n. Symbol Rb
A soft silvery-white metallic element of the alkali group that ignites spontaneously in air and reacts violently with water, used in photocells and as a getter in the manufacture of vacuum tubes. Atomic number 37; atomic weight 85.47; melting point 39.30°C; boiling point 688°C; specific gravity (solid) 1.532; valence 1, 2, 3, 4. See Periodic Table.
[From Latin rūbidus, red; see reudh- in the Appendix of Indo-European roots.]
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