Argon Facts

Argon is the 18th element of the periodic table. These argon facts contain chemical and physical data along with general information and history.

Element cell for Argon

Argon Periodic Table Cell

Basic Argon Facts

Name: Argon

Atomic Number: 18

Element Symbol: Ar

Group: 18

Period: 3

Block: p

Element Family: Noble Gas

Atomic Mass: 39.948(1)

Electron Configuration: [Ne]3s23p6 (shorthand) or 1s22s22p63s23p(full)

Discovery: Lord Rayleigh and Sir William Ramsay in 1894

Rayleigh noticed a difference in density between nitrogen obtained from the air and nitrogen obtained through chemical reactions. He also noticed oxygen was the same density, no matter how you obtained it. Ramsay heard of this problem and began to collaborate with Rayleigh. Ramsay designed a method to remove oxygen, carbon dioxide and nitrogen from a volume of air. Once finished, he found there was still a small amount of gas remaining. The leftover gas did not react with any other chemical. His spectral analysis showed the gas was an unknown element.

Name Origin: Argon gas was completely unreactive with other chemicals, almost like argon was too lazy to react. Ramsay and Rayleigh named argon from the Greek word argos meaning lazy or inactive.


Natural argon is comprised of three stable isotopes: 36Ar, 38Ar and 40Ar. Twenty one radioactive isotopes exist ranging from 30Ar to 53Ar.

Argon-36 is a stable isotope containing 18 neutrons. 0.3336% of natural argon is argon-35.

Argon-37 is a stable isotope containing 20 neutrons. .0629% of natural argon is argon-38.

Argon-40 is a stable isotope containing 22 neutrons. 99.6035% of natural argon is argon-40.

Argon-39 is a radioactive isotope containing 21 neutrons. It is formed when cosmic radiation interacts with atmospheric argon-40. Argon-39 decays by β- decay into 39K with a half-life of 269 years and can be found naturally in trace quantities.

Argon Ice

Small sample of melting solid argon. Credit: Deglr6328/Creative Commons

Physical Data

Density:  0.001633 g/cm3

Melting Point: 83.81 K ​(−189.34 °C, ​−308.81 °F)

Boiling Point: 87.302 K ​(−185.848 °C, ​−302.526 °F)

Triple Point: 83.8058 K at ​68.89 kPa

Critical Point: 150.687 K at 4.863 MPa

State at 20ºC: Gas

Heat of Fusion: 1.18 kJ/mol

Heat of Vaporization: 6.53 kJ/mol

Molar Heat Capacity: 20.85 J/mol·K

Chlorine Atom

Electron shell configuration of a chlorine atom.

Atomic Data

Atomic Radius: 1.88 Å

Covalent Radius: 1.06 Å

Van der Waals Radius:  1.88 Å

Electron Affinity: not stable

Electronegativity: unknown

1st Ionization Energy: 1520.571 kJ/mol

2nd Ionization Energy: 2665.857 kJ/mol

3rd Ionization Energy: 3930.81 kJ/mol

4th Ionization Energy: 5770.79 kJ/mol

5th Ionization Energy: 7238.33 kJ/mol

6th Ionization Energy: 8781.034 kJ/mol

7th Ionization Energy: 11995.347 kJ/mol

8th Ionization Energy: 13841.79 kJ/mol

Oxidation States: +7, +5, +1, -1 (common), +6, +6, +2 (uncommon)

Argon Discharge Tube

Argon in a discharge tube. Argon emits a violet light when ionized. Credit: Alchemisthp/Creative Commons

Fun Argon Facts

  • Argon is a colorless, odorless gas at room temperature. When ionized, argon emits a distinctive violet glow.
  • Argon is produced industrially by cryogenically distilling air.
  • Argon only accounts for 0.94% by volume of the gasses in the atmosphere. Yet, it is the third most plentiful gas in air.
  • Argon is the go-to gas when an inert environment is needed.
  • Argon is used in fire-suppression systems. The argon displaces the oxygen in a room and combustion stops.
  • Argon is added to incandescent lights to protect the filament from oxygen. It is also widely used in fluorescent bulbs.
  • Double paned windows use argon between the panes to act as an insulator.
  • Argon has no known biological role.
  • Argon-39 is used much like carbon-14 to date water and ice samples.
  • Prior to 1957, the element symbol for argon was A. The IUPAC changed it to the Ar we know today.

Learn more about elements on the periodic table.

Argon Facts
Last modified: May 31st, 2015 by Todd Helmenstine