Nonmetallic material, or in nontechnical terms anonmetal, refers to materials which are notmetals. Depending upon context it is used in slightly different ways. In everyday life it would be a generic term for those materials such as plastics, wood or ceramics which are not typical metals such as the iron alloys used in bridges. In some areas of chemistry, particularly theperiodic table, it is used for just thosechemical elements which are not metallic atstandard temperature and pressure conditions. It is also sometimes used to describe broad classes of dopant atoms in materials. In general usage in science, it refers to materials which do not have electrons that can readily move around, more technically there are no available states at theFermi energy, the equilibrium energy of electrons. For historical reasons there is a very different definition ofmetals in astronomy, with just hydrogen and helium as nonmetals. The term may also be used as a negative of the materials of interest such as inmetallurgy ormetalworking.

Variations in the environment, particularly temperature and pressure can change a nonmetal into a metal, and vica versa; this is always associated with some major change in the structure, aphase transition. Other external stimuli such as electric fields can also lead to a local nonmetal, for instance in certainsemiconductor devices. There are also manyphysical phenomena which are only found in nonmetals such aspiezoelectricity orflexoelectricity.

The original approach to conduction and nonmetals was a band-structure withdelocalized electrons (i.e. spread out in space). In this approach a nonmetal has agap in theenergy levels of the electrons at theFermi level.^{[1]: Chpt 8 & 19} In contrast, a metal would have at least one partially occupied band at the Fermi level;^[1] in a semiconductor or insulator there are no delocalized states at the Fermi level, see for instanceAshcroft and Mermin.^[1] These definitions are equivalent to stating that metals conduct electricity atabsolute zero, as suggested byNevill Francis Mott,^[2]: 257 and the equivalent definition at other temperatures is also commonly used as in textbooks such asChemistry of the Non-Metals byRalf Steudel^[3] and work onmetal–insulator transitions.^[4][5]

In early work^[6][7] this band structure interpretation was based upon a single-electron approach with the Fermi level in the band gap as illustrated in the Figure, not including a complete picture of themany-body problem where bothexchange andcorrelation terms can matter, as well asrelativistic effects such asspin-orbit coupling. A key addition by Mott andRudolf Peierls was that these could not be ignored.^[8] For instance,nickel oxide would be a metal if a single-electron approach was used, but in fact has quite a large band gap.^[9] As of 2024 it is more common to use an approach based upondensity functional theory where the many-body terms are included.^[10][11] Rather than single electrons, the filling involvesquasiparticles called orbitals, which are the single-particle like solutions for a system with hundreds to thousands of electrons. Although accurate calculations remain a challenge, reasonable results are now available in many cases.^[12][13]

It is also common to nuance somewhat the early definitions ofAlan Herries Wilson and Mott. As discussed by both thechemistPeter Edwards and colleagues,^[15] as well asFumiko Yonezawa,^{[2]: 257–261}it is also important in practice to consider the temperatures at which both metals and nonmetals are used. Yonezawa provides a general definition:^[2]: 260

When a material 'conducts' and at the same time 'the temperature coefficient of the electric conductivity of that material is not positive under a certain environmental condition,' the material is metallic under that environmental condition. A material which does not satisfy these requirements is not metallic under that environmental condition.

Band structure definitions of metallicity are the most widely used, and apply both to single elements such as insulating boron^[16] as well as compounds such asstrontium titanate.^[17] (There are many compounds which have states at the Fermi level and are metallic, for instancetitanium nitride.^[18]) There are many experimental methods of checking for nonmetals by measuring theband gap, or by ab-initio quantum mechanical calculations.^[19]

An alternative inmetallurgy is to consider variousmalleable alloys such assteel,aluminium alloys and similar as metals, and other materials as nonmetals;^[20] fabricating metals is termedmetalworking,^[21] but there is no corresponding term for nonmetals. A loose definition such as this is often the common usage, but can also be inaccurate. For instance, in this usage plastics are nonmetals, but in fact there are (electrically) conducting polymers^[22][23] which should formally be described as metals. Similar, but slightly more complex, many materials which are (nonmetal) semiconductors behave like metals when they contain a high concentration ofdopants, being calleddegenerate semiconductors.^[24] A general introduction to much of this can be found in the 2017 book byFumiko Yonezawa^{[2]: Chpt 1}

The termnonmetal (chemistry) is also used for those elements which are not metallic in their normal ground state; compounds are sometimes excluded from consideration. Some textbooks use the termnonmetallic elements such as theChemistry of the Non-Metals byRalf Steudel,^[25]: 4 which also uses thegeneral definition in terms of conduction and the Fermi level.^[25]: 154 The approach based upon the elements is often used in teaching to help students understand the periodic table of elements,^[26] although it is ateaching oversimplification.^[27][28] Those elements towards the top right of the periodic table are nonmetals, those towards the center (transition metal andlanthanide) and the left are metallic. An intermediate designationmetalloid is used for some elements.

The term is sometimes also used when describingdopants of specific elements types in compounds, alloys or combinations of materials, using the periodic table classification. For instance metalloids are often used in high-temperature alloys,^[29] and nonmetals inprecipitation hardening in steels and other alloys.^[30] Here the description implicitly includes information on whether the dopants tend to beelectron acceptors that lead tocovalently bonded compounds rather thanmetallic bonding or electron acceptors.

A quite different approach is used inastronomy where the termmetallicity is used for all elements heavier than helium, so the only nonmetals are hydrogen and helium. This is a historical anomaly. In 1802,William Hyde Wollaston^[31] noted the appearance of a number of dark features in the solar spectrum.^[32] In 1814,Joseph von Fraunhofer independently rediscovered the lines and began to systematically study and measure theirwavelengths, and they are now calledFraunhofer lines. He mapped over 570 lines, designating the most prominent with the letters A through K and weaker lines with other letters.^[33][34][35]

About 45 years later,Gustav Kirchhoff andRobert Bunsen^[36] noticed that several Fraunhofer lines coincide with characteristicemission lines identifies in the spectra of heated chemical elements.^[37] They inferred that dark lines in the solar spectrum are caused byabsorption bychemical elements in the solar atmosphere.^[38] Their observations^[39] were in the visible range where the strongest lines come from metals such as Na, K, Fe.^[40] In the early work on the chemical composition of the sun the only elements that were detected in spectra were hydrogen and various metals,^{[41]: 23–24} with the termmetallic frequently used when describing them.^{[41]: Part 2} In contemporary usage all the extra elements beyond just hydrogen and helium are termed metallic.

TheastrophysicstCarlos Jaschek, and the stellar astronomer andspectroscopist Mercedes Jaschek, in their bookThe Classification of Stars, observed that:^[42]

Stellar interior specialists use 'metals' to designate any element other than hydrogen and helium, and in consequence ‘metal abundance’ implies all elements other than the first two. For spectroscopists this is very misleading, because they use the word in the chemical sense. On the other handphotometrists, who observe combined effects of all lines (i.e. without distinguishing the different elements) often use this word 'metal abundance', in which case it may also include the effect of the hydrogen lines.

There are many cases where an element or compound is metallic under certain circumstances, but a nonmetal in others. One example ismetallic hydrogen which forms under very high pressures.^[44] There are many other cases as discussed by Mott,^[4] Inada et al^[5] and more recently by Yonezawa.^[2]

There can also be local transitions to a nonmetal, particularly insemiconductor devices. One example is afield-effect transistor where an electric field can lead to a region where there are no electrons at the Fermi energy (depletion zone).^[45][46]

Nonmetals have a wide range of properties, for instance the nonmetaldiamond is the hardest known material, while the nonmetalmolybdenum disulfide is a solid lubricants used in space.^[47] There are some properties specific to them not having electrons at the Fermi energy. The main ones, for which more details are available in the links are:^{[1]: Chpt 27-29 [48]}

• Dielectric polarization,^[49] approximately equivalent to alignment of local dipoles with an electric field, as incapacitors.
• Electrostriction,^[50] a change in volume due to an electric field, or more accuratelypolarization density.
• Flexoelectricity, where there is a coupling between strain gradients and polarization.^[51] This plays a role in the generation ofstatic electricity due to thetriboelectric effect.^[52]
• Piezoelectricity, a coupling between polarization and linear strains.^[50]
• A decreased resistance with temperature, due to having more carriers (viaFermi–Dirac statistics) available in partially occupied higher energy bands^[1][53]
• Increased conductivity when illuminated with light orultraviolet radiation, calledphotoconductivity. This is similar to the effect of temperature, but with the photons exciting electrons into partially occupied states.^[54]
• Transmit electric fields as in the capacitor figure above; in a metal there iselectric-field screening that prevents this beyond very small distances, seeClassical Electrodynamics.^[55]

• Abundance of the chemical elements – Measure of the relative occurrences of chemical elements in a given context
• Charge-transfer insulators – Nonmetal due to charge transfer between atoms
• Charge transport mechanisms – Models for electric current flow
• Dielectric strength – Degree of insulation
• Electrical conduction – Measure of a substance's ability to resist or conduct electric currentPages displaying short descriptions of redirect targets
• Kondo insulator – Strongly correlated system with a narrow band gap at low temperatures
• List of data references for chemical elements
• List of manufacturing processes – Manufacturing processes
• List of materials properties
• List of states of matter – States of Matter
• Metallicity distribution function – Distribution within a group of stars of the ratio of iron to hydrogen in a star
• Mott insulator – Materials classically predicted to be conductors, that are actually insulators
• Superconductor-insulator transition – Type of quantum phase transitionPages displaying short descriptions of redirect targets
• Topological insulator – State of matter with insulating bulk but conductive boundary

• Chemical physics
• Condensed matter physics
• Materials science
• Metallurgy
• Nonmetals
• Periodic table
• Solid-state chemistry

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