Argon is the third-most abundant gas in the Earth’s atmosphere, at 0.934% (9340 ppmv). Argon is mostly used as an inert shielding gas in welding and other high-temperature industrial processes where ordinarily unreactive substances become reactive; for example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning.
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Argon is produced industrially by the fractional distillation of liquid air. a process that separates liquid nitrogen, which boils at 77.3 K, from argon, which boils at 87.3 K, and liquid oxygen, which boils at 90.2 K.
Protons and Neutrons in Argon
The total number of neutrons in the nucleus of an atom is called the neutron number of the atom and is given the symbol N. Neutron number plus atomic number equals atomic mass number: N+Z=A. The difference between the neutron number and the atomic number is known as the neutron excess: D = N – Z = A – 2Z.
For stable elements, there is usually a variety of stable isotopes. Isotopes are nuclides that have the same atomic number and are therefore the same element, but differ in the number of neutrons. Mass numbers of typical isotopes of Argon are 36; 38; 40.
Main Isotopes of Argon
Nearly all of the argon in the Earth’s atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in the Earth’s crust. Argon has 26 known isotopes, from 29Ar to 54Ar and 1 isomer (32mAr), of which three are stable (36Ar, 38Ar, and 40Ar). On the Earth, 40Ar makes up 99.6% of natural argon.
Argon-36 is composed of 18 protons, 18 neutrons, and 18 electrons.
Argon-38 is composed of 18 protons, 20 neutrons, and 18 electrons.
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Argon-40 is composed of 18 protons, 22 neutrons, and 18 electrons. Almost all of the argon in the Earth’s atmosphere is the product of 40K decay, since 99.6% of Earth atmospheric argon is 40Ar.