Argon Facts (Atomic Number 18 or Ar)

Argon Chemical and Physical Properties

Argon glows violet in an electric field.
Argon glows violet in an electric field. pslawinski,

Argon is a noble gas with element symbol Ar and atomic number 18. It is best known for its use as an inert gas and for making plasma globes.

Fast Facts: Argon

  • Element Name: Argon
  • Element Symbol: Ar
  • Atomic Number: 18
  • Atomic Weight: 39.948
  • Appearance: Colorless inert gas
  • Group: Group 18 (Noble Gas)
  • Period: Period 3
  • Discovery: Lord Rayleigh and William Ramsay (1894)


Argon was discovered by Sir William Ramsay and Lord Rayleigh in 1894 (Scotland). Prior to the discovery, Henry Cavendish (1785) suspected some unreactive gas occurred in air. Ramsay and Rayleigh isolated argon by removing the nitrogen, oxygen, water, and carbon dioxide. They found the remaining gas was 0.5% lighter than nitrogen. The emission spectrum of the gas did not match that of any known element.

Electron Configuration

[Ne] 3s2 3p6

Word Origin

The word argon comes from the Greek word argos, which means inactive or lazy. This refers to the extremely low chemical reactivity of argon.


There are 22 known isotopes of argon ranging from Ar-31 to Ar-51 and Ar-53. Natural argon is a mixture of three stable isotopes: Ar-36 (0.34%), Ar-38 (0.06%), Ar-40 (99.6%). Ar-39 (half-life = 269 yrs) is to determine the age of ice cores, ground water and igneous rocks.


Under ordinary conditions, argon is a colorless, odorless, and flavorless gas. The liquid and solid forms are transparent, resembling water or nitrogen. In an electric field, ionized argon produces a characteristic lilac to violet glow.


Argon has a freezing point of -189.2°C, boiling point of -185.7°C, and density of 1.7837 g/l. Argon is considered to be a noble or inert gas and does not form true chemical compounds, although it does form a hydrate with a dissociation pressure of 105 atm at 0°C. Ion molecules of argon have been observed, including (ArKr)+, (ArXe)+, and (NeAr)+. Argon forms a clathrate with b hydroquinone, which is stable yet without true chemical bonds. Argon is two and a half times more soluble in water than nitrogen, with approximately the same solubility as oxygen. Argon's emission spectrum includes a characteristic set of red lines.


Argon is used in electric lights and in fluorescent tubes, photo tubes, glow tubes, and in lasers. Argon is used as an inert gas for welding and cutting, blanketing reactive elements, and as a protective (nonreactive) atmosphere for growing crystals of silicon and germanium.


Argon gas is prepared by fractionating liquid air. The Earth's atmosphere contains 0.94% argon. Mars' atmosphere contains 1.6% Argon-40 and 5 ppm Argon-36.


Because it is inert, argon is considered to be non-toxic. It is a normal component of air that we breathe every day. Argon is used in blue argon laser to repair eye defects and kill tumors. Argon gas may replace nitrogen in underwater breathing mixtures (Argox) to help reduce the incidence of decompression sickness. Although argon is non-toxic, it is considerably more dense than air. In an enclosed space, it may present an asphyxiation risk, particularly near ground level.

Element Classification

Inert Gas

Density (g/cc)

1.40 (@ -186 °C)

Melting Point (K)


Boiling Point (K)



Colorless, tasteless, odorless noble gas

Atomic Radius (pm): 2-

Atomic Volume (cc/mol): 24.2

Covalent Radius (pm): 98

Specific Heat (@20°C J/g mol): 0.138

Evaporation Heat (kJ/mol): 6.52

Debye Temperature (K): 85.00

Pauling Negativity Number: 0.0

First Ionizing Energy (kJ/mol): 1519.6

Lattice Structure: Face-Centered Cubic

Lattice Constant (Å): 5.260

CAS Registry Number: 7440–37–1

Argon Trivia

  • The first noble gas to be discovered was argon.
  • Argon glows violet in a gas discharge tube. It is the gas found in plasma balls.
  • William Ramsay, in addition to argon, discovered all the noble gases except radon. This earned him the 1904 Noble Prize in Chemistry.
  • The original atomic symbol for argon was A. In 1957, the IUPAC changed the symbol to the current Ar.
  • Argon is the 3rd most common gas in Earth's atmosphere.
  • Argon is produced commercially by fractional distillation of air.
  • Substances are stored in argon gas to prevent interactions with the atmosphere.


  • Brown, T. L.; Bursten, B. E.; LeMay, H. E. (2006). J. Challice; N. Folchetti, eds. Chemistry: The Central Science (10th ed.). Pearson Education. pp. 276 & 289. ISBN 978-0-13-109686-8.
  • Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). Boca Raton, FL: CRC Press. p. 4.121. ISBN 1439855110.
  • Shuen-Chen Hwang, Robert D. Lein, Daniel A. Morgan (2005). "Noble Gases". Kirk Othmer Encyclopedia of Chemical Technology. Wiley. pp. 343–383.
  • Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 0-8493-0464-4.
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Helmenstine, Anne Marie, Ph.D. "Argon Facts (Atomic Number 18 or Ar)." ThoughtCo, Aug. 26, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 26). Argon Facts (Atomic Number 18 or Ar). Retrieved from Helmenstine, Anne Marie, Ph.D. "Argon Facts (Atomic Number 18 or Ar)." ThoughtCo. (accessed March 8, 2021).