18
  Ar  
39.948000
Argon

Name: Argon
Symbol: Ar
Atomic Number: 18
Atomic Weight: 39.948000
Family: Noble Gases
CAS RN: 7440-37-1
Description: An inert colorless, tasteless and odorless gas.
State (25C): Gas
Oxidation states: 0

Molar Volume: 24.2 cm3/mole
Valence Electrons: 3p6

Boiling Point:  87.45K, -185.7C, -302.3F
Melting Point:
83.96K, -189.19C, -308.54F
Electrons Energy Level: 2, 8, 8
Isotopes: 18 + 3 Stable
Heat of Vaporization: 6.447 kJ/mol
Heat of Fusion: 1.188 kJ/mol
Density: 1.7824 g/L @ 273K & 1atm
Specific Heat: 0.52 J/gK
Atomic Radius: 0.88
Ionic Radius: unknown
Electronegativity: N/A (Pauling); 3.2 (Allrod Rochow)
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 in an experiment in which they removed all of the oxygen and nitrogen from the air.  Argon was also encountered in 1882 through independent research of H.F. Newall and W.N. Hartley. Each observed new lines in the color spectrum of air but were unable to identify the element responsible for the lines. Argon became the first member of the noble gases to be discovered.  Argon is third in abundance in the earth's atmosphere (about 1% by volume).  Until 1957 the symbol for Argon (Ar) was A.

Argon constitutes 0.934% by volume and 1.29% by mass of the Earth's atmosphere, and air is the primary raw material used by industry to produce purified argon products. Argon is isolated from air by fractionation, most commonly by cryogenic fractional distillation, a process that also produces purified nitrogen, oxygen, neon, krypton and xenon.

The Martian atmosphere in contrast contains 1.6% of argon-40 and 5 ppm of argon-36.   The Mariner spaceprobe fly-by of the planet Mercury in 1973 found that Mercury has a very thin atmosphere with 70% argon, believed to result from releases of the gas as a decay product from radioactive materials on the planet. In 2005, the Huygens probe also discovered the presence of argon-40 on Titan, the largest moon of Saturn.

Argon has been used in ordinary incandescent lighting and other applications in which diatomic nitrogen is not sufficiently inert.  Argon will not react with the filament of light bulbs even at high temperatures.

2
He
4.002
10
Ne
20.17
18
Ar
39.94
36
Kr
83.80
54
Xe
131.3
86
Rn
222.0
118
Uuo
293.0

Applications

1s2 2s2p6 3s2p6

Compounds

Argon’s complete octet of electrons indicates full s and p subshells.  This full outer energy level makes argon very stable and extremely resistant to bonding with other elements. Before 1962, argon and the other noble gases were considered to be chemically inert and unable to form compounds; however, compounds of the heavier noble gases have since been synthesized. The creation of argon hydrofluoride (HArF), a metastable compound of argon with fluorine and hydrogen, was first reported by researchers at the University of Helsinki in 2000.  By shining ultraviolet light onto frozen argon containing a small amount of hydrogen fluoride, argon hydrofluoride (HArF) was formed.  It is stable up to 40 degrees kelvin (-233C).

Although the neutral ground-state chemical compounds of argon are presently limited to HArF, argon can form clathrates with water when atoms of it are trapped in a lattice of the water molecules. Also argon-containing ions, such as ArH+ and excited state complexes as ArF are well known.  Theoretical calculations on computers have shown several argon compounds that should be stable but for which no synthesis routes are currently known.

Argon has approximately the same solubility in water as oxygen gas and is 2.5 times more soluble in water than nitrogen gas.  This highly stable chemical element is colorless, odorless, tasteless and nontoxic in both its liquid and gaseous forms.

1s2
2s2 2p6
3s2 3p6

Isotopes

The main isotopes of argon found on Earth are 40Ar, 36Ar, and 38Ar.   Naturally occurring 40K with a half-life of 1.250109 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.

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Isotope Atomic Mass Half-Life
Ar30 30.022 <20 ns
Ar31 31.012 15.1 ms
Ar32 31.9977 98 ms
Ar33 32.9899 173 ms
Ar34 33.9803 844.5 ms
Ar35 34.9753 1.775 seconds
Ar36 35.9675 Stable
Ar37 36.9668 35.04 days
Ar38 37.9627 Stable
Ar39 38.9643 269 years
Ar40 39.9624 Stable
Ar41 40.9645 109.34 minutes
Ar42 41.963 32.9 years
Ar43 42.9657 5.37 minutes
Ar44 43.9654 11.87 minutes
Ar45 44.9681 21.48 seconds
Ar46 45.9681 8.4 seconds
Ar47 46.972 ~700 ms
Ar48 47.975  
Ar49 48.982 =>170 ns
Ar50 49.986 =>170 ns
Ar51 50.993 >200 ns
Ar52 51.998  
Ar53 53.01  

Argon - (Gr. argos, inactive), Its presence in air was suspected by Cavendish in 1785, discovered by Lord Rayleigh and Sir William Ramsay in 1894.  The gas is prepared by fractionation of liquid air, the atmosphere containing 0.94% argon.  The atmosphere of Mars contains 1.6% of 40Ar and 5 p.p.m. of 36Ar. Argon is two and one half times as soluble in water as nitrogen, having about the same solubility as oxygen. It is recognized by the characteristic lines in the red end of the spectrum. It's used in electric light bulbs and in fluorescent tubes at a pressure of about 400 Pa, and in filling photo tubes, glow tubes, etc. Argon is also used as an inert gas shield for arc welding and cutting, as a blanket for the production of titanium and other reactive elements, and as a protective atmosphere for growing silicon and germanium crystals.  Argon is colorless and odorless, both as a gas and liquid.  It is available in high-purity form. Commercial argon is available at a cost of about 30 per cubic foot.  Argon is considered to be a very inert gas and is not known to form true chemical compounds, as do krypton, xenon, and radon.  However, it does form a hydrate having a dissociation pressure of 105 atm at 0'C.  Ion molecules such as (ArKr)+, (ArXe)', (NeAr)+ have been observed spectroscopically.  Argon also forms a clathrate with P-hydroquinone.  This clathrate is stable and can be stored for a considerable time, but a true chemical bond does not exist.  Van der Waals' forces act to hold the argon.  Naturally occurring argon is a mixture of three isotopes.  Eighteen other radioactive isotopes are now known to exist.  Commercial argon is priced at about $70/300 cu. ft. or 8.5 cu. meters.


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Argon Data

 

Atomic Structure

Atomic Radius (): 0.88
Atomic Volume cm3/mol : 28.5cm3/mol
Covalent Radius: 0.98
Crystal Structure: Cubic face centered
Ionic Radius: unknown

Chemical Properties

Electrochemical Equivalents: unknown
Electron Work Function: unknown
Electronegativity: N/A (Pauling); 3.2 (Allrod Rochow)
Heat of Fusion: 1.188 kJ/mol
Incompatibilities: unknown
First Ionization Potential: 15.759
Second Ionization Potential: 27.629
Third Ionization Potential: 40.74
Valence Electron Potential: unknown
Ionization Energy (eV): 15.760 eV

Physical Properties

Atomic Mass Average: 39.948
Boiling Point: 87.45K, -185.7C, -302.3F
Melting Point: 83.96K, -189.19C, -308.54F
Heat of Vaporization: 6.447 kJ/mol
Coefficient of Lineal Thermal Expansion/K-1: N/A
Electrical Conductivity: unknown
Thermal Conductivity: 0.0001772 W/cmK
Density: 1.7824 g/L @ 273K & 1atm
Enthalpy of Atomization: unknown
Enthalpy of Fusion: 1.18 kJ/mole
Enthalpy of Vaporization: 6.43 kJ/mole
Flammability Class: unknown
Molar Volume: 24.2 cm3/mole
Optical Refractive Index: 1.000281
Relative Gas Density (Air=1): unknown
Specific Heat: 0.52 J/gK
Vapor Pressure: unknown
Estimated Crustal Abundance: 3.5 milligrams per kilogram
Estimated Oceanic Abundance: 4.510-1 milligrams per liter


(Gr. argos, inactive) Its presence in air was suspected by Cavendish in 1785, discovered by Lord Rayleigh and Sir William Ramsay in 1894. The gas is prepared by fractionation of liquid air, the atmosphere containing 0.94% argon. The atmosphere of Mars contains 1.6% of 40Ar and 5 p.p.m. of 36Ar. Argon is two and one half times as soluable in water as nitrogen, having about bhe same solubility as oxygen. It is used in electric light bulbs and in fluorescent tubes at a pressure of about 400 Pa. and in filling photo tubes, glow tubes, etc. Argon is also used as an inert gas shield for arc welding and cutting, as blanket for the production of titanium and other reactive elements, and as a protective atmosphere for growing silicon and germanium crystals. Argon is colorless and odorless, both as a gas and liquid. Argon is considered to be a very inert gas and is not known to form true chemical compounds, as do kypton, xenon, and radon. Naturally occurring argon is a mixture of three isotopes.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond