a common metallic element of the alkali group, in nature always occuring combined, as in common salt, in albite, etc. it is isolated as a soft, waxy, white, unstable metal, so readily oxidized that it combines violently with water, and to be preserved must be kept under petroleum or some similar liquid. sodium is used combined in many salts, in the free state as a reducer, and as a means of obtaining other metals (as magnesium and aluminium) is an important commercial product. symbol na  (natrium). atomic weight 23. specific gravity 0.97.