Sodium (Na) is the second element in the alkali metal group, with an atomic number of 11 and an atomic weight of 22.9898 amu. Its melting point 208°F (97.8°C) and boiling point 1,621.4°F (883°C) are both lower than those of lithium, a trend that continues in the alkali metal group; as the atomic mass and size increase, the melting and boiling points decrease. Humphry Davy first isolated sodium metal by passing electricity through molten sodium hydroxide in 1807. It occurs naturally, in compound form, in relatively large amounts—about 20,000 parts per million in the earth's crust, plus a large concentration in seawater. Sodium chloride (or common salt) is one of the most common compounds on Earth, followed closely by sodium carbonate (also called soda ash or washing soda). Both of these are obtained now largely by mining.
Sodium compounds of various kinds are vital to industry. Sodium nitrite is a principle ingredient in gunpowder. The pulp and paper industry uses large amounts of sodium hydroxide, sodium carbonate, and sodium sulfate; the latter helps dissolve the lignin from wood pulp in the Kraft process so it can be made into cardboard and brown paper. In addition to paper pulping, sodium carbonate is used by power companies to absorb sulfur dioxide, a serious pollutant, from smokestack gases. Sodium carbonate is also important to the glass and detergent industries. Sodium hydroxide is one of the top 10 industrially produced chemicals, heavily used in manufacturing. Sodium chloride is used in foods, in water softeners, and as a deicer for roads and sidewalks. Sodium bicarbonate (baking soda) is produced for the food industry as well.
Many useful chemicals and processes, particularly those of the "chlor-alkali" industry, can trace their production back to sodium chloride. Passing electricity through a concentrated salt water solution (the electrolysis of brine) produces sodium hydroxide and chlorine gas. Electrolysis of molten sodium chloride, on the other hand, yields elemental sodium and chlorine gas. Sodium sulfate is prepared in large quantities by reacting sulfuric acid with sodium chloride.
Biochemically, sodium is a vital nutrient, although excesses of it can aggravate high blood pressure. Sodium compounds regulate nerve transmission, alter membrane permeability, and perform myriad other tasks for living organisms.