Doping with this element makes an amorphous phase that improves the selectivity of iron-vanadium-oxide catalysts. Heating this element with platinum or gold yields a stable ionic platinide or auride. This element’s chloride names the crystal structure made of two interpenetrating cubic lattices, which occurs when the cation and anion have about the same radius. This element’s chloride is used in density-gradient (*) centrifugation (10[1])because of its high solubility and density. This element’s melting point is slightly below gallium’s, the lowest of any stable non-mercury metallic element. The ground-state hyperfine transition frequency of this element’s 133 isotope is used to define the second, so it is used in atomic clocks. For 10 points, name this most electropositive element, the heaviest stable alkali metal. ■END■