rubidium
Concise Oxford English Dictionary © 2008 Oxford University Press:
rubidium /rʊˈbɪdɪəm/
▶noun the chemical element of atomic number 37, a rare soft silvery reactive metal of the alkali metal group. (Symbol: Rb)
– origin C19: mod. L., from L. rubidus ‘red’ (with ref. to lines in its spectrum).
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