Copper

Copper's atoms hold a full 3d subshell and a single 4s electron ([Ar]3d10 4s1). That lone, loosely held electron moves freely through the metal, so copper conducts electricity better than any metal except silver. Copper is also one of the few metals that is not silvery: electrons in its filled d band absorb blue and green light, leaving the reddish-orange reflection we see. Its common oxidation states are +1 and +2, and blue Cu2+ ions color many of its salts and minerals.

Copper sits below hydrogen in the reactivity series, so it does not react with dilute non-oxidizing acids and can occur as the native metal, which is one reason it was among the first metals people used. Today around 60 percent of copper goes into electrical wiring, motors, generators and electronics. Alloyed with tin it becomes bronze, and with zinc, brass. In air, copper slowly forms a green patina of basic copper salts that protects the metal beneath.

Discovery

People have used copper for more than 10,000 years: a copper pendant from northern Iraq dates to about 8700 BCE. Smelting copper from ore began around 5000 BCE, and mixing it with tin started the Bronze Age around 3300 BCE. Much of the Roman Empire's copper came from Cyprus, so the metal was called aes Cyprium, metal of Cyprus, later shortened to cuprum, the source of the symbol Cu and the English word copper.