Polonium (pronounced /pɵˈloʊniəm/, po-LOH-nee-əm) is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie and Pierre Curie. A rare and highly radioactive metalloid,1 polonium is chemically similar to bismuth2 and tellurium, and it occurs in uranium ores. Polonium has been studied for possible use in heating spacecraft. It is unstable; all isotopes of polonium are radioactive. Polonium is very volatile; it will almost completely vaporize at room temperatures.
Polonium 210 (also called Radium F) is an alpha emitter - most recently used to kill Alexander Valterovich Litvinenko, he died November 23, 2006.
Polonium - wikipedia
Los Alamos National Labs