, from ²⁹Ar to ⁵⁴Ar and 1

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Is ...

(^32mAr), of which three are

stable A stable is a building in which livestock, especially horses, are kept. It most commonly means a building that is divided into separate stalls for individual animals and livestock. There are many different types of stables in use today; the ...

(³⁶Ar, ³⁸Ar, and ⁴⁰Ar). On the Earth, ⁴⁰Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are ³⁹Ar with a half-life of 268 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 of less than two hours, and most less than one minute. The least stable is ²⁹Ar with a half-life of approximately seconds. The naturally occurring ⁴⁰K, with a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ...

of 1.248 years, decays to stable ⁴⁰Ar by

electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. ...

(10.72%) and by

positron emission Positron emission, beta plus decay, or β+ decay is a subtype of radioactive decay called beta decay, in which a proton inside a radionuclide nucleus is converted into a neutron while releasing a positron and an electron neutrino (). Positron ...

(0.001%), and also transforms to stable ⁴⁰Ca via

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

(89.28%). These properties and ratios are used to determine the age of rocks through potassium–argon dating. Despite the 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 ⁴⁰K 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% ³⁸Ar and mostly (85%) ³⁶Ar. Similarly, the ratio of the three isotopes ³⁶Ar:³⁸Ar:⁴⁰Ar in the atmospheres of the outer planets is measured to be 8400:1600:1. In the Earth's

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...

, radioactive ³⁹Ar (half-life 268(8) years) is made by

cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...

activity, primarily from ⁴⁰Ar. In the subsurface environment, it is also produced through

neutron capture Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons ...

by ³⁹K or

alpha emission Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an at ...

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...

. 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, 40}Ar. 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, 40}Ar. Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon. Lighter radioactive isotopes can decay to different elements (usually

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

) while heavier ones decay to

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...

. ³⁶Ar, in the form of argon hydride, was detected in the

Crab Nebula The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus. The common name comes from William Parsons, 3rd Earl of Rosse, who observed the object in 1842 u ...

supernova remnant A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...

during 2013. This was the first time a noble molecule was detected in

outer space Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, pred ...

. Radioactive ³⁷Ar is a synthetic radionuclide that is created from the

by ⁴⁰Ca followed by an

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be prod ...

emission as a result of subsurface nuclear explosions. It has a half-life of 35 days.

List of isotopes

, - , ²⁹Ar , style="text-align:right" , 18 , style="text-align:right" , 11 , , ~ s , 2p , ²⁷S , , , , - , ³⁰Ar , style="text-align:right" , 18 , style="text-align:right" , 12 , 30.02247(22) , <10 ps , 2p , ²⁸S , 0+ , , , - , rowspan=4, ³¹Ar , rowspan=4 style="text-align:right" , 18 , rowspan=4 style="text-align:right" , 13 , rowspan=4, 31.01216(22)# , rowspan=4, 15.1(3) ms , β⁺, p (68.3%) , ³⁰S , rowspan=4, 5/2+ , rowspan=4, , rowspan=4, , - , β⁺ (22.63%) , ³¹Cl , - , β⁺, 2p (9.0%) , ²⁹P , - , β⁺, 3p (0.07%) , ²⁸Si , - , rowspan=2, ³²Ar , rowspan=2 style="text-align:right" , 18 , rowspan=2 style="text-align:right" , 14 , rowspan=2, 31.9976378(19) , rowspan=2, 98(2) ms , β⁺ (64.42%) , ³²Cl , rowspan=2, 0+ , rowspan=2, , rowspan=2, , - , β⁺, p (35.58%) , ³¹S , - , style="text-indent:1em" , ^32mAr , colspan="3" style="text-indent:2em" , 5600(100) keV , unknown , , , 5−# , , , - , rowspan=2, ³³Ar , rowspan=2 style="text-align:right" , 18 , rowspan=2 style="text-align:right" , 15 , rowspan=2, 32.9899255(4) , rowspan=2, 173.0(20) ms , β⁺ (61.3%) , ³³Cl , rowspan=2, 1/2+ , rowspan=2, , rowspan=2, , - , β⁺, p (38.7%) , ³²S , - , ³⁴Ar , style="text-align:right" , 18 , style="text-align:right" , 16 , 33.98027009(8) , 843.8(4) ms , β⁺ , ³⁴Cl , 0+ , , , - , ³⁵Ar , style="text-align:right" , 18 , style="text-align:right" , 17 , 34.9752577(7) , 1.7756(10) s , β⁺ , ³⁵Cl , 3/2+ , , , - , ³⁶Ar , style="text-align:right" , 18 , style="text-align:right" , 18 , 35.967545105(29) , colspan=3 align=center,

Observationally Stable Stable nuclides are nuclides that are not radioactive and so (unlike radionuclides) do not spontaneously undergo radioactive decay. When such nuclides are referred to in relation to specific elements, they are usually termed stable isotopes. Th ...

Believed to undergo double electron capture to ³⁶S (lightest theoretically unstable nuclide for which no evidence of radioactivity has been observed) , 0+ , 0.003336(4) , , - , ³⁷Ar , style="text-align:right" , 18 , style="text-align:right" , 19 , 36.96677631(22) , 35.011(19) d , EC , ³⁷Cl , 3/2+ , , , - , ³⁸Ar , style="text-align:right" , 18 , style="text-align:right" , 20 , 37.96273210(21) , colspan=3 align=center, Stable , 0+ , 0.000629(1) , , - , ³⁹ArUsed in

argon–argon dating Argon–argon (or 40Ar/39Ar) dating is a radiometric dating method invented to supersede potassiumargon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer ...

, style="text-align:right" , 18 , style="text-align:right" , 21 , 38.964313(5) , 268(8) y , β⁻ , ³⁹K , 7/2− , Trace

Cosmogenic nuclide Cosmogenic nuclides (or cosmogenic isotopes) are rare nuclides (isotopes) created when a high-energy cosmic ray interacts with the nucleus of an '' in situ'' Solar System atom, causing nucleons (protons and neutrons) to be expelled from the atom ...

, , - , ⁴⁰ArUsed in

and potassium–argon dating , style="text-align:right" , 18 , style="text-align:right" , 22 , 39.9623831238(24) , colspan=3 align=center, Stable , 0+ , 0.996035(4)Generated from ⁴⁰K in rocks. These ratios are terrestrial. Cosmic abundance is far less than ³⁶Ar. , , - , ⁴¹Ar , style="text-align:right" , 18 , style="text-align:right" , 23 , 40.9645006(4) , 109.61(4) min , β⁻ , ⁴¹K , 7/2− , , , - , ⁴²Ar , style="text-align:right" , 18 , style="text-align:right" , 24 , 41.963046(6) , 32.9(11) y , β⁻ , ⁴²K , 0+ , Trace , , - , ⁴³Ar , style="text-align:right" , 18 , style="text-align:right" , 25 , 42.965636(6) , 5.37(6) min , β⁻ , ⁴³K , 5/2(−) , , , - , ⁴⁴Ar , style="text-align:right" , 18 , style="text-align:right" , 26 , 43.9649238(17) , 11.87(5) min , β⁻ , ⁴⁴K , 0+ , , , - , ⁴⁵Ar , style="text-align:right" , 18 , style="text-align:right" , 27 , 44.9680397(6) , 21.48(15) s , β⁻ , ⁴⁵K , (5/2,7/2)− , , , - , ⁴⁶Ar , style="text-align:right" , 18 , style="text-align:right" , 28 , 45.9680374(12) , 8.4(6) s , β⁻ , ⁴⁶K , 0+ , , , - , rowspan=2, ⁴⁷Ar , rowspan=2 style="text-align:right" , 18 , rowspan=2 style="text-align:right" , 29 , rowspan=2, 46.9727681(12) , rowspan=2, 1.23(3) s , β⁻ (99.8%) , ⁴⁷K , rowspan=2, (3/2−) , rowspan=2, , rowspan=2, , - , β⁻, n (0.2%) , ⁴⁶K , - , ⁴⁸Ar , style="text-align:right" , 18 , style="text-align:right" , 30 , 47.97608(33) , 415(15) ms , β⁻ , ⁴⁸K , 0+ , , , - , ⁴⁹Ar , style="text-align:right" , 18 , style="text-align:right" , 31 , 48.98155(43)# , 236(8) ms , β⁻ , ⁴⁹K , 3/2−# , , , - , ⁵⁰Ar , style="text-align:right" , 18 , style="text-align:right" , 32 , 49.98569(54)# , 106(6) ms , β⁻ , ⁵⁰K , 0+ , , , - , ⁵¹Ar , style="text-align:right" , 18 , style="text-align:right" , 33 , 50.99280(64)# , 60# ms 200 ns, β⁻ , ⁵¹K , 3/2−# , , , - , ⁵²Ar , style="text-align:right" , 18 , style="text-align:right" , 34 , 51.99863(64)# , 10# ms , β⁻ , ⁵²K , 0+ , , , - , rowspan=2, ⁵³Ar , rowspan=2 style="text-align:right" , 18 , rowspan=2 style="text-align:right" , 35 , rowspan=2, 53.00729(75)# , rowspan=2, 3# ms , β⁻ , ⁵³K , rowspan=2, (5/2−)# , rowspan=2, , rowspan=2, , - , β⁻, n , ⁵²K , - , ⁵⁴Ar , style="text-align:right" , 18 , style="text-align:right" , 36 , , , β⁻ , ⁵⁴K , 0+ , ,

References

External links

Argon isotopes data from ''The Berkeley Laboratory Isotopes Project's''
{{Authority control Argon

List of isotopes

See also

References

External links