Polonium is a chemical element with symbol Po and atomic number 84, discovered in 1898 by Marie and Pierre Curie. A rare and highly radioactive element with no stable isotopes, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Applications of polonium are sparse and include: heaters in space probes; antistatic devices; or as sources of neutrons and alpha particles. Because of positioning in the periodic table, polonium is sometimes classified as a metalloid. Other sources say that on the basis of properties and behavior, it is "unambiguously a metal".