Metal
A metal is something you can know by sight and touch. Polish it or fracture it, and a lustrous surface appears. Bend it, and it gives without shattering. Run a current through it, and electricity flows freely. Run heat along it, and warmth travels fast. These behaviors trace back to one quiet fact: metals keep electrons available at the Fermi level, while nonmetallic materials do not. The story of metals begins around 11,000 years ago with copper, and it has not stopped widening since. Sodium, the first light metal, joined the list in 1809. By summer of 2024, palladium and platinum were trading at slightly less than half the price of gold. Along the way, the word itself splits into a chemist's strict definition and a builder's loose one. So what makes a length of steel and a glowing plasma in a white dwarf star members of the same family? Why are some metals brittle when nearly all the rest can be drawn into wire? And how did a material as familiar as aluminium stay a jewelry curiosity until 1886? The answers run from inside the atom to the core of the Earth.
Cool a metal to absolute zero and it still conducts electricity. That single fact comes from delocalized states sitting at the Fermi energy. When no voltage is applied, electrons in a material carry many different momenta that average to zero. Apply a voltage, and some electrons shift to states with slightly higher momentum along the field while others slow. The result is a net drift velocity, and that drift is electric current.
The Pauli exclusion principle sets the rule that makes this possible. No two electrons can share the same quantum state, so an electron can only move to a higher-momentum state if that state is empty. In metals, exactly such empty delocalized states wait at energies near the highest occupied levels. Semiconductors like silicon and nonmetals like strontium titanate lack this convenience. They carry an energy gap between the filled valence band and the empty conduction band, and a small electric field cannot push electrons across it.
Elemental metals span a wide range of conductivity, from 6.9 times ten cubed siemens per centimeter for manganese to 6.3 times ten to the fifth for silver. A semiconducting metalloid like boron sits far below at 1.5 times ten to the minus sixth. Heating usually drops a metal's conductivity, because jostling atoms obstruct the flowing electrons. Plutonium breaks the pattern. Between roughly minus 175 and plus 125 degrees Celsius its conductivity rises with heat, accompanied by an unusually large thermal expansion and a shift from monoclinic to face-centered cubic structure near 100 degrees Celsius. Such oddities in the transuranic elements are blamed on relativistic and spin effects that simple models miss.
The same conducting electrons also carry heat, which is why metals warm so evenly. The empirical Wiedemann-Franz law captures this link, holding that the ratio of thermal to electrical conductivity rises with temperature by a constant that is nearly the same across many metals. Predicting these numbers from scratch starts with the free electron model and climbs toward density functional theory once the ion lattice can no longer be ignored.
Hammer a metal and it flattens. Pull it and it stretches into wire. This malleability and ductility come from metallic bonding, which points in no particular direction. The Peierls stress stays low, dislocations move easily, and many combinations of planes and directions open up for plastic deformation. Because the atoms pack closely, the Burgers vector of each dislocation is small, so little energy is needed to create one.
Table salt shows the opposite case. In an ionic compound the Burgers vectors are far larger, and moving a dislocation costs far more energy, which is why such crystals cleave instead of bending. Within a metal's elastic range, Hooke's law describes the springiness well: stress stays linearly proportional to strain up to the proportional limit. Push past that, and the metal yields for good.
Temperature stirs the hidden defects inside a metal. Grain boundaries, point vacancies, line and screw dislocations, stacking faults, and twins can all shift with heat, in crystalline and non-crystalline metals alike. Internal slip, creep, and metal fatigue may follow. The atoms themselves usually settle into one of three arrangements: body-centered cubic, where each atom sits at the center of a cube of eight; or face-centered cubic and hexagonal close-packed, where each atom touches twelve neighbors and only the layer stacking differs. Some metals trade one structure for another as the temperature changes.
A handful of elemental metals refuse to bend at all. Beryllium, chromium, manganese, gallium, and bismuth are brittle, and arsenic and antimony join them if counted as metals. A low ratio of bulk to shear modulus, called Pugh's criterion, flags this intrinsic brittleness. Brittleness arises when dislocations struggle to move, often paired with large Burgers vectors and few slip planes to travel along.
A metal need not be a single element at all. It may be iron, an alloy like stainless steel, or even a molecular compound such as polymeric sulfur nitride. Most pure metals are too soft, too brittle, or too reactive to use as they are. Mixing metals with other elements tunes the result toward whatever is wanted: more ductile, harder, corrosion-resistant, or simply a better color and luster.
Iron's alloys dominate everything else by quantity and by value. Steel, stainless steel, cast iron, tool steel, and alloy steel all begin with iron and carbon, where rising carbon levels trade away ductility and toughness. Add silicon and the result is cast iron. Add more than ten percent chromium along with nickel and molybdenum, and the carbon steel becomes stainless steel that resists corrosion. The most demanding parts, such as jet engines, may draw on alloys containing more than ten elements.
Copper alloys reach back into prehistory, and bronze gave the Bronze Age its name. Today copper alloys matter most in electrical wiring. The alloys of aluminium, titanium, and magnesium arrived much more recently, held back by chemical reactivity that forces electrolytic extraction. Their reward is a high strength-to-weight ratio, prized in aerospace and some automotive work, with magnesium offering electromagnetic shielding as a bonus.
Beyond the elements and their blends sit stranger members. Conducting ceramics qualify as metals because they too carry delocalized states at the Fermi level. Titanium nitride is one, with electrical conductivity rivaling elemental metals, finding use in orthopedic devices and as a wear-resistant coating. Some polymers conduct as well, their aromatic regions behaving much like graphite, so their conduction runs in strongly preferred directions. Liquid mercury conducts, and so does a plasma, whose charged particles echo the electrons inside elemental metals, especially within white dwarf stars.
Ferrous comes from the Latin word for containing iron, and it splits the whole family in two. Ferrous metals run from pure wrought iron to alloys like steel, and they are often magnetic, though not always. Non-ferrous metals and alloys lack any appreciable iron.
Refractory metals resist heat and wear with stubborn endurance. The common definition gathers niobium, molybdenum, tantalum, tungsten, and rhenium, each melting above 2000 degrees Celsius and staying hard at room temperature. Compounds like titanium nitride sometimes earn the same label. White metals take a different role entirely, a range of white-colored alloys with low melting points used for decoration. In Britain, the fine art trade writes white metal in auction catalogues for foreign silver that lacks British Assay Office marks but is understood, and priced, as silver.
Base metals oxidize or corrode easily, reacting with dilute hydrochloric acid to give a chloride and hydrogen. Iron, nickel, lead, and zinc fit the label, and copper counts too even though it does not react with that acid. Noble metals stand opposite, resisting corrosion and oxidation, and they often double as precious metals owing to perceived rarity. Gold, platinum, silver, rhodium, iridium, and palladium belong here. In alchemy and numismatics the contrast becomes base against precious, the latter prized for economic value. Gold, silver, platinum, and palladium each hold an ISO 4217 currency code, while most coins today are struck from base metals of low intrinsic worth.
Half-metals and semimetals sit at the family's odd corners. A half-metal conducts electrons of one spin while insulating the other, first described in 1983 to explain manganese-based Heusler alloys. Every half-metal is ferromagnetic or ferrimagnetic, though most ferromagnets are not half-metals. A semimetal carries a small energy overlap between conduction and valence bands without overlapping in momentum space, giving it both holes and electrons in modest numbers. Arsenic, antimony, bismuth, gray tin, and graphite are the classic examples, alongside compounds like mercury telluride.
Metallic elements up to the neighborhood of iron are forged inside stars. Light elements from hydrogen to silicon fuse in sequence, pouring out light and heat while building heavier nuclei. Past iron the arithmetic flips, since fusing such nuclei would consume energy rather than release it. Heavier elements instead form by neutron capture, in two modes named for their pace.
The s-process, where the s stands for slow, spaces single captures years or decades apart, letting unstable nuclei beta decay between them. Stable cadmium-110 absorbs neutrons until it reaches cadmium-115, which decays to nearly stable indium-115, whose half-life is 30,000 times the age of the universe. The chain climbs on through indium-116 to tin-116 and beyond, stopping at bismuth because polonium and astatine decay too quickly to continue. The r-process, the rapid one, captures neutrons faster than nuclei can decay, skipping that zone of instability to build thorium and uranium. Stars later shed much of their mass, and neutron star mergers add more, seeding the interstellar medium with heavy elements that gravity gathers into new stars and planets.
The Earth's crust runs about 25 percent metallic elements by weight, and 80 percent of those are light metals such as sodium, magnesium, and aluminium. Metallic elements sort themselves by chemical loyalty. Lithophiles love oxygen and rock, mostly s-block, reactive d-block, and f-block elements living in low-density silicate minerals. Chalcophiles favor sulfur, the less reactive d-block elements and period 4 to 6 p-block metals, hiding in insoluble sulfide minerals and sinking deeper as the crust solidified.
Gold follows a third path as a siderophile, an iron-loving element that bonds with neither oxygen nor sulfur. At the Earth's formation, the most noble of metals sank into the core in dense alloys, which is why gold is rare. That core, mostly iron and rotating as fluid, is thought to generate the planet's magnetic field. Stretched into a column with a five square meter footprint, the core would stand nearly 700 light years tall, and the field it produces shields the atmosphere and its ozone layer from the solar wind and cosmic rays.
Bauxite is one of the ores that feed the world's metals, found by prospecting and then explored and examined as deposits. Mines fall into surface pits worked by heavy excavation and subsurface workings underground. Sometimes the selling price makes even a low-concentration source worth digging.
Once the ore is out, the metal must be pried from it. Pyrometallurgy uses high heat to turn ore into raw metal, while hydrometallurgy does the same with aqueous chemistry. An ionic ore is usually smelted, heated with a reducing agent, and iron and many common metals reduce with carbon. Aluminium and sodium have no practical reducing agent, so they yield only to electrolysis. Sulfide ores take a preliminary step, roasted in air to become oxides before reduction.
Demand for metals tracks economic growth, since they fill infrastructure, construction, manufacturing, and consumer goods. Through the 20th century the variety of metals in use grew quickly, and nations like China and India now push that demand higher. More of the world's metal stock now sits above ground in use rather than below as reserves. Copper in use in the United States rose from 73 grams to 238 grams per person between 1932 and 1999.
Metals can be used again and again, which saves energy and spares the environment. Recycling aluminium from old material saves 95 percent of the energy needed to make it fresh from bauxite. Yet global recycling stays low. In 2010 the International Resource Panel, hosted by the United Nations Environment Programme, called society's metal stocks huge mines above ground. The report warned that recycling rates for rare metals in mobile phones, hybrid car battery packs, and fuel cells are so low that, without sharp improvement, these critical metals could vanish from modern technology.
Copper may have been first, noticed for its color, weight, and malleability, with gold, silver, meteoric iron, and lead following in prehistory. Brass came from smelting copper and zinc ores together, even though pure zinc waited until the 13th century to be isolated. Meteoric iron carrying nickel sometimes outperformed any industrial steel made up to the 1880s. Bronze of copper and arsenic appears earliest on the Iranian plateau in the fifth millennium BCE, and tin became the main partner only in the late third millennium BCE. The earliest known steel, nearly 4,000 years old, comes from Kaman-Kalehoyuk in Anatolia, dated to 1800 BCE. From about 500 BCE the sword-makers of Toledo, Spain added wolframite to their iron, and the resulting Toledo steel armed Roman legions after Hannibal used it in the Punic Wars.
Writers slowly pinned down what a metal even was. Around 340 BCE, in Book III of his Meteorology, Aristotle divided what lay underground into metals and minerals. Arabic and medieval alchemists held that every metal blended a principle of sulfur, the combustible father, with a principle of mercury, the liquid mother, and believed all metals were ripening toward gold underground. Albertus Magnus is thought to have isolated arsenic in 1250 by heating soap with arsenic trisulfide. The first systematic text on mining and metallurgy was Vannoccio Biringuccio's De la Pirotechnia of 1540, followed sixteen years later by Georgius Agricola's De Re Metallica in 1556. In his earlier De Natura Fossilium of 1546, Agricola counted six traditional metals, gold, silver, copper, iron, tin, and lead, while arguing that quicksilver and bismuth deserved the name too.
The count climbed faster as chemistry matured. Platinum, the third precious metal, was found in Ecuador between 1736 and 1744 by the Spanish astronomer Antonio de Ulloa and the mathematician Jorge Juan y Santacilia, with Ulloa writing the first scientific description in 1748. In 1789 Martin Heinrich Klaproth isolated an oxide he took for uranium itself, though Eugene-Melchior Peligot prepared the first uranium metal only in 1841, and Henri Becquerel found radioactivity using uranium in 1896. Cerium, the first lanthanide, was discovered in 1803 at Bastnas, Sweden by Jons Jakob Berzelius and Wilhelm Hisinger, and independently by Klaproth.
The last stable metals came hard. By 1900 only elements 71, 72, and 75 remained below lead, the heaviest stable metal. Von Welsbach proved in 1906 that old ytterbium hid element 71, and Urbain's name lutetium prevailed. Rhenium, element 75, was correctly named in 1925 by Walter Noddack, Ida Eva Tacke, and Otto Berg. Hafnium, element 72, was the last stable element discovered, located in Norwegian zircon by Coster and Hevesy in 1922. By the end of World War II scientists had synthesized neptunium, plutonium, curium, and americium, and curium and americium were by-products of the Manhattan project that built the first atomic bomb in 1945.
Henry Bessemer's process of 1855 opened the modern era of steel, turning pig iron into cheap steel in great quantity, so mild steel replaced wrought iron almost everywhere. The Gilchrist-Thomas process improved it by lining the converter with a basic material to strip out phosphorus. Stainless steel took longer. Clark and Woods patented an alloy in 1872 that would now count as stainless, and Pierre Berthier had noted the corrosion resistance of iron-chromium alloys back in 1821, but industrialization waited until 1912 in England, Germany, and the United States.
Light metals upended old assumptions. Every elemental metal known before 1809 was relatively dense, and heaviness was treated as a defining trait, until sodium, potassium, and strontium arrived from 1809 onward and behaved as metals despite their lightness. Aluminium was discovered in 1824, yet large-scale production waited until 1886, after which falling prices put it into jewelry, eyeglass frames, tableware, and foil, and World War I governments demanded it for light, strong airframes. Pure titanium was first prepared in 1910 but stayed in the laboratory until 1932, then entered military aviation in the early 1950s with aircraft like the F-100 Super Sabre and the Lockheed A-12 and SR-71. Scandium metal first appeared in 1937, with aluminium-scandium alloy production beginning in 1971.
After World War II the inventions grew exotic. Superalloys of iron, nickel, cobalt, and chromium, with touches of tungsten and other metals, held their strength above 650 degrees Celsius in high-performance engines. The first metallic glass, an alloy of gold and silicon, was produced at Caltech in 1960, and its disordered atomic structure still conducts electricity well, with ferromagnetic versions cutting losses in high-efficiency transformers. Shape-memory alloys, first seen in 1932 in a gold-cadmium alloy, took off after the effect turned up in a nickel-titanium alloy in 1962. In 1984 the Israeli metallurgist Dan Shechtman found an aluminium-manganese alloy with five-fold symmetry that defied crystallographic convention, a quasicrystal that took two years to publish and earned him the Nobel Prize in Chemistry in 2011.
The newest families push toward many elements at once. High-entropy alloys such as AlLiMgScTi mix five or more metals in nearly equal parts, a name coined by the Taiwanese scientist Jien-Wei Yeh, and research into them accelerated sharply in the 2010s. Complex metallic alloys build vast unit cells, like NaCd2 with 348 sodium and 768 cadmium atoms, a structure Linus Pauling began describing in 1923 and only solved in 1955. MAX phases combine an early transition metal, an A-group element, and either carbon or nitrogen, as in Ti3SiC2, polishing to a metallic luster and resisting thermal shock. Icosahedrite, the first quasicrystal found in nature, was discovered in 2009, proof that the catalogue opened by copper 11,000 years ago is still gaining members.
Common questions
What is the scientific definition of a metal?
A metal is a material that shows a lustrous appearance when polished or fractured and conducts electricity and heat relatively well. These properties come from having electrons available at the Fermi level, unlike nonmetallic materials. A metal can be a chemical element such as iron, an alloy such as stainless steel, or a molecular compound such as polymeric sulfur nitride.
Why are most metals malleable and ductile?
Most metals are malleable and ductile because metallic bonding is non-directional, which keeps the Peierls stress low and lets dislocations move easily. Close-packed atoms give dislocations small Burgers vectors, so little energy is needed to deform the metal. By contrast, ionic compounds like table salt have much larger Burgers vectors and cleave instead of bending.
When did the history of refined metals begin?
The history of refined metals is thought to begin with the use of copper about 11,000 years ago. Gold, silver, iron as meteoric iron, lead, and brass were in use before bronze first appeared in the fifth millennium BCE. Sodium, the first light metal, was discovered in 1809.
What is the difference between base metals and noble metals?
Base metals oxidize or corrode easily, such as iron, nickel, lead, and zinc, and many react with dilute hydrochloric acid to form a chloride and hydrogen. Noble metals like gold, platinum, silver, rhodium, iridium, and palladium resist corrosion and oxidation. Noble metals tend to be precious metals, often because of perceived rarity.
How are heavy metallic elements formed in stars?
Metallic elements up to around iron are made by stellar nucleosynthesis, where light elements from hydrogen to silicon fuse and release energy. Heavier elements form by neutron capture in the slow s-process and the rapid r-process. The r-process can skip the zone of instability at bismuth to build heavier elements such as thorium and uranium.
Why is recycling metals important and how much energy does it save?
Metals are inherently recyclable and can be used repeatedly, which saves energy and reduces environmental impact. Recycling aluminium from old material saves 95 percent of the energy needed to make it from bauxite ore. In 2010 the International Resource Panel warned that low recycling rates for rare metals in mobile phones, hybrid car batteries, and fuel cells could make these critical metals unavailable.
What new types of metal alloys were developed after World War II?
After World War II metallurgists developed superalloys, bulk metallic glasses, shape-memory alloys, quasicrystalline alloys, complex metallic alloys, high-entropy alloys, and MAX phases. The first metallic glass, a gold-silicon alloy, was produced at Caltech in 1960. Dan Shechtman discovered an aluminium-manganese quasicrystal in 1984 and won the Nobel Prize in Chemistry in 2011.