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The main isotopes of argon (Ar) found on Earth are 40Ar, 36Ar, and 38Ar. Naturally occurring 40K with a half-life of 1.250×109 years, decays to stable 40Ar (11.2%) by electron capture and by positron emission, and also transforms to stable 40Ca (88.8%) via beta decay. These properties and ratios are used to determine the age of rocks through potassium-argon dating.[1]
In the Earth's atmosphere, 39Ar is made by cosmic ray activity, primarily with 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 natAr [2]. 37Ar is created from the decay of 40Ca as a result of subsurface nuclear explosions. It has a half-life of 35 days.[1] The content of 42Ar in the Earth's atmosphere is lower than 6×10-21 parts per part of natAr [3].
Standard atomic mass: 39.948(1) u.
Table
nuclide
symbol |
Z(p) |
N(n) |
isotopic mass (u)
|
half-life |
nuclear
spin |
representative
isotopic
composition
(mole fraction) |
range of natural
variation
(mole fraction) |
| excitation energy |
| 30Ar |
18 |
12 |
30.02156(32)# |
<20 ns |
0+ |
|
|
| 31Ar |
18 |
13 |
31.01212(22)# |
14.4(6) ms |
5/2(+#) |
|
|
| 32Ar |
18 |
14 |
31.9976380(19) |
98(2) ms |
0+ |
|
|
| 32mAr |
5600(100)# keV |
? |
5-# |
|
|
| 33Ar |
18 |
15 |
32.9899257(5) |
173.0(20) ms |
1/2+ |
|
|
| 34Ar |
18 |
16 |
33.9802712(4) |
844.5(34) ms |
0+ |
|
|
| 35Ar |
18 |
17 |
34.9752576(8) |
1.775(4) s |
3/2+ |
|
|
| 36Ar |
18 |
18 |
35.967545106(29) |
STABLE |
0+ |
0.003365(30) |
|
| 37Ar |
18 |
19 |
36.96677632(22) |
35.04(4) d |
3/2+ |
|
|
| 38Ar |
18 |
20 |
37.9627324(4) |
STABLE |
0+ |
0.000632(5) |
|
| 39Ar |
18 |
21 |
38.964313(5) |
269(3) a |
7/2- |
|
|
| 40Ar |
18 |
22 |
39.9623831225(29) |
STABLE |
0+ |
0.996003(30) |
|
| 41Ar |
18 |
23 |
40.9645006(4) |
109.61(4) min |
7/2- |
|
|
| 42Ar |
18 |
24 |
41.963046(6) |
32.9(11) a |
0+ |
|
|
| 43Ar |
18 |
25 |
42.965636(6) |
5.37(6) min |
(5/2-) |
|
|
| 44Ar |
18 |
26 |
43.9649240(17) |
11.87(5) min |
0+ |
|
|
| 45Ar |
18 |
27 |
44.9680400(6) |
21.48(15) s |
(1/2,3/2,5/2)- |
|
|
| 46Ar |
18 |
28 |
45.96809(4) |
8.4(6) s |
0+ |
|
|
| 47Ar |
18 |
29 |
46.97219(11) |
1.23(3) s |
3/2-# |
|
|
| 48Ar |
18 |
30 |
47.97454(32)# |
0.48(40) s |
0+ |
|
|
| 49Ar |
18 |
31 |
48.98052(54)# |
170(50) ms |
3/2-# |
|
|
| 50Ar |
18 |
32 |
49.98443(75)# |
85(30) ms |
0+ |
|
|
| 51Ar |
18 |
33 |
50.99163(75)# |
60# ms [>200 ns] |
3/2-# |
|
|
| 52Ar |
18 |
34 |
51.99678(97)# |
10# ms |
0+ |
|
|
| 53Ar |
18 |
35 |
53.00494(107)# |
3# ms |
(5/2-)# |
|
|
Notes
- The isotopic composition refers to that in air.
- Geologically exceptional samples are known in which the isotopic composition lies outside the reported range. The uncertainty in the atomic mass may exceed the stated value for such specimens.
- Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
- Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.
References
- ^ a b "40Ar/39Ar dating and errors". Retrieved on 2007-03-07.
- ^ P. Benetti et al., Measurement of the specific activity of 39Ar in natural argon. Nucl. Instr. Meth. A 574 (2007) 83 [1].
- ^ V. D. Ashitkov et al., New experimental limit on the 42Ar content in the Earth’s atmosphere. Nucl. Instr. Meth. A 416 (1998) 179 [2].
- Isotope masses from Ame2003 Atomic Mass Evaluation by G. Audi, A.H. Wapstra, C. Thibault, J. Blachot and O. Bersillon in Nuclear Physics A729 (2003).
- Isotopic compositions and standard atomic masses from Atomic weights of the elements. Review 2000 (IUPAC Technical Report). Pure Appl. Chem. Vol. 75, No. 6, pp. 683-800, (2003) and Atomic Weights Revised (2005).
- Half-life, spin, and isomer data selected from these sources. Editing notes on this article's talk page.
- Audi, Bersillon, Blachot, Wapstra. The Nubase2003 evaluation of nuclear and decay properties, Nuc. Phys. A 729, pp. 3-128 (2003).
- National Nuclear Data Center, Brookhaven National Laboratory. Information extracted from the NuDat 2.1 database (retrieved Sept. 2005).
- David R. Lide (ed.), Norman E. Holden in CRC Handbook of Chemistry and Physics, 85th Edition, online version. CRC Press. Boca Raton, Florida (2005). Section 11, Table of the Isotopes.
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