Ar-40 is Argon's most common isotope because 40 is closest to its atomic mass which is 39.948 u ± 0.001 u.
Abundance and Isotopes
Abundance earth’s crust: 3.5 parts per million by weight, 1.8 parts per million by moles
Abundance solar system: 0.01 percent by weight, 3.3 parts per million by moles
Isotopes: 18 whose half-lives are known, mass numbers 30 to 47. Of these, three are stable. They are found naturally in the percentages shown: Ar-36 (0.337%), Ar-38 (0.063%) and Ar-40 (99.600%).
Argon is produced when K-40 present naturally in the earth’s crust undergoes radioactive decay to Ar-40. The argon makes its way into the atmosphere. Argon is produced commercially by fractional distillation of liquefied air with (for high purity argon) catalytic burning of left over traces of oxygen.
Away from Earth, Ar-36 is the most abundant isotope, synthesized in the silicon burning phase of stars with a mass of about 11 or more Earth suns.
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