Tuesday 20 November 2012

Argon

Argon

Vial of glowing ultrapure argon. Original size in cm: 1 x 5. Source: images-of-elements.com Vial of glowing ultrapure argon. Original size in cm: 1 x 5. Source: images-of-elements.com
Published:
Updated: October 29, 2012, 11:25 am
Topics: Physics & Chemistry Environmental Chemistry Atmospheric Science Chemical Engineering
This article has been reviewed by the following Topic Editor: Lori Zaikowski
Argon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table (noble gases). Argon is present in the Earth's atmosphere at 0.94%. Argon's full outer electron shell makes it stable and resistant to molecular bonding with other elements. Its triple point temperature of 83.8058 K is a defining fixed point in the International Temperature Scale of 1990.
Previous Element: Chlorine

Next Element: Potassium
18

Ar

39.948
Physical Properties
Color colorless
Phase at Room Temp. gas
Density (g/cm3) 0.0018
Hardness (Mohs) ---
Melting Point (K) 84
Boiling Point (K) 87.3
Heat of Fusion (kJ/mol) 1.2
Heat of Vaporization (kJ/mol) 6.5
Heat of Atomization (kJ/mol) 0
Thermal Conductivity (J/m sec K) 0.02
Electrical Conductivity (1/mohm cm) 0
Source air
Atomic Properties
Electron Configuration [Ne]3s23p6
Number of Isotopes 3
Electron Affinity (kJ/mol) ---
First Ionization Energy (kJ/mol) 1520.5
Second Ionization Energy (kJ/mol) 2665.8
Third Ionization Energy (kJ/mol) 3930.8
Electronegativity ---
Polarizability (Å3) 1.586
Atomic Weight 39.948
Atomic Volume (cm3/mol) 28.5
Ionic Radius2- (pm) ---
Ionic Radius1- (pm) ---
Atomic Radius (pm) 98
Ionic Radius1+ (pm) ---
Ionic Radius2+ (pm) ---
Ionic Radius3+ (pm) ---
Common Oxidation Numbers ---
Other Oxid. Numbers ---
Abundance
In Earth's Crust (mg/kg) 3.5
In Earth's Ocean (mg/L) 4.5×10-1
In Human Body (%) 0%
Regulatory / Health
CAS Number 7440-37-1
OSHA Permissible Exposure Limit No limits
OSHA PEL Vacated 1989 No limits
NIOSH Recommended Exposure Limit No limits
Sources:
Mineral Information Institute
Jefferson Accelerator Laboratory
EnvironmentalChemistry.com		  

Argon is colorless, odorless, tasteless and nontoxic in both its liquid and gaseous forms. Argon is inert under most conditions and forms no confirmed stable compounds at room temperature. Although argon is a noble gas, it has been found to have the capability of forming some compounds. For example, the creation of the molecule argon fluorohydride (HArF), a metastable compound of argon with fluorine and hydrogen, was reported by researchers at the University of Helsinki in 2000. Argon-containing ions and excited state complexes, such as ArH+ and ArF, respectively, are known to exist. Theoretical calculations have shown several argon compounds that should be stable but for which no synthesis routes are currently known.

Argon (Greek meaning "idle" or “lazy” in reference to its chemical inactivity) was suspected to be present in air by Henry Cavendish in 1785 but was not discovered until 1894 by Lord Rayleigh and Sir William Ramsay in Scotland in an experiment in which they removed all of the oxygen and nitrogen from a sample of air. They had determined that nitrogen produced from chemical compounds was one-half percent lighter than nitrogen from the atmosphere. The difference seemed insignificant, but it was important enough to attract their attention for many months. They concluded that there was another gas in the air mixed in with the nitrogen. Argon was also encountered in 1882 through independent research of H.F. Newall and W.N. Hartley. Each observed new lines in the color spectrum of air but were unable to identify the element responsible for the lines. Argon became the first member of the noble gases to be discovered. The symbol for argon is now Ar, but up until 1957 it was A.

Argon constitutes 0.934% by volume and 1.29% 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.The Martian atmosphere in contrast contains approximately 2% argon by volume. In 2005, the Huygens probe also discovered the presence of argon-40 on Titan, the largest moon of Saturn.

The main isotopes of argon found on Earth are 40Ar (99.6%), 36Ar (0.34%), and 38Ar (0.06%). Naturally occurring 40K with a half-life of 1.25×109 years, decays to stable 40Ar (11.2%) by electron capture and positron emission, and also to stable 40Ca (88.8%) via beta decay. These properties and ratios are used to determine the age of rocks. In the Earth's atmosphere, 39Ar is made by cosmic ray activity, primarily with 40Ar. In the subsurface environment, it is also produced through neutron capture by 39K or alpha emission by calcium. 37Ar is created from the decay of 40Ca as a result of subsurface nuclear explosions. It has a half-life of 35 days.

Argon’s complete octet of electrons indicates full s and p subshells. This full outer energy level makes argon very stable and extremely resistant to bonding with other elements. Before 1962, argon and the other noble gases were considered to be chemically inert and unable to form compounds; however, compounds of the heavier noble gases have since been synthesized.

Argon is produced industrially by the partial distillation of liquid air, a process that separates liquid nitrogen, which boils at 77.4 K, from argon, which boils at 87.3 K and oxygen, which boils at 90.2 K.

There are several different reasons why argon is used in particular applications:
An argon & mercury vapor discharge tube. (Source: Pslawinski) An argon & mercury vapor discharge tube. (Source: Pslawinski)
An inert gas is needed. In particular, argon is the cheapest alternative when diatomic nitrogen is not sufficiently inert.
  • Low thermal conductivity is required.
  • The electronic properties (ionization and/or the emission spectrum) are necessary.
Other noble gases would probably work as well in most of these applications, but argon is by far the cheapest. Argon is inexpensive since it is a byproduct of the production of liquid oxygen and liquid nitrogen, both of which are used on a large industrial scale. The other noble gases (except helium) are produced this way as well, but argon is the most plentiful since it has the highest concentration in the atmosphere. The bulk of argon applications arise simply because it is inert and relatively cheap.
The numerous industrial uses of argon include:
  • As a fill gas in incandescent lighting, because argon will not react with the filament of light bulbs even at high temperatures.
  • As an inert gas shield in many forms of welding, including metal inert gas welding and tungsten inert gas welding. For metal inert gas welding argon is often mixed with CO2.
  • Cylinders containing argon gas for use in extinguishing fire without damaging server equipment.
  • As the gas of choice for the plasma used in ICP spectroscopy.
  • As the gas of choice (in ionised form) for sputter coating of specimens for Scanning Electron Microscopy.
  • As a protective atmosphere for growing silicon and germanium crystals, and in partial pressure heat treat furnaces.
  • By museum conservators to protect old materials or documents, which are prone to gradual oxidation in the presence of air.
  • To keep open bottles of wine from oxidizing, and in a number of dispensing units and keeper cap systems.
  • To preserve leftover finishes like varnish, polyurethane, paint, and other chemicals during storage. Commercially available in an aerosol can for this purpose (US Patent 6629402).
  • In winemaking to top off barrels, displacing oxygen and thus preventing the wine from turning to vinegar during the aging process.
  • As an atmosphere in graphite electric furnaces, to keep graphite from oxidizing.
  • An unusual application is as an asphyxiant in the poultry industry, either for mass culling following disease outbreaks, or as a means of slaughter more humane than the electric bath. Argon's relatively high density causes it to remain close to the ground during gassing. Its non-reactive nature makes it suitable in a food product, and since it replaces oxygen at a cellular level in the dead bird, argon also enhances shelf life.
  • It is used for thermal insulation in energy efficient windows.
  • Argon is also used in technical scuba diving to inflate a dry suit, because it is inert and has low thermal conductivity.
  • Argon is also valuable for the specific way it ionizes and emits light. It is used in plasma globes and calorimetry in experimental particle physics. Blue argon lasers are used in surgery to weld arteries, destroy tumors, and to correct eye defects. In microelectronics, argon ions has applications for sputtering.
  • Argon-39, with a half-life of 269 years, has been used for a number of applications, primarily ice core and ground water dating.
Medical uses of argon include:
  • In the pharmaceutical industry to top off bottles of intravenous drug preparations (for example intravenous paracetamol), again displacing oxygen and therefore prolonging the drug's shelf-life.
  • Cryosurgery procedures such as cryoablation use liquefied argon to destroy cancer cells. In surgery it is used in a procedure called "argon enhanced coagulation" which is a form of argon plasma beam electrosurgery. The procedure carries a risk of producing gas embolism in the patient and has resulted in the death of one person via this type of accident.

Although argon is non-toxic, it does not satisfy the body's need for oxygen and is thus an asphyxiant. In confined spaces, it is known to result in death due to asphyxiation. Argon is 39% more dense than air and is considered highly dangerous in closed areas. It is also difficult to detect because it is colorless, odorless, and tasteless.


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