rubidium(n = noun.substance)atomic number 37, rb - 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;
A 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. [1913 Webster]