 = Cadmium = 
 
 Cadmium is a chemical element with symbol Cd and atomic number 48 . This soft , bluish @-@ white metal is chemically similar to the two other stable metals in group 12 , zinc and mercury . Like zinc , it demonstrates oxidation state + 2 in most of its compounds , and like mercury , it has a lower melting point than other transition metals . 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 . The average concentration of cadmium in Earth 's crust is between 0 @.@ 1 and 0 @.@ 5 parts per million ( ppm ) . It was discovered in 1817 simultaneously by Stromeyer and Hermann , both in Germany , as an impurity in zinc carbonate . 
 Cadmium occurs as a minor component in most zinc ores and is a <unk> of zinc production . Cadmium was used for a long time as a corrosion @-@ resistant plating on steel , and cadmium compounds are used as red , orange and yellow pigments , to colour glass , and to stabilize plastic . Cadmium use is generally decreasing because it is toxic ( it is specifically listed in the European <unk> of <unk> <unk> ) and nickel @-@ cadmium batteries have been replaced with nickel @-@ metal hydride and lithium @-@ ion batteries . One of its few new uses is cadmium telluride solar panels . 
 Although cadmium has no known biological function in higher organisms , a cadmium @-@ dependent carbonic <unk> has been found in marine diatoms . 
 
 = = Characteristics = = 
 
 
 = = = Physical properties = = = 
 
 Cadmium is a soft , malleable , <unk> , bluish @-@ white divalent metal . It is similar in many respects to zinc but forms complex compounds . Unlike most other metals , cadmium is resistant to corrosion and is used as a protective plate on other metals . As a bulk metal , cadmium is <unk> in water and is not flammable ; however , in its powdered form it may burn and release toxic fumes . 
 
 = = = Chemical properties = = = 
 
 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 . 
 Cd + <unk> + 2 <unk> → <unk> ( <unk> ) 2 
 The structures of many cadmium complexes with <unk> , amino acids , and <unk> have been determined . 
 
 = = = Isotopes = = = 
 
 Naturally occurring cadmium is composed of 8 isotopes . Two of them are radioactive , and three are expected to decay but have not done so under laboratory conditions . The two natural radioactive isotopes are <unk> ( beta decay , half @-@ life is 7 @.@ 7 × 1015 years ) and <unk> ( two @-@ neutrino double beta decay , half @-@ life is 2 @.@ 9 × <unk> years ) . The other three are <unk> , <unk> ( both double electron capture ) , and <unk> ( double beta decay ) ; only lower limits on these half @-@ lives have been determined . At least three isotopes – <unk> , <unk> , and <unk> – are stable . Among the isotopes that do not occur naturally , the most long @-@ lived are <unk> with a half @-@ life of 462 @.@ 6 days , and <unk> with a half @-@ life of 53 @.@ 46 hours . All of the remaining radioactive isotopes have half @-@ lives of less than 2 @.@ 5 hours , and the majority have half @-@ lives of less than 5 minutes . Cadmium has 8 known meta states , with the most stable being <unk> ( t1 / 2 