sodium, a metallic chemical element; symbol Na [Lat. natrium ]; at. no. 11; at. wt. 22.98977; m.p. 97.81°C; b.p. 892.9°C; sp. gr. 0.971 at 20°C; valence +1. Sodium is a soft, silver-white metal. Extremely reactive chemically, it is one of the alkali metals in Group 1 of the periodic table. Like potassium, which it closely resembles, it oxidizes rapidly in air; it also reacts violently with water, liberating hydrogen (which may ignite) and forming the hydroxide. It must be stored out of contact with air and water and should be handled carefully. Sodium combines directly with the halogens. The metal is usually prepared by electrolysis of the fused chloride (the Downs process); formerly, the chief method of preparation was by electrolysis of the fused hydroxide (the Castner process). Metallic sodium has limited use. It is used in sodium arc lamps for street lighting; pure or alloyed with potassium, it has found use as a heat-transfer liquid, e.g., in certain nuclear reactors. It is used principally in the manufacture of tetraethyl lead (a gasoline antiknock compound) and of sodamide, NaNH2, sodium cyanide, NaCN, sodium peroxide, Na2O2, and sodium hydride, NaH. Sodium compounds are extensively used in industry and for many nonindustrial purposes. Among the most important compounds are chloride (common salt, NaCl), bicarbonate (baking soda, NaHCO3), carbonate (soda ash, or washing soda, Na2CO3), hydroxide (caustic soda, or lye, NaOH), nitrate (Chile saltpeter, NaNO3), thiosulfate (hypo, Na2S2O3·5H2O), phosphates, and borax (Na2B4O7·10H2O). Sodium hydroxide is used wherever a cheap alkali is needed, for example, in making soap. Substances containing sodium impart a characteristic yellow color to a flame. Because of its activity sodium is not found uncombined in nature. It occurs abundantly and widely distributed in its compounds, which are present in rocks and soil, in the oceans, in salt lakes, in mineral waters, and in deposits in various parts of the world. Sodium compounds are found in the tissues of plants and animals. Sodium is an essential element in the diet, but some people must limit the amount of sodium in their food for medical reasons. Discovery of sodium is usually credited to Sir Humphry Davy, who prepared the metal from its hydroxide in 1807; its compounds have been known since antiquity.