COPPER

Copper Pipe

Physical Properties

Atomic Number: 29
Mass Number: 63.546
Electron Configuration: 1s²2s²2p⁶3s²3p⁶4s¹3d¹⁰
Crystal Structure: Face-Centered Cubic
Density: 8.96 g/cm³
Melting Point: 1083 ^oC
Important Oxidation States: +1, +2
Standard Reduction Potential: +0.34 volts (Cu²⁺ + 2e^- = Cu)

Properties of Elemental Copper
Copper is the next to the last member of the first-row transition metals. Copper is one of the two exceptions to the writing of electron configurations. One would expect copper to be an s²d⁹ ion. Instead, one electron is "borrowed" from the 4s orbital to completely fill the 3d orbitals. Copper metal is therefore paramagnetic due to the unpaired electron in the 4s orbital. Copper has the lustor generally associated with metals but exhibits a characteristic red color. Copper is an excellent electrical conductor and is used to make electrical wires. Copper is commonly alloyed with zinc to make brass and with tin to form bronze. Copper has a positive reduction potential, meaning that it is fairly easily reduced. Since reversing an electrochemical reaction changes the sign of the potential, copper metal is not easily oxidized and is fairly unreactive. For excample, copper metal will not react with hydrochloric acid like many of the more reactive metals. However, it will react with concentrated nitric aicd (see the photo below). Because of this unreactivity, copper can be found in nature in the elemental form and it has been widely used since ancient times. Today most copper is obtained from sulfide ores. Some common copper minerals are cuprite (Cu₂O), Chalcocite (Cu₂S), and Chalcopyrite (CuFeS₂).

Copper Metal Reacting with Nitric Acid Copper metal does not react with concentraed hydrochloric acid. It does, however, react with concentrated nitric acid. In this photo, a small piece of copper wire has been dropped into a beaker of nitric acid. The green color is due to the presence of the copper(II) ion. The brown gas is nitrogen dioxide being formed from the reduction of the nitrate ion. For a video of this reaction see the reactions page.

Copper Flame Test In a flame test, copper ions emit deeply colored blue-green light. Anyone who has ever welded or worked with copper pipe may have noticed their torch flame turn this same shade of blue-green. The flame is much more bluish than the flame test for baruim.

The Chemistry of Copper(I)
The two most common ions formed by copper are the +1 (cuprous) and +2 (cupric).The cuprous ion is the less stable of the two oxidation states and is easily oxidized. Therefore, any work done with copper(I) compounds must be carried out in an inert atmosphere. Since electrons are lost first out of the 4s orbital, the electron configuration of the cuprous ion is 4s⁰3d¹⁰ and the it is therefore diamagnetic. As a consequence of the d¹⁰ configuration, one would not expect the cuprous ion to be colored, and the compounds CuCl, CuBr, and CuI are white in color. Copper(I) oxide is red and color and is sometimes used as a pigment. In this case, the color arises from a different mechanism. Many copper minerals contain copper in the +1 oxidation state. For example, cuprite is copper(I) oxide.

Cuprite The mineral Cuprite, Cu2O Photo courtesy of R. Weller, Cochise College

The Chemistry of Copper(II)
Removal of a second electron results in the a 4s⁰3d⁹ configuration. Therefore the cupric ion is paramagnetic. The cupric ion tends to exhibit square planar or distorted octahedral geometries. In the distorted octahedral geometry, two of the ligands form a square planar geometry and the remaining two bonds are elongated. Many copper compounds are blue or green in color. Crystals of copper sulfate pentahydrate are a characteristic blue color. Copper(II) chloride is yellow when anhydrous but forms a dihydrate that is bluish-green in color. In relatively dilute solution the copper(II) ion is pale blue in color. Upon the addition of ammonia, the color darkens considerably due to the formation of the tetraammine complex. Copper(II) salts are sometimes used as fungicides.

Copper Sulfate Pentahydrate This photo shows blue crystals of copper sulfate pentahydrate. Most copper compounds are blue or bluish green in color. In solution the copper(II) ion is pale blue in color.

A Chemical Test for Copper(II) In relatively dilute colution, copper(II) ions are pale blue in color, as shown in the bottle at the left. When several drops of concentrated ammonia are added, the solution turns a much deeper shade of blue due to the formation of the tetraammine copper(II) ion.