Argon

This article is about a chemical element. For other uses, see Argon (disambiguation).

18 chlorine ← argon → potassium Ne ↑ Ar ↓ Kr periodic table
General
Name, Symbol, Number	argon, Ar, 18
Chemical series	noble gases
Group, Period, Block	18, 3, p
Appearance	colorless
Atomic mass	39.948(1) g/mol
Electron configuration	[Ne] 3s2 3p6
Electrons per shell	2, 8, 8
Physical properties
Phase	gas
Density	(0 °C, 101.325 kPa) 1.784 g/L
Melting point	83.80 K (-189.35 °C, -308.83 °F)
Boiling point	87.30 K (-185.85 °C, -302.53 °F)
Heat of fusion	1.18 kJ/mol
Heat of vaporization	6.43 kJ/mol
Heat capacity	(25 °C) 20.786 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K   47 53 61 71 87
Atomic properties
Crystal structure	cubic face centered
Oxidation states	0
Electronegativity	no data (Pauling scale)
Ionization energies (more)	1st: 1520.6 kJ/mol
	2nd: 2665.8 kJ/mol
	3rd: 3931 kJ/mol
Atomic radius	71 pm
Atomic radius (calc.)	71 pm
Covalent radius	97 pm
Van der Waals radius	188 pm
Miscellaneous
Magnetic ordering	nonmagnetic
Thermal conductivity	(300 K) 17.72 mW/(m·K)
Speed of sound	(gas, 27 °C) 323 m/s
CAS registry number	7440-37-1
Notable isotopes
Main article: Isotopes of argon iso NA half-life DM DE (MeV) DP 36Ar 0.337% Ar is stable with 18 neutrons 37Ar syn 35 d ε  ? 37Cl 38Ar 0.063% Ar is stable with 20 neutrons 39Ar syn 269 y β- 0.565 39K 40Ar 99.600% Ar is stable with 22 neutrons 42Ar syn 32.9 y β- 0.600 42K
References

Argon is the chemical element in the periodic table that has the symbol Ar and atomic number 18. The third noble gas, in group 18, argon makes up less than 1% of the Earth's atmosphere, making it the most common noble gas on Earth.

Contents 1 Notable characteristics 2 Applications 3 History 4 Occurrence 5 Compounds 6 Isotopes 7 References 8 External links

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 few known true chemical compounds that contain argon, which is 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 as of yet.

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.

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:

• Argon is used as an inert gas shield in many forms of welding, including metal inert gas welding and tungsten inert gas welding.
• as a non-reactive blanket in the manufacture of titanium and other reactive elements.
• as a protective atmosphere for growing silicon and germanium crystals.
• as a gas for use in plasma globes.
• as a gas for use in energy efficient windows.
• 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 use liquefied argon to destroy cancer cells.
• Liquid argon is used in calorimetry in experimental particle physics.
• Argon is used in technical SCUBA diving to inflate the dry suit, because it is inert and has low thermal conductivity.

History

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.

Occurrence

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 argon-40 and 5 ppm of argon-36. In 2005, the Huygens probe also discovered the presence of Ar-40 on Titan, the largest moon of Saturn [1].

Compounds

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): see http://pubs.acs.org/cen/80th/noblegases.html in its paragraph starting "Many recent findings". It is stable up to 40°K.

Isotopes

The main isotopes of argon found on Earth are ⁴⁰Ar, ³⁶Ar, and ⁴⁰Ar. Naturally occurring ⁴⁰K with a half-life of 1.250 x 10⁹ years, decays to stable ⁴⁰Ar (11.2%) by electron capture and by positron emission, and also transforms 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.

References

• Los Alamos National Laboratory – Argon

The content of this page is retrieved from http://en.wikipedia.org/wiki/Argon under GFDL