Science Fair Project Encyclopedia
|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|
|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|
|State of matter||gas (nonmagnetic)|
|Melting point||83.8 K (−153.7 ºC / −308.7 °F)|
|Boiling point||87.3 K (−150.2 ºC / −302.4 °F)|
|Heat of vaporization||6.447 kJ/mol|
|Heat of fusion||1.188 kJ/mol|
|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|
|SI units & STP are used except where noted.|
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.
- 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.
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 .
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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