Nonmetallic material, or in nontechnical terms a nonmetal, refers to materials which are not . 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 the periodic table, it is used for just those which are not metallic at standard 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 the Fermi energy, the equilibrium energy of electrons. For historical reasons there is a very different definition of Metallicity, with just hydrogen and helium as nonmetals. The term may also be used as a negative of the materials of interest such as in metallurgy or metalworking.
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, a phase transition. Other external stimuli such as electric fields can also lead to a local nonmetal, for instance in certain semiconductor devices. There are also many physical phenomena which are only found in nonmetals such as piezoelectricity or flexoelectricity.
General definition
The original approach to conduction and nonmetals was a band-structure with delocalized electrons (i.e. spread out in space). In this approach a nonmetal has a
band gap in the
band structure of the electrons at the
Fermi level.
In contrast, a metal would have at least one partially occupied band at the Fermi level;
in a semiconductor or insulator there are no delocalized states at the Fermi level, see for instance Ashcroft and Mermin.
These definitions are equivalent to stating that metals conduct electricity at
absolute zero, as suggested by Nevill Francis Mott,
and the equivalent definition at other temperatures is also commonly used as in textbooks such as
Chemistry of the Non-Metals by
Ralf Steudel and work on metal–insulator transitions.
In early work
- 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
as well as compounds such as strontium titanate.
(There are many compounds which have states at the Fermi level and are metallic, for instance
titanium nitride.
) There are many experimental methods of checking for nonmetals by measuring the
band gap, or by ab-initio quantum mechanical calculations.
Functional definition
An alternative in
metallurgy is to consider various
malleable alloys such as
steel,
and similar as metals, and other materials as nonmetals;
fabricating metals is termed
metalworking,
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
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 of
dopants, being called degenerate semiconductors.
A general introduction to much of this can be found in the 2017 book by
Fumiko Yonezawa
Periodic table elements
The term
Nonmetal 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 term
nonmetallic elements such as the
Chemistry of the Non-Metals by
Ralf Steudel,
which also uses the general definition in terms of conduction and the Fermi level.
The approach based upon the elements is often used in teaching to help students understand the periodic table of elements,
although it is a
Lie-to-children.
Those elements towards the top right of the periodic table are nonmetals, those towards the center (
transition metal and
lanthanide) and the left are metallic. An intermediate designation
metalloid is used for some elements.
The term is sometimes also used when describing 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,
Nonmetals in astronomy
A quite different approach is used in
astronomy where the term
metallicity 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
[Melvyn C. Usselman: William Hyde Wollaston Encyclopædia Britannica, retrieved 31 March 2013] noted the appearance of a number of dark features in the solar spectrum.
[William Hyde Wollaston (1802) "A method of examining refractive and dispersive powers, by prismatic reflection," Philosophical Transactions of the Royal Society, 92: 365–380; see especially p. 378.] In 1814, Joseph von Fraunhofer independently rediscovered the lines and began to systematically study and measure their
, and they are now called
Fraunhofer lines. He mapped over 570 lines, designating the most prominent with the letters A through K and weaker lines with other letters.
[Joseph Fraunhofer (1814 - 1815) "Bestimmung des Brechungs- und des Farben-Zerstreuungs - Vermögens verschiedener Glasarten, in Bezug auf die Vervollkommnung achromatischer Fernröhre" (Determination of the refractive and color-dispersing power of different types of glass, in relation to the improvement of achromatic telescopes), Denkschriften der Königlichen Akademie der Wissenschaften zu München (Memoirs of the Royal Academy of Sciences in Munich), 5: 193–226; see especially pages 202–205 and the plate following page 226.]
About 45 years later, Gustav Kirchhoff and Robert Bunsen[See:
]
-
Gustav Kirchhoff (1859) die Fraunhofer'schen Linien" (On Fraunhofer's lines), Monatsbericht der Königlichen Preussische Akademie der Wissenschaften zu Berlin (Monthly report of the Royal Prussian Academy of Sciences in Berlin), 662–665.
-
Gustav Kirchhoff (1859) "Ueber das Sonnenspektrum" (On the sun's spectrum), Verhandlungen des naturhistorisch-medizinischen Vereins zu Heidelberg (Proceedings of the Natural History / Medical Association in Heidelberg), 1 (7) : 251–255.
noticed that several Fraunhofer lines coincide with characteristic emission lines identifies in the spectra of heated chemical elements.
The Carlos Jaschek, and the stellar astronomer and spectroscopist Mercedes Jaschek, in their book The Classification of Stars, observed that:
- 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 hand photometrists, 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.
Metal-insulator transition
There are many cases where an element or compound is metallic under certain circumstances, but a nonmetal in others. One example is metallic hydrogen which forms under very high pressures.
There are many other cases as discussed by Mott,
Inada et al
and more recently by Yonezawa.
There can also be local transitions to a nonmetal, particularly in semiconductor devices. One example is a field-effect transistor where an electric field can lead to a region where there are no electrons at the Fermi energy (depletion zone).
Properties specific to nonmetals
Nonmetals have a wide range of properties, for instance the nonmetal
diamond is the hardest known material, while the nonmetal molybdenum disulfide is a solid lubricants used in space.
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:
-
Dielectric,
-
Electrostriction,
-
Flexoelectricity, where there is a coupling between strain gradients and polarization.
-
Piezoelectricity, a coupling between polarization and linear strains.
-
A decreased resistance with temperature, due to having more carriers (via Fermi–Dirac statistics) available in partially occupied higher energy bands
-
Increased conductivity when illuminated with light or ultraviolet radiation, called photoconductivity. This is similar to the effect of temperature, but with the photons exciting electrons into partially occupied states.
-
Transmit electric fields as in the capacitor figure above; in a metal there is electric-field screening that prevents this beyond very small distances, see Classical Electrodynamics.
See also
