atomic number 21
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|Noun||1.||atomic number 21 - a white trivalent metallic element; sometimes classified in the rare earth group; occurs in the Scandinavian mineral thortveitite|
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.
thortveitite - a mineral consisting of scandium yttrium silicate; a source of scandium