sodium UK /ˈsəʊ.di.əm/US /ˈsoʊ.di.əm/ noun3 •The chemical element (symbol Na) with an atomic number of 11 and atomic weight of 22.990. It is a soft, waxy, silvery, reactive alkali metal that is never found unbound in nature. •An atom of this element. •[before a noun] Employing sodium. formssodiums