Rubidium

A rare metallic element. 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, 85.2.

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