Argon

Argon is a chemical element. Its symbol is argon is Ar and its atomic number is 18. It is part of the noble gas group. It is an odorless and tasteless gas. Argon is the third-most abundant gas in the Earth's atmosphere.

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]
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	Ar: 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	Ar: 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%.

The name "argon" is from the Greek word ἀργός meaning "lazy" or "inactive" because it does not react with other chemicals.

Chemistry

Argon atoms are found in air. About 1% of the Earth's atmosphere (the air around us) is argon. It is not toxic and it does not burn easily. Not many compounds can be made from Argon. However, some compounds have been formed, like argon fluorohydride (HArF). Argon is a heavier gas than helium, so if a balloon is filled with argon, it would fall to the ground.

History

Argon was first removed from air in 1894 by Lord Rayleigh and Sir William Ramsay at University College London by removing oxygen, carbon dioxide, water, and nitrogen from a sample of clean air.

Isotopes

The main isotopes of argon found on Earth are ⁴⁰Ar (99.6%), ³⁶Ar (0.34%), and ³⁸Ar (0.06%). The most abundant isotope of argon is ⁴⁰Ar. But other planets have different isotopes of argon.

Production

Argon is gotten industrially by the fractional distillation of liquid air. About 700,000 tonnes of argon are produced worldwide every year.

⁴⁰Ar is made by the decay of ⁴⁰K with a half-life of 1.25 billion years by electron capture or positron emission. Because of this, it is used in knowing how old rocks are.

Uses

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. It can also be used in neon lights for a purple color. Argon is used in the poultry industry to asphyxiate birds. Argon is sometimes used for extinguishing (ending) fires.

Liquid argon is used in neutrino experiments and looking for dark matter. Argon is used to preserve food. Argon is sometimes used as the propellant in aerosol cans. Argon is also used as a preservative for such products as varnish, polyurethane, and paint. Argon has been used by athletes as a doping agent and has been banned as use. It is used to know how old rocks are.

Argon Media

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  A small piece of rapidly melting solid argon

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  A: test-tube, B: dilute alkali, C: U-shaped glass tube, D: platinum electrode

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  Captioned "Argon", caricature of Lord Rayleigh in Vanity Fair, 1899

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  Space-filling model of argon fluorohydride

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

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

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

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  Argon gas-discharge lamp forming "Ar", the symbol for argon

Related pages

• List of common elements
• Periodic table