Copper

Copper,  ₂₉Cu

Dendritic copper crystals made by electrolysis.
General properties
Name, symbol	Copper, Cu
Appearance	Reddish metallic
Copper in the periodic table
↑ Cu ↓ Silver Nickel ← Copper → Zinc
Atomic number	29
Standard atomic weight (Ar)	63.546(3)
Group, block	, d-block
Period	period 4
Electron configuration	[Ar] 3d10 4s1
per shell	2, 8, 18, 1
Physical properties
Reddish metallic
Phase	Solid
Melting point	1357.77 K ​(1084.62 °C, ​1984.32 °F)
Boiling point	2835 K ​(2562 °C, ​​4643 °F)
Density near r.t.	8.96 g/cm3
when liquid, at	8.02 g/cm3
Heat of fusion	13.26 kJ/mol
Heat of	300.4 kJ/mol
Molar heat capacity	24.44 J/(mol·K)
pressure
Atomic properties
Oxidation states	−2, +1, +2, +3, +4 ​​(a mildly basic oxide)
Electronegativity	Pauling scale: 1.90
energies	1st: 745.5 kJ/mol 2nd: 1957.9 kJ/mol 3rd: 3555 kJ/mol
Atomic radius	empirical: 128 pm
Covalent radius	132±4 pm
Van der Waals radius	140 pm
Miscellanea
Crystal structure	​Face-centered cubic (fcc)
Speed of sound thin rod	3810 m/s (at ) (annealed)
Thermal expansion	16.5 µm/(m·K) (at 25 °C)
Thermal conductivity	401 W/(m·K)
Electrical resistivity	16.78·10-9 Ω·m (at 20 °C)
Magnetic ordering	Diamagnetic
Young's modulus	110–128 GPa
Shear modulus	48 GPa
Bulk modulus	140 GPa
Poisson ratio	0.34
Mohs hardness	3.0
Vickers hardness	343–369 MPa
Brinell hardness	235–878 MPa
CAS Registry Number	7440-50-8
History
Naming	After Cyprus, principal mining place in Roman era (Cyprium)
Middle East (9000 BC)

Copper is a transition metal with the symbol Cu and the atomic number 29. It is a widely available commodity and can be obtained in numerous forms. It is an amphoteric metal that exists in two common oxidation states, +1 and +2, and can also exist in a +3 and a +4 state. Copper itself is chemically resistant, but its oxidation states show tendencies to react. It can be plated out of solution or replaced by a more active metal.

Properties

Physical properties

Copper is a reddish metal that is highly malleable and ductile. It can take a high polish, but often appears dull. Older pieces may be coated with the black copper(II) oxide. It has extremely high conductance of both heat and electricity.

Chemical properties

Copper is notable for its varied chemistry. Copper metal will not dissolve in strong acids, but the addition of an oxidizer such as hydrogen peroxide permits the dissolution of the metal in most acids.

Two copper compounds, tetraamminecopper(II) sulfate and tetrachlorocupric acid.

Nitric acid is a sufficiently strong oxidizer to dissolve copper metal. In both cases copper will exist in the +2 (cupric) state, the most common for copper. Compounds of copper(II) are generally bluish.

When a sufficiently strong reducer, such as iodide or ascorbic acid, is used to oxidize copper, it may enter the +1 (cuprous) state. Copper(I) compounds are generally insoluble in water and highly reducing. Most of them are white, though copper(I) chloride may vary in color due to various impurities, and copper(I) oxide is red, or yellow when prepared by certain routes.

Copper(II) ions turn an ordinary flame blue or green.

Copper also has extensive coordination chemistry in both the +1 and +2 states. While copper(I) compounds are insoluble in water, the proper ligand, such as ammonia, cyanide, thiosulfate, or chloride, the copper(I) ion becomes soluble in water. Copper(II) compounds undergo dramatic color changes with the addition of proper ligands: with chloride, a green complex forms (though a red complex, trichlorocuprate, is known to exist); with ammonia and ethylenediamine, a dark blue complex forms, with bromide, a maroon complex forms, and with ascorbic acid, a yellow complex forms.

Copper(III) compounds can be made in the amateur chemist's lab with sodium persulfate and potassium periodate.

One copper(IV) compound is known to exist, but is inaccessible to the amateur due to the requirement of fluorine gas.

Availability

28 gram 3N copper bar

Copper is a common coinage material, and can be found in United States pennies made before 1983. These are an alloy of 95% copper and 5% tin.

Several types of wiring contain copper metal. These wires may be glazed and will require removal before use.

Many electronics contain copper, such as transistors (the back metal plate that is in contact with the heat sink or covered by the plastic case), diodes (the wire), pins (though most are brass), rectifiers, PCBs, computer chips, on rare occasions some heat sinks are made of copper (or brass) etc. Some transistor heat sinks sometimes have an extra copper plate, for better heat dispersion.

Preparation

Copper metal powder can be made from copper(II) sulfate, available as root killer for septic systems, with the addition of either zinc metal or aluminium metal and a chloride source (such as table salt).

Copper powder can be made by reducing a copper salt, such as copper sulfate with iron. If impure iron, such as steel is used, the carbon from the steel will contaminate the solution and may slightly passivate the metal. However the black powder also contains copper(II) oxide, which too is black. The copper powder will require washing, though a simple decantation will suffice, as copper is denser than both contaminants.

Under certain circumstances, a copper foil may be produced instead of powder, usually if copper(II) chloride is used, in an acidic solution.

Clean, superfine, and air-stable copper powder can be created by adding ascorbic acid (vitamin C) to a solution of copper sulfate followed by boiling for a time. An excess of ascorbic acid yields the copper powder and a yellow solution; the composition of this solution has yet to be determined.

Projects

• Illustrations of Le Chatelier's principle
• Growing crystals
• Copper soap
• Copper alloys

A piece of copper, showing some minor oxidation

Handling

Safety

Copper metal poses little toxicity and clean copper metals are known to have antiseptic properties, thought not as powerful as silver.

Copper compounds are moderately toxic. They are irritant on contact with the eyes, mucous or inhaled.

Storage

Copper metal will develop a thin layer of oxide on its surface, that is usually not a problem and can be easily removed. However, if the copper metal was cleaned with acid before and hasn't been thoroughly washed, there is a risk of forming a patina. Copper powder is more sensitive and in moist air will rapidly oxidize to CuO. It is best stored in closed containers, either in inert gas (CO₂ is enough) or under water or any other solvent. In case of the latter, there is the problem when drying the powder, as it may oxidize if done at a temperature too high in open air.

Hygroscopic copper compounds should be stored in sealed containers, to prevent them from absorbing the water from air.

Disposal

Special disposal procedures are required with copper compounds. All copper waste should be converted to the +2 state with peroxide and converted to the carbonate. This waste should never be dumped down the drain: rather it should be taken to the proper disposal facility or recycled.

Recycling copper back to metallic form is easy: dissolve the copper(II) carbonate in a suitable acid, such as sulfuric acid, and then add in a reducing agent. Iron, zinc or aluminium are a good choice as they're cheap and available in large quantities. If aluminum is used, a chloride source should be added to promote replacement and break the protective Al₂O₃ layer.

See also

• Copper compounds