A chemical element, Cu, atomic number 29, atomic weight 63.546. Copper is located in the periodic table between nickel and zinc in the first row of transition elements and in the same subgroup as the other so-called coinage metals, silver and gold. Copper, a nonferrous metal, is the twentieth most abundant element present in the Earth's crust, at an average level of 68 parts per million (0.11 kg/metric ton). See also: Periodic table; Transition elements
Copper metal and copper alloys have considerable technological importance because of their combined electrical, mechanical, and physical properties. Although some pure copper metal is present in nature, commercial copper is obtained by reduction of the copper compounds in ores (Fig. 1) followed by electrochemical refining. The rich chemistry of copper is restricted mostly to the valence states Cu(I) and Cu(II); compounds containing Cu(0), Cu(III), and Cu(IV) are uncommon. Soluble copper salts are potent bacteriocides and algicides at low levels and toxic to humans in large doses. Yet copper is an essential trace element that is present in various metalloproteins required for the survival of plants and animals. See also: Antimicrobial agents; Copper alloys; Electrometallurgy
Copper is a comparatively heavy metal. The density of the pure solid is 8.96 g/cm3 at 20°C. The density of commercial copper varies with method of manufacture, averaging 8.90–8.92 g/cm3 in cast refinery shapes, 8.93 g/cm3 for annealed tough-pitch copper, and 8.94 g/cm3 for oxygen-free copper. The density of liquid copper is 8.22 g/cm3 near the freezing point.
The melting point of copper is 1083.0 ∓ 0.1°C. Its normal boiling point is 2595°C. The coefficient of linear expansion of copper is 1.65 × 10−5/°C at 20°C.
The electrical resistivity of copper in the usual volumetric unit, that of a cube measuring 1 cm in each direction, is 1.6730 × 10−6 ohm · cm at 20°C. Only silver has a greater volumetric conductivity than copper. On a relative basis in which silver is rated 100, copper is 94, aluminum 57, and iron 16.
The mass resistivity of pure copper for a length of 1 m weighing 1 g at 20°C is 0.14983 ohm. The conductivity of copper on the mass basis is surpassed by several light metals, notably aluminum. The relative values are 100 for aluminum, 50 for copper, and 44 for silver.
Copper's high conductivity makes it an ideal material for a number of electrical and electronics applications. By far the largest use of copper is for wire and cable in the electrical industry (Fig. 2). In electronics, copper is used for the metal layers for integrated circuits and the conductors for printed circuit boards. In the building industry copper is used for roofing, flashing, downspouts, and plumbing. See also: Conductor (electricity); Electrical conductivity of metals; Integrated circuits; Printed circuit board
Copper-containing proteins provide diverse biochemical functions, including copper uptake and transport (ceruloplasmin), copper storage (metallothionen), protective roles (superoxide dismutase), catalysis of substrate oxygenation (dopamine β-monooxygenase), biosynthesis of connective tissue (lysyl oxidase), terminal oxidases for oxygen metabolism (cytochrome c oxidase), oxygen transport (hemocyanin), and electron transfer in photosynthetic pathways (plastocyanin). See also: Bioinorganic chemistry; Enzyme