chlorine – argon Ne Ar Kr       Full table
General
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 (unknown)
Crystal structure	cubic face centered
Physical properties
State of matter	gas (nonmagnetic)
Melting point	83.8 K ("308.7 °F)
Boiling point	87.3 K ("302.4 °F)
Molar volume	22.56 ×10-6 m3/mol
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
Miscellaneous
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
SI units & STP are used except where noted.

Argon is a 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 chemically inert element is colorless and odorless in both its liquid and gaseous forms. There are no known true chemical compounds that contain argon. The creation of argon hydroflouride (HArF) was reported by researchers at the University of Helsinki in 2000. A highly unstable compound with fluorine has been reported but not yet proven. 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.

Applications

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.

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

History

Argon (Greek argos meaning "lazy") 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.

Occurrence

This gas is isolated through liquid air fractionation since the atmosphere contains only 0.94% volume of argon (1.29% mass). The Martian atmosphere in contrast contains 1.6% of Ar-40 and 5 ppm Ar-36.

Isotopes

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 decays to stable Ca-40 (88.8%) by negatron emission. 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.

Reference

• Los Alamos National Laboratory – Argon (http://periodic.lanl.gov/elements/18.html)