Cadmium (atomic symbol Cd) is a very soft metal, similar to zinc. It is atomic number 48, group 12 and period 6, positioned directly under zinc on the periodic table. It is one of the 38 elements in groups 3 through 12 known as transition metals. In general, the transition metals are all ductile, malleable and conductive. Cadmium and cadmium compounds are highly toxic, whereas zinc is not. The element received its name from the Latin word “cadmia” or Greek “kadmeia,” which both mean “calamine.” Confusingly, calamine (ZnO) is actually a compound of zinc, also known as zinc oxide.
Cadmium was independently discovered by two chemists at roughly the same time, but credit goes to Friedrich Stromeyer. In 1817, Stromeyer found the new element present in zinc compounds. Zinc oxide was, and still is, used to treat a variety of skin conditions, but Stromeyer found that the pharmacies in the town of Hildesheim, Germany were using zinc carbonate instead, and he did not understand why.
Stromeyer heated a number of carbonate samples to convert them to oxide. The white carbonate should have remained white, but it instead turned a yellow-orange. Usually, yellow-orange coloration meant contamination by iron or lead, but he could not find traces of either. He then went to the factory selling the carbonate to the pharmacies, and found that they noted the same discoloration when trying to produce zinc oxide. Unable to determine the cause, they sold the carbonate instead. After careful study, he determined the contamination was an undiscovered element. He named it cadmium because it had been first noted in the zinc oxide, or calamine. In 1818, Karl Hermann noted a yellow coloration in several samples of zinc sulfide and also concluded it to be a new element.
Like the other transition metals, cadmium is malleable and ductile. It is soft enough to cut with a knife and is bluish-white in color. It conducts electricity well and is resistant to corrosion. Due to its toxicity and environmental concerns it is rarely produced in laboratories. When needed, it is isolated from zinc ores like sphlalerite (ZnS), where it is most often found. The only mineral with any substantial amount of cadmium is greenockite (CdS), but it is not plentiful enough to mine profitably. Cadmium is also a byproduct of copper and lead refining.
Use of cadmium is also limited because of its toxicity. It is sometimes electroplated to materials to prevent corrosion, but more often zinc is used for that purpose. The most common use of cadmium is in rechargeable nickel-cadmium batteries. It also appears as an alloy with other metals to make bearings and solder. Cadmium compounds were used in the picture tubes of black and white televisions, and then used for blue and green phosphors in color televisions. Cadmium sulphide is used in yellow pigments and cadmium selenide in reds, the color created sometimes called cadmium red. A promising use of cadmium is in solar panels. It can be compounded with tellurium to make low-cost photovoltaic modules.
