po·lo·ni·um (p?-l??n?-?m)n. Symbol Po 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.[From Medieval Latin Pol?nia, Poland (the native country of Marie Curie, who discovered this element along with her husband Pierre).]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.polonium (p??l??n??m) n (Elements & Compounds) a very rare radioactive element that occurs in trace amounts in uranium ores. The isotope polonium-210 is produced artificially and is used as a lightweight power source in satellites and to eliminate static electricity in certain industries. Symbol: Po; atomic no: 84; half-life of most stable isotope, 209Po: 103 years; valency: ?2, 0, 2, 4, or 6; relative density (alpha modification): 9.32; melting pt: 254°C; boiling pt: 962°C[C19: New Latin, from Medieval Latin Pol?nia Poland; named in honour of the Polish nationality of its discoverer, Marie Curie]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014po?lo?ni?um (p??lo? ni ?m) n. a radioactive chemical element discovered by Pierre and Marie Curie in 1898. Symbol: Po; at. no.: 84; at. wt.: about 210. [