Argon

Argon

Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 (noble gases) of the periodic table. Argon is the third most common gas in the Earth's atmosphere, at 0.93% (9,300 ppm), making it approximately 23.8 times as abundant as next most common atmospheric gas, carbon dioxide (390 ppm), and more than 500 times as abundant as the next most common noble gas, neon (18 ppm). Nearly all of this argon is radiogenic argon-40 derived from the decay of potassium-40 in the Earth's crust. In the universe, argon-36 is by far the most common argon isotope, being the preferred argon isotope produced by stellar nucleosynthesis in supernovas.

The name "argon" is derived from the Greek word αργον meaning "lazy" or "the inactive one", a reference to the fact that the element undergoes almost no chemical reactions. The complete octet (eight electrons) in the outer atomic shell makes argon stable and resistant to bonding with other elements. Its triple point temperature of 83.8058 K is a defining fixed point in the International Temperature Scale of 1990.

Argon is produced industrially by the fractional distillation of liquid air. Argon is mostly used as an inert shielding gas in welding and other high-temperature industrial processes where ordinarily non-reactive substances become reactive; for example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning. Argon gas also has uses in incandescent and fluorescent lighting, and other types of gas discharge tubes. Argon makes a distinctive blue-green gas laser.

Occurrence

Argon constitutes 0.934% by volume and 1.28% by mass of the Earth's atmosphere, and air is the primary raw material used by industry to produce purified argon products. Argon is isolated from air by fractionation, most commonly by cryogenic fractional distillation, a process that also produces purified nitrogen, oxygen, neon, krypton and xenon.

Symbol		Ar
Atomic Number		18
Atomic Weight		39.948
Oxidation States		0
Electronegativity, Pauling
State at RT		Gas, Nonmetal
Melting Point, K		83.8
Boiling Point, K		87.3

Interesting Facts about Argon

• Lord Rayleigh said: “Argon must not be deemed rare. A large hall may easily contain a greater weight of it than a man can carry.” On a planetary scale, we can calculate that Earth’s atmosphere holds 65 trillion metric tons of argon. That’s more than 9 metric tons of argon per person on Earth.
• Until 1957, argon’s chemical symbol was A. In 1957, IUPAC agreed that the symbol should change to Ar. Argon was not the only element whose symbol changed in 1957. IUPAC also changed mendelevium from Mv to Md.
• Most people are familiar with carbon dating, which uses the decay of the radioactive carbon-14 isotope to find the ages of things that were once alive. Carbon-14′s half-life is about 5730 years and the technique is not useful for material more than about 60 thousand years old. Potassium-argon and argon-argon dating allow us to date rocks that are much older than this. Potassium-40 decays to argon-40 and calcium-40, with a half-life of 1.25 billion years. The ratio of potassium-40 to argon-40 trapped in rock can be used to determine how long it is since the rock has solidified. More recently, the ratio of argon-39 to argon-40 has been used in precision dating.
• The vast majority of argon on Earth comes from the radioactive decay of potassium-40, producing stable argon-40. Over 99% of Earth’s argon is argon-40.
• Away from Earth, argon-36 is the most abundant isotope, synthesized in the silicon burning phase of stars with a mass of about 11 or more Earth suns. During silicon burning, an alpha-particle adds to a silicon-32 nucleus to make sulfur-36, which can add another alpha-particle to become argon-36, some of which can become calcium-40, etc.

Appearance and Characteristics

Harmful effects:

Argon is considered to be non-toxic.

Characteristics:

• Argon is a noble gas. It is colorless, odorless and extremely unreactive.
• It is, however, not completely inert – photolysis of hydrogen fluoride in a solid argon matrix at 7.5 kelvin yields argon fluorohydride, HArF.
• Argon forms no stable compounds at room temperature.

Uses of Argon

• As a result of its unreactiveness, argon is used in light bulbs to protect the filament and to provide an unreactive atmosphere in the vicinity of welding.
• It is also used in the semi-conductor industry to provide an inert atmosphere for silicon and germanium crystal growth.
• Argon is used in medical lasers, in ophthalmology for example to correct eye defects such as blood vessel leakage, retinal detachment, glaucoma and macular degeneration.
• Argon has low thermal conductivity and is used as the gas between the glass panes in high-efficiency double and triple glazing.