Copper
Copper is a chemical element with symbol Cu (from Latin:cuprum) and atomic number 29. It is a soft, malleable and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a reddish-orange color.
Identity
- Atomic mass
- 63.546 u
- Category
- Transition metals
- Group
- 11
- Period
- 4
- Block
- d
- Oxidation States
- +1, +2
- Phase (25 °C)
- Solid
- Appearance
- red-orange metallic luster
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s1 3d10
- Short configuration
- [Ar] 3d10 4s1
- Electrons per shell
- 2, 8, 18, 1
- Electronegativity (Pauling)
- 1.90
- Electron affinity
- 119 kJ/mol
- 1st ionization energy
- 746 kJ/mol
- Ionization energies
- 746 · 1958 · 3555 · 5536 · 7700… kJ/mol
Physical properties
- Melting point
- 1084.6 °C (1358 K)
- Boiling point
- 2561.8 °C (2835 K)
- Density
- 8.96 g/cm³
- Molar heat capacity
- 24.4 J/(mol·K)
Chemical behavior of copper
Copper has an anomalous electronic configuration ([Ar] 3d¹⁰ 4s¹), where an electron from the 4s orbital promotes to the 3d subshell to achieve quantum stability of a completely filled d subshell. Its characteristic oxidation states are +1 (cuprous) and +2 (cupric), with the Cu²⁺ ion being the most stable and common in aqueous solution due to its high hydration enthalpy. It belongs to group 11 (coinage metals) and presents low general chemical reactivity, resisting atmospheric corrosion through the gradual formation of a passive basic salt patina.
Reactivity
2 Cu + O₂ → 2 CuO
On heating to redness in air, copper oxidises on the surface forming a black copper(II) oxide layer; at lower temperatures it can form reddish copper(I) oxide.
Cu + 2 AgNO₃ → Cu(NO₃)₂ + 2 Ag
It displaces metals with lower reduction potential such as silver from their saline solutions, precipitating pure metallic silver needles while the solution takes an intense blue tone.
2 Cu + O₂ + CO₂ + H₂O → Cu₂CO₃(OH)₂
In humid atmospheres with carbon dioxide, over the years it develops the classic green patina of basic copper carbonate (malachite), which protects the metal from further corrosion.
Cu + 4 HNO₃ → Cu(NO₃)₂ + 2 NO₂ + 2 H₂O
It is not attacked by non-oxidising acids such as hydrochloric acid, but it dissolves quickly with concentrated nitric acid, releasing dense brown nitrogen dioxide fumes.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Copper(II) sulfate pentahydrate | CuSO₄·5H₂O | Blue vitriol; classic agricultural fungicide (Bordeaux broth), electroplating electrolyte and chemical reagent |
| Copper(II) oxide | CuO | Ceramic black pigment, catalyst in chemical synthesis and material precursor in superconducting ceramics |
| Copper(II) chloride | CuCl₂ | Mordant in textile dyeing, industrial catalyst in oxidation processes and blue pyrotechnic colourant |
| Copper(I) oxide | Cu₂O | Anti-fouling paints for ship hulls, red pigment in stained glass and photovoltaic semiconductor |
Electron shell distribution
View in the atom builderIsotopes
Natural copper is composed of two stable isotopes: copper-63 (⁶³Cu, with 34 neutrons and 69.15% abundance) and copper-65 (⁶⁵Cu, with 36 neutrons and 30.85%). Several tens of synthetic radioisotopes are known, among which copper-64 (⁶⁴Cu, half-life of 12.7 hours) is particularly valued in nuclear medicine as a theranostic radiopharmaceutical combining positron emission for PET and beta radiation for focused radiotherapy.
Occurrence and production
It is a relatively scarce element (about 50-60 ppm in the Earth's crust), but concentrated by hydrothermal processes. Although it can appear as native copper free, the vast majority is extracted from sulphide minerals such as Chalcopyrite (CuFeS₂), bornite and chalcocite, or from oxidised minerals such as malachite and azurite. The world's largest deposits are exploited by open-pit mining in the Andes, especially in Chile and Peru. Its final refining is carried out by flotation, melting and electrolytic electrodeposition of cathodes with 99.99% purity.
History and origin of the name
It is one of the first metals used by humanity, with usage testimonies exceeding 10,000 years. Its easy cold forging and subsequent reduction marked the transition to the Chalcolithic and the Bronze Age (when alloyed with tin). The ancient Romans obtained much of their reserves from the Cyprus, naming it 'aes cyprium' (Cyprus metal), a phrase that evolved to 'cuprum', from which its universal chemical symbol Cu derives.
Applications and relevance of copper
Uses of the element
Due to its exceptional electrical and thermal conductivity (only surpassed by silver), ductility and fatigue resistance, copper is the basic metal of electrification: electrical wiring, motor and generator windings, printed circuit boards, telephony and telecommunications. It is also massively used in water and gas pipes, heat exchangers and architectural clothing.
Uses of its compounds
Copper sulfate is widely used in agriculture as a fungicide and algicide, and as a veterinary vitamin-mineral supplement. Cuprous oxide is incorporated into paints to prevent molluscs and algae from fixing on ship hulls; other copper complexes serve as catalysts in the petrochemical industry.
Health, safety and environment
Biological role
It is an essential trace element for animals and plants. It acts as a catalytic cofactor in vital electron transport enzymes such as cytochrome c oxidase and superoxide dismutase, and participates in the biosynthesis of haemoglobin, myelin and collagen. Its homeostasis is strictly regulated by transporter proteins such as ceruloplasmin.
Safety
In solid metallic form it is inert and biologically safe, presenting notable antimicrobial properties on contact. However, inhalation of fine dusts or soluble copper salts at high doses can cause acute gastroenteritis, liver damage and haemolysis; foundry fumes cause the so-called metal fever.
Environment
Although it is a nutrient at low concentrations, free Cu²⁺ ion is very toxic to aquatic invertebrates, fish and algae in rivers and estuaries. Mining requires containment pools for tailings to prevent acid leaching and dispersion of heavy metals into river basins.
Frequently asked questions about Copper
Why does copper have such a characteristic reddish colour?
Almost all metals are silver or grey because they reflect all visible wavelengths of light equally. In copper, the distribution of its electrons absorbs part of the blue and green light from the spectrum. As those cold colours are not reflected, the reflected light reaches our eyes enriched in red, orange and golden tones.
What is Wilson's disease and what is its relation to copper?
Wilson's disease is a rare hereditary disorder that prevents the body from correctly eliminating excess copper. As a result, this metal can accumulate especially in the liver and brain and cause serious damage if not treated. Symptoms vary, but can include liver problems, movement disorders or behavioural alterations. Early diagnosis and adequate medical treatment help control copper accumulation.
What are the advantages of recycling copper compared to primary mining?
Copper is 100% recyclable without any loss of its mechanical, electrical or purity properties. Melting and refining scrap copper requires up to 85% less energy than processing virgin mineral from the mine. It also avoids crushing tonnes of rock for each kilogram of metal, eliminates the sulphur dioxide emissions associated with smelting and drastically reduces water consumption and the generation of mining waste.
How is radioactive copper used in cancer research?
Some radioactive copper isotopes, such as copper-64, are studied in nuclear medicine. They can be bound to molecules designed to locate certain tissues or processes in the body, which allows imaging techniques to investigate diseases, including some tumours. Possible therapeutic applications are also being studied, but not all these methods are routine treatments. Their usefulness depends on the type of tumour and the isotope combined with the molecule.
Why does copper kill bacteria and viruses on surfaces?
Copper has a very effective natural antimicrobial capacity. When a microorganism settles on a copper surface, the metal's ions quickly perforate the protective membrane of the bacterium or the envelope of the virus. Once inside, these ions damage its vital proteins and destroy its genetic material, eliminating the germs in a few hours without the need for disinfectants.
Why is bronze usually harder than pure copper?
Pure copper is relatively soft and can deform easily. When mixed with tin, bronze is obtained, an alloy that is usually harder and more resistant to wear. It also melts and moulds differently, so it was very useful for making tools, bells, sculptures and other pieces. Its exact properties depend on the proportion of tin and how the metal has been worked.
Why is the Statue of Liberty green if it is made of copper plates?
When the Statue of Liberty was installed in 1886 in New York, it shone with bright copper colour. Over decades, continuous exposure to moist sea air, oxygen, carbon dioxide and sulphur compounds produced successive oxidation reactions. These transformations formed a superficial mineral layer of malachite, brochantite and antlerite. This green patina does not weaken the internal structure; rather, it acts as an impermeable shield that stops the corrosion of the underlying copper.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.