Sodium is an alkali metal element denoted by the atomic symbol Na. It has an atomic number of 11 and the average atomic weight of its isotopes is 22.98977. Sodium is a silvery-white metal that is so soft it can be cut with an ordinary knife. The element is so highly reactive that it explodes when it comes into contact with water.

Sodium is the sixth most abundant element in the earth's crust, which contains about 2.8 percent sodium, and the second most abundant element dissolved in seawater. It never occurs freely in nature but is widely found in deposits of sodium compounds such as soda, borax (sodium borate), and saltpeter (sodium nitrate). Many huge deposits of salt were left behind when ancient seas evaporated.

Sodium's discovery depended on the invention of the electrical battery around 1800 by Italian physicist Alessandro Volta. Before then, many substances were suspected to contain more than...