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Chemical element, symbol Ar and atomic number 18 / From Wikipedia, the free encyclopedia

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Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas.[8] Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abundant as water vapor (which averages about 4000 ppmv, but varies greatly), 23 times as abundant as carbon dioxide (400 ppmv), and more than 500 times as abundant as neon (18 ppmv). Argon is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust.

Quick facts: Argon, Pronunciation, Appearance, Standard at...
Argon, 18Ar
Vial containing a violet glowing gas
Pronunciation/ˈɑːrɡɒn/ (AR-gon)
Appearancecolorless gas exhibiting a lilac/violet glow when placed in an electric field
Standard atomic weight Ar°(Ar)
Argon in the periodic table


Atomic number (Z)18
Groupgroup 18 (noble gases)
Periodperiod 3
Block  p-block
Electron configuration[Ne] 3s2 3p6
Electrons per shell2, 8, 8
Physical properties
Phase at STPgas
Melting point83.81 K (−189.34 °C, −308.81 °F)
Boiling point87.302 K (−185.848 °C, −302.526 °F)
Density (at STP)1.784 g/L
when liquid (at b.p.)1.3954 g/cm3
Triple point83.8058 K, 68.89 kPa[3]
Critical point150.687 K, 4.863 MPa[3]
Heat of fusion1.18 kJ/mol
Heat of vaporization6.53 kJ/mol
Molar heat capacity20.85[4] J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K)   47 53 61 71 87
Atomic properties
Oxidation states0
ElectronegativityPauling scale: no data
Ionization energies
  • 1st: 1520.6 kJ/mol
  • 2nd: 2665.8 kJ/mol
  • 3rd: 3931 kJ/mol
  • (more)
Covalent radius106±10 pm
Van der Waals radius188 pm
Color lines in a spectral range
Spectral lines of argon
Other properties
Natural occurrenceprimordial
Crystal structure face-centered cubic (fcc)
Face-centered cubic crystal structure for argon
Thermal conductivity17.72×10−3  W/(m⋅K)
Magnetic orderingdiamagnetic[5]
Molar magnetic susceptibility−19.6×10−6 cm3/mol[6]
Speed of sound323 m/s (gas, at 27 °C)
CAS Number7440-37-1
Discovery and first isolationLord Rayleigh and William Ramsay (1894)
Isotopes of argon
Main isotopes[7] Decay
abun­dance half-life (t1/2) mode pro­duct
36Ar 0.334% stable
37Ar trace 35 d ε 37Cl
38Ar 0.0630% stable
39Ar trace 268 y β 39K
40Ar 99.6% stable
41Ar trace 109.34 min β 41K
42Ar synth 32.9 y β 42K
Symbol_category_class.svg Category: Argon
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Nearly all argon in Earth's atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in Earth's crust. In the universe, argon-36 is by far the most common argon isotope, as it is the most easily produced by stellar nucleosynthesis in supernovas.

The name "argon" is derived from the Greek word ἀργόν, neuter singular form of ἀργός meaning 'lazy' or 'inactive', as a reference to the fact that the element undergoes almost no chemical reactions. The complete octet (eight electrons) in the outer atomic shell makes argon 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.

Argon is extracted industrially by the fractional distillation of liquid air. It is mostly used as an inert shielding gas in welding and other high-temperature industrial processes where ordinarily unreactive substances become reactive; for example, an argon atmosphere is used in graphite electric furnaces to prevent the graphite from burning. It is also used in incandescent, fluorescent lighting, and other gas-discharge tubes. It makes a distinctive blue-green gas laser. It is also used in fluorescent glow starters.

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