Metal

A metal is a material that, when polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as opposed to nonmetallic materials which do not. Metals are typically ductile and malleable.
A metal may be a chemical element, such as iron, an alloy, such as stainless steel, or a molecular compound, such as polymeric sulfur nitride. The general science of metals is called metallurgy, a subtopic of materials science; aspects of their electronic and thermal properties are also within the scope of condensed matter physics and solid-state chemistry, as it is a multidisciplinary topic. In colloquial use, materials such as steel alloys are referred to as metals, while others, such as polymers, wood, or ceramics are nonmetallic materials.
A metal conducts electricity at a temperature of absolute zero, which is a consequence of delocalized states at the Fermi energy. Many elements and compounds become metallic under high pressures; for example, iodine gradually becomes a metal at a pressure between 40 and 170 thousand times atmospheric.
When discussing the periodic table and some chemical properties, the term metal is often used to denote those elements which in pure form and at standard conditions are metals in the sense of electrical conduction mentioned above. The related term metallic may also be used for types of dopant atoms or alloying elements.
The strength and resilience of some metals has led to their frequent use in, for example, high-rise building and bridge construction, as well as most vehicles, many home appliances, tools, pipes, and railroad tracks. Precious metals were historically used as coinage, but in the modern era, coinage metals have extended to at least 23 of the chemical elements. There is also extensive use of multi-element metals such as titanium nitride or degenerate semiconductors in the semiconductor industry.
The history of refined metals is thought to begin with the use of copper about 11,000 years ago. Gold, silver, iron, lead, and brass were likewise in use before the first known appearance of bronze in the fifth millennium BCE. Subsequent developments include the production of early forms of steel; the discovery of sodium—the first light metal—in 1809; the rise of modern alloy steels; and, since the end of World War II, the development of more sophisticated alloys.

Properties

Form and structure

Most metals are shiny and lustrous, at least when polished, or fractured. Sheets of metal thicker than a few micrometres appear opaque, but gold leaf transmits green light. This is due to the freely moving electrons which reflect light.
Although most elemental metals have higher densities than nonmetals, there is a wide variation in their densities, lithium being the least dense and osmium the most dense. Some of the 6d transition metals are expected to be denser than osmium, but their known isotopes are too unstable for bulk production to be possible. Magnesium, aluminium and titanium are light metals of significant commercial importance. Their respective densities of 1.7, 2.7, and 4.5 g/cm³ can be compared to those of the older structural metals, like iron at 7.9 and copper at 8.9 g/cm³. The most common lightweight metals are aluminium and magnesium alloys.
Metals are typically malleable and ductile, deforming under stress without cleaving. The nondirectional nature of metallic bonding contributes to the ductility of most metallic solids, where the Peierls stress is relatively low allowing for dislocation motion, and there are also many combinations of planes and directions for plastic deformation. Due to their having close packed arrangements of atoms the Burgers vector of the dislocations are fairly small, which also means that the energy needed to produce one is small. In contrast, in an ionic compound like table salt, the Burgers vectors are much larger and the energy to move a dislocation is far higher. Reversible elastic deformation in metals can be described well by Hooke's law for the restoring forces, where the stress is linearly proportional to the strain, up to the proportional limit of the material.
A temperature change may lead to the movement 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 metals. Internal slip, creep, and metal fatigue may also ensue.
The atoms of simple metallic substances are often in one of three common crystal structures, namely body-centered cubic, face-centered cubic, and hexagonal close-packed. 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.
Many other metals with different elements have more complicated structures, such as rock-salt structure in titanium nitride or perovskite in some nickelates.

Electrical and thermal

The electronic structure of metals makes them good conductors of electricity. In general, electrons in a material all have different momenta, which average to zero when there is no external voltage. In metals, when a voltage is applied, some electrons shift to states with slightly higher momentum in the direction of the electric field, while others slow down slightly. This creates a net drift velocity that leads to an electric current. This involves small changes in which wavefunctions the electrons are in, changing to those with the higher momenta. According to the Pauli exclusion principle, no two electrons can occupy the same quantum state. Therefore, for the electrons to shift to higher-momentum states, such states must be unoccupied. In metals, these empty delocalized electron states are available at energies near the highest occupied levels, as shown in the Figure.
By contrast, semiconductors like silicon and nonmetals like strontium titanate have an energy gap between the highest filled electron states and the lowest empty states. A small electric field is insufficient to excite electrons across this gap, making these materials poor electrical conductors. However, semiconductors can carry some current when doped with elements that introduce additional partially occupied energy states, or when thermal excitation enables electrons to cross the energy gap.
The elemental metals have electrical conductivity values of from 6.9 × 10³ S/cm for manganese to 6.3 × 10⁵ S/cm for silver. In contrast, a semiconducting metalloid such as boron has an electrical conductivity 1.5 × 10⁻⁶ S/cm. Typically, the electrical conductivity of metals decreases with heating because the increased thermal motion of the atoms makes it harder for electrons to flow. Exceptionally, plutonium's electrical conductivity increases when heated in the temperature range of around −175 to +125 °C, with anomalously large thermal expansion coefficient and a phase change from monoclinic to face-centered cubic near 100 °C. This behavior, along with similar phenomena observed in other transuranic elements, is attributed to more complex relativistic and spin interactions which are not captured in simple models.
All of the metallic alloys as well as conducting ceramics and polymers are metals by the same definition; for instance titanium nitride has delocalized states at the Fermi level. They have electrical conductivities similar to those of elemental metals. Liquid forms are also metallic conductors or electricity, for instance mercury. In normal conditions no gases are metallic conductors. However, a plasma is a metallic conductor and the charged particles in a plasma have many properties in common with those of electrons in elemental metals, particularly for white dwarf stars.
Metals are relatively good conductors of heat, which in metals is transported mainly by the conduction electrons. At higher temperatures the electrons can occupy slightly higher energy levels given by Fermi–Dirac statistics. These have slightly higher momenta and can pass on thermal energy. The empirical Wiedemann–Franz law states that in many metals the ratio between thermal and electrical conductivities is proportional to temperature, with a proportionality constant that is roughly the same for all metals.
The contribution of a metal's electrons to its heat capacity and thermal conductivity, and the electrical conductivity of the metal itself can be approximately calculated from the free electron model. However, 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 structure and binding energy of a metal. Various models are applicable, the simplest being the nearly free electron model. Modern methods such as density functional theory are typically used.

Chemical

The elements which form metals usually form cations through electron loss. Most will react with oxygen in the air to form oxides over various timescales which depend upon whether the native oxide forms a passivation layer that acts as a diffusion barrier. Some others, like palladium, platinum, and gold, do not react with the atmosphere at all; gold can form compounds where it gains an electron. The oxides of elemental metals are often basic. However, oxides with very high oxidation states such as CrO₃, Mn₂O₇, and OsO₄ often have strictly acidic reactions; and oxides of the less electropositive metals such as BeO, Al₂O₃, and PbO, can display both basic and acidic properties. The latter are termed amphoteric oxides.

Periodic table distribution of elemental metals

The elements that form exclusively metallic structures under ordinary conditions are shown in yellow on the periodic table below. The remaining elements either form covalent network structures, molecular covalent structures, or remain as single atoms. Astatine, francium, and the elements from fermium onwards are shown in gray because they are extremely radioactive and have never been produced in bulk. Theoretical and experimental evidence suggests that these uninvestigated elements should be metals, except for oganesson which DFT calculations indicate would be a semiconductor.
The situation changes with pressure: at extremely high pressures, all elements are expected to metallize. Arsenic has both a stable metallic allotrope and a metastable semiconducting allotrope at standard conditions. A similar situation affects carbon : graphite is metallic, but diamond is not.