- Cadmium is a chemical element with the symbol Cd and atomic number 48, belonging to Group 12 of the periodic table along with zinc and mercury.
- It is a soft, ductile, silvery-white metal that is relatively rare in the Earth’s crust, occurring at an average abundance of about 0.1 parts per million.
- Cadmium has a density of 8.65 g/cm³, a melting point of 321.1 °C, and a boiling point of 767 °C. It is stable in air and water at ambient temperatures, developing a thin oxide layer that protects it from rapid corrosion.
- Cadmium is not found in its native metallic form; instead, it is usually a minor byproduct of zinc, lead, and copper refining, most commonly obtained from sphalerite (ZnS) ores, where it substitutes for zinc in the crystal lattice.
- The element was discovered in 1817 independently by Friedrich Stromeyer and Karl Samuel Leberecht Hermann in Germany, during the analysis of zinc carbonate. Its name comes from the Latin word cadmia (meaning calamine, a zinc ore) and the Greek kadmeia (referring to the mineral from the region of Thebes, or Cadmea). Stromeyer identified cadmium as a distinct metal when he noticed a yellow oxide forming upon heating impure zinc carbonate, which could not be explained by zinc alone.
- Cadmium’s chemical behavior resembles that of zinc, but it is generally more reactive and toxic. It readily forms divalent compounds (Cd²⁺) and reacts with nonmetals to produce salts such as cadmium sulfide, cadmium chloride, and cadmium oxide. These compounds exhibit a range of colors and properties, many of which have found industrial and technological uses. Cadmium metal is soft enough to be cut with a knife and is malleable and ductile, making it easy to work into alloys. It has good corrosion resistance, particularly in alkaline environments, and this property led to its historical use in electroplating to protect steel from rust.
- Industrial applications of cadmium have been diverse. It has been used in nickel–cadmium (Ni–Cd) rechargeable batteries, as a protective coating in aerospace and marine hardware, and in specialized low-melting alloys. Cadmium compounds, particularly cadmium sulfide and cadmium selenide, have been used as pigments (bright yellows, oranges, and reds) and in semiconductors for photovoltaic cells, photodetectors, and quantum dots. Cadmium telluride (CdTe) has become especially important in thin-film solar technology due to its near-optimal bandgap for sunlight absorption. However, many of these uses have been curtailed or replaced with safer alternatives due to health and environmental concerns.
- Cadmium is highly toxic to humans and animals. It is a cumulative poison that accumulates primarily in the kidneys and liver, and long-term exposure can cause kidney damage, bone demineralization (Itai-itai disease in Japan being a notorious example), respiratory issues, and cancer. Cadmium compounds are classified as human carcinogens by agencies such as the International Agency for Research on Cancer (IARC). Occupational exposure typically occurs through inhalation of dust or fumes during smelting, welding, or battery production, while environmental exposure can result from industrial emissions, improper waste disposal, and contaminated food or water.
- Environmental concerns with cadmium are significant because the metal is persistent and bioaccumulative. Once released into the environment, cadmium does not break down and can cycle through soils, water, and living organisms for decades. Regulations in many countries restrict cadmium use, mandate strict industrial emission controls, and require safe recycling or disposal of cadmium-containing products. Current global production is closely tied to zinc refining, with annual output in the tens of thousands of metric tons, most of which goes into battery manufacturing.
