The Online Encyclopedia and Dictionary







Name, Symbol, Number argon, Ar, 18
Chemical series noble gases
Group, Period, Block 18 (VIIIA), 3, p
Density, Hardness 1.784 kg/m3 (273 K), NA
Appearance Colorless
Atomic properties
Atomic weight 39.948 amu
Atomic radius (calc.) no data (71 pm)
Covalent radius 97 pm
van der Waals radius 188 pm
Electron configuration [Ne]3s2 3p6
e- 's per energy level 2, 8, 8
Oxidation states (Oxide) 0, +2 (in rare cases)
Crystal structure cubic face centered
Physical properties
State of matter gas (nonmagnetic)
Melting point 83.8 K (−153.7 C / −308.7 F)
Boiling point 87.3 K (−150.2 C / −302.4 F)
Heat of vaporization 6.447 kJ/mol
Heat of fusion 1.188 kJ/mol
Vapor pressure NA
Speed of sound 319 m/s at 293.15 K
Electronegativity no data (Pauling scale)
Specific heat capacity 520 J/(kg*K)
Electrical conductivity no data
Thermal conductivity 0.01772 W/(m*K)
1st ionization potential 1520.6 kJ/mol
2nd ionization potential 2665.8 kJ/mol
3rd ionization potential 3931 kJ/mol
4th ionization potential 5771 kJ/mol
5th ionization potential 7238 kJ/mol
6th ionization potential 8781 kJ/mol
7th ionization potential 11995 kJ/mol
8th ionization potential 13842 kJ/mol
Most stable isotopes
iso NA half-life DM DE MeV DP
36Ar 0.336% Ar is stable with 18 neutrons
38Ar 0.063% Ar is stable with 20 neutrons
39Ar {syn.} 269 y β- 0.565 39K
40Ar 99.6% Ar is stable with 22 neutrons
42Ar {syn} 32.9 y β- 0.600 42K
SI units & STP are used except where noted.
Argon was also a codename used for the KH-5 reconnaissance satellite.

Argon is the chemical element in the periodic table that has the symbol Ar and atomic number 18. The third noble gas, in period 8, argon makes up about 1% of the Earth's atmosphere.


Notable characteristics

Argon is 2.5 times as soluble in water as nitrogen which is approximately the same solubility as oxygen. This highly stable chemical element is colorless and odorless in both its liquid and gaseous forms. There are no known true chemical compounds that contain argon, one of the reasons it was formerly called an inert gas. The creation of argon hydrofluoride (HArF), a highly unstable compound of argon with fluorine, was reported by researchers at the University of Helsinki in 2000, but has not been confirmed.

Although no chemical compounds of argon are presently confirmed, argon can form clathrates with water when atoms of it are trapped in a lattice of the water molecules. Theoretical calculations on computers have shown several Argon compounds that should be stable but for which no synthesis routes are currently known.


It is used in lighting since it will not react with the filament in a lightbulb even under high temperatures and other cases where diatomic nitrogen is an unsuitable (semi-)inert gas. Other uses;

  • Used as an inert gas shield in many forms of welding, including mig and tig (where the "I" stands for inert).
  • as a non-reactive blanket in the manufacture of titanium and other reactive elements.
  • as a protective atmosphere for growing silicon and germanium crystals.
  • Argon-39 has been used for a number of applications, primarily ice coring. It has also been used for ground water dating.
  • Cryosurgery procedures such as cryoablation uses liquefied argon to destroy cancer cells.

Argon is also used in technical SCUBA diving to inflate the dry suit, due to its nonreactive, heat isolating effect.


Argon (Greek argos meaning "inactive") 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.


This gas is isolated through liquid air fractionation since the atmosphere contains only 0.934% volume of argon (1.29% mass). The Martian atmosphere in contrast contains 1.6% of Ar-40 and 5 ppm Ar-36. In 2005, the Huygens probe also discoverd the presence of Ar-40 on Titan, the largest moon of Saturn [1].


Before 1962, argon and the other noble gases were generally considered to be chemically inert and not able to form compounds. However, since then, scientists have been able to force the heavier noble gases to form compounds. In 2000, the first argon compounds were formed by researchers at the University of Helsinki. By shining ultraviolet light onto frozen argon containing a small amount of hydrogen fluoride, they were able to form argon hydrofluoride (HArF).


The main isotopes of argon found on Earth are Ar-40, Ar-36, and Ar-38. Naturally occurring K-40 with a half-life of 1.250 x 109 years, decays to stable Ar-40 (11.2%) by electron capture and by positron emission, and also transforms to stable Ca-40 (88.8%) via beta decay. These properties and ratios are used to determine the age of rocks.

In the Earth's atmosphere, Ar-39 is made by cosmic ray activity, primarily with Ar-40. In the subsurface environment, it is also produced through neutron-capture by K-39 or alpha emission by calcium. Argon-37 is created from the decay of calcium-40 as a result of subsurface nuclear explosions. It has a half-life of 35 days.


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Last updated: 10-24-2005 05:55:37
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