A radioactive metallic element, atomic no. 92, atomic wt. 238.0289, occurring mainly in pitchblende and notable for its two isotopes: 238U and 235U (99.2745% and 0.720%, respectively, the rest being made up by 234U), 235U being the first substance ever shown capable of supporting a self-sustaining chain reaction. [G. myth. character, Uranus]
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ura·ni·um yu̇-'rā-nē-əm n 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 symbol U see ELEMENT (table)
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ura·ni·um (U) (u-raґne-əm) [L. Uranus a planet] a hard and heavy radioactive metallic element; atomic number, 92; atomic weight, 238.03; specific gravity, 18.68. Naturally occurring uranium is composed of three isotopes of mass numbers 234, 235, and 238. Uranium 235 separated from U 238 undergoes fission with slow neutrons, giving up neutrons which can join the nucleus of U 238 to form neptunium, which in turn decays by beta particle emission to form plutonium. Cf. neptunium and plutonium.Medical dictionary. 2011.