1. A naturally radioactive metallic element, occurring in minute quantities as a product of radium disintegration and produced by bombarding bismuth or lead with neutrons. it has 27 isotopes ranging in mass number from 192 to 218, of which po 210, with a half-life of 138.39 days, is the most readily available. atomic number 84; melting point 254°C; boiling point 962°C; specific gravity 9.32; valence 2, 4. see table at element.

1. a chemical element (symbol po) with atomic number 84.

1. A radioactive chemical element, discovered by M. and mme. curie in pitchblende, and originally called radium F. it has atomic number 84 and an atomic weight of 210. it is a very rare natural element, having an abundance in uranium ores only 0.2% that of radium. it is closely related chemically to bismuth. it emits only alpha rays, and has a half-life of 138 days. it is thus more unstable than radium, and a milligram of polonium emits as many alpha particles as 5 grams of radium. twenty-seven isotopes are known, with atomic masses from 192 to 218. at present a more practical method of preparation than isolation from ores is the preparation by neutron bombardment of bismuth in a nuclear reactor, and it may be obtained commercially by users having an appropriate permit.

1. A radioactive substance discovered in pitchblende by M. and mme. curie in 1898: named in honor of poland, the native country of mme. curie.

1. a radioactive metallic element that is similar to tellurium and bismuth; occurs in uranium ores but can be produced by bombarding bismuth with neutrons in a nuclear reactor