URANIUM

[ura.ni.um] n, often attrib [NL, fr. Uranus] (ca. 1797): a silvery heavy radioactive polyvalent metallic element that is found esp. in pitchblende and uraninite and exists naturally as a mixture of three isotopes of mass number 234, 235, and 238 in the proportions of 0.006 percent, 0.71 percent, and 99.28 percent respectively