Although cadmium usually have an oxidation state of + 2 , it also exist in the + 1 state . Cadmium and its congeners are not always considered transition metals , in that they do not have partly filled d or f electron shells in the elemental or common oxidation states . Cadmium burns in air to form brown amorphous cadmium oxide ( CdO ) ; the crystalline form of this compound is a dark red which change color when heated , similar to zinc oxide . Hydrochloric acid , sulfuric acid , and nitric acid dissolve cadmium by forming cadmium chloride ( CdCl2 ) , cadmium sulfate ( CdSO4 ) , or cadmium nitrate ( Cd ( NO3 ) 2 ) . The oxidation state + 1 can be produced by dissolving cadmium in a mixture of cadmium chloride and aluminium chloride , forming the Cd22 + cation , which is similar to the Hg22 + cation in mercury ( I ) chloride .