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Name , symbol , atomic number Argon, Ar, 18
Element category Noble gases
Group , period , block 18 , 3 , p
Appearance colorless gas
CAS number 7440-37-1
EC number 231-147-0
ECHA InfoCard 100.028.315
Mass fraction of the earth's envelope 3.6  ppm
Atomic mass 39.948 (1) u
Covalent radius 106 pm
Van der Waals radius 188 pm
Electron configuration [ Ne ] 3 s 2 3 p 6
1. Ionization energy 15th.759 611 7 (5) eV 1 520.57 kJ / mol
2. Ionization energy 27.62967 (12) eV2 665.86 kJ / mol
3. Ionization energy 40.735 (12) eV3 930 kJ / mol
4. Ionization energy 59.58 (18) eV5 749 kJ / mol
5. Ionization energy 74.84 (17) eV7 221 kJ / mol
Physical state gaseous
Crystal structure Cubic area-centered
density 1.784 kg m −3 at 273 K.
magnetism diamagnetic ( Χ m = −1.1 10 −8 )
Melting point 83.8 K (−189.3 ° C)
boiling point 87.15 K (−186 ° C)
Molar volume (solid) 22.56 · 10 −6 m 3 · mol −1
Heat of evaporation 6.52 kJ / mol
Heat of fusion 1.18 kJ mol −1
Speed ​​of sound 319 m s −1 at 293.15 K.
Thermal conductivity 0.01772 W m −1 K −1
isotope NH t 1/2 ZA ZE (M eV ) ZP
35 ares {syn.} 1.775 s ε 5.965 35 cl
36 ares 0.336% Stable
37 ares {syn.} 35.04 d ε 0.813 37 Cl
38 ares 0.063% Stable
39 ares in traces 269 a β - 0.565 39 K
40 ares 99.6% Stable
41 ares {syn.} 109.34 min β - 2,492 41 K
42 ares {syn.} 32.9 a β - 0.600 42 K
For other isotopes see list of isotopes
safety instructions
GHS labeling of hazardous substances
04 - gas bottle


H and P phrases H: 280
P: 403
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Argon ( ancient Greek ἀργός argós “inactive, inert”) is a chemical element with the symbol Ar (until 1957 only A) and the atomic number 18. In the periodic table it is in the 8th main group or the 18th  IUPAC group and therefore counts to the noble gases . Like the other noble gases, it is a colorless, extremely inert, monatomic gas . In many properties such as melting and boiling point or density , it stands between the lighter neon and the heavier krypton .

Argon is the most common noble gas found on earth, its proportion in the atmosphere is around 0.934%. This makes argon the third most common constituent of the earth's atmosphere , after nitrogen and oxygen . This is largely due to the decay of the 40 K isotope of potassium, producing 40 Ar.

Argon was the first noble gas to be discovered and extracted as a substance, hence the name that basically fits every noble gas. Helium (from the Greek helios for "sun") was previously only detected spectroscopically in sunlight and in earthly samples and neon was only discovered later. Argon was found in 1894 by Lord Rayleigh and William Ramsay through fractional distillation of liquid air. As the cheapest noble gas, argon is used in large quantities as a protective gas, for example in welding and in the production of some metals, but also as a filling gas for incandescent lamps .


Lord Rayleigh

Henry Cavendish , who researched the reactivity of air in 1783, found the first evidence of argon, which was discovered later . It generated electrical discharges in a certain amount of air that was enriched with oxygen in a ratio of 5: 3. Nitrogen and oxygen reacted with one another and the resulting nitrogen oxides could be washed out. A small amount of unreacted gas always remained. However, Cavendish did not realize that it was a different element and did not continue his experiments.

After John William Strutt, 3rd Baron Rayleigh , had determined the density of nitrogen isolated from air in 1892, he noticed that nitrogen obtained from ammonia had a lower density. There has been various speculations about this finding; Sun said James Dewar , there must be a N 3 , so a nitrogen analog of ozone action. Rayleigh repeated Cavendish's experiments by creating electrical sparks in an air-filled glass ball, causing nitrogen and oxygen to react. After confirming Cavendish's result of an unreactive residue, William Ramsay examined it more closely from 1894 onwards by transferring it over hot magnesium . Since magnesium reacts with nitrogen to form nitride , it was able to remove more nitrogen from the mixture. He noticed an increase in density and finally found a previously unknown, inert gas. On January 31, 1895, Ramsay and Rayleigh finally announced the discovery of the new element, which they called argon after the ancient Greek ἀργός argos , "sluggish" . When William Ramsay continued to study argon isolated from the air from 1898, he discovered three other elements in it, the noble gases neon , krypton and xenon .

The gas found its first technical applications in the electrical industry : Among other things, rectifiers based on glow discharge in argon, the so-called Tungar tubes, were manufactured .


Argon is one of the more common elements in the universe, and its frequency is comparable to sulfur and aluminum . It is the third most common noble gas in the universe after helium and neon . In this case, there is a primordial argon, which approximately in the sun or gas planet like Jupiter is found, mainly of the isotopes 36 Ar and 38 Ar, while the third stable isotope, 40 Ar, there occurs only in a small amount. The ratio of 36 Ar to 38 Ar is about 5.7.

On the other hand, argon is the most common noble gas on earth. It makes up 0.934% of the volume of the atmosphere (excluding water vapor ), making it the third most common constituent of the atmosphere after nitrogen and oxygen . The composition of terrestrial argon differs considerably from that of primordial argon in space. It consists of more than 99% of the isotope 40 Ar, which was created by the decay of the potassium isotope 40 K. The primordial isotopes, on the other hand, are only present in small quantities.

Since argon is created by the decay of potassium in the earth's crust, it is also found in rocks. When rocks melt in the earth's mantle, the argon gasses out, but so does the helium that is produced in other decays. It therefore mainly accumulates in the basalts of the oceanic crust . The argon is released from the rocks into the groundwater . Therefore , argon is dissolved in spring water , especially when it comes from great depths.

Extraction and presentation

The pure argon is extracted exclusively from the air, usually in the context of air liquefaction in the Linde process . The argon is not separated from the main air components in the main rectification column of the process, but in a separate argon column. In this, raw argon is first produced by rectification , which still contains around 3–5% oxygen and 1% nitrogen.

The raw argon is then purified in further stages. The gas mixture is first warmed to room temperature and compressed to 4-6  bar . In order to remove the remaining oxygen, hydrogen is then injected, which reacts with the oxygen to form water on noble metal catalysts. After this has been removed, the argon, which accumulates at the lower end of the column, is separated from the remaining nitrogen in a further column, so that argon with a purity of 99.9999% (argon 6.0) can be produced.

Other sources for the production of argon are the production of ammonia in the Haber-Bosch process and the production of synthesis gas , for example for the production of methanol . In these processes, which use air as a starting material, argon and other noble gases accumulate in the production process and can be isolated from the gas mixture. As with the Linde process, the various gases are separated from one another by adsorption or rectification, thus producing pure argon.


Physical Properties

Argon is colorless even in the solid and liquid state.
cubic close packing of solid argon, a  = 526 pm

Under normal conditions, argon is a monoatomic, colorless and odorless gas that condenses at 87.15 K (−186 ° C) and solidifies at 83.8 K (−189.3 ° C). Like the other noble gases apart from helium, argon crystallizes in a cubic close packing of spheres with the lattice parameter a  = 526  pm at 4 K.

Like all noble gases, argon only has closed shells ( noble gas configuration ). This explains why the gas is always monatomic and the reactivity is low.

With a density of 1.784 kg / m 3 at 0 ° C and 1013 hPa, argon is heavier than air, so it sinks. In the phase diagram , the triple point is at 83.8 K and 689 hPa, the critical point at 150.86 K, 4896 kPa and a critical density of 0.536 g / cm 3 .

Argon is somewhat soluble in water. A maximum of 53.6 ml of argon can dissolve in one liter of water at 0 ° C and normal pressure.

Chemical properties

As a noble gas, argon hardly reacts with other elements or compounds. So far only the experimentally shown argon fluorohydride HArF is known, which is obtained by photolysis of hydrogen fluoride in an argon matrix at 7.5 K and identified by means of new lines in the infrared spectrum. Above 27 K it decomposes. According to calculations, other compounds of argon should be metastable and relatively difficult to decompose; however, these could not be represented experimentally so far. Examples are the chlorine analog of the argon fluorohydride HArCl, but also compounds in which the proton has been replaced by other groups, such as FArCCH as an organic argon compound and FArSiF 3 with an argon-silicon bond.

Argon forms some clathrates in which it is physically trapped in voids in a surrounding crystal. An argon hydrate is stable at −183 ° C, but the rate of formation is very slow because recrystallization has to take place. If the ice is mixed with chloroform , the clathrate already forms at −78 ° C. A clathrate of argon in hydroquinone is also stable .


A total of 23 isotopes and a further core isomer of argon are known. Of these, three, namely the isotopes 36 Ar, 38 Ar and 40 Ar, are stable and occur in nature. 40 ares predominate with a share of 99.6% of the natural terrestrial isotope mixture. 36 Ar and 38 Ar are rare at 0.34% and 0.06%, respectively. Of the unstable isotopes, 39 Ar with 269 years and 42 Ar with 32.9 years have the longest half-lives . All other isotopes have short half-lives ranging from less than 10 ps at 30 Ar to 35.04 days at 37 Ar.

40 Ar is used to determine the age of rocks ( potassium-argon dating ). This takes advantage of the fact that unstable 40 K, which is contained in these, slowly decays to 40 Ar. The more potassium has broken down to argon, the older the rock. The short-lived isotope 41 Ar can be used to check gas pipelines. By passing 41 Ar through , the efficiency of ventilation or the tightness of a pipe can be determined.

List of argon isotopes

Biological importance

Like the other noble gases, argon has no biological significance due to its inertia and is also non-toxic. In higher concentrations, it has a suffocating effect by displacing the oxygen. At pressures of more than 24 bar it has a narcotic effect .


Argon gas bottles in a fire extinguishing system

Argon is the cheapest noble gas that is available in large quantities and is used in many areas. In 1998 production was around two billion m³ or two km³ worldwide. Most of the argon is used as a protective gas . It is used whenever the cheaper nitrogen cannot be used. These include, above all, welding processes for metals that react with nitrogen at high temperatures, such as titanium , tantalum and tungsten . Argon is also used as the inert gas in metal inert gas welding and tungsten inert gas welding, which are used, for example, when welding aluminum alloys or high-alloy steels . It is also used in metallurgy as a protective gas, for example for the production of titanium, high-purity silicon or melt refining and for degassing metal melts.

Argon is a food additive (E 938) and is used as a propellant and protective gas in the packaging of food and wine production.

Argon is mainly used as a gaseous extinguishing agent for the protection of property, especially in electrical and IT systems, and acts by displacing oxygen. Pure argon or a gas mixture together with nitrogen is used for this purpose.

In analysis, argon is used as a carrier and protective gas for gas chromatography and inductively coupled plasma ( ICP-MS , ICP-OES ).

Incandescent lamps are often filled with argon-nitrogen mixtures because a gas filling reduces the sublimation of the filament. Argon has a lower thermal conductivity than lighter gases, but it is cheaper than other heavier and therefore even less thermally conductive gases such as krypton or xenon. An advantage of the lower thermal conductivity is a higher possible annealing temperature and thus higher light yield. It is also used as a filling gas for insulating glass panes because of its low thermal conductivity . Argon is also used as a luminous gas in gas discharge lamps with a typical violet color. If a little mercury is added, the color changes to blue. Furthermore, argon is the laser medium in argon-ion lasers .

Argon discharge tube.jpg

Argon gas discharge tubes of various designs

In the field of steel production , argon plays a particularly important role in the field of secondary metallurgy. With the argon purging, the steel alloy can be degassed and homogenized at the same time, in particular the undesired, dissolved nitrogen is removed from the melt.

When diving argon is - particularly in the use of helium containing Trimix as the breathing gas - used to dry suits to fill or to tare it. The low thermal conductivity of the gas is also used to delay the cooling down of the suit wearer.

Argon has been on the doping list of the World Anti-Doping Agency (WADA) since May 2014 . The lack of oxygen resulting from inhalation of argon obviously activates the formation of the body's own erythropoietin (EPO). Xenon is also on the doping list for the same reason .


Web links

Commons : Argon  - Collection of images, videos and audio files
Wiktionary: Argon  - explanations of meanings, word origins, synonyms, translations

Individual evidence

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This article was added to the list of excellent articles on March 7, 2010 in this version .