Argon

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

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 m W/(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 a chemical element in the periodic table. It has the symbol Ar and atomic number 18. The third noble gas, in group 18, argon makes up about 1% of the Earth's atmosphere, making it the most common noble gas on Earth.

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 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.
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 uses liquefied argon to destroy cancer cells.

Argon is also 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 Ar-40 and 5 ppm Ar-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.htmll in its paragraph starting "Many recent findings". It is stable up to 40° K.

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 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.