What Is Argon?
Argon is the third most abundant gas in the Earth’s atmosphere, after nitrogen and oxygen, except for water vapor.
It belongs to group 18 of the periodic table of elements and is called noble gas. However, it is also called “rare gas” due to its age when it was chemically difficult to separate and extract.
Argon is present in the atmosphere at about 0.93%. Among the noble gases, argon has the largest ratio in air. In modern times, argon is obtained by cooling air to a cryogenic temperature of about -200°C and separating it from nitrogen and oxygen.
Uses of Argon
Argon is widely used in manufacturing as an inert gas because it does not react easily with other substances. A typical application is as a shielding gas for welding.
By using a shielding gas consisting of Argon plus a few percent of oxygen, the arc can be focused locally, enabling high-quality welding processes. Argon is also often used as an atmospheric gas in the manufacture of silicon wafers, the material used in semiconductor substrates.
Using an inert gas atmosphere that does not react easily with other substances, it is also possible to produce high-quality silicon with a purity of 99.99%. Argon is also widely used as an atmospheric gas in refining furnaces for manufacturing stainless steel.
Properties of Argon
At room temperature and pressure, argon is a colorless, odorless gas. It is called an inert gas because it does not easily cause chemical reactions with other substances. Its triple point is 83.8058K.
Structure of Argon
Argon is a noble gas of group 18, one of the third periodic elements, with the element symbol Ar and atomic weight 39.95. It has no valence electrons in its outermost shell and satisfies the octet rule, making it chemically stable in structure. The electron configuration is [Ne] 3s2 3p6.
Specific gravity is 1.65 in the solid state at -233°C and 1.39 in the liquid state at -186°C. Specific gravity relative to air is 1.38. In the solid state, it has a face-centered cubic structure.
Most argon on Earth has a mass number of 40 (40Ar) and was formed by the decay of potassium-40 (40K) in the earth’s crust. 40Ar has a half-life of 1.25 x 109 years. On the other hand, Argon-36 (36Ar) is the most abundant isotope in the universe and was produced by supernova explosions. There are three stable isotopes of Argon: 36Ar, 38Ar, and 40Ar.
Other Information on Argon
1. Purification of Argon
Fractional distillation of liquid air will yield a single unit of argon. Besides repeated fractional distillation, argon can be purified by fractional adsorption with activated carbon, heating with alkali metals, gas chromatography, and fractional evaporation.
2. Isotopes of Argon
There are 25 known isotopes of argon. In the Earth’s atmosphere, 40Ar is by far the most abundant.
39Ar and 40Ar are produced by the action of cosmic rays. On the Earth’s surface, 39Ar is produced by neutron capture of 39K and other processes. The measured percentage of39Ar in natural argon is (8.0 ± 0.6) x 10-16 g/g.
In addition, atmospheric nuclear testing produces 37Ar, which has a half-life of 35 days. The fraction of 42Ar in the Earth’s atmosphere is about 6 x 10-21.
3. Compounds of Argon
Because the single atoms of argon satisfy the octet rule, for a long time, there were no known compounds of argon bound to other atoms. In 2000, however, argon fluorohydride was synthesized for the first time. The chemical formula is HArF, which can be synthesized by mixing argon, hydrogen fluoride, and cesium iodide and irradiating the mixture with ultraviolet light at 7.5K.
Argon fluorohydride is stable below -256°C, but above -256°C, it decomposes into argon and hydrogen fluoride. Argon does not form stable inclusion compounds, except for hydrate crystals and quinol molecular compounds.
Specifically, it forms 8Ar-46H2O with water and the inclusion compound Ar-3C6H4(OH)2 with hydroquinone.