Ba
This element's pure form, which is not found in nature, is used to remove trace gases from vacuum tubes.  Combined with sulfate it is used as a filter for rubber, plastics and resins and since it does not dissolve in water can be ingested to produce X-rays of digestive tract.  It is present in witherite and its salts give a characteristic green color in the flame test.  FTP, name this alkaline earth metal with atomic number 56 and chemical symbol Ba [B-A].
This element's pure form, which is not found in nature, is used to remove trace gases from vacuum tubes.  
Barium