Argon-40 is a naturally occurring, stable isotope of the chemical element argon, which has 22 neutrons in its atomic nucleus in addition to the element-specific 18 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 40.
The discovery was made through mass spectrometric analysis of natural Ar gas; it was announced in 1920 [1].
See also: List of individual Argon isotopes (and general data sources).
General data
Name of the isotope:Argon-40; Ar-40Symbol:40Ar or 4018ArMass number A:40 (= number of nucleons)Atomic number Z:18 (= number of protons)Neutrons N:22Nucleon pairing (Z - N):even - evenNuclear ratio (N/Z ratio):1.2222222222222 (= neutron-proton ratio)Neutron excess (N-Z):4Isotopic mass:39.962383120(23) u (atomic weight of Argon-40)Nuclide mass:39.9525088 u (calculated nuclear mass without electrons)Mass excess:-35.0399 MeVMass defect:0.369095608 u (per nucleus)Nuclear binding energy:343.81034428 MeV (per nucleus) 8.59525861 MeV (average binding energy per nucleon)Separation energy:SN = 9.869(5) MeV (first neutron) SP = 12.2587(17) MeV (first proton)Half-life:stableSpin and parity: (nuclear angular momentum)0+Charge radius:3.4274(26) femtometer fmMirror nucleus:Titanium-40Year of discovery:1920
Comparison of the natural Argon isotopes including isotopic abundance (mole fraction of the isotope mixture in percent):
Atomic Mass ma
Quantity
Half-life
Spin
Argon Isotopic mixture
39.88 u
100 %
Isotope 36Ar
35.967545106(29) u
0.3336(210) %
stable
0+
Isotope 38Ar
37.96273210(21) u
0.0629(70) %
stable
0+
Isotope 39Ar
38.964313(5) u
[trace]
268(8) years
7/2-
Isotope 40Ar
39.962383120(23) u
99.6035(250) %
stable
0+
Isotope 42Ar
41.963046(6) u
[trace]
32.9(11) years
0+
Isotones and Isobars
The following table shows the atomic nuclei that are isotonic (same neutron number N = 22) and isobaric (same nucleon number A = 40) with Argon-40. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.
