Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Cadmium, symbol Cd, silvery-white metallic element that can easily be shaped. The atomic number of cadmium is 48; the element is one of the transition elements in group 12 (or IIb) of the periodic table (see Periodic Law).


Cadmium was discovered in 1817 by the German chemist Friedrich Stromeyer, who found it in incrustations in zinc furnaces. The element ranks about 65th in natural abundance among the elements in the earth's crust. Cadmium melts at 321° C (610° F), boils at 765° C (1409° F), and has a specific gravity of 8.64; the atomic weight of cadmium is 112.41. When heated, cadmium burns in air with a bright light, forming the oxide CdO.

Cadmium occurs as the principal constituent of a mineral only in the rare greenockite. Almost the entire cadmium output of the United States is obtained as a by-product in the refining of zinc ores, chiefly from Missouri and Montana. It is also obtained from the zinc ores of Silesia. Fractional distillation or electrolysis is used to separate the cadmium and zinc.


Cadmium may be electrolytically deposited as a coating on metals, chiefly iron or steel, on which it forms a chemically resistant coating. Cadmium lowers the melting point of metals with which it is alloyed; it is used with lead, tin, and bismuth in the manufacture of fusible metals for automatic sprinkler systems, fire alarms, and electric fuses. An alloy of cadmium with lead and zinc is used as a solder for iron. Cadmium salts are used in photography and in the manufacture of fireworks, rubber, fluorescent paints, glass, and porcelain. Cadmium has been used as a control or shielding material in atomic energy plants because of its high absorption of low-energy neutrons. Cadmium sulfide is employed in a type of photovoltaic cell (see Solar Energy), and nickel-cadmium batteries are in common use for specialized purposes.

Cadmium sulfate is used as an astringent. Cadmium sulfide, formed as a bright yellow precipitate when hydrogen sulfide is passed through a solution of cadmium salt, is an important pigment known as cadmium yellow. The selenide CdSe is also used as a pigment. Cadmium and solutions of its compounds are highly toxic, with cumulative effects similar to those of mercury poisoning.