osmium (Os) /oz″mē·əm/ [Gk, osme, odor] , a hard, grayish, pungent-smelling metallic element. Its atomic number is 76; its atomic mass is 190.2. Used to produce alloys of extreme hardness, it is highly toxic and is slowly converted to the highly volatile, pungent-smelling, extremely toxic osmium tetroxide (OsO4) in air.
