A radioactive metallic element, occurring naturally in small quantities as a product of radium disintegration and produced synthetically by bombarding bismuth or lead with neutrons. Most isotopes decay by alpha-particle emission; the most stable are Po-208 and Po-209, with half-lives of 2.9 years and 102 years, respectively. Po-210, with a half-life of 138.4 days, is the most readily available isotope and is extremely toxic. Atomic number 84; melting point 254°C; boiling point 962°C; specific gravity 9.20; valence 2, 4, 6. See Periodic Table.