rubidium
nounA 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. cross-reference: Periodic Table.
nounChemical symbol, Rb; atomic weight, 85.25. A metal belonging to the group of elements which includes lithium, sodium, potassium, and cæsium: so named from the reddish tint of its salts.
nounA rare metallic element of the alkali metal series, atomic number 37. It occurs quite widely, but in small quantities, and always combined. It is isolated as a soft yellowish white metal, analogous to potassium in most of its properties. Symbol Rb. Atomic weight, 85.48.
nounA
a soft silvery metallic element of the alkali metal group; burns in air and reacts violently in water; occurs in carnallite and lepidolite and pollucite