COPPER

Atomic Number: 29
Atomic Weight: 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)

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).

Cuprite and Chalcopyrite Photos of cuprite (left) and chalcopyrite (right),. Cuprite is reddish in color; chalcopyrite is golden in appearance, similar to iron pyrite. Photos courtesy of R. Weller, Cochise College

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.

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.

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, four of the ligands form a square planar geometry and the remaining two bonds are elongated.

Most copper compounds are blue or green in color. One is the copper sulfate pentahydrate pictued below. Another example is copper chloride dihydrate, which is bluish green in color. The greenish color is due to the coordination of the chloride ion, probably proudcing some tetrachlorocuprate(II).

When dissolved in water most copper compounds form the hexaaquocopper(II) ion is, which is pale blue in color. A wide range of colors can be produced when different ligands are added. For example, the addition of concentrated ammonia ammonia results results in the formation of a blue-violet complex ion that is more intensely colored than the original solution. A similar reaction occurs with ethylenediamine. Addition of the nitrite ion results in the formation of a green complex ion. Last, addition of hydrochloric acid results in the formation of yellow complex ion.

Two Copper Compounds Copper sulfate pentahydrate (left) has a bluish color; copper chloride dihydrate (right) is blue-green due to coordination of the chloride ions.

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.