atomic number 21


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Noun1.atomic number 21 - a white trivalent metallic elementatomic 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
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Most of the rare earths consist of the elements between the lanthanum (atomic number 57) and lutetium (atomic number 71), with two outliers: scandium (atomic number 21) and yttrium (atomic number 39).