# Isotopes of argon

Argon ($_{18}$Ar) has 26 known isotopes, from $^{29}$Ar to $^{54}$Ar, of which three are stable ($^{36}$Ar, $^{38}$Ar, and $^{40}$Ar). On Earth, $^{40}$Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are $^{39}$Ar with a half-life of 268 years, $^{42}$Ar with a half-life of 32.9 years, and $^{37}$Ar 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 naturally occurring $^{40}$K, with a half-life of 1.248×10$^9$ years, decays to stable $^{40}$Ar by electron capture (10.72%) and by positron emission (0.001%), and also to stable $^{40}$Ca via beta decay (89.28%). These properties and ratios are used to determine the age of rocks through potassium–argon dating.
Despite the trapping of $^{40}$Ar in many rocks, it can be released by melting, grinding, and diffusion. Almost all argon in the Earth's atmosphere is the product of $^{40}$K decay, since 99.6% of Earth's atmospheric argon is $^{40}$Ar, whereas in the Sun and presumably in primordial star-forming clouds, argon consists of < 15% $^{38}$Ar and mostly (85%) $^{36}$Ar. Similarly, the ratio of the isotopes $^{36}$Ar:$^{38}$Ar:$^{40}$Ar in the atmospheres of the outer planets is measured to be 8400:1600:1.
In the Earth's atmosphere, radioactive $^{39}$Ar (half-life 268(8) years) is made by cosmic ray activity, primarily from $^{40}$Ar. In the subsurface environment, it is also produced through neutron capture by $^{39}$>K or alpha emission by calcium. The content of $^{39}$Ar 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 $^{42}$Ar (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) while heavier ones decay to potassium.
$^{36}$Ar, in the form of argon hydride, was detected in the Crab Nebula supernova remnant during 2013. This was the first time a noble molecule was detected in outer space.
$^{37}$Ar is a synthetic radionuclide that is created via neutron capture of $^{40}$Ca followed by alpha particle emission, as a result of subsurface nuclear explosions. It has a half-life of 35 days.

## List of isotopes

| Nuclide | Z | N | Isotopic mass (Da) | Half-life | Decay mode | Daughter isotope | Spin and parity | Natural abundance (mole fraction) | Natural abundance (mole fraction) |
| Nuclide | Excitation energy | Excitation energy | Excitation energy | Half-life | Decay mode | Daughter isotope | Spin and parity | Normal proportion | Range of variation |
| $^{29}$Ar | 18 | 11 | 29.04076(47)# | | 2p | $^{27}$S | 5/2+# | | |
| $^{30}$Ar | 18 | 12 | 30.02369(19)# | <10 ps | 2p | $^{28}$S | 0+ | | |
| $^{31}$Ar | 18 | 13 | 31.01216(22)# | 15.0(3) ms | β$^+$, p (68.3%) | $^{30}$S | 5/2+ | | |
| $^{31}$Ar | 18 | 13 | 31.01216(22)# | 15.0(3) ms | β$^+$ (22.63%) | $^{31}$Cl | 5/2+ | | |
| $^{31}$Ar | 18 | 13 | 31.01216(22)# | 15.0(3) ms | β$^+$, 2p (9.0%) | $^{29}$P | 5/2+ | | |
| $^{31}$Ar | 18 | 13 | 31.01216(22)# | 15.0(3) ms | β$^+$, 3p (0.07%) | $^{28}$Si | 5/2+ | | |
| $^{31}$Ar | 18 | 13 | 31.01216(22)# | 15.0(3) ms | β$^+$, p, α? (<0.38%) | $^{26}$Si | 5/2+ | | |
| $^{31}$Ar | 18 | 13 | 31.01216(22)# | 15.0(3) ms | β$^+$, α? (<0.03%) | $^{27}$P | 5/2+ | | |
| $^{31}$Ar | 18 | 13 | 31.01216(22)# | 15.0(3) ms | 2p? (<0.03%) | $^{29}$S | 5/2+ | | |
| $^{32}$Ar | 18 | 14 | 31.9976378(19) | 98(2) ms | β$^+$ (64.42%) | $^{32}$Cl | 0+ | | |
| $^{32}$Ar | 18 | 14 | 31.9976378(19) | 98(2) ms | β$^+$, p (35.58%) | $^{31}$S | 0+ | | |
| $^{33}$Ar | 18 | 15 | 32.98992555(43) | 173.0(20) ms | β$^+$ (61.3%) | $^{33}$Cl | 1/2+ | | |
| $^{33}$Ar | 18 | 15 | 32.98992555(43) | 173.0(20) ms | β$^+$, p (38.7%) | $^{32}$S | 1/2+ | | |
| $^{34}$Ar | 18 | 16 | 33.980270092(83) | 846.46(35) ms | β$^+$ | $^{34}$Cl | 0+ | | |
| $^{35}$Ar | 18 | 17 | 34.97525772(73) | 1.7756(10) s | β$^+$ | $^{35}$Cl | 3/2+ | | |
| $^{36}$Ar | 18 | 18 | 35.967545106(28) | Observationally Stable | Observationally Stable | Observationally Stable | 0+ | 0.003336(210) | |
| $^{37}$Ar | 18 | 19 | 36.96677630(22) | 35.011(19) d | EC | $^{37}$Cl | 3/2+ | Trace | |
| $^{38}$Ar | 18 | 20 | 37.96273210(21) | Stable | Stable | Stable | 0+ | 0.000629(70) | |
| $^{39}$Ar | 18 | 21 | 38.9643130(54) | 268.2+3.1 −2.9 y | β$^−$ | $^{39}$K | 7/2− | 8×10$^{−16}$ | |
| $^{40}$Ar | 18 | 22 | 39.9623831220(23) | Stable | Stable | Stable | 0+ | 0.996035(250) | |
| $^{41}$Ar | 18 | 23 | 40.96450057(37) | 109.61(4) min | β$^−$ | $^{41}$K | 7/2− | Trace | |
| $^{42}$Ar | 18 | 24 | 41.9630457(62) | 32.9(11) y | β$^−$ | $^{42}$K | 0+ | | |
| $^{43}$Ar | 18 | 25 | 42.9656361(57) | 5.37(6) min | β$^−$ | $^{43}$K | 5/2(−) | | |
| $^{44}$Ar | 18 | 26 | 43.9649238(17) | 11.87(5) min | β$^−$ | $^{44}$K | 0+ | | |
| $^{45}$Ar | 18 | 27 | 44.96803973(55) | 21.48(15) s | β$^−$ | $^{45}$K | (5/2−,7/2−) | | |
| $^{46}$Ar | 18 | 28 | 45.9680392(25) | 8.4(6) s | β$^−$ | $^{46}$K | 0+ | | |
| $^{47}$Ar | 18 | 29 | 46.9727671(13) | 1.23(3) s | β$^−$ (>99.8%) | $^{47}$K | (3/2)− | | |
| $^{47}$Ar | 18 | 29 | 46.9727671(13) | 1.23(3) s | β$^−$, n? (<0.2%) | $^{46}$K | (3/2)− | | |
| $^{48}$Ar | 18 | 30 | 47.976001(18) | 415(15) ms | β$^−$ (62%) | $^{48}$K | 0+ | | |
| $^{48}$Ar | 18 | 30 | 47.976001(18) | 415(15) ms | β$^−$, n (38%) | $^{47}$K | 0+ | | |
| $^{49}$Ar | 18 | 31 | 48.98169(43)# | 236(8) ms | β$^−$ | $^{49}$K | 3/2−# | | |
| $^{49}$Ar | 18 | 31 | 48.98169(43)# | 236(8) ms | β$^−$, n (29%) | $^{48}$K | 3/2−# | | |
| $^{49}$Ar | 18 | 31 | 48.98169(43)# | 236(8) ms | β$^−$, 2n? | $^{47}$K | 3/2−# | | |
| $^{50}$Ar | 18 | 32 | 49.98580(54)# | 106(6) ms | β$^−$ (63%) | $^{50}$K | 0+ | | |
| $^{50}$Ar | 18 | 32 | 49.98580(54)# | 106(6) ms | β$^−$, n (37%) | $^{49}$K | 0+ | | |
| $^{50}$Ar | 18 | 32 | 49.98580(54)# | 106(6) ms | β$^−$, 2n? | $^{48}$K | 0+ | | |
| $^{51}$Ar | 18 | 33 | 50.99303(43)# | 30# ms [>200 ns] | β$^−$? | $^{51}$K | 1/2−# | | |
| $^{51}$Ar | 18 | 33 | 50.99303(43)# | 30# ms [>200 ns] | β$^−$, n? | $^{50}$K | 1/2−# | | |
| $^{51}$Ar | 18 | 33 | 50.99303(43)# | 30# ms [>200 ns] | β$^−$, 2n? | $^{49}$K | 1/2−# | | |
| $^{52}$Ar | 18 | 34 | 51.99852(64)# | 40# ms [>620 ns] | β$^−$? | $^{52}$K | 0+ | | |
| $^{52}$Ar | 18 | 34 | 51.99852(64)# | 40# ms [>620 ns] | β$^−$, n? | $^{51}$K | 0+ | | |
| $^{52}$Ar | 18 | 34 | 51.99852(64)# | 40# ms [>620 ns] | β$^−$, 2n? | $^{50}$K | 0+ | | |
| $^{53}$Ar | 18 | 35 | 53.00729(75)# | 20# ms [>620 ns] | β$^−$? | $^{53}$K | 5/2−# | | |
| $^{53}$Ar | 18 | 35 | 53.00729(75)# | 20# ms [>620 ns] | β$^−$, n? | $^{52}$K | 5/2−# | | |
| $^{53}$Ar | 18 | 35 | 53.00729(75)# | 20# ms [>620 ns] | β$^−$, 2n? | $^{51}$K | 5/2−# | | |
| $^{54}$Ar | 18 | 36 | 54.01348(86)# | 5# ms [>400 ns] | β$^−$? | $^{54}$K | 0+ | | |
| $^{54}$Ar | 18 | 36 | 54.01348(86)# | 5# ms [>400 ns] | β$^−$, n? | $^{53}$K | 0+ | | |
| $^{54}$Ar | 18 | 36 | 54.01348(86)# | 5# ms [>400 ns] | β$^−$, 2n? | $^{52}$K | 0+ | | |
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* ↑ $^m$Ar – Excited nuclear isomer.
* ↑ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
* ↑ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
* ↑ Modes of decay: EC:Electron capture n:Neutron emission p:Proton emission
* ↑ Bold symbol as daughter – Daughter product is stable.
* ↑ ( ) spin value – Indicates spin with weak assignment arguments.
* ↑ # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
* ↑ Believed to undergo double electron capture to $^{36}$S (lightest theoretically unstable nuclide for which no evidence of radioactivity has been observed)
* 1 2 3 Cosmogenic nuclide
* ↑ Used in argon–argon dating
* ↑ Used in argon–argon dating and potassium–argon dating
* ↑ Generated from $^{40}$K in rocks. These ratios are terrestrial. Cosmic abundance is far less than $^{36}$Ar.