atomic number 90
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|Noun||1.||atomic number 90 - a soft silvery-white tetravalent radioactive metallic element; isotope 232 is used as a power source in nuclear reactors; occurs in thorite and in monazite sands|
metal, metallic element - any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc.
monazite - a reddish-brown mineral containing rare earth metals; an important source of thorium and cerium
thorite - a radioactive mineral consisting of thorium silicate; it is a source of thorium that is found in coarse granite