Metal
Definition
A metal (from Greek μέταλλον métallon, "mine, quarry, metal") is a material that, when freshly prepared or fractured, has a lustrous appearance, and which is a relatively good conductor of heat and electricity, and is usually malleable (it can be hammered into thin sheets) or ductile (it can be drawn into a wire). A metal may be a chemical element such as gold, or an alloy such as stainless steel, or a compound alloy such as nickel aluminide.
In physics, a metal is regarded as any substance capable of conducting electricity at a temperature of absolute zero.
In chemistry, two elements that would otherwise qualify as brittle metals—arsenic and antimony—are commonly instead recognised as metalloids, on account of their predominately non-metallic chemistry. Around 95 of the 118 elements in the periodic table are metals; the others are nonmetals or metalloids, though elements near the boundaries of each category have been assigned variably to either (hence the lack of an exact count).
Astrophysicists use the term "metal" to refer collectively to all chemical elements in a star that are heavier than the lightest two, hydrogen and helium, and not just traditional metals. A star fuses lighter atoms, mostly hydrogen and helium, to make heavier atoms over its lifetime. Used in that sense, the metallicity of an astronomical object is the proportion of its matter made up of the heavier chemical elements.
Many elements and compounds that are not normally classified as metals become metallic under high pressures. For example, physicists were able to keep hydrogen in its solid state under more than 3 million times the atmospheric pressure and deduce its metallic properties.
The strength and resilience of some metals has led to their frequent use in high-rise building and bridge construction, as well as most vehicles, many home appliances, tools, pipes, non-illuminated signs and railroad tracks. Precious metals were historically used as coinage.
In the periodic table
The elements which are usually considered to be metals under ordinary conditions are shown in yellow on the periodic table below. The remaining elements are either metalloids (B, Si, Ge, As, Sb, and Te being commonly recognised as such in chemistry) or nonmetals. Astatine (At) is usually classified as either a nonmetal or a metalloid; it has been predicted to be a metal. It is here shown as a metalloid.
Metals–nonmetals in the periodic table | ||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | |||||||||||||||
| Group → | ||||||||||||||||||||||||||||||||
| ↓ Period | ||||||||||||||||||||||||||||||||
| 1 | H | He | ||||||||||||||||||||||||||||||
| 2 | Li | Be | B | C | N | O | F | Ne | ||||||||||||||||||||||||
| 3 | Na | Mg | Al | Si | P | S | Cl | Ar | ||||||||||||||||||||||||
| 4 | K | Ca | Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn | Ga | Ge | As | Se | Br | Kr | ||||||||||||||
| 5 | Rb | Sr | Y | Zr | Nb | Mo | Tc | Ru | Rh | Pd | Ag | Cd | In | Sn | Sb | Te | I | Xe | ||||||||||||||
| 6 | Cs | Ba | La | Ce | Pr | Nd | Pm | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu | Hf | Ta | W | Re | Os | Ir | Pt | Au | Hg | Tl | Pb | Bi | Po | At | Rn |
| 7 | Fr | Ra | Ac | Th | Pa | U | Np | Pu | Am | Cm | Bk | Cf | Es | Fm | Md | No | Lr | Rf | Db | Sg | Bh | Hs | Mt | Ds | Rg | Cn | Nh | Fl | Mc | Lv | Ts | Og |
Metal Metalloid Nonmetal Unknown propertiesBackground color shows metal–metalloid–nonmetal trend in the periodic table | ||||||||||||||||||||||||||||||||
Structure and bonding
The atoms of metallic substances are typically arranged in one of three common crystal structures, namely body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp). In bcc, each atom is positioned at the center of a cube of eight others. In fcc and hcp, each atom is surrounded by twelve others, but the stacking of the layers differs. Some metals adopt different structures depending on the temperature.
Atoms of metals readily lose their outer shell electrons, resulting in a free flowing cloud of electrons throughout the metallic lattice. The solid state of the metal is the net result of electrostatic interactions between each atom and the electron cloud. This type of interaction is called a metallic bond.
Crystal structures for some metals are listed in the table below
| Metal | Crystal Structure | Atomic Number | Atomic Radius (nm) |
|---|---|---|---|
| Aluminium | FCC | 13 | 0.1431 |
| Cadmium | HCP | 48 | 0.1490 |
| Chromium | BCC | 24 | 0.1249 |
| Cobalt | HCP | 27 | 0.1253 |
| Copper | FCC | 29 | 0.1278 |
| Gold | FCC | 79 | 0.1442 |
| Iron (α) | BCC | 26 | 0.1241 |
| Lead | FCC | 82 | 0.1750 |
| Molybdenum | BCC | 42 | 0.1363 |
| Nickel | FCC | 28 | 0.1246 |
| Platinum | FCC | 78 | 0.1387 |
| Silver | FCC | 47 | 0.1445 |
| Tantalum | BCC | 73 | 0.1430 |
| Titanium (α) | HCP | 22 | 0.1445 |
| Tungsten | BCC | 74 | 0.1371 |
| Zinc | HCP | 30 | 0.1332 |
Properties
Physical
Metals are shiny and lustrous, at least when freshly prepared or fractured. Sheets of metal thicker than a few micrometres appear opaque, but gold leaftransmits green light. Metals are relatively good conductors of electricity and heat. Typically they are malleable and ductile, deforming under stress without cleaving.
Although most metals have higher densities than most nonmetals, there is wide variation in their densities, lithium being the least dense solid element and osmium the most dense. The alkali and alkaline earth metals in groups 1 and 2 are referred to as light metals because they have low density. The high density of most metals is due to their tightly packed crystalline structures.
The strength of metallic bonds for different metals reaches a maximum around the center of the transition metal series, as those elements have large amounts of delocalized electrons in tight binding type metallic bonds. However, other factors (such as atomic radius, nuclear charge, number of bonds orbitals, overlap of orbital energies and crystal form) are involved as well.
Mechanical
Mechanical properties of metals include ductility, i.e. their capacity for elastic deformation. The nondirectional nature of metallic bonding is thought to contribute significantly to the ductility of most metallic solids. In contrast, when the planes of an ionic bond slide past one another, such as may occur in a crystal of table salt, the resultant change in location shifts ions of the same charge into close proximity, resulting in the cleavage of the crystal; such a shift is not observed in a covalently bonded crystal, such as a diamond, where fracture and crystal fragmentation occurs. Reversible elastic deformation in metals can be described by Hooke's Law for restoring forces, where the stress is linearly proportional to the strain.
Forces larger than the elastic limit, or heat, may cause a permanent (irreversible) deformation of the object, known as plastic deformation or plasticity. An applied force may be a tensile (pulling) force, a compressive (pushing) force, or a shear, bending or torsion (twisting) force. A temperature change may affect the movement or displacement of structural defects in the metal such as grain boundaries, point vacancies, line and screw dislocations, stacking faults and twins in both crystalline and non-crystalline solids. Internal slip, creep, and metal fatigue may ensue.
Electrical
The good electrical conductivities of metals originate from the fact that they readily lose their outer shell electrons. Broadly, the forces holding an individual atom’s outer shell electrons in place are weaker than the attractive forces on the same electrons arising from interactions between the atoms in the solid or liquid metal. The electrons involved become delocalised and the atomic structure of a metal can effectively be visualised as a collection of atoms embedded in a cloud of relatively mobile electrons.
The electrical conductivity of a metal, as well as the contribution of its electrons to its heat capacity and thermal conductivity, can be calculated from the free electron model, albeit this does not take into account the detailed structure of the metal's ion lattice.
Taking into account the positive potential caused by the arrangement of the ion cores enables consideration of the electronic band structureand binding energy of a metal. Various mathematical models are applicable, the simplest being the nearly free electron model.
Chemical
Metals are usually inclined to form cations through electron loss, reacting with oxygen in the air to form oxides over various timescales (iron rusts over years, while potassium burns in seconds). Examples:
- 4 Na + O2 → 2 Na2O (sodium oxide)
- 2 Ca + O2 → 2 CaO (calcium oxide)
- 4 Al + 3 O2 → 2 Al2O3 (aluminium oxide).
The transition metals (such as iron, copper, zinc, and nickel) are slower to oxidize because they form a passivating layer of oxide that protects their interiors. Others, like palladium, platinum and gold, do not react with the atmosphere at all. Some metals form a barrier layer of oxide on their surface which cannot be penetrated by further oxygen molecules and thus retain their shiny appearance and good conductivity for many decades (like aluminium, magnesium, some steels, and titanium). The oxides of metals are generally basic, as opposed to those of nonmetals, which are acidic. Exceptions are largely oxides with very high oxidation states such as CrO3, Mn2O7, and OsO4, which have strictly acidic reactions.
Painting, anodizing or plating metals are good ways to prevent their corrosion. However, a more reactive metal in the electrochemical series must be chosen for coating, especially when chipping of the coating is expected. Water and the two metals form an electrochemical cell, and if the coating is less reactive than the underlying metal, the coating actually promotescorrosion.
Alloys
An alloy is a mixture of two or more elements in which the main component is a metal. Most pure metals are either too soft, brittle or chemically reactive for practical use. Combining different ratios of metals as alloys modifies the properties of pure metals to produce desirable characteristics. The aim of making alloys is generally to make them less brittle, harder, resistant to corrosion, or have a more desirable color and luster. Of all the metallic alloys in use today, the alloys of iron (steel, stainless steel, cast iron, tool steel, alloy steel) make up the largest proportion both by quantity and commercial value. Iron alloyed with various proportions of carbon gives low, mid and high carbon steels, with increasing carbon levels reducing ductility and toughness. The addition of silicon will produce cast irons, while the addition of chromium, nickel and molybdenum to carbon steels (more than 10%) results in stainless steels.
Other significant metallic alloys are those of aluminium, titanium, copper and magnesium. Copper alloys have been known since prehistory—bronzegave the Bronze Age its name—and have many applications today, most importantly in electrical wiring. The alloys of the other three metals have been developed relatively recently; due to their chemical reactivity they require electrolytic extraction processes. The alloys of aluminium, titanium and magnesium are valued for their high strength-to-weight ratios; magnesium can also provide electromagnetic shielding. These materials are ideal for situations where high strength-to-weight ratio is more important than material cost, such as in aerospace and some automotive applications.
Alloys specially designed for highly demanding applications, such as jet engines, may contain more than ten elements.
Categories
- The table of metallic elements in this section shows how metals can be categorised in the discipline of chemistry. The rest of the section lists some categories of metals that are used more generally.
Base metal
In chemistry, the term base metal is used informally to refer to a metal that is easily oxidized or corroded, such as reacting easily with dilute hydrochloric acid (HCl) to form a metal chloride and hydrogen. Examples include iron, nickel, lead and zinc. Copper is considered a base metal as it is oxidized relatively easily, although it does not react with HCl. The term base metal is commonly used in opposition to noble metal.
In alchemy, a base metal was a common and inexpensive metal, as opposed to precious metals, mainly gold and silver. A longtime goal of the alchemists was the transmutation of base metals into precious metals.
In numismatics, coins of the past derived their value primarily from their precious metal content. Most modern currencies are fiat currency (they need no intrinsic value), allowing the coins to be made of base metal.
Ferrous metal
The term "ferrous" is derived from the Latin word meaning "containing iron". This can include pure iron, such as wrought iron, or an alloy such as steel. Ferrous metals are often magnetic, but not exclusively.
Heavy metal
A heavy metal is any relatively dense metal or metalloid. More specific definitions have been proposed, but none have obtained widespread acceptance. Some heavy metals have niche uses, or are notably toxic; some are essential in trace amounts.
Noble metal
Noble metals are metals that are resistant to corrosion or oxidation, unlike most base metals. They tend to be precious metals, often due to perceived rarity. Examples include gold, platinum, silver, rhodium, iridium and palladium.
Precious metal
A precious metal is a rare metallic chemical element of high economic value.
Chemically, the precious metals are less reactive than most elements, have high luster and high electrical conductivity. Historically, precious metals were important as currency, but are now regarded mainly as investment and industrial commodities. Gold, silver, platinum and palladium each have an ISO 4217 currency code. The best-known precious metals are gold and silver. While both have industrial uses, they are better known for their uses in art, jewelry, and coinage. Other precious metals include the platinum group metals: ruthenium, rhodium, palladium, osmium, iridium, and platinum, of which platinum is the most widely traded.
The demand for precious metals is driven not only by their practical use, but also by their role as investments and a store of value. Palladium and platinum are, as of fall 2017, valued at about three quarters the price of gold. Silver is substantially less expensive than these metals, but is often traditionally considered a precious metal in light of its role in coinage and jewelry.
Abundance and occurrence
| Metals in the Earth's crust: | |||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| abundance and main occurrence or source, by weight | |||||||||||||||||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | ||
| 1 | H | He | |||||||||||||||||
| 2 | Li | Be | B | C | N | O | F | Ne | |||||||||||
| 3 | Na | Mg | Al | Si | P | S | Cl | Ar | |||||||||||
| 4 | K | Ca | Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn | Ga | Ge | As | Se | Br | Kr | |
| 5 | Rb | Sr | Y | Zr | Nb | Mo | Ru | Rh | Pd | Ag | Cd | In | Sn | Sb | Te | I | Xe | ||
| 6 | Cs | Ba | La |  | Hf | Ta | W | Re | Os | Ir | Pt | Au | Hg | Tl | Pb | Bi | |||
| 7 |  | ||||||||||||||||||
| Ce | Pr | Nd | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu | ||||||
| Th | U | |||||||||||||||||
Most abundant (up to (82000 ppm) | Rare (0.01–0.99 ppm) | ||||||||||||||||||
Abundant (100–999 ppm) | Very rare (0.0001–0.0099 ppm) | ||||||||||||||||||
Uncommon (1–99 ppm) | |||||||||||||||||||
| Metals left of the dividing line occur (or are sourced) mainly as lithophiles; those to the right, as chalcophiles except gold (a siderophile) and tin (a lithophile). | |||||||||||||||||||
The Earth's crust is made of approximately 25% of metals by weight, of which 80% are light metals such as sodium, magnesium, and aluminium. Nonmetals (~75%) make up the rest of the crust. Despite the overall scarcity of some heavier metals such as copper, they can become concentrated in economically extractable quantities as a result of mountain building, erosion, or other geological processes.
Metals are primarily found as lithophiles (rock-loving) or chalcophiles (ore-loving). Lithophile metals are mainly the s-block elements, the more reactive of the d-block elements. and the f-block elements. They have a strong affinity for oxygen and mostly exist as relatively low density silicate minerals. Chalcophile metals are mainly the less reactive d-block elements, and the period 4–6 p-block metals. They are usually found in (insoluble) sulfide minerals. Being denser than the lithophiles, hence sinking lower into the crust at the time of its solidification, the chalcophiles tend to be less abundant than the lithophiles.
On the other hand, gold is a siderophile, or iron-loving element. It does not readily form compounds with either oxygen or sulfur. At the time of the Earth's formation, and as the most noble (inert) of metals, gold sank into the core due to its tendency to form high-density metallic alloys. Consequently, it is a relatively rare metal. Some other (less) noble metals—molybdenum, rhenium, the platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, and platinum), germanium, and tin—can be counted as siderophiles but only in terms of their primary occurrence in the Earth (core, mantle and crust), rather the crust. These metals otherwise occur in the crust, in small quantities, chiefly as chalcophiles (less so in their native form).
Extraction
Metals are often extracted from the Earth by means of mining ores that are rich sources of the requisite elements, such as bauxite. Ore is located by prospecting techniques, followed by the exploration and examination of deposits. Mineral sources are generally divided into surface mines, which are mined by excavation using heavy equipment, and subsurface mines.
Once the ore is mined, the metals must be extracted, usually by chemical or electrolytic reduction. Pyrometallurgy uses high temperatures to convert ore into raw metals, while hydrometallurgy employs aqueous chemistry for the same purpose. The methods used depend on the metal and their contaminants.
When a metal ore is an ionic compound of that metal and a non-metal, the ore must usually be smelted—heated with a reducing agent—to extract the pure metal. Many common metals, such as iron, are smelted using carbon as a reducing agent. Some metals, such as aluminium and sodium, have no commercially practical reducing agent, and are extracted using electrolysisinstead.
Sulfide ores are not reduced directly to the metal but are roasted in air to convert them to oxides.
Recycling
Demand for metals is closely linked to economic growth. During the 20th century, the variety of metals uses in society grew rapidly. Today, the development of major nations, such as China and India, and advances in technologies, are fuelling ever more demand. The result is that mining activities are expanding, and more and more of the world's metal stocks are above ground in use, rather than below ground as unused reserves. An example is the in-use stock of copper. Between 1932 and 1999, copper in use in the US rose from 73g to 238g per person.
Metals are inherently recyclable, so in principle, can be used over and over again, minimizing these negative environmental impacts and saving energy. For example, 95% of the energy used to make aluminium from bauxite ore is saved by using recycled material. Levels of metals recycling are generally low. In 2010, the International Resource Panel, hosted by the United Nations Environment Programme (UNEP) published reports on metal stocks that exist within society and their recycling rates.
The report authors observed that the metal stocks in society can serve as huge mines above ground. They warned that the recycling rates of some rare metals used in applications such as mobile phones, battery packs for hybrid cars and fuel cells are so low that unless future end-of-life recycling rates are dramatically stepped up these critical metals will become unavailable for use in modern technology.
Metallurgy
Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures (alloys). Metallurgy is also the technology of metals: the way in which science is applied to the extraction and production of metals, and the engineering of metal components for use in products for manufacturers and consumers.
Uses
Metals are present in nearly all aspects of modern life. Iron may be the most common as it accounts for 90% of all refined metals. Platinum may be the most ubiquitous given it is said to be found in, or used to produce, 20% of all consumer goods
Some metals and metal alloys possess high structural strength per unit mass, making them useful materials for carrying large loads or resisting impact damage. Metal alloys can be engineered to have high resistance to shear, torque and deformation. However the same metal can also be vulnerable to fatigue damage through repeated use or from sudden stress failure when a load capacity is exceeded. The strength and resilience of metals has led to their frequent use in high-rise building and bridge construction, as well as most vehicles, many appliances, tools, pipes, non-illuminated signs and railroad tracks.
Metals are good conductors, making them valuable in electrical appliances and for carrying an electric current over a distance with little energy lost. Electrical power grids rely on metal cables to distribute electricity. Home electrical systems, for the most part, are wired with copper wire for its good conducting properties.
The thermal conductivity of metals is useful for containers to heat materials over a flame. Metals are also used for heat sinks to protect sensitive equipment from overheating.
The high reflectivity of some metals is important in the construction of mirrors, including precision astronomical instruments. This last property can also make metallic jewelry aesthetically appealing.
Some metals have specialized uses; radioactive metals such as uranium and plutonium are used in nuclear power plants to produce energy via nuclear fission. Mercury is a liquid at room temperature and is used in switches to complete a circuit when it flows over the switch contacts. Shape memory alloy is used for applications such as pipes, fasteners and vascular stents.
Metals can be doped with foreign molecules—organic, inorganic, biological and polymers. This doping entails the metal with new properties that are induced by the guest molecules. Applications in catalysis, medicine, electrochemical cells, corrosion and more have been developed.
Trade
The World Bank reports that China was the top importer of ores and metals in 2005 followed by the United States and Japan.
History
The nature of metals has fascinated humans for many centuries, because these materials provided people with tools of unsurpassed properties both in war and in their preparation and processing. Pure gold and silver have been known to humans since the Stone Age. Lead and silver were fused from their ores as early as the fourth millennium BC.
Ancient Latin and Greek writers such as Theophrastus, Pliny the Elder in his Natural History, or Pedanius Dioscorides, did not try to classify metals. The ancient Europeans never attained the concept "metal" as a distinct elementary substance with fixed, characteristic chemical and physical properties. Following Empedocles, all substances within the sublunary sphere were assumed to vary in their constituent classical elements of earth, water, air and fire. Following the Pythagoreans, Plato assumed that these elements could be further reduced to plane geometrical shapes (triangles and squares) bounding space and relating to the regular polyhedra in the sequence earth:cube, water:icosahedron, air:octahedron, fire:tetrahedron. However, this philosophical extension did not become as popular as the simple four elements, after it was rejected by Aristotle. Aristotle also rejected the atomic theory of Democritus, since he classified the implied existence of a vacuum necessary for motion as a contradiction (a vacuum implies nonexistence, therefore cannot exist). Aristotle did, however, introduce underlying antagonistic qualities (or forces) of dry vs. wet and cold vs. heat into the composition of each of the four elements. The word "metal" originally meant "mines" and only later gained the general meaning of products from materials obtained in mines. In the first centuries A.D. a relation between the planets and the existing metals was assumed as Gold:Sun, Silver:Moon, Electrum:Jupiter, Iron:Mars, Copper:Venus, Tin:Mercury, Lead:Saturn. After electrum was determined to be a combination of silver and gold, the relations Tin:Jupiter and Mercury:Mercury were substituted into the previous sequence.
Arabic and medieval alchemists believed that all metals, and in fact, all sublunar matter, were composed of the principle of sulfur, carrying the combustible property, and the principle of mercury, the mother of all metals and carrier of the liquidity or fusibility, and the volatility properties. These principles were not necessarily the common substances sulfur and mercury found in most laboratories. This theory reinforced the belief that the all metals were destined to become gold in the bowels of the earth through the proper combinations of heat, digestion, time, and elimination of contaminants, all of which could be developed and hastened through the knowledge and methods of alchemy. Paracelsus added the third principle of salt, carrying the nonvolatile and incombustible properties, in his tria prima doctrine. These theories retained the four classical elements as underlying the composition of sulfur, mercury and salt.
The first systematic text on the arts of mining and metallurgy was De la Pirotechnia by Vannoccio Biringuccio, which treats the examination, fusion, and working of metals. Sixteen years later, Georgius Agricola published De Re Metallica in 1555, a clear and complete account of the profession of mining, metallurgy, and the accessory arts and sciences, as well as qualifying as the greatest treatise on the chemical industry through the sixteenth century. He gave the following description of a metal in his De Natura Fossilium (1546).