Copper

Copper (IPA: /'k?p?/, /'k?p??/) is a chemical element in the periodic table that has the symbol Cu (Latin: cuprum) and atomic number 29. It is a ductile metal with excellent electrical conductivity, and finds extensive use as an electrical conductor, heat conductor, as a building material, and as a component of various alloys.

Copper is an essential trace nutrient to all high plants and animals. In animals, including humans, it is found primarily in the bloodstream, as a co-factor in various enzymes, and in copper-based pigments. However, in sufficient amounts, copper can be poisonous and even fatal to organisms.

Copper has played a significant part in the history of mankind, which has used the easily accessible uncompounded metal for nearly 10,000 years. Civilizations in places such as Iraq, China, Egypt, Greece, India and the Sumerian cities all have early evidence of using copper. During the Roman Empire, copper was principally mined on Cyprus, hence the origin of the name of the metal as Cyprium, "metal of Cyprus", later shortened to Cuprum. A number of countries, such as Chile and the United States, still have sizable reserves of the metal which are extracted through large open pit mines. Nevertheless, the price of copper has risen rapidly, increasing 500% from a 60-year low in 1999, largely due to increased demand. The Earth has an estimated 61 years of copper reserves remaining.

Conductivity

Copper has a high electrical and thermal conductivity, second only to silver among pure metals at room temperature.

Colour

Copper is a reddish-colored metal; it has its characteristic color because of its band structure.

In its liquefied state, a pure copper surface without ambient light appears somewhat greenish, a characteristic shared with gold. When liquid copper is in bright ambient light, it retains some of its pinkish luster.

Location in the periodic table

Copper occupies the same family of the periodic table as silver and gold, since they each have one s-orbital electron on top of a filled electron shell. This similarity in electron structure makes them similar in many characteristics. All have very high thermal and electrical conductivity, and all are malleable metals.

Corrosion properties

Pure water and air

Copper is a metal that does not react with water (H2O), but the oxygen of the air will react slowly at room temperature to form a layer of copper oxide on copper metal.

It can be seen that copper in "pure" water is more noble than hydrogen. As a result it does not corrode in oxygen free water and the corrosion rate in oxygenated water is low.

The Pourbaix diagram for copper in pure water, perchloric acid or sodium hydroxide. It is important to note that in contrast to the oxidation of iron by wet air that the layer formed by the reaction of air with copper has a protective effect against further corrosion. On old copper roofs a green layer of copper carbonate can often be seen.

Sulfide media

Copper metal does react with hydrogen sulfide and sulfide containing solutions. A series of different copper sulfides can form on the surface of the copper metal.

The Pourbaix diagram for copper in a sulfide containing aqueous medium. Note that the copper sulfide area of the plot is very complex due to the existence of many different sulfides, a close up is also provided to make the graph more clear. It is clear that the copper is now able to corrode even without the need for oxygen as the copper is now less noble than hydrogen. This can be observed in every day life when copper metal surfaces tarnish after exposure to air which contains sulfur compounds.

The Pourbaix diagram for copper in a sulfide containing aqueous medium.

Ammonia media

Copper does react with oxygen-containing ammonia solutions because the ammonia forms water-soluble copper complexes. The formation of these complexes causes the corrosion to become more thermodynamically favoured than the corrosion of copper in an identical solution that does not contain the ammonia.

The Pourbaix diagram for copper in 10 M ammonia solution

Chloride media

Copper does react with a combination of oxygen and hydrochloric acid to form a series of copper chlorides. It is interesting to note that if copper(II) chloride (green/blue) is boiled with copper metal (with little or no oxygen present) then white copper(I) chloride will be formed.

The Pourbaix diagram for copper in a chloride solution

Isotopes

There are two stable isotopes, 63Cu and 65Cu, along with a couple dozen radioisotopes. The vast majority of radioisotopes have half lives on the order of minutes or less; the longest lived, 67Cu, has a half life of 61.8 hours.

Alloys

Numerous copper alloys exist, many with important historical and contemporary uses. Speculum metal and bronze are alloys of copper and tin. Brass is an alloy of copper and zinc. Monel metal, also called cupronickel, is an alloy of copper and nickel. While the metal "bronze" usually refers to copper-tin alloys, it also is a generic term for any alloy of copper, such as Aluminium bronze, silicon bronze, and manganese bronze.

Germicidal effect

Copper is germicidal, via the oligodynamic effect. For example, brass doorknobs disinfect themselves of many bacteria within a period of eight hours. This effect is useful in many applications.

Applications

Copper is malleable and ductile, a good conductor of heat and, when very pure, a good conductor of electricity.

The purity of copper is expressed as 4N for 99.99% pure or 7N for 99.99999% pure. The numeral gives the number of nines after the decimal point when expressed as a decimal (e.g. 4N means 0.9999, or 99.99%).