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colorless gas with lilac/violet emission
General properties
Name, symbol, number argon, Ar, 18
Element category noble gases
Group, period, block 183, p
Standard atomic weight 39.948(1)g·mol−1
Electron configuration [Ne] 3s2 3p6
Electrons per shell 2, 8, 8 (Image)
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
Triple point 83.8058 K (-189°C), 69 kPa
Critical point 150.87 K, 4.898 MPa
Heat of fusion 1.18 kJ·mol−1
Heat of vaporization 6.43 kJ·mol−1
Specific heat capacity (25 °C) 20.786 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K   47 53 61 71 87
Atomic properties
Oxidation states 0
Electronegativity no data (Pauling scale)
Ionization energies
1st: 1520.6 kJ·mol−1
2nd: 2665.8 kJ·mol−1
3rd: 3931 kJ·mol−1
Covalent radius 106±10 pm
Van der Waals radius 188 pm
Crystal structure face-centered cubic
Magnetic ordering diamagnetic[1]
Thermal conductivity (300 K) 17.72x10-3  W·m−1·K−1
Speed of sound (gas, 27 °C) 323 m/s
CAS registry number 7440–37–1
Most stable isotopes
Main article: Isotopes of argon
iso NA half-life DM DE (MeV) DP
36Ar 0.337% 36Ar is stable with 18 neutrons
37Ar syn 35 d ε 0.813 37Cl
38Ar 0.063% 38Ar is stable with 20 neutrons
39Ar trace 269 y β 0.565 39K
40Ar 99.600% 40Ar is stable with 22 neutrons
41Ar syn 109.34 min β 2.49 41K
42Ar syn 32.9 y β 0.600 42K
Lord Rayleigh's method for the isolation of argon, based on an experiment of Henry Cavendish's. The gases are contained in a test-tube (A) standing over a large quantity of weak alkali (B), and the current is conveyed in wires insulated by U-shaped glass tubes (CC) passing through the liquid and round the mouth of the test-tube. The inner platinum ends (DD) of the wire receive a current from a battery of five Grove cells and a Ruhmkorff coil of medium size.

Argon (pronounced /ˈɑrɡɒn/) is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table (noble gases). Argon is the third most common gas in the Earth's atmosphere, at 0.93% -- making it more common than carbon dioxide. It is the third most abundant gas and the most frequently used of the noble gases. The complete octet in the outer atomic shell makes it stable and resistant to bonding with other elements. Its triple point temperature of 83.8058 K is a defining fixed point in the International Temperature Scale of 1990.



A small piece of rapidly melting argon ice.

Argon has approximately the same solubility in water as oxygen gas and is 2.5 times more soluble in water than nitrogen gas. Argon is colorless, odorless, and nontoxic as a solid, liquid, and gas. Argon is inert under most conditions and forms no confirmed stable compounds at room temperature.

Although argon is a noble gas, it has been found to have the capability of forming some compounds. For example, the creation of argon fluorohydride (HArF), a marginally stable compound of argon with fluorine and hydrogen, was reported by researchers at the University of Helsinki in 2000.[2] 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.[3] Also argon-containing ions and excited state complexes, such as ArH+ and ArF, respectively, are known to exist. Theoretical calculations have predicted several argon compounds that should be stable,[4] but for which no synthesis routes are currently known.


Argon (αργος, Greek meaning "inactive", in reference to its chemical inactivity)[5][6][7] was suspected to be present in air by Henry Cavendish in 1785 but was not isolated until 1894 by Lord Rayleigh and Sir William Ramsay in Scotland in an experiment in which they removed all of the oxygen, carbon dioxide, water and nitrogen from a sample of clean air.[8][9] They had determined that nitrogen produced from chemical compounds was one-half percent lighter than nitrogen from the atmosphere. The difference seemed insignificant, but it was important enough to attract their attention for many months. They concluded that there was another gas in the air mixed in with the nitrogen.[10] 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. The symbol for argon is now Ar, but up until 1957 it was A.[11]


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.[12]


The main isotopes of argon found on Earth are 40Ar (99.6%), 36Ar (0.34%), and 38Ar (0.06%). Naturally occurring 40K with a half-life of 1.25 × 109 years, decays to stable 40Ar (11.2%) by electron capture and positron emission, and also to stable 40Ca (88.8%) via beta decay. These properties and ratios are used to determine the age of rocks.[13]

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 neutron spallation of 40Ca as a result of subsurface nuclear explosions. It has a half-life of 35 days.[13]

Argon is notable in that its isotopic composition varies greatly between different locations in the solar system. Where the major source of argon is the decay of potassium-40 in rocks, Argon-40 will be the dominant isotope, as it is on earth. Argon produced directly by stellar nucleosynthesis, in contrast, is dominated by the alpha process nuclide, argon-36. Correspondingly, solar argon contains 84.6% argon-36 based on solar wind measurements. [14]

The predominance of radiogenic argon-40 is responsible for the fact that the standard atomic weight of terrestrial argon is greater than that of the next element, potassium. This was puzzling at the time when argon was discovered, since Mendeleev had placed the elements in his periodic table in order of atomic weight, although the inertness of argon implies that it must be placed before the reactive alkali metal potassium. Henry Moseley later solved this problem by showing that the periodic table is actually arranged in order of atomic number. (See History of the periodic table).

The Martian atmosphere contains 1.6% of argon-40 and 5 ppm of argon-36. The Mariner space probe 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.[15]


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. In August 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, argon fluorohydride (HArF) was formed.[2][16] It is stable up to 40 kelvin (−233 °C).



Argon is produced industrially by the fractional distillation of liquid air, a process that separates liquid nitrogen, which boils at 77.3 K, from argon, which boils at 87.3 K and oxygen, which boils at 90.2 K. About 700,000 tons of argon are produced worldwide every year. [17]

In radioactive decays

40Ar, the most abundant isotope of argon, is produced by the decay of 40K with a half-life of 1.25 × 109 years by electron capture or positron emission. Because of this, it is used in potassium-argon dating to determine the age of rocks.


Cylinders containing argon gas for use in extinguishing fire without damaging server equipment

There are several different reasons why argon is used in particular applications:

  • An inert gas is needed. In particular, argon is the cheapest alternative when diatomic nitrogen is not sufficiently inert.
  • Low thermal conductivity is required.
  • The electronic properties (ionization and/or the emission spectrum) are necessary.

Other noble gases would probably work as well in most of these applications, but argon is by far the cheapest. Argon is inexpensive since it is a byproduct of the production of liquid oxygen and liquid nitrogen, both of which are used on a large industrial scale. The other noble gases (except helium) are produced this way as well, but argon is the most plentiful since it has the highest concentration in the atmosphere. The bulk of argon applications arise simply because it is inert and relatively cheap.

Industrial processes

Argon is used in some high-temperature industrial processes, where ordinarily non-reactive substances become reactive. For example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning.

For some of these processes, the presence of nitrogen or oxygen gases might cause defects within the material. Argon is used in various types of metal inert gas welding such as tungsten inert gas welding, as well as in the processing of titanium and other reactive elements. An argon atmosphere is also used for growing crystals of silicon and germanium.

Argon is an asphyxiant in the poultry industry, either for mass culling following disease outbreaks, or as a means of slaughter more humane than the electric bath. Argon's relatively high density causes it to remain close to the ground during gassing. Its non-reactive nature makes it suitable in a food product, and since it replaces oxygen within the dead bird, argon also enhances shelf life.[18]

Argon is sometimes used for extinguishing fires where damage to equipment is to be avoided (see photo).


A sample of caesium is packed under argon to avoid reactions with air

Argon is used to displace oxygen- and moisture-containing air in packaging material to extend the shelf-lives of the contents. Aerial oxidation, hydrolysis, and other chemical reactions which degrade the products are retarded or prevented entirely. Bottles of high-purity chemicals and certain pharmaceutical products are available in sealed bottles or ampoules packed in argon. In wine making, argon is used to top-off barrels to avoid the aerial oxidation of ethanol to acetic acid during the aging process.

Argon is also available in aerosol-type cans, which may be used to preserve compounds such as varnish, polyurethane, paint, etc. for storage after opening.[19]

Since 2001 the American National Archives stores important national documents such as the Declaration of Independence and the Constitution within argon-filled cases to retard their degradation. Using argon reduces gas leakage, compared with the helium used in the preceding five decades. [20]

Laboratory equipment

Gloveboxes are typically filled with argon, which recirculates over scrubbers to maintain an oxygen- and moisture-free atmosphere

Argon may be used as the inert gas within Schlenk lines and gloveboxes. The use of argon over comparatively less expensive dinitrogen is preferred where nitrogen may react.

Argon may be used as the carrier gas in gas chromatography and in electrospray ionization mass spectrometry; it is the gas of choice for the plasma used in ICP spectroscopy. Argon is preferred for the sputter coating of specimens for scanning electron microscopy. Argon ions are also used for sputtering in microelectronics.

Medical use

Cryosurgery procedures such as cryoablation use liquefied argon to destroy cancer cells. In surgery it is used in a procedure called "argon enhanced coagulation" which is a form of argon plasma beam electrosurgery. The procedure carries a risk of producing gas embolism in the patient and has resulted in the death of one person via this type of accident.[21] Blue argon lasers are used in surgery to weld arteries, destroy tumors, and to correct eye defects.[22] It has also used experimentally to replace nitrogen in the breathing or decompression mix, to speed the elimination of dissolved nitrogen from the blood.[23] See Argox (breathing gas).


Incandescent lights are filled with argon, to preserve the filaments at high temperature from oxidation. It is used for the specific way it ionizes and emits light, such as in plasma globes and calorimetry in experimental particle physics. Gas-discharge lamps filled with argon provide blue light. Argon is also used for the creation of blue laser light.

Miscellaneous uses

It is used for thermal insulation in energy efficient windows.[24] Argon is also used in technical scuba diving to inflate a dry suit, because it is inert and has low thermal conductivity.[25]

Compressed argon is allowed to expand, to cool the seeker heads of the AIM-9 Sidewinder missile, and other missiles that use cooled thermal seeker heads. The gas is stored at high pressure.[26]

Argon-39, with a half-life of 269 years, has been used for a number of applications, primarily ice core and ground water dating. Also, potassium-argon dating is used in dating igneous rocks.


Although argon is non-toxic, it does not satisfy the body's need for oxygen and is thus an asphyxiant. Argon is 25% more dense than air and is considered highly dangerous in closed areas. It is also difficult to detect because it is colorless, odorless, and tasteless. In confined spaces, it is known to result in death due to asphyxiation. A 1994 incident in Alaska that resulted in one fatality highlights the dangers of argon tank leakage in confined spaces, and emphasizes the need for proper use, storage and handling.[27]

See also


  1. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
  2. ^ a b "HArF! Argon's not so noble after all - researchers make argon fluorohydride". 
  3. ^ Belosludov, V. R.; Subbotin, O S; Krupskii, D S; Prokuda, O V; Belosludov, R V; Kawazoe, Y (2006). "Microscopic model of clathrate compounds". J. Phys.: Conf. Ser. 29: 1. doi:10.1088/1742-6596/29/1/001. 
  4. ^ Cohen, Arik; Lundell, Jan; Gerber, R. Benny (2003). "First compounds with argon–carbon and argon–silicon chemical bonds". The Journal of Chemical Physics 119: 6415. doi:10.1063/1.1613631. 
  5. ^ Hiebert, E. N. (1963). "In Noble-Gas Compounds". in Hyman, H. H.. Historical Remarks on the Discovery of Argon: The First Noble Gas. Chicago, IL: University of Chicago Press. pp. 3–20. 
  6. ^ Travers, M. W. (1928). The Discovery of the Rare Gases. London: Edward Arnold & Co.. pp. 1–7. 
  7. ^ Lord Rayleigh (1895). "Argon: A New Constituent of the Atmosphere". Chemical News 71,: 51–58. 
  8. ^ Lord Rayleigh; Ramsay, William (1894 - 1895). "Argon, a New Constituent of the Atmosphere". Proceedings of the Royal Society of London 57 (1): 265–287. doi:10.1098/rspl.1894.0149. 
  9. ^ William Ramsay. "Nobel Lecture in Chemistry, 1904". 
  10. ^ "About Argon, the Inert; The New Element Supposedly Found in the Atmosphere". The New York Times. Retrieved 2009-02-01. 
  11. ^ Holden, Norman E. (12). "History of the Origin of the Chemical Elements and Their Discoverers". National Nuclear Data Center (NNDC). 
  12. ^ "Argon, Ar". Retrieved 2007-03-08. 
  13. ^ a b "40Ar/39Ar dating and errors". Retrieved 2007-03-07. 
  14. ^ Lodders, Katharina (2008). "the solar argon abundance". The Astrophysical Journal 674: 607. doi:10.1086/524725. 
  15. ^ "Seeing, touching and smelling the extraordinarily Earth-like world of Titan". European Space Agency. 21. 
  16. ^ Bartlett, Neil. "The Noble Gases". Chemical & Engineering News. 
  17. ^ "Periodic Table of Elements: Argon – Ar". Retrieved 2008-09-12. 
  18. ^ Fletcher, D. L.. "Symposium: Recent Advances in Poultry Slaughter Technology Slaughter Technology". Retrieved 2010-01-01. 
  19. ^ Zawalick, Steven Scott "Method for preserving an oxygen sensitive liquid product" U.S. Patent 6,629,402 Issue date: Octover 7, 2003
  20. ^ "Schedule for Renovation of the National Archives Building". Retrieved 2009-07-07. 
  21. ^ "Fatal Gas Embolism Caused by Overpressurization during Laparoscopic Use of Argon Enhanced Coagulation". MDSR. 24. 
  22. ^ Fujimoto, James; Rox Anderson, R. (2006). "Tissue Optics, Laser-Tissue Interaction, and Tissue Engineering" (pdf). Biomedical Optics. pp. 77–88. Retrieved 2007-03-08. 
  23. ^ Pilmanis Andrew A, Balldin UI, Webb James T, Krause KM (December 2003). "Staged decompression to 3.5 psi using argon-oxygen and 100% oxygen breathing mixtures". Aviation, Space, Environmental Medicine 74 (12): 1243–50. PMID 14692466. 
  24. ^ "Energy-Efficient Windows". Retrieved 2009-08-01. 
  25. ^ Nuckols ML, Giblo J, Wood-Putnam JL. (September 15-18, 2008). "Thermal Characteristics of Diving Garments When Using Argon as a Suit Inflation Gas.". Proceedings of the Oceans 08 MTS/IEEE Quebec, Canada Meeting (MTS/IEEE). Retrieved 2009-03-02. 
  26. ^ "Description of Aim-9 Operation". Retrieved 2009-02-01. 
  27. ^ Middaugh, John (1994-06-23). "Welder's Helper Asphyxiated in Argon-Inerted Pipe (FACE AK-94-012)". State of Alaska Department of Public Health. Retrieved 2009-02-01. 

Further reading

  • Triple point pressure: 69 kPa - "Section 4, Properties of the Elements and Inorganic Compounds; Melting, boiling, triple, and critical temperatures of the elements". CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. 2005. 

External links

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

ARGON (from the Gr. it-, privative, and g pyov, work; hence meaning "inert"), a gaseous constituent of atmospheric air. For more than a hundred years before 1894 it had been supposed that the composition of the atmosphere was thoroughly known. Beyond variable quantities of moisture and traces of carbonic acid, hydrogen, ammonia, &c., the only constituents recognized were nitrogen and oxygen. The analysis of air was conducted by determining the amount of oxygen present and assuming the remainder to be nitrogen. Since the time of Henry Cavendish no one seemed even to have asked the question whether the residue was, in truth, all capable of conversion into nitric acid.

The manner in which this condition of complacent ignorance came to be disturbed is instructive. Observations undertaken mainly in the interest of Prout's law, and extending over many years, had been conducted to determine afresh the densities of the principal gases - hydrogen, oxygen and nitrogen. In the latter case, the first preparations were according to the I See Victor Loret, "Les flutes egyptiennes antiques," Journal Asiatique, 8eme serie, tome xiv., Paris, 1889, pp. 129, 130 and 132.

Catalogue descriptif et analytique du musee du Conservatoire Royal de Bruxelles (Ghent, 1880), p. 141.

3 A Descriptive Catalogue of the Musical Instruments in the South Kensington Museum, by Carl Engel (London, 18 74), p. 143.

convenient method devised by Vernon Harcourt, in which air charged with ammonia is passed over red-hot copper. Under the influence of the heat the atmospheric oxygen unites with the hydrogen of the ammonia, and when the excess of the latter is removed with sulphuric acid, the gas properly desiccated should be pure nitrogen, derived in part from the ammonia, but principally from the air. A few concordant determinations of density having been effected, the question was at first regarded as disposed of, until the thought occurred that it might be desirable to try also the more usual method of preparation in which the oxygen is removed by actual oxidation of copper without the aid of ammonia. Determinations made thus were equally concordant among themselves, but the resulting density was about 10 1 6 - 0 part greater than that found by Harcourt's method (Rayleigh, Nature, vol. xlvi. p. 512, 1892). Subsequently when oxygen was substituted for air in the first method, so that all (instead of about one-seventh part) of the nitrogen was derived from ammonia, the difference rose to 2%. Further experiment only brought out more clearly the diversity of the gases hitherto assumed to be identical. Whatever were the means employed to rid air of accompanying oxygen, a uniform value of the density was arrived at, and this value was z % greater than that appertaining to nitrogen extracted from compounds such as nitrous oxide, ammonia and ammonium nitrite. No impurity, consisting of any known substance, could be discovered capable of explaining an excessive weight in the one case, or a deficiency in the other. Storage for eight months did not disturb the density of the chemically extracted gas, nor had the silent electric discharge any influence upon either quality. ("On an Anomaly encountered in determining the Density of Nitrogen Gas," Proc. Roy. Soc., April 1894.) At this stage it became clear that the complication depended upon some hitherto unknown body, and probability inclined to the existence of a gas in the atmosphere heavier than nitrogen, and remaining unacted upon during the removal of the oxygen - a conclusion afterwards fully established by Lord Rayleigh and Sir William Ramsay. The question which now pressed was as to the character of the evidence for the universally accepted view that the so-called nitrogen of the atmosphere was all of one kind, that the nitrogen of the air was the same as the nitrogen of nitre. Reference to Cavendish showed that he had already raised this question in the most distinct manner, and indeed, to a certain extent, resolved it. In his memoir of 1785 he writes: "As far as the experiments hitherto published extend, we scarcely know more of the phlogisticated part of our atmosphere than that it is not diminished by lime-water, caustic alkalies, or nitrous air; that it is unfit to support fire or maintain life in animals; and that its specific gravity is not much less than that of common air; so that, though the nitrous acid, by being united to phlogiston, is converted into air possessed of these properties, and consequently, though it was reasonable to suppose, that part at least of the phlogisticated air of the atmosphere consists of this acid united to phlogiston, yet it may fairly be doubted whether the whole is of this kind, or whether there are not in reality many different substances confounded together by us under the name of phlogisticated air. I therefore made an experiment to determine whether the whole of a given portion of the phlogisticated air of the atmosphere could be reduced to nitrous acid, or whether there was not a part of a different nature to the rest which would refuse to undergo that change. The foregoing experiments indeed, in some measure, decided this point, as much the greatest part of air let up into the tube lost its elasticity; yet, as some remained unabsorbed, it did not appear for certain whether that was of the same nature as the rest or not. For this purpose I diminished a similar mixture of dephlogisticated [oxygen] and common air, in the same manner as before [by sparks over ], till it was reduced to a small part of its original bulk. I then, in order to decompound as much as I could of the phlogisticated air [nitrogen] which remained in the tube, added some dephlogisticated air to it and continued the spark until no further diminution took place. Having by these means condensed as much as I could of the phlogisticated air, I let up some solution of liver of sulphur to absorb the dephlogisticated air; after which only a small bubble of air remained unabsorbed, which certainly was not more than of the bulk of the dephlogisticated air let up into the tube; so that, if there be any part of the dephlogisticated air of our atmosphere which differs from the rest, and cannot be reduced to nitrous acid, we may safely conclude that it is not more than 7a part of the whole." Although, as was natural, Cavendish was satisfied with his result, and does not decide whether the small residue was genuine, it is probable that his residue was really of a different kind from the main bulk of the "phlogisticated air," and contained the gas afterwards named argon.

The announcement to the British Association in 1894 by Rayleigh and Ramsay of a new gas in the atmosphere was received with a good deal of scepticism. Some doubted the discovery of a new gas altogether, while others denied that it. was present in the atmosphere. Yet there was nothing inconsistent with any previously ascertained fact in the asserted presence of i o ho of a non-oxidizable gas about half as heavy again as nitrogen. The nearest approach to a difficulty lay in the behaviour of liquid air, from which it was supposed, as the event. proved erroneously, that such a constituent would separate itself in the solid form. The evidence of the existence of a new gas (named Argon on account of its chemical inertness), and a statement of many of its properties, were communicated to the Royal Society (see Phil. Trans, clxxxvi. p. 187) by the discoverers in January 1895. The isolation of the new substance by removal of nitrogen from air was effected by two distinct methods. Of these the first is merely a development of that of Cavendish. The gases were contained in a test-tube A (fig. 1) standing over a large quantity of weak alkali B, and the current was conveyed in wires insulated by U-shaped glass tubes CC passing through the liquid and round the mouth of the test-tube. The inner platinum ends DD of the wire may be sealed into the glass insulating tubes, but reliance should not be placed upon these sealings. In order to secure tightness in spite of cracks, mercury was placed in the bends. With a battery of five Grove cells and a Ruhmkorff coil of medium size, a somewhat short spark, or arc, of about 5 mm. was found to be more favourable than a longer one. When the mixed gases were in the right proportion, the rate of absorption was about 3 o c.c. per hour, about: thirty times as fast as Cavendish could work with the electrical machine of his day. Where it is available, an alternating electric current is much superior to a battery and break. This combination, introduced by W. Spottiswoode, allows the absorption in the apparatus of fig. r to be raised to about 80 c.c. per hour, and the method is very convenient for the purification. of small quantities of argon and for determinations of the amount present in various samples of gas, e.g. in the gases expelled from solution in water. A convenient adjunct to this apparatus is a small voltameter, with the aid of which oxygen or hydrogen. can be introduced at pleasure. The gradual elimination of the= nitrogen is tested at a moment's notice with a miniature spectroscope. For this purpose a small Leyden jar is connected as usual. to the secondary terminals, and if necessary the force of the discharge is moderated by the insertion of resistance in the primary circuit. When with a fairly wide slit the yellow line is no longer visible, the residual nitrogen may be considered to have fallen below 2 or 3%. During this stage the oxygen should be in considerable excess. When the yellow line of nitrogen has disappeared, and no further contraction seems to be in progress, the oxygen maybe removed by cautious introduction of hydrogen. The spectrum may now be further examined with a more powerful. instrument. The most conspicuous group in the argon spectrum at atmospheric pressure is that first recorded by A. Schuster (fig. 2). Water vapour and excess of oxygen in moderation do not interfere seriously with its visibility. It is of interest to note that the argon spectrum may be fully developed by operating upon a miniature scale, starting with only 5 c.c. of air (Phil. Mag. vol. i. p. 10 3, 1901) .

The development of Cavendish's method upon a large scale FIG. I.

involves arrangements different from what would at first be expected. The transformer working from a public supply should give about 6000 volts on open circuit, although when the electric flame is established the voltage on the platinums is only from 1600 to 2000. No sufficient advantage is attained by raising the pressure of the gases above atmosphere, but a capacious vessel is necessary. This may consist of a glass sphere of 50 litres' capacity, into the neck of which, presented downwards, the necessary tubes are fitted. The whole of the interior surface is washed with a fountain of alkali, kept in circulation by means of a small centrifugal pump. In this apparatus, and with about one horse-power utilized at the transformer, the absorption of gas is 21 litres per hour ("The Oxidation of Nitrogen Gas," Trans. Chem. Soc., 1897).

In one experiment, specially undertaken for the sake of measurement, the total air employed was 9250 c.c., and the oxygen consumed, manipulated with the aid of partially deaerated water, amounted to 10,820 c.c. The oxygen contained in the air would be 1942 c.c.; so that the quantities of atmospheric nitrogen and of total oxygen which enter into combination would be 7308 c.c. and 12,762 c.c. respectively. This corresponds to N+1 7 5 0, the oxygen being decidedly in excess of the proportion required to form nitrous acid. The argon ultimately found was 75 o c.c., or a little more than I % of the atmospheric nitrogen used. A subsequent determination over mercury by A. M. Kellas (Proc. Roy. Soc. lix. p. 66, 1895) gave 1 186 c.c. as the amount of argon present in loo c.c. of mixed atmospheric nitrogen and argon. In the earlier stages of the inquiry, when it was important to meet the doubts which had been expressed as to the presence of the new gas in the atmosphere, blank experiments were executed in which air was replaced by nitrogen from ammonium nitrite. The residual argon, derived doubtless from the water used to manipulate the gases, was but a small 43 44 45 46 47 48 49 5°00 Argon Red Zinc Hydrogen Hy F FIG. 2.

fraction of what would have been obtained from a corresponding quantity of air.

The other method by which nitrogen may be absorbed on a considerable scale is by the aid of magnesium. The metal in the form of thin turnings is charged into hard glass or iron tubes heated to a full red in a combustion furnace. Into this air, previously deprived of oxygen by red-hot copper and thoroughly dried, is led in a continuous stream. At this temperature the nitrogen combines with the magnesium, and thus the argon is concentrated. A still more potent absorption is afforded by calcium prepared in situ by heating a mixture of magnesium dust with thoroughly dehydrated quick-lime. The density of argon, prepared and purified by magnesium, was found by Sir William Ramsay to be 19.941 on the O = 16 scale. The volume actually weighed was 163 c.c. Subsequently large-scale operations with the same apparatus as had been used for the principal gases gave an almost identical result (19.940) for argon prepared with oxygen.

Argon is soluble in water at 12° C. to about 4.0%, that is, it is about 22 times more soluble than nitrogen. We should thus expect to find it in increased proportion in the dissolved gases of rain-water. Experiment has confirmed this anticipation. The weight of a mixture of argon and nitrogen prepared from the dissolved gases showed an excess of 24 mg. over the weight of true nitrogen, the corresponding excess for the atmospheric mixture being only I I mg. Argon is contained in the gases liberated by many thermal springs, but not in special quantity. The gas collected from the King's Spring at Bath gave only a %, i.e. half the atmospheric proportion.

The most remarkable physical property of argon relates to the constant known as the ratio of specific heats. When a gas is warmed one degree, the heat which must be supplied depends upon whether the operation is conducted at a constant volume or at a constant pressure, being greater in the latter case. The ratio of specific heats of the principal gases is I. 4, which, according to the kinetic theory, is an indication that an important fraction of the energy absorbed is devoted to rotation or vibration. If, as for Boscovitch points, the whole energy is translatory, the ratio of specific heats must be I. 67. This is precisely the number found from the velocity of sound in argon as determined by Kundt's method, and it leaves no room for any sensible energy of rotatory or vibrational motion. The same value had previously been found for mercury vapour by Kundt and Warburg, and had been regarded as confirmatory of the monatomic character attributed on chemical grounds to the mercury molecule. It may be added that helium has the same character as argon in respect of specific heats (Ramsay, Proc. Roy. Soc. 1. p. 86, 1895).

The refractivity of argon is 961 of that of air. This low refractivity is noteworthy as strongly antagonistic to the view at one time favoured by eminent chemists that argon was a condensed form of nitrogen represented by N3. The viscosity of argon is I. 21, referred to air, somewhat higher than for oxygen, which stands at the head of the list of the principal gases ("On some Physical Properties of Argon and Helium," Proc. Roy. Soc. vol. lix. p. 198, 1896).

The spectrum shows remarkable peculiarities. According to circumstances, the colour of the light obtained from a Plucker vacuum tube changes "from red to a rich steel blue," to use the words of Crookes, who first described the phenomenon. A third spectrum is distinguished by J. M. Eder and Edward Valenta. The red spectrum is obtained at moderately low 'pressures (5 mm.) by the use of a Ruhmkorff coil without a jar or air-gap. The red lines at 7056 and 6965 (Crookes) are characteristic. The blue spectrum is best seen at a somewhat lower pressure (I mm. to 2 . 5 mm.), and usually requires a Leyden jar to be connected to the secondary terminals. In some conditions very small causes effect a transition from the one spectrum to the other. The course of electrical events attending the operation of a Ruhmkorff coil being extremely complicated, special interest attaches to some experiments conducted by John Trowbridge and T. W. Richards, in which the source of power was a secondary battery of 5000 cells. At a pressure of 1 mm. the red glow of argon was readily obtained with a voltage of 2000, but not with much less. After the discharge was once started, the difference of potentials at the terminals of the tube varied from 630 volts upwards.







ture, Cent.








- 146.0

35 o

- 194.4

- 214.0


- 121.0


- 187.0

- 189.6

Oxygen .

- 118.8


- 182.7


The introduction of a capacity between the terminals of the Geissler tube, for example two plates of metal 1600 sq. cm. in area separated by a glass plate I cm. thick, made no difference in the red glow so long as the connexions were good nd the condenser was quiet. As soon as a spark-gap was introduced, or the condenser began to emit the humming sound peculiar to it, the beautiful blue glow so characteristic of argon immediately appeared. (Phil. Mag. xliii. p. 77, 1897.) The behaviour of argon at low temperatures was investigated by K. S. Olszewski (Phil. Trans., 18 95, p. 2 53). The following results are extracted from the table given by him: - The smallness of the interval between the boiling and freezing points is noteworthy.

From the manner of its preparation it was clear at an early stage that argon would not combine with magnesium or calcium at a red heat, nor under the influence of the electric discharge with oxygen, hydrogen or nitrogen. Numerous other attempts to induce combination also failed. Nor does it appear that any well-defined compound of argon has yet been prepared. It was found, however, by M. P. E. Berthelot that under the influence of the silent electric discharge, a mixture of benzene vapour and argon underwent contraction, with formation of a gummy product from which the argon could be recovered.







(air = I)







I 98






Boiling points

c. 6° 1





at 760 mm.





Critical tern-







68° abs.




Critical pres-










Weight of ic.c.



I 212



of liquid




The facts detailed in the original memoir led to the conclusion that argon was an element or a mixture of elements, but the question between these alternatives was left open. The behaviour on liquefaction, however, seemed to prove that in the latter case either the proportion of the subordinate constituents was small, or else that the various constituents were but little contrasted. An attempt, somewhat later, by Ramsay and J. Norman Collie to separate argon by diffusion into two parts, which should have different densities or refractivities, led to no distinct effect. More recently Ramsay and M. W. Travers have obtained evidence of the existence in the atmosphere of three new gases, besides helium, to which have been assigned the names of neon, krypton and xenon. These gases agree with argon in respect of the ratio of the specific heats and in being non-oxidizable under the electric spark. As originally defined, argon included small proportions of these gases, but it is now preferable to limit the name to the principal constituent and to regard the newer gases as "companions of argon." The physical constants associated with the name will scarcely be changed, since the proportion of the "companions" is so small. Sir William Ramsay considers that probably the volume of all of them taken together does not exceed part of that of the argon. The physical properties of these gases are given in the following table (Proc. Roy. Soc. lxvii. p. 331, 1900): - The glow obtained in vacuum tubes is highly characteristic, whether as seen directly or as analysed by the spectroscope.

Argon as a Percentage

Percentage of

Percentage of

of the Nitrogen and




3 0


1 9













Now that liquid air is available in many laboratories, it forms an advantageous starting-point in the preparation of argon. Being less volatile than nitrogen, argon accumulates relatively as liquid air evaporates. That the proportion of oxygen increases at the same time is little or no drawback. The following analyses (Rayleigh, Phil. Mag., June 1903) of the vapour arising from liquid air at various stages of the evaporation will give an idea of the course of events: - (R.)

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Chemical element
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Argon n.

  1. argon (a chemical element with an atomic number of 18).

Simple English

Full table
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
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
Most stable isotopes
iso NA half-life DM DE MeV DP
36Ar 0.336% Ar is stable with 18 neutrons
38Ar 0.063% Ar is stable with 20 neutrons
39Ar {syn.} 269 y β- 0.565 39K
40Ar 99.6% Ar is stable with 22 neutrons
42Ar {syn} 32.9 y β- 0.600 42K
SI units & STP are used except where noted.

Argon is a chemical element. The symbol for argon is Ar, and its atomic number (or proton number) is 18. It is a noble gas and no electrons or protons can be lost or gained from this atom.

Argon atoms are found in air. About 1% of the Earth's atmosphere (the air around us) is argon.

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