Argon-40

Properties and data of the isotope ⁴⁰Ar.

Contents

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:S_N = 9.869(5) MeV (first neutron)
S_P = 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

Parent Nuclides

Direct parent isotopes are: ⁴⁰Cl, ⁴⁰K.

Occurrence

Comparison of the natural Argon isotopes including isotopic abundance (mole fraction of the isotope mixture in percent):

	Atomic Mass m_a	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) a	7/2-
Isotope ⁴⁰Ar	39.962383120(23) u	99.6035(250) %	stable	0+
Isotope ⁴²Ar	41.963046(6) u	[trace]	32.9(11) a	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.

Z	Isotone N = 22	Isobar A = 40
9	³¹F
10	³²Ne
11	³³Na
12	³⁴Mg	⁴⁰Mg
13	³⁵Al	⁴⁰Al
14	³⁶Si	⁴⁰Si
15	³⁷P	⁴⁰P
16	³⁸S	⁴⁰S
17	³⁹Cl	⁴⁰Cl
18	⁴⁰Ar	⁴⁰Ar
19	⁴¹K	⁴⁰K
20	⁴²Ca	⁴⁰Ca
21	⁴³Sc	⁴⁰Sc
22	⁴⁴Ti	⁴⁰Ti
23	⁴⁵V	⁴⁰V
24	⁴⁶Cr
25	⁴⁷Mn
26	⁴⁸Fe
27	⁴⁹Co
28	⁵⁰Ni