atomic number 55
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|Noun||1.||atomic number 55 - a soft silver-white ductile metallic element (liquid at normal temperatures); the most electropositive and alkaline metal|
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.
cesium 137 - a radioactive isotope of cesium used in radiation therapy
pollucite - a rare mineral that is an important source of cesium