Cadmium (48)

Cadmium is a soft, bluish-white metal and is easily cut with a knife. It is similar in many respects to zinc. Interestingly, a characteristic cadmium "scream" is heard on bending a cadmium bar (better than the lambs!). Cadmium and its compounds are highly toxic. Silver solder, which contains cadmium, should be handled with care (this along with lead makes it a health hazard concern).

•Name: Cadmium
•Symbol: Cd
•Atomic number: 48
•Atomic weight: 112.411
•Standard state: solid at 298 K
•CAS Registry ID: 7440-43-9
•Group in periodic table: 12
•Period in periodic table: 5
•Block in periodic table: d-block
•Color: silvery grey metallic
•Classification: Metallic

Historical information

Cadmium was discovered by Friedrich Strohmeyer at 1817 in Germany. Origin of name is somewhat confusing, from the Latin word "cadmia" meaning "calamine" (zinc carbonate, ZnCO3) and from the Greek word "kadmeia" with the same meaning. Cadmium was discovered by Friedrich Stromeyer in 1817 from an impurity in some samples of zinc carbonate, ZnCO3. He noted that these particular samples changed color on heating, which pure zinc carbonate does not. He was persistent enough to follow this observation through and he eventually isolated some cadmium metal by roasting and reduction of the sulfide.

Physical properties

•Melting point: 594.22 [or 321.07 °C (609.93 °F)] K
•Boiling point: 1040 [or 767 °C (1413 °F)] K
•Density of solid: 8650 kg m-3

Orbital properties

•Ground state electron configuration: [Kr].4d10.5s2
•Shell structure: 2.8.18.18.2
•Term symbol: 1S0

Isolation

It is rare that preparation of cadmium in the laboratory should be required because of environmental concerns about cadmium. The isolation of cadmium is associated with zinc recovery as cadmium is an impurity in zinc ores. Most zinc production is based upon sulfide ores. These are roasted in industrial plants to form zinc oxide, ZnO. This may be reduced with carbon to form zinc metal, but in practice ingenious technology is required to ensure that the resulting zinc does not contain oxide impurities.

ZnO + C → Zn + CO

ZnO + CO → Zn + CO2

CO2 + C → 2CO

After this process, zinc may be refined by distillation under vacuum and this process also allows the separation of any cadmium present in the crude zinc.

The other type of extraction of zinc is electrolytic. Dissolution of crude zinc oxide, ZnO, in sulphuric acid gives zinc sulphate, ZnSO4 in solution. Before electrolysis to produce zinc, the cadmium impurity and is removed as a precipitate by the addition of zinc dust as cadmium sulphate