Cadmium << KAD mee uhm >> is a soft, silvery-white metallic element used for plating and in alloys (mixtures of metals). The German chemist Friedrich Stromeyer discovered it in 1817. For every billion atoms of Earth’s crust, only about 30 are cadmium. It occurs with zinc minerals and is obtained as a by-product of zinc refining. China is the leading producer of cadmium.
Cadmium is poisonous. People have become seriously ill or have died soon after breathing cadmium dust or fumes of cadmium oxide. Small amounts of cadmium entering the body over long periods may also damage the kidneys and deform bones. Some people fear that hazardous amounts of cadmium have reached the environment from widespread industrial use of the metal.
Industry uses cadmium in alloys for high-speed bearings, and as a protective coating for other metals. Cadmium is often used instead of zinc for galvanizing iron and steel—that is, for coating them to prevent rust. Cadmium provides poorer long-term protection than zinc. But it keeps a brighter color for longer periods. Cadmium rods are used in nuclear reactors to control nuclear reactions. Nickel-cadmium and silver-cadmium batteries, which can be recharged, are used in watches, calculators, and many other small devices.
Cadmium has the symbol Cd. Its atomic number (number of protons in its nucleus) is 48. Its relative atomic mass is 112.411. An element’s relative atomic mass equals its mass (amount of matter) divided by 1/12 of the mass of carbon 12, the most abundant form of carbon. The density of cadmium is 8.65 grams per cubic centimeter at 20 °C. Cadmium melts at 320.9 °C and boils at 765 °C. Chemists classify cadmium as a transition metal . For information on the position of cadmium on the periodic table, see the article Periodic table .
