Clay bird shaped ritual vessel archmus Heraklion, 2300-1900 BCE, one of the earlier uses of nonmetallic materials.
Nonmetallic material, or in nontechnical terms a nonmetal, refers to materials which are not
metals. 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
chemical elements 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
metals 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 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.
A nonmetal has a
gap in the
energy levels of the electrons at the
Fermi level.[1]: Chpt 8 & 19 In contrast, a metal has at least one partially occupied band at the
Fermi level;[1] in a semiconductor or insulator there are no states at the Fermi level, see for instance
Ashcroft and Mermin.[1] These definitions are equivalent to stating that metals conduct electricity at
absolute zero, as suggested by
Nevill Francis Mott,[2]: 257 and the equivalent definition at other temperatures is also commonly used as in textbooks such as Chemistry of the Non-Metals by
Ralf Steudel[3] and work on
metal–insulator transitions.[4][5]
Band structure definitions of metallicity are the most widely used, and apply both to single elements such as insulating boron[6] as well as compounds such as
strontium titanate.[7] (There are many compounds which have states at the Fermi level and are metallic, for instance
titanium nitride.[8]) There are many experimental methods of checking for nonmetals by measuring the
band gap, or by ab-initio quantum mechanical calculations.[9]
Functional definition
A turret lathe operator machining metallic parts for transport planes in the 1940s.
An alternative in
metallurgy is to consider various
malleable alloys such as
steel,
aluminium alloys and similar as metals, and other materials as nonmetals;[10] fabricating metals is termed
metalworking,[11] but there is no corresponding term for nonmetals. A loose definition such as this is often the common useage, but can also be inaccurate. For instance, in this useage plastics are nonmetals, but in fact there are (electrically) conducting polymers[12][13] which should be described formally 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.[14] A general introduction to much of this can be found in the 2017 book by
Fumiko Yonezawa[2]: Chpt 1
The term
nonmetal (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 term nonmetallic elements such as the Chemistry of the Non-Metals by
Ralf Steudel,[15]: 4 which also uses the
general definition in terms of conduction and the Fermi level.[15]: 154 The approach based upon the elements is often used in teaching to help students understand the periodic table of elements,[16] although it is a
teaching oversimplification.[17][18] 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
dopants 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,[19] and nonmetals in
precipitation hardening in steels and other alloys.[20] Here the description implicitly includes information on whether the dopants tend to be
electron acceptors that lead to
covalently bonded compounds rather than
metallic bonding or electron acceptors.
Solar spectrum with Fraunhofer lines as it appears visually.
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[21] noted the appearance of a number of dark features in the solar spectrum.[22] In 1814,
Joseph von Fraunhofer independently rediscovered the lines and began to systematically study and measure their
wavelengths, 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.[23][24][25]
About 45 years later,
Gustav Kirchhoff and
Robert Bunsen[26] noticed that several Fraunhofer lines coincide with characteristic
emission lines identifies in the spectra of heated chemical elements.[27] They inferred that dark lines in the solar spectrum are caused by
absorption by
chemical elements in the solar atmosphere.[28] Their observations[29] were in the visible range where the strongest lines come from metals such as Na, K, Fe and thus all the extra elements beyond just hydrogen and helium were termed metallic.
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.
Small changes in positions and d-levels lead to a metal-insulator transition in
vanadium dioxide.[31]
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.[32] There are many other cases as discussed by Mott,[4] Inada et al[5] and more recently by Yonezawa.[2]
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.[35] 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 [36]
A decreased resistance with temperature, due to having more carriers (via
Fermi–Dirac statistics) available in partially occupied higher energy bands[1][41]
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.[42]
^
abcdeAshcroft, Neil W.; Mermin, N. David (1976). Solid state physics. Fort Worth Philadelphia San Diego [etc.]: Saunders college publ.
ISBN978-0-03-083993-1.
Gustav Kirchhoff (1859)
"Ueber 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.