Copper ( /ˈkɒpər/ KOP-ər) is a chemical element with the symbol Cu (Latin: cuprum) and atomic number 29. It is a ductile metal, with very high thermal and electrical conductivity. Pure copper is rather soft and malleable, and a freshly exposed surface has a reddish-orange color. It is used as a thermal conductor, an electrical conductor, a building material, and a constituent of various metal alloys.
Copper metal and alloys have been used for thousands of years. In the Roman era, copper was principally mined on Cyprus, hence the origin of the name of the metal as Cyprium, "metal of Cyprus", later shortened to Cuprum.
Like many other metals, copper is easily recyclable. However, the fraction of copper in active use is steadily increasing, and its total quantity available on Earth may be barely sufficient to allow all countries to reach developed world levels of copper usage. Some countries, such as Chile and the United States, still have sizeable reserves of unmined metal which are extracted through large open pit mines.
Copper compounds are commonly encountered as salts of Cu2+, which often impart blue or green colors to minerals such as turquoise and have been widely used historically as pigments. Copper metal architectural structures and statuary eventually corrode to acquire a characteristic green patina. Copper as both metal and pigmented salt, has a significant presence in decorative art.
Copper(II) ions (Cu2+) are soluble in water, where they function at low concentration as bacteriostatic substances, fungicides, and wood preservatives. In sufficient amounts, copper salts can be poisonous to higher organisms as well. However, despite universal toxicity at high concentrations, the Cu2+ ion at lower concentrations is an essential trace nutrient to all higher plant and animal life. In animals, including humans, it is found widely in tissues, with concentration in liver, muscle, and bone. It functions as a co-factor in various enzymes and in copper-based pigments.