 Although cadmium usually has an oxidation state of + 2 , it also exists in the + 1 state . Cadmium and its <unk> 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 <unk> cadmium oxide ( <unk> ) ; the crystalline form of this compound is a dark red which changes color when heated , similar to zinc oxide . <unk> acid , sulfuric acid , and nitric acid dissolve cadmium by forming cadmium chloride ( <unk> ) , cadmium sulfate ( <unk> ) , or cadmium nitrate ( Cd ( <unk> ) 2 ) . The oxidation state + 1 can be produced by dissolving cadmium in a mixture of cadmium chloride and aluminium chloride , forming the <unk> + cation , which is similar to the <unk> + cation in mercury ( I ) chloride . 
