Barium

Barium is a chemical element with symbol Ba and atomic number 56. Because of its high chemical reactivity barium is never found in nature as a free element. The most common naturally occurring minerals of barium are barite (barium sulfate, BaSO4) and witherite (barium carbonate, BaCO3), both being insoluble in water.
Barium is a soft, silvery-white metal, with a slight golden shade when ultrapure. The silvery-white color of barium metal rapidly vanishes upon oxidation in air yielding a dark gray oxide layer. Barium has a medium specific weight and good electrical conductivity.
Because of the high reactivity of the metal, toxicological data are available only for compounds.[25] Water-soluble barium compounds are poisonous. At low doses, barium ions act as a muscle stimulant, whereas higher doses affect the nervous system, causing cardiac irregularities, tremors, weakness, anxiety, dyspnea and paralysis.
Barium is not carcinogenic, and it does not bioaccumulate. However, inhaled dust containing insoluble barium compounds can accumulate in the lungs, causing a benign condition called baritosis.