Barium

Barium is a metallic chemical element (atomic number 56).  In it's pure state, it is a silver colored metal.  However, because it is highly reactive, it is never found in this state in nature.  
Barium sulphate is used as a contrast material in studies of the digestive tract (such as a barium enema)
However, any barium compound that is water-soluble is toxic.  The toxicity of the metal itself is not well understood due to its highly reactive nature.  Inhaled barium compounds can lead to baritosis if they accumulate in the lungs.  Certain barium ions can also block potassium ion channels in the nervous system.
Barium at Wikipedia
Category:Chemistry