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F

Ne

--

Cl

Ar

--

Br

Kr

--

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Element Fact
Fluorine is the most reactive element, which reacts practically with all organic and inorganic substances, including metals, glass and ceramics, often with violent effects.

Argon, Ar

argon sample

Atomic number: 18
Atomic mass: 39.948(1)
Natural abundance: -
Isotopes:
36Ar (35.9675463) - 0.337%
38Ar (37.9627322) - 0.063%
40Ar (39.962383123 - 99.6%

History and Production

From Greek word argos, meaning inactive. Its existence in air was suspected by Cavendish in 1785. However, it was not until 1894 that Lord Rayleigh and Sir W. Ramsey recognized it as an element. The gas is isolated via fractionation and distillation of air. Argon is mainly used as an inert-atmosphere in high-temperature metallurgical processes such as arc-welding, cutting, and in the production of some reactive elements such as titanium. It is also used in providing an inert environment for growing silicon and germanium crystals. It is used in various lighting equipment such as light bulbs, fluorescent tubes etc.

Physical Data

It exists as the monoatomic entity in the atmosphere (0.94% by volume). Both liquid and gas are colorless and odorless. It is very inert and is not known to produce any real compound.

Interatomic distance: 360.0 pm (van der Waal diameter)
Melting point: -189.35C
Boiling point: -185.35C
Thermal conductivity/Wm-1K-1: 0.0177 (27C), 0.0165 (0C)
Density/kgm-3: 1380 (b.p.), 1.784 (0C)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 0 kJ/mol
Gibbs free energy of formation: -
Entropy: 154.8 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Ne] 3s2 3p6 = [Ar]
Term symbol: 1S0
Electron affinity: (not stable)   Electronegativity (Pauline): -
Ionization energy (first, second, third): 1520.57, 2665.86, 3930.82 kJ/mol

Chemical property

Argon's outer-most electron shell has already achieved the most stable configuration and hence the atom is particularly resistant to chemical reaction. There are no known stable compound of argon. However, along with other heavier noble gasses, water molecules can be arranged to form cavity to encapsulate argon under high pressure. These species are known as clathrates. Argon does not form chemical bonds, rather it is weakly held via van der Waals forces with the water molecules.



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