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Natrium, commonly known as sodium, is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white alkali metal in Group 1 of the periodic table. Sodium is highly reactive, quickly tarnishing in air and reacting vigorously with water to form sodium hydroxide and hydrogen gas.

Occurrence and production: In nature sodium is never found free in rocks; it occurs principally in compounds such as halite and in seawater. Industrial production relies on electrolysis of molten sodium chloride (the Downs cell) to yield elemental sodium and chlorine gas; alternative chlor-alkali processes produce chlorine and sodium hydroxide from brine, but not metallic sodium.

Properties: Sodium has a melting point of 97.72 C and a boiling point of 883 C. It is lightly dense with a density of about 0.97 g/cm3 at room temperature and is extremely reactive, soft enough to be cut with a knife.

Applications: It is used to produce chlorine and sodium hydroxide, as a reducing agent in organic synthesis, in sodium-vapor lamps and for heat transfer in some industrial processes. In biology, sodium ions regulate fluid balance and nerve function and are essential in diets (in appropriate amounts). Sodium compounds also contribute to glassmaking and certain chemical syntheses.

Safety: Sodium metal is stored under oil or inert atmosphere to prevent contact with air and moisture. It readily reacts with water and air, with fire hazards for finely divided metal.

Etymology and history: The name sodium derives from the Latin natrium; the symbol Na reflects this. Sodium was first isolated in 1807 by Humphry Davy through electrolysis of molten salts.