Barium
Barium is too reactive as a metal, so it is not found in the earth as a metal. It is found in chemical compounds. Barium only occurs in one oxidation state: +2. Most barium compounds are colorless. The ones that dissolve in water or stomach acid are very toxic. Barium sulfate is well known because it does not dissolve in water or acids. Barium compounds are quite heavy. Barium compounds put out a greenish flame when heated red-hot.