Cadmium is a transition metal. The transition metals are the elements found in Rows 4 through 7 between Groups 2 and 13 in the periodic table, a chart that shows how elements are related. Cadmium was discovered by German chemist Friedrich Stromeyer (1776-1835) in 1817. It is found most commonly in ores of zinc.
Cadmium is a soft metal that is easily cut with a knife. It resembles zinc in many of its physical and chemical properties. However, it is much less abundant in the Earth's crust than zinc.
By far the most important use of cadmium in the United States is in the production of nicad ( nickel -cadmium), or rechargeable, batteries. It is also used in pigments, coatings and plating, manufacture of plastic products, and alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals.
Group 12 (IIB)
Caution must be used when handling cadmium and its compounds, as they are toxic to humans and animals. They present a threat to the environment because of their many applications.
Discovery and naming
In addition to being a professor at Göttingen University, Stromeyer was a government official responsible for inspecting pharmacies in the state of Hanover, Germany. On one inspection trip, he found that many pharmacies were stocking a compound of zinc called zinc carbonate (ZnCO 3 ) instead of the usual zinc oxide (ZnO). Zinc oxide is still readily available in pharmacies today.
Stromeyer was told that the supplier had problems making zinc oxide from
zinc carbonate and had offered the substitution. The normal process was to
heat zinc carbonate to produce zinc oxide:
The supplier explained that zinc carbonate turned yellow when heated. Normally, a yellow color meant that iron was present as an impurity. The supplier found no iron in his zinc carbonate, but it was still yellow. Pharmacies would not buy yellow zinc oxide, so the supplier sold white zinc carbonate instead.
Stromeyer analyzed the odd yellow zinc carbonate. What he discovered was a new element—cadmium. The cadmium caused the zinc carbonate to turn yellow when heated. The name comes from the ancient term for zinc oxide, cadmia. Zinc oxide is still available in pharmacies today. It is sold under the name of calamine lotion. Calamine lotion is a popular remedy for stopping the itch of sunburn or bug bites.
Cadmium is a shiny metal with a bluish cast (shade) to it. It is very soft and can almost be scratched with a fingernail. Its melting point is 321°C (610°F) and its boiling point is 765°C(1,410°F). The density of cadmium is 8.65 grams per cubic centimeter.
An interesting property of cadmium is its effect in alloys. In combination with certain metals, it lowers the melting point. Some common low-melting-point alloys are Lichtenberg's metal, Abel's metal, Lipowitz' metal, Newton's metal, and Wood's metal.
Cadmium reacts slowly with
in moist air at room temperatures, forming cadmium oxide:
Cadmium does not react with water, though it reacts with most acids.
Occurrence in nature
The abundance of cadmium in the Earth's crust is estimated to be about 0.1 to 0.2 parts per million. It ranks in the lower 25 percent of the elements in terms of abundance in the earth.
The only important ore of cadmium is greenockite, or cadmium sulfide (CdS). Most cadmium is obtained as a by-product of zinc refinement.
The largest producers of cadmium in 1996 were Canada, Japan, Belgium, the United States, China, Kazakhstan, and Germany.
Eight naturally occurring isotopes of cadmium exist. They are cadmium-106, cadmium-108, cadmium-110, cadmium-111, cadmium-112, cadmium-113, cadmium-114, and cadmium-116. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
About 20 radioactive isotopes of cadmium are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
One isotope of cadmium, cadmium-109, is sometimes used to analyze metal alloys. It provides a way of keeping track of the alloys in stock and sorting different forms of scrap metal from each other.
Most cadmium is obtained as a by-product from zinc refinement. Cadmium and zinc melt at different temperatures, providing one way of separating the two metals. As a liquid mixture of zinc and cadmium is cooled, zinc becomes a solid first. It can be removed from the mixture, leaving liquid cadmium behind.
Cadmium and fire sprinklers
A common low-melting-point cadmium alloy is Wood's metal. This alloy melts at 70°C (158°F), and is used in fire sprinkler systems as a plug. When the temperature rises above 70°C (158°F), the plug melts and falls out. This opens up the water line and activates the sprinkler. Out sprays the water!
Today, about 70 percent of cadmium produced worldwide is used in nickel-cadmium (nicad) batteries. Nicad batteries are rechargeable.
At one time, the most important use of cadmium was in the electroplating of steel. Electroplating is a process by which a thin layer of one metal is deposited on the surface of a second metal. An electric current is passed through a solution containing
In the last 30 years, the use of cadmium for electroplating has dropped by about 70 percent due to environmental concerns. Discarded electroplated steel puts cadmium into the environment. Alternative coating methods are usually used now.
Today, about 70 percent of cadmium produced worldwide is used in nickel-cadmium (nicad) batteries. Nicad batteries can be used over and over. When a nicad battery has lost some or all of its power, it is inserted into a unit that plugs into an electrical outlet. Electricity from the outlet recharges the battery.
Nicad batteries are used in a large variety of appliances, including compact disc players, cellular telephones, pocket recorders, handheld power tools, cordless telephones, Laptop computers, camcorders, and scanner radios. Two French automobile manufacturers are exploring the possibility of using nicad batteries in electric cars.
A popular use of cadmium compounds is as coloring agents. The two compounds most commonly used are cadmium sulfide (CdS) and cadmium selenide (CdSe). The sulfide is yellow, orange, or brown, while the selenide is red. These compounds are used to color paints and plastics. There is concern about possible environmental effects of using cadmium for this purpose. However, no satisfactory substitutes have been found.
The major sources of cadmium in humans are cigarette smoking, certain foods (such as shellfish, liver and kidney meats), coal burning, and contaminated water. Those most at risk for high intake of cadmium are people who work directly with the metal. Manufacturing plants where batteries are made use cadmium as a fine powder where it can easily be inhaled. Workers must be careful in handling cadmium.
There is growing concern about the dangers of cadmium in the environment. Rechargeable batteries are made with cadmium and nickel. Cadmium can escape from landfills (where trash is buried) and get into the ground and groundwater. From there, it can become part of the food and water that humans and animals ingest.
Low levels of cadmium cause nausea, vomiting, and diarrhea. Inhaled, cadmium dust causes dryness of the throat, choking, headache, and pneumonia-like symptoms.
Manufacturing plants where batteries are made use cadmium as a fine powder where it can easily be inhaled. Workers must be careful in handling cadmium.
The effects of extensive cadmium exposure is not known, but are thought to include heart and kidney disease, high blood pressure, and cancer. A cadmium poisoning disease called itai-itai, Japanese for "ouch-ouch," causes aches and pains in the bones and joints. In the 1970s, a number of cases of itai-itai were reported in Japan when waste from a zinc refinery got into the public water supply. Those wastes contained cadmium compounds.