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Science / Periodic Table of Elements / Argon (Ar): Atomic number: 18, Atomic mass: 39.948 g.mol -1, Electronegativity: unknown, Density: 1.78.10 -3 g.cm -3 at 0 °C, Melting point: -189 °C, Boiling point: -185.7 °C, Vanderwaals radius: 0.192 nm, Ionic radius: unknown, Isotopes: 6, Electronis shell: [Ne] 3s23p6, Energy of first ionisation: 1520 kJ.mol -1, Energy of second ionisation: 2665.8 kJ.mol -1, Energy of third ionisation: 3931 kJ.mol -1, Discovered by: Sir Ramsay in 1894. Argon was suspected to be present in air by Henry Cavendish in 1785 but wasn't discovered until 1894 by Lord Rayleigh and Sir William Ramsay. Argon is the third noble gas, in period 8, and it makes up about 1% of the Earth's atmosphere. Argon has approximately the same solubility as oxygen and it is 2.5 times as soluble in water as nitrogen . This chemically inert element is colorless and odorless in both its liquid and gaseous forms. It is not found in any compounds. This gas is isolated through liquid air fractionation since the atmosphere contains only 0.94% argon. The Martian atmosphere in contrast contains 1.6% of Ar-40 and 5 ppm Ar-36. World production exceeds 750.000 tonnes per year, the supply is virtually inexhaustible. Applications: Argon does not react with the filament in a lightbulb even under high temperatures, so is used in lighting and in other cases where diatomic nitrogen is an unsuitable (semi-)inert gas. Argon is perticularly important for the metal industry, being used as an inert gas shield in arc welding and cutting. Other uses incude non-reactive blanket in the manufacture of titanium and other reactive elements and as a protective atmosphere for growing silicon and germanium crystals. Argon-39 has been used for a number of applications, primarily ice coring. It has also been used for ground water dating. Argon is also used in technical SCUBA diving to inflate the drysuit, due to its nonreactive, heat isolating effect. Argon as the gap between the panes of glass provides better insulation because it is a poorer conductor of heat than ordinary air. The most exotic use of argon is in the tyre of luxury cars. Argon in the environment: In earth's atmosphere, Ar-39 is made by cosmic ray activity, primarily with Ar-40. In the subsurface environment, it is also produced through neutron-capture by K-39 or alpha emission by calcium. Argon-37 is produced from the decay of calcium-40, the result of subsurface nuclear explosions. It has a half-life of 35 days.
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