Argon

Argon,  ₁₈Ar

General properties
Pronunciation	/ˈɑːrɡɒn/ ​(AR-gon)
Appearance	colorless gas exhibiting a lilac/violet glow when placed in an electric field
Standard atomic weight (Ar, standard)	[39.792, 39.963] conventional: 39.948
Argon in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Ne ↑ Ar ↓ Kr chlorine ← argon → potassium
Atomic number (Z)	18
Group	group 18 (noble gases)
Period	period 3
Block	p-block
Element category	noble gas
Electron configuration	[Ne] 3s2 3p6
Electrons per shell	2, 8, 8
Physical properties
Phase at STP	gas
Melting point	83.81 K ​(−189.34 °C, ​−308.81 °F)
Boiling point	87.302 K ​(−185.848 °C, ​−302.526 °F)
Density (at STP)	1.784 g/L
when liquid (at b.p.)	1.3954 g/cm3
Triple point	83.8058 K, ​68.89 kPa
Critical point	150.687 K, 4.863 MPa
Heat of fusion	1.18 kJ/mol
Heat of vaporization	6.53 kJ/mol
Molar heat capacity	20.85 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 states	0
Electronegativity	Pauling scale: no data
Ionization energies	1st: 1520.6 kJ/mol 2nd: 2665.8 kJ/mol 3rd: 3931 kJ/mol (more)
Covalent radius	106±10 pm
Van der Waals radius	188 pm
Spectral lines of argon
Other properties
Natural occurrence	primordial
Crystal structure	​face-centered cubic (fcc)
Speed of sound	323 m/s (gas, at 27 °C)
Thermal conductivity	17.72×10−3  W/(m·K)
Magnetic ordering	diamagnetic
Magnetic susceptibility	−19.6·10−6 cm3/mol
CAS Number	7440-37-1
History
Discovery and first isolation	Lord Rayleigh and William Ramsay (1894)
Main isotopes of argon
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct 36Ar 0.334% stable 37Ar syn 35 d ε 37Cl 38Ar 0.063% stable 39Ar trace 269 y β− 39K 40Ar 99.604% stable 41Ar syn 109.34 min β− 41K 42Ar syn 32.9 y β− 42K
36 Ar and 38 Ar content may be as high as 2.07% and 4.3% respectively in natural samples. 40 Ar is the remainder in such cases, whose content may be as low as 93.6%.

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.

Argon is often used when welding steel and similar work, to push away the air around the weld, so the oxygen in the air can't join with the metal being welded.

Related pages

• List of common elements
• Periodic table