A metallic element having two allotropic forms: a hard, extremely brittle, lustrous, bluish-white, crystalline material and a gray amorphous form. It is used in a wide variety of alloys, especially with lead in battery plates, and in the manufacture of flame-proofing compounds, paint, semiconductor devices, and ceramic products. Atomic number 51; atomic weight 121.76; melting point 630.63°C; boiling point 1,587°C; specific gravity 6.68; valence 3, 5. See Periodic Table.