Atomic number 29

Copper

Cu
+ phase
− phase

Shells Orbitals 3d 4s

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29 electrons in 4 shells: 2, 8, 18, 1.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Transition metal
Period: 4
Group: 11
Block: d
Standard atomic weight: 63.546(3) Da
Electron configuration: [Ar] 3d¹⁰ 4s¹
Phase at 25 °C: solid
Melting point: 1084.6 °C
Boiling point: 2562 °C
Density: 8.933 g/cm³
Electronegativity (Pauling): 1.9
Discovered: Prehistoric times, in use for at least 11,000 years

Neighbours in the table

Ni Zn

Four shells hold copper’s 29 electrons, 2, 8, 18 and 1: [Ar] 3d¹⁰ 4s¹. Each shell divides into subshells, levels of slightly different energy, and the usual filling order would put a pair in 4s and nine electrons in 3d. Copper instead moves one of that pair across and completes the 3d subshell. OpenStax traces the exception to two subshells of very similar energy, where small effects change the filling order. A filled subshell apparently brings extra stability, and no simple rule predicts where such swaps occur.

When copper forms ions that lone 4s electron goes first: transition elements give up their outer s electrons before any d electron, though the s electrons were added first. PubChem gives +2 and +1 as its common oxidation states, a bookkeeping number that treats every bond as ionic; the Cu²⁺ ion colours water blue. Nobody discovered copper: Jefferson Lab’s text on the PubChem record has it in use for at least 11,000 years and won from ore for at least 7,000.

Los Alamos calls it reddish and lustrous, and ranks it behind only silver as an electrical conductor. A magnet is not drawn to it. OpenStax’s physics volume lists copper among diamagnetic materials, whose atoms carry no permanent magnetic moment and take on only a small induced one, pointing against the field.

Where it is found

PubChem’s element record puts copper at 60 milligrams in a kilogram of crust and 2.5 × 10⁻⁴ milligrams in a litre of seawater. Los Alamos says copper occasionally occurs native, as the uncombined metal. The most important ores are the sulfides, covellite (CuS) among them, with oxides such as tenorite (CuO) and hydroxycarbonates such as malachite (Cu₂(OH)₂CO₃) found less often.

The US Geological Survey’s estimate for 2024 puts Chile first, at 5.3 million metric tons of copper content, then Congo (Kinshasa) at 3.3 million and Peru at 2.6 million. The world total is about 23 million metric tons. The United States mined 1.1 million, some 70 per cent of it in Arizona. Scrap provided about 35 per cent of the country’s supply.

From those ores the metal is won by smelting, leaching and electrolysis. Copper of 99.999 per cent purity or better is sold commercially. PubChem’s record adds a prospecting tool: the proportions of copper-63 and copper-65 vary measurably between natural materials, and those ratios are used to rank prospects and focus drilling.

Why it matters

Los Alamos names the electrical industry as one of copper’s greatest users, and wiring and plumbing as its household forms. Aluminium conducts at about 60 per cent of copper’s level, yet it replaces copper in transmission lines, where its light weight matters more. Optical fibre substitutes for it in telecommunications, plastics in water pipe.

Pure copper is too soft for most purposes, and people found about 5,000 years ago that mixing in another metal strengthens it. Bronze, copper with up to 25 per cent tin, was the first alloy anyone made. Brass, copper with 5 to 45 per cent zinc, came about 2,500 years ago, and the Romans first used it widely. It still goes into musical instruments, screws and hardware that must resist corrosion.

Delivered from France, the Statue of Liberty was brown, the colour of its copper skin. Oxygen turned the surface first to red copper(I) oxide, Cu₂O, then to black copper(II) oxide. Carbon dioxide, water and sulfur trioxide from coal smoke then converted that oxide to two carbonates, one green and one blue, and a green sulfate: the patina. The patina seals the copper beneath it, a process called passivation; rust, by contrast, flakes off and exposes fresh iron.

Properties

Properties of copper Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	29
Standard atomic weight	63.546(3) Da	Da	IUPAC/CIAAW 2024
Melting point	1084.6 °C	°C	Los Alamos: 1084.6 °C
Boiling point	2562 °C	°C	Los Alamos: 2562 °C
Density	8.933 g/cm³	g/cm³
Electronegativity (Pauling)	1.9
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰
Phase at 25 °C	solid

Safety

Copper in wire, pipe and coins is safe to handle; its safety summary cites a review that found little evidence of a serious industrial hazard from the metal. The summary’s cautions belong to the workplace and to water: dusts and mists carry a NIOSH exposure limit, and fumes can bring on metal fume fever. Copper or its salts can set off contact dermatitis in susceptible people, and copper powder is classed as very toxic to aquatic life. The PubChem Laboratory Chemical Safety Summary and the NIOSH card for copper dusts and mists are the sheets to consult.

Questions

Is copper a transition metal?

By IUPAC's definition, not quite. That definition asks for partially filled d orbitals, and a copper atom's 3d subshell is full. OpenStax therefore notes that copper, silver and gold are not technically transition elements. The term is used for the whole d block anyway, and a broader definition covers any element that has, or readily forms, partially filled d orbitals. Either way, the d orbitals finish filling with the copper family, group 11.

Why is copper such a good conductor of electricity?

Copper is a metallic solid. In OpenStax's picture its atoms sit as nuclei in a sea of delocalised electrons that are free to move, and every such solid conducts heat and electricity well. The periodic-trends chapter adds that conductivity and malleability depend on having electrons that are easy to remove. Los Alamos ranks copper second only to silver as an electrical conductor.

Are pennies made of copper?

Older ones are. PubChem's safety data for copper put the change at 1982: United States pennies minted before then are made of copper, and those minted since are zinc with a copper coating. Jefferson Lab gives the reason coins used copper at all, its resistance to corrosion by air, moisture and seawater. Los Alamos adds that every American coin is a copper alloy.

Does the human body need copper?

Yes, in small amounts. OpenStax's anatomy text lists it among the trace minerals. Seafood, organ meats, nuts, legumes and chocolate are among the foods that supply it. Two plasma proteins, hephaestin and ceruloplasmin, contain copper. They oxidise iron from the +2 to the +3 state, the form transferrin can carry, so a shortage of copper slows the delivery of iron for haem and shows up as anaemia.

How is copper extracted from its ore?

Concentrated sulfide ore, OpenStax says, is roasted to drive off part of its sulfur as sulfur dioxide, then heated with limestone, which carries off iron and silica as slag. Air blown through the molten copper(I) sulfide converts some of it to copper(I) oxide, and the two react to leave metal. Trapped air blisters give blister copper its name. Cast into plates, it becomes the anodes of electrolytic refining, which also recovers silver.

Compounds containing copper

CuSO₄ Copper(II) sulfate 6 atoms in one formula unit of an ionic lattice.