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Liquid fluorine

Fluorine is an extremely reactive chemical element with atomic number 9 (pictured in liquid form at cryogenic temperatures). A highly toxic pale yellow gas at standard conditions, it was first described in 1529 as its principal source fluorite, a mineral added as a flux for smelting, and named after the Latin verb fluo meaning "flow". As the lightest halogen and most electronegative element, it is difficult to separate from its compounds, and several early experimenters died or were injured. The process employed for its modern production low-temperature electrolysis remains the same as that used by Henri Moissan in 1886 to achieve its first isolation. The high costs of refining fluorine gas lead most commercial uses, such as aluminium refining, insulation and refrigeration, to use its compounds; uranium enrichment is the free element's largest application. Fluorine is a part of some pharmaceuticals and appears as the fluoride ion in toothpaste, but has no known metabolic role in mammals; a few plants possess fluorine-containing poisons to deter herbivores. Fluorocarbon gases are usually potent greenhouse gases and organofluorine compounds persist in the environment. (Full article...)

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The radian is the standard unit of angular measure, used in many areas of mathematics. An angle's measurement in radians is numerically equal to the length of a corresponding arc of a unit circle, so one radian is just under 57.3 degrees (when the arc length is equal to the radius); a full circle corresponds to an angle of 2 radians. In the SI, the radian has the symbol "rad"; it was a supplementary unit until that category was abolished in 1995, and is now considered a derived unit.

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