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Element Fact
Selenium is an essential trace element in the diets that acts as antioxidants.

Fluorine, F

fluorine sample

Atomic number: 9
Atomic mass: 18.9984032(5)
Natural abundance: 554 ppm
19F (18.9984032) - 100.0%

History and Production

From Latin fluere, flowing which is related to the early use of fluorspar (CaF2). Due to the extreme reactivity nature of the element, many attempts had been made to isolate the gas, some ended in tragedy. In 1886, H. Moissan finally isolated fluorine by electrolysis of potassium hydrogen fluoride/hydrogen fluoride, after over 70 years of unsuccessful attempts by others. He was later awarded the Nobel Prize for chemistry in 1906. Nowadays, it is produced using methods based on Moissan's original procedure. The generator is made of a mild-steel pot (acting as cathode) while the anode consists of a central compacted, ungraphitized carbon rod. Fluorine and its compounds are used in the production of uranium, high-temperature plastics, dielectrics. Hydrofluoric acid is used for glass etching. In the form of cryolite, Na3AlF6, it is used in the production of aluminium.

Physical Data

fluorspar, a fluorite mineral Fluorine is the most electronegative and reactive of all elements, hence the difficulty of isolation in the early years. It is a pale yellow, very corrosive gas which react practically with all organic and inorganic substances, including metals, glass (as shown above), ceramics, water etc., often with violent effects. Fluorine has a pungent smell, even at a concentration of 20 ppb (parts per billion), which is below the safe working level. Fluorine occurs in nature mainly in the form of fluorite, CaF2, such as fluorspar (left) and fluorapatite, Ca5(PO4)3F. Cryolite is a rare mineral and is only commercially available in Greenland. Cryolite that is used in the production of aluminium are in fact largely synthetic.

Interatomic distance: 141.8 pm
Melting point: -219.62C
Boiling point: -188.12C
Thermal conductivity/Wm-1K-1: 0.0279 (27C)
Density/kgm-3: 1516 (b.p.), 1.696 (0C)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 79.4 kJ/mol
Gibbs free energy of formation: 126.6 kJ/mol
Entropy: 62.3 J/mol K
Heat capacity: 22.7 J/mol K

Electronic data

Electronic configuration: [He] 2s2 2p5
Term symbol: 2P3/2
Electron affinity: 328.1651 kJ/mol   Electronegativity (Pauline): 3.98
Ionization energy (first, second, third): 1681.05, 3374.17, 6050.45 kJ/mol

Chemical properties

Fluorine is the most reactive of all elements. Except helium, argon and neon, it forms compound with all other elements, often with violent effects. If heated, it can react with even gold, platinum and xenon. Fluorine reacts slowly with graphite and with diamond only above 700C. However, some metals such as copper and nickel form a protective layer of fluorides that prevent further reaction. For this reason, vessels of these materials are used for handling the gas.

The reactivity is due to its small size and that the dissociation energy of F-F bond is much smaller than the bonds that fluorine forms with other elements.

Test for fluorine (as fluoride ion)
(1) Concentrated sulfuric acid liberates hydrogen fluoride which attacks glass, forming silicon tetrafluoride. The latter is hydrolysed to gel-like silicic acid by water and appears turbid.
(2) Calcium nitrate solution gives a white precipitate of calcium fluoride.
(3) Addition of sodium chloride and lead nitrate will give lead chlorofluoride precipitate, PbClF. This can be filtered off and weighed to estimate the fluoride.

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