Isotopes of argon

Argon (₁₈Ar) has 26 known isotopes, from ²⁹Ar to ⁵⁴Ar, of which three 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 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.

Isotopes of argon (₁₈Ar)

Main isotopes[1] Decay abun­dance half-life (t1/2) mode pro­duct 36Ar 0.334% stable 37Ar trace 35 d ε 37Cl 38Ar 0.0630% stable 39Ar trace 268 y β− 39K 40Ar 99.6% stable 41Ar trace 109.34 min β− 41K 42Ar synth 32.9 y β− 42K
Standard atomic weight Ar°(Ar)
	[39.792, 39.963][2] 39.95±0.16 (abridged)[3]
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The naturally occurring ⁴⁰K, with a half-life of 1.248×10⁹ years, decays to stable ⁴⁰Ar by electron capture (10.72%) and by positron emission (0.001%), and also to stable ⁴⁰Ca via beta decay (89.28%). These properties and ratios are used to determine the age of rocks through potassium–argon dating.^[4]

Despite the trapping of ⁴⁰Ar in many rocks, it can be released by melting, grinding, and diffusion. Almost all argon in the Earth's atmosphere is the product of ⁴⁰K decay, since 99.6% of Earth's 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 isotopes ³⁶Ar:³⁸Ar:⁴⁰Ar in the atmospheres of the outer planets is measured to be 8400:1600:1.^[5]

In the Earth's atmosphere, radioactive ³⁹Ar (half-life 268(8) years) is made by cosmic ray activity, primarily from ⁴⁰Ar. In the subsurface environment, it is also produced through neutron capture by ³⁹>K 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, 40}Ar.^[6] 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.^[7] Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon.^[8] Lighter radioactive isotopes can decay to different elements (usually chlorine) while heavier ones decay to potassium.

³⁶Ar, in the form of argon hydride, was detected in the Crab Nebula supernova remnant during 2013.^[9][10] This was the first time a noble molecule was detected in outer space.^[9][10]

³⁷Ar is a synthetic radionuclide that is created via neutron capture of ⁴⁰Ca followed by alpha particle emission, as a result of subsurface nuclear explosions. It has a half-life of 35 days.^[4]

List of isotopes

Nuclide [n 1]	Z	N	Isotopic mass (Da)[11] [n 2][n 3]	Half-life[1]	Decay mode[1] [n 4]	Daughter isotope [n 5]	Spin and parity[1] [n 6][n 7]	Natural abundance (mole fraction)
	Excitation energy							Normal proportion[1]	Range of variation
29Ar[12]	18	11	29.04076(47)#		2p	27S	5/2+#
30Ar	18	12	30.02369(19)#	<10 ps	2p	28S	0+
31Ar	18	13	31.01216(22)#	15.0(3) ms	β+, p (68.3%)	30S	5/2+
					β+ (22.63%)	31Cl
					β+, 2p (9.0%)	29P
					β+, 3p (0.07%)	28Si
					β+, p, α? (<0.38%)	26Si
					β+, α? (<0.03%)	27P
					2p? (<0.03%)	29S
32Ar	18	14	31.9976378(19)	98(2) ms	β+ (64.42%)	32Cl	0+
					β+, p (35.58%)	31S
33Ar	18	15	32.98992555(43)	173.0(20) ms	β+ (61.3%)	33Cl	1/2+
					β+, p (38.7%)	32S
34Ar	18	16	33.980270092(83)	846.46(35) ms	β+	34Cl	0+
35Ar	18	17	34.97525772(73)	1.7756(10) s	β+	35Cl	3/2+
36Ar	18	18	35.967545106(28)	Observationally Stable[n 8]			0+	0.003336(210)
37Ar	18	19	36.96677630(22)	35.011(19) d	EC	37Cl	3/2+	Trace[n 9]
38Ar	18	20	37.96273210(21)	Stable			0+	0.000629(70)
39Ar[n 10]	18	21	38.9643130(54)	268.2+3.1 −2.9 y[13]	β−	39K	7/2−	8×10−16[14][n 9]
40Ar[n 11]	18	22	39.9623831220(23)	Stable			0+	0.996035(250)[n 12]
41Ar	18	23	40.96450057(37)	109.61(4) min	β−	41K	7/2−	Trace[n 9]
42Ar	18	24	41.9630457(62)	32.9(11) y	β−	42K	0+
43Ar	18	25	42.9656361(57)	5.37(6) min	β−	43K	5/2(−)
44Ar	18	26	43.9649238(17)	11.87(5) min	β−	44K	0+
45Ar	18	27	44.96803973(55)	21.48(15) s	β−	45K	(5/2−,7/2−)
46Ar	18	28	45.9680392(25)	8.4(6) s	β−	46K	0+
47Ar	18	29	46.9727671(13)	1.23(3) s	β− (>99.8%)	47K	(3/2)−
					β−, n? (<0.2%)	46K
48Ar	18	30	47.976001(18)	415(15) ms	β− (62%)	48K	0+
					β−, n (38%)	47K
49Ar	18	31	48.98169(43)#	236(8) ms	β−	49K	3/2−#
					β−, n (29%)	48K
					β−, 2n?	47K
50Ar	18	32	49.98580(54)#	106(6) ms	β− (63%)	50K	0+
					β−, n (37%)	49K
					β−, 2n?	48K
51Ar	18	33	50.99303(43)#	30# ms [>200 ns]	β−?	51K	1/2−#
					β−, n?	50K
					β−, 2n?	49K
52Ar	18	34	51.99852(64)#	40# ms [>620 ns]	β−?	52K	0+
					β−, n?	51K
					β−, 2n?	50K
53Ar	18	35	53.00729(75)#	20# ms [>620 ns]	β−?	53K	5/2−#
					β−, n?	52K
					β−, 2n?	51K
54Ar	18	36	54.01348(86)#	5# ms [>400 ns]	β−?	54K	0+
					β−, n?	53K
					β−, 2n?	52K
This table header & footer: view

1. ^mAr – Excited nuclear isomer.
2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. Modes of decay:

   EC:	Electron capture
   n:	Neutron emission
   p:	Proton emission

5. Bold symbol as daughter – Daughter product is stable.
6. ( ) spin value – Indicates spin with weak assignment arguments.
7. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
8. Believed to undergo double electron capture to ³⁶S (lightest theoretically unstable nuclide for which no evidence of radioactivity has been observed)
9. Cosmogenic nuclide
10. Used in argon–argon dating
11. Used in argon–argon dating and potassium–argon dating
12. Generated from ⁴⁰K in rocks. These ratios are terrestrial. Cosmic abundance is far less than ³⁶Ar.