polonium

 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.  [Webster 1913 Suppl.]