Barium (Greek barys, “heavy”), symbol Ba, soft, silvery, highly reactive metallic element. Barium is in group 2 (or IIa) of the periodic table (see Periodic Law) and is one of the alkaline earth metals. The atomic number of barium is 56. Barium was first recognized as an element in 1808 by the English scientist Sir Humphry Davy.

The element reacts vigorously with water; it is rapidly corroded by moist air. In fact, the element is so reactive that it occurs in nature only as a compound. Its most important compounds are the minerals barium sulfate and barium carbonate (witherite), BaCO₃.

Barium makes up about 1/2000th of Earth’s crust. The atomic weight of barium is 137.33. The element melts at 727°C (1341°F), boils at 1897°C (3447°F), and has a specific gravity of 3.5.

Barium metal has few practical applications, although it has sometimes been used in coating electrical conductors in electronic apparatus and in automobile ignition systems. Barium sulfate (BaSO₄) has been used as a filler for rubber products, in paint, and in linoleum. Barium nitrate has been used in fireworks, and barium carbonate in rat poisons. A form of barium sulfate, which is opaque to X rays, has been used for the X-ray examination of the gastrointestinal tract.