Nonmetal

In chemistry, a nonmetal is a chemical element that generally lacks a predominance of metallic properties; they range from colorless gases (like hydrogen) to shiny solids (like carbon, as graphite). The electrons in nonmetals behave differently from those in metals. With some exceptions, those in nonmetals are fixed in place, resulting in nonmetals usually being poor conductors of heat and electricity and brittle or crumbly when solid. The electrons in metals are generally free moving and this is why metals are good conductors and most are easily flattened into sheets and drawn into wires. Nonmetal atoms tend to attract electrons in chemical reactions and to form acidic compounds.

Extract of periodic table showing how often each element is classified as a nonmetal:
 14  effectively always[n 1]  3  frequently[n 2]  6  sometimes (metalloids)[n 3]
Nearby metals are shown in a gray font.[n 4]
There is no precise definition of a nonmetal; which elements are counted as such varies.
Hydrogen is usually in group 1 (per the below full table) but can be in group 17 (per the above extract).[n 5]
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Two nonmetals, hydrogen and helium, make up about 99% of ordinary matter in the observable universe by mass. Five nonmetallic elements, hydrogen, carbon, nitrogen, oxygen and silicon, largely make up the Earth's crust, atmosphere, oceans and biosphere.

Most nonmetals have biological, technological or domestic roles or uses. Living organisms are composed almost entirely of the nonmetals hydrogen, oxygen, carbon, and nitrogen. Nearly all nonmetals have individual uses in medicine and pharmaceuticals; lighting and lasers; and household items.

While the term non-metallic dates from as far back as 1566, there is no widely agreed precise definition of a nonmetal. Some elements have a marked mixture of metallic and nonmetallic properties; which of these borderline cases are counted as nonmetals can vary depending on the classification criteria. Fourteen elements are effectively always recognized as nonmetals and up to about nine more are frequently to sometimes added, as shown in the periodic table extract.

Definition and applicable elements

A nonmetal is a chemical element deemed to lack a preponderance of metallic properties such as luster, deformability, good thermal and electrical conductivity, and the capacity to form a basic (rather than acidic) oxide.[8] Since there is no rigorous definition of a nonmetal,[9][10][11] some variation exists among sources as to which elements are classified as such. The decisions involved depend on which property or properties are regarded as most indicative of nonmetallic or metallic character.[12]

Although Steudel,[13] in 2020, recognised twenty-three elements as nonmetals, any such list is open to challenge.[1] Fourteen almost always recognized are hydrogen, oxygen, nitrogen, and sulfur; the highly reactive halogens fluorine, chlorine, bromine, and iodine; and the noble gases helium, neon, argon, krypton, xenon, and radon (see e.g. Larrañaga et al).[1] The authors recognized carbon, phosphorus and selenium as nonmetals; Vernon[2] had earlier reported that these three elements were instead sometimes counted as metalloids. The elements commonly recognized as metalloids namely boron; silicon and germanium; arsenic and antimony; and tellurium are sometimes counted as an intermediate class between the metals and the nonmetals when the criteria used to distinguish between metals and nonmetals are inconclusive.[14] At other times they are counted as nonmetals in light of their nonmetallic chemistry.[4]

Of the 118 known elements[15] no more than about 20% are regarded as nonmetals.[16] The status of a few elements is less certain. Astatine, the fifth halogen, is often ignored on account of its rarity and intense radioactivity;[17] theory and experimental evidence suggest it is a metal.[18] The superheavy elements copernicium (Z = 112), flerovium (114), and oganesson (118) may turn out to be nonmetals; their status has not been confirmed.[19]

General properties

Physical
Physical properties apply to elements in their most stable forms in ambient conditions
Variety in color and form
of some nonmetallic elements
Boron in its β-rhombohedral phase
Metallic appearance of carbon as graphite
Blue color of liquid oxygen
Pale yellow liquid fluorine in a cryogenic bath
Sulfur as a yellow powder
Liquid bromine at room temperature
Metallic appearance of iodine under white light
Liquefied xenon

About half of nonmetallic elements are gases; most of the rest are shiny solids. Bromine, the only liquid, is so volatile that it is usually topped by a layer of its fumes; sulfur is the only colored solid nonmetal. The fluid nonmetals have very low densities, melting points and boiling points, and are poor conductors of heat and electricity.[20] The solid elements have low densities, are brittle or crumbly with low mechanical and structural strength,[21] and poor to good conductors.[n 6]

The internal structures and bonding arrangements of the nonmetals explain their differences in form. Those existing as discrete atoms (e.g. xenon) or molecules (e.g. oxygen, sulfur and bromine) have low melting and boiling points as they are held together by weak London dispersion forces acting between their atoms or molecules.[25] Many are gases at room temperature. Nonmetals that form giant structures, such as chains of up to 1,000 atoms (e.g. selenium),[26] sheets (e.g. carbon) or 3D lattices (e.g. silicon), have higher melting and boiling points, as it takes more energy to overcome their stronger covalent bonds, so they are all solids. Those closer to the left side of the periodic table, or further down a column, often have some weak metallic interactions between their molecules, chains, or layers, consistent with their proximity to the metals; this occurs in boron,[27] carbon,[28] phosphorus,[29] arsenic,[30] selenium,[31] antimony,[32] tellurium,[33] and iodine.[34]

Nonmetallic elements are either shiny, colored, or colorless. For boron, graphitic carbon, silicon, black phosphorus, germanium, arsenic, selenium, antimony, tellurium and iodine, their structures feature varying degrees of delocalised electrons that scatter incoming visible light, resulting in a shiny appearance.[35] The colored nonmetals (sulfur, fluorine, chlorine, bromine) absorb some colours (wavelengths) and transmit the complementary colours. For chlorine, its "familiar yellow-green colour...is due to a broad region of absorption in the violet and blue regions of the spectrum".[36][n 7] For the colorless nonmetals (hydrogen, nitrogen, oxygen, and the noble gases) their electrons are held sufficiently strongly such that no absorption occurs in the visible part of the spectrum, and all visible light is transmitted.[38]

The electrical and thermal conductivities of nonmetals and the brittle nature of the solids are likewise related to their internal arrangements. Whereas good conductivity and plasticity (malleability, ductility) are ordinarily associated with the presence of free moving and uniformly distributed electrons in metals[39] the electrons in nonmetals typically lack such mobility.[40] Among the nonmetallic elements, good electrical and thermal conductivity occurs only in carbon, arsenic and antimony.[n 8] Good thermal conductivity otherwise occurs only in boron, silicon, phosphorus, and germanium;[22] such conductivity is transmitted though vibrations of the crystalline lattices of these elements.[41] Moderate electrical conductivity occurs in boron, silicon, phosphorus, germanium, selenium, tellurium and iodine.[n 9] Plasticity occurs in limited circumstances only in carbon, phosphorus, sulfur, and selenium.[n 10]

The physical differences between metals and nonmetals arise from internal and external atomic forces. Internally, the positive charge arising from the protons in an atom's nucleus acts to hold the atom's outer electrons in place. Externally, the same electrons are subject to attractive forces from the protons in nearby atoms. When the external forces are greater than, or equal to, the internal force, outer electrons are expected to become free to move between atoms, and metallic properties are predicted. Otherwise nonmetallic properties are expected.[48]

Chemical
Some chemistry-based typical
differences between metals and nonmetals[49]
AspectMetalsNonmetals
Electronegativity Lower than nonmetals,
with some exceptions[50]		 Moderate to very high
Chemical
bonding
Seldom form
covalent bonds		 Frequently form
covalent bonds
Metallic bonds (alloys)
between metals		 Covalent bonds
between nonmetals
Ionic bonds between nonmetals and metals
Oxidation
states		 Positive Negative or positive
Oxides Basic in lower oxides;
increasingly acidic
in higher oxides		 Acidic;
never basic[51]
In aqueous
solution[52]		 Exist as cations Exist as anions
or oxyanions

Nonmetals have moderate to high values of electronegativity[53] and tend to form acidic compounds. For example, the solid nonmetals (including metalloids) react with nitric acid to form either an acid, or an oxide that has acidic properties predominating.[n 11]

They tend to gain or share electrons when they react, unlike metals which tend to donate electrons. Given the stability of the electron configurations of the noble gases (which have full outer shells), nonmetals generally gain enough electrons to give them the electron configuration of the following noble gas, whereas metals tend to lose electrons sufficient to leave them with the electron configuration of the preceding noble gas. For nonmetallic elements this tendency is summarized in the duet and octet rules of thumb (and for metals there is a less rigorously predictive 18-electron rule).[56]

Nonmetals mostly have higher ionization energies, electron affinities, electronegativity values, and standard reduction potentials than metals. In general, the higher these values the more nonmetallic is the element.[57]

The chemical differences between metals and nonmetals largely arise from the attractive force between the positive nuclear charge of an individual atom and its negatively charged outer electrons. From left to right across each period of the periodic table the nuclear charge increases as the number of protons in the atomic nucleus increases.[58] There is an associated reduction in atomic radius[59] as the increasing nuclear charge draws the outer electrons closer to the core.[60] In metals, the effect of the nuclear charge is generally weaker than for nonmetallic elements. In bonding, metals therefore tend to lose electrons, and form positively charged or polarized atoms or ions whereas nonmetals tend to gain those same electrons due to their stronger nuclear charge, and form negatively charged ions or polarized atoms.[61]

The number of compounds formed by nonmetals is vast.[62] The first ten places in a "top 20" table of elements most frequently encountered in 895,501,834 compounds, as listed in the Chemical Abstracts Service register for November 2, 2021, were occupied by nonmetals. Hydrogen, carbon, oxygen and nitrogen were collectively found in the majority (80%) of compounds. Silicon, a metalloid, was in 11th place. The highest rated metal, with an occurrence frequency of 0.14%, was iron, in 12th place.[63] A few examples of nonmetal compounds are: boric acid (H
3BO
3), used in ceramic glazes; selenocysteine (C
3H
7NO
2Se), the 21st amino acid of life;[64] phosphorus sesquisulfide (P4S3), in strike anywhere matches; and teflon ((C
2F
4)n),[65] as used in non-stick coatings for pans and other cookware.

Complications
Periodic table highlighting the first row of each block.[n 12] Helium (He), as a noble gas, is normally shown over neon (Ne) with the rest of the noble gases. The elements within scope of this article are inside the thick black borders. The status of oganesson (Og, element 118) is not yet known.
Electronegativity values of the group 16 chalcogen elements showing a W-shaped alternation or secondary periodicity going down the group

Complicating the chemistry of the nonmetals are the anomalies seen in the first row of each periodic table block. These anomalies are prominent in hydrogen, boron (whether as a nonmetal or metalloid), carbon, nitrogen, oxygen and fluorine. In later rows they manifest as secondary periodicity or non-uniform periodic trends going down most of the p-block groups,[66] and unusual oxidation states in the heavier nonmetals.

First row anomaly

Starting with hydrogen, the first row anomaly largely arises from the electron configurations of the elements concerned. Hydrogen is noted for the different ways it forms bonds. It most commonly forms covalent bonds. It can lose its single electron in aqueous solution, leaving behind a bare proton with tremendous polarizing power.[67] This consequently attaches itself to the lone electron pair of an oxygen atom in a water molecule, thereby forming the basis of acid-base chemistry.[68] A hydrogen atom in a molecule can form a second, weaker, bond with an atom or group of atoms in another molecule. Such bonding, "helps give snowflakes their hexagonal symmetry, binds DNA into a double helix; shapes the three-dimensional forms of proteins; and even raises water's boiling point high enough to make a decent cup of tea."[69]

Hydrogen and helium, and boron to neon have unusually small atomic radii. This occurs because the 1s and 2p subshells have no inner analogues (i.e., there is no zero shell and no 1p subshell) and they therefore experience no electron repulsion effects, unlike the 3p, 4p and 5p subshells of heavier elements.[70] Ionization energies and electronegativities among these elements are consequently higher than would otherwise be expected, having regard to periodic trends. The small atomic radii of carbon, nitrogen, and oxygen facilitate the formation of double or triple bonds.[71]

While it would normally be expected that hydrogen and helium, on electron configuration consistency grounds, would be located atop the s-block elements, the first row anomaly in these two elements is strong enough to warrant alternative placements. Hydrogen is occasionally positioned over fluorine, in group 17 rather than over lithium in group 1. Helium is regularly positioned over neon, in group 18, rather than over beryllium, in group 2.[72]

Secondary periodicity

Immediately after the first row of d-block metals, scandium to zinc, the 3d electrons in the p-block elements i.e., gallium (a metal), germanium, arsenic, selenium, and bromine, are not as effective at shielding the increased positive nuclear charge. A similar effect accompanies the appearance of fourteen f-block metals between barium and lutetium, ultimately resulting in smaller than expected atomic radii for the elements from hafnium (Hf) onwards.[73] The net result, especially for the group 13–15 elements, is that there is an alternation in some periodic trends going down groups 13 to 17.[74]

Unusual oxidation states

The larger atomic radii of the heavier group 15–18 nonmetals enable higher bulk coordination numbers, and result in lower electronegativity values that better tolerate higher positive charges. The elements involved are thereby able to exhibit oxidation states other than the lowest for their group (that is, 3, 2, 1, or 0) for example in phosphorus pentachloride (PCl5), sulfur hexafluoride (SF6), iodine heptafluoride (IF7), and xenon difluoride (XeF2).[75]

Subclasses
Modern periodic table extract showing nonmetal subclasses. H is usually shown in group 1 but can instead be in group 17.[n 13]
† moderately strong oxidising agent ‡ strong oxidising agent[n 14]

Approaches to classifying nonmetals may involve from as few as two subclasses to up to six or seven. For example, the Encyclopædia Britannica periodic table recognizes noble gases, halogens, and other nonmetals, and splits the elements commonly recognized as metalloids between "other metals" and "other nonmetals".[87] The Royal Society of Chemistry periodic table instead uses a different color for each of its eight main groups, and nonmetals can be found in seven of these.[88]

From right to left in periodic table terms, three or four kinds of nonmetals are more or less commonly discerned. These are:

  • the relatively inert noble gases;
  • a set of chemically strong halogen elements—fluorine, chlorine, bromine and iodine—sometimes referred to as nonmetal halogens[89] (the term used here) or stable halogens;[90]
  • a set of unclassified nonmetals, including elements such as hydrogen, carbon, nitrogen, and oxygen, with no widely recognized collective name; and
  • the chemically weak nonmetallic metalloids[91] sometimes considered to be nonmetals and sometimes not.[n 15]

Since the metalloids occupy "frontier territory",[93] where metals meet nonmetals, their treatment varies from author to author. Some consider them separate from both metals and nonmetals; some regard them as nonmetals[94] or as a sub-class of nonmetals.[95] Other authors count some of them as metals, for example arsenic and antimony, due to their similarities to heavy metals.[96][n 16] Metalloids are here treated as nonmetals in light of their chemical behavior, and for comparative purposes.

Aside from the metalloids, some boundary fuzziness and overlapping (as occurs with classification schemes generally)[97] can be discerned among the other nonmetal subclasses. Carbon, phosphorus, selenium, iodine border the metalloids and show some metallic character, as does hydrogen. Among the noble gases, radon is the most metallic and begins to show some cationic behavior, which is unusual for a nonmetal.[98]

Noble gases
Main article: Noble gas
A small (about 2 cm long) piece of rapidly melting argon ice

Six nonmetals are classified as noble gases: helium, neon, argon, krypton, xenon, and the radioactive radon. In conventional periodic tables they occupy the rightmost column. They are called noble gases in light of their characteristically very low chemical reactivity.[99]

They have very similar properties, with all of them being colorless, odorless, and nonflammable. With their closed outer electron shells the noble gases have feeble interatomic forces of attraction resulting in very low melting and boiling points.[100] That is why they are all gases under standard conditions, even those with atomic masses larger than many normally solid elements.[101]

Chemically, the noble gases have relatively high ionization energies, nil or negative electron affinities, and relatively high electronegativities. Compounds of the noble gases number in the hundreds although the list continues to grow,[102] with most of these involving oxygen or fluorine combining with either krypton, xenon or radon.[103]

In periodic table terms, an analogy can be drawn between the noble gases and noble metals such as platinum and gold, with the latter being similarly reluctant to enter into chemical combination.[104] As a further example, xenon, in the +8 oxidation state, forms a pale yellow explosive oxide, XeO4, while osmium, another noble metal, forms a yellow strongly oxidizing oxide, OsO4. There are parallels too in the formulas of the oxyfluorides: XeO2F4 and OsO2F4, and XeO3F2 and OsO3F2.[105]

About 1015 tonnes of noble gases are present in the Earth's atmosphere.[106] Helium is additionally found in natural gas to the extent of as much as 7%.[107] Radon diffuses out of rocks, where it is formed during the natural decay sequence of uranium and thorium.[108] In 2014 it was reported that the Earth's core may contain about 1013 tons of xenon, in the form of stable XeFe3 and XeNi3 intermetallic compounds. This may explain why "studies of the Earth's atmosphere have shown that more than 90% of the expected amount of Xe is depleted."[109]

Nonmetal halogens
See also: Halogen
A cluster of purple fluorite CaF
2, a fluorine mineral, between two quartzes

While the nonmetal halogens are markedly reactive and corrosive elements, they can be found in such mundane compounds as toothpaste (NaF); ordinary table salt (NaCl); swimming pool disinfectant (NaBr); or food supplements (KI). The word "halogen" means "salt former".[110]

Physically, fluorine and chlorine are pale yellow and yellowish green gases; bromine is a reddish-brown liquid (usually topped by a layer of its fumes); and iodine, under white light, is a metallic-looking[76] solid. Electrically, the first three are insulators while iodine is a semiconductor (along its planes).[111]

Chemically, they have high ionization energies, electron affinities, and electronegativity values, and are mostly relatively strong oxidizing agents.[112] Manifestations of this status include their corrosive nature.[113] All four exhibit a tendency to form predominately ionic compounds with metals[114] whereas the remaining nonmetals, bar oxygen, tend to form predominately covalent compounds with metals.[n 17] The reactive and strongly electronegative nature of the nonmetal halogens represents the epitome of nonmetallic character.[118]

In periodic table terms, the counterparts of the highly nonmetallic halogens in group 17 are the highly reactive alkali metals, such as sodium and potassium, in group 1.[119] Most of the alkali metals, as if in imitation of the nonmetal halogens, are known to form –1 anions (something that rarely occurs among metals).[120]

The nonmetal halogens are found in salt-related minerals. Fluorine occurs in fluorite (CaF2), a widespread mineral. Chlorine, bromine and iodine are found in brines. Exceptionally, a 2012 study reported the presence of 0.04% native fluorine (F
2) by weight in antozonite, attributing these inclusions as a result of radiation from the presence of tiny amounts of uranium.[121]

Unclassified nonmetals
Selenium conducts electricity around 1,000 times better when light falls on it, a property used in light-sensing applications.[122]

After the nonmetallic elements are classified as either noble gases, halogens or metalloids (following), the remaining seven nonmetals are hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur and selenium. In their most stable forms, three are colorless gases (H, N, O); three have a metal-like appearance (C, P, Se); and one is yellow (S). Electrically, graphitic carbon is a semimetal along its planes[123] and a semiconductor in a direction perpendicular to its planes;[124] phosphorus and selenium are semiconductors;[125] and hydrogen, nitrogen, oxygen, and sulfur are insulators.[n 18]

They are generally regarded as being too diverse to merit a collective examination,[127] and have been referred to as other nonmetals,[128] or more plainly as nonmetals, located between the metalloids and the halogens.[129] Consequently, their chemistry tends to be taught disparately, according to their four respective periodic table groups,[130] for example: hydrogen in group 1; the group 14 nonmetals (carbon, and possibly silicon and germanium); the group 15 nonmetals (nitrogen, phosphorus, and possibly arsenic and antimony); and the group 16 nonmetals (oxygen, sulfur, selenium, and possibly tellurium). Other subdivisions are possible according to the individual preferences of authors.[n 19]

Hydrogen, in particular, behaves in some respects like a metal and in others like a nonmetal.[132] Like a metal it can (first) lose its single electron;[133] it can stand in for alkali metals in typical alkali metal structures;[134] and is capable of forming alloy-like hydrides, featuring metallic bonding, with some transition metals.[135] On the other hand, it is an insulating diatomic gas, like a typical nonmetal, and in chemical reactions has a tendency to eventually attain the electron configuration of helium.[136] It does this by way of forming a covalent or ionic bond[135] or, if it has lost its electron, attaching itself to a lone pair of electrons.[137]

Some or all of these nonmetals nevertheless have several shared properties. Most of them, being less reactive than the halogens,[138] can occur naturally in the environment.[139] They have prominent biological[140][141] and geochemical roles.[127] While their physical and chemical character is "moderately non-metallic", on a net basis,[127] all of them have corrosive aspects. Hydrogen can corrode metals. Carbon corrosion can occur in fuel cells.[142] Acid rain is caused by dissolved nitrogen or sulfur. Oxygen corrodes iron via rust. White phosphorus, the most unstable form, ignites in air and produces phosphoric acid residue.[143] Untreated selenium in soils can give rise to corrosive hydrogen selenide gas.[144] When combined with metals, the unclassified nonmetals can form high hardness (interstitial or refractory) compounds,[145] on account of their relatively small atomic radii and sufficiently low ionization energy values.[127] They show a tendency to bond to themselves, especially in solid compounds.[146][127] Diagonal periodic table relationships among these nonmetals echo similar relationships among the metalloids.[147][148]

In periodic table terms, a geographic analogy is seen between the unclassified nonmetals and transition metals. The unclassified nonmetals occupy territory between the strongly nonmetallic halogens on the right and the weakly nonmetallic metalloids on the left. The transition metals occupy territory, "between the virulent and violent metals on the left of the periodic table, and the calm and contented metals to the right ... [and] ... form a transitional bridge between the two".[149]

Unclassified nonmetals typically occur in elemental forms (oxygen, sulfur) or are found in association with either of these two elements:[150]

  • Hydrogen occurs in the world's oceans as a component of water, and in natural gas as a component of methane and hydrogen sulfide.[151]
  • Carbon occurs in limestone, dolomite, and marble, as carbonates.[152] Less well known is carbon as graphite, which mainly occurs in metamorphic silicate rocks[153] as a result of the compression and heating of sedimentary carbon compounds.[154]
  • Oxygen is found in the atmosphere; in the oceans as a component of water; and in the crust as oxide minerals.
  • Phosphorus minerals are widespread, usually as phosphorus-oxygen phosphates.[155]
  • Elemental sulfur can be found in or near hot springs and volcanic regions in many parts of the world; sulfur minerals are widespread, usually as sulfides or oxygen-sulfur sulfates.[156]
  • Selenium occurs in metal sulfide ores, where it partially replaces the sulfur; elemental selenium is occasionally found.[157]

Metalloids
Main article: Metalloid
A crystal of realgar, also known as "ruby sulphur" or "ruby of arsenic", an arsenic sulfide mineral As4S4

The six elements more commonly recognized as metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium, each having a metallic appearance. On a standard periodic table, they occupy a diagonal area in the p-block extending from boron at the upper left to tellurium at lower right, along the dividing line between metals and nonmetals shown on some tables.[2]

They are brittle and poor to good conductors of heat and electricity. Boron, silicon, germanium and tellurium are semiconductors. Arsenic and antimony have the electronic structures of semimetals although both have less stable semiconducting forms.[2]

Chemically the metalloids generally behave like (weak) nonmetals. Among the nonmetallic elements they tend to have the lowest ionization energies, electron affinities, and electronegativity values, and are relatively weak oxidizing agents. They further demonstrate a tendency to form alloys with metals.[2]

In periodic table terms, to the left of the weakly nonmetallic metalloids are an indeterminate set of weakly metallic metals (such as tin, lead and bismuth)[158] sometimes referred to as post-transition metals.[159] Dingle explains the situation this way:

... with 'no-doubt' metals on the far left of the table, and no-doubt non-metals on the far right ... the gap between the two extremes is bridged first by the poor (post-transition) metals, and then by the metalloids—which, perhaps by the same token, might collectively be renamed the 'poor non-metals'.[160]

The metalloids tend to be found in forms combined with oxygen or sulfur or (in the case of tellurium) gold or silver.[150] Boron is found in boron-oxygen borate minerals including in volcanic spring waters. Silicon occurs in the silicon-oxygen mineral silica (sand). Germanium, arsenic and antimony are mainly found as components of sulfide ores. Tellurium occurs in telluride minerals of gold or silver. Native forms of arsenic, antimony and tellurium have been reported.[161]

Allotropes
Brownish crystals of buckminsterfullerene (С60), a semiconducting allotrope of carbon

Most nonmetallic elements exist in allotropic forms. Carbon, for example, occurs as graphite, diamond and other forms. Such allotropes may exhibit physical properties that are more metallic or less nonmetallic.[162]

Among the nonmetal halogens, and unclassified nonmetals:

  • Iodine is known in a semiconducting amorphous form.[163]
  • Graphite, the standard state of carbon, is a fairly good electrical conductor. The diamond allotrope of carbon is clearly nonmetallic, being translucent and an extremely poor electrical conductor.[164] Carbon is known in several other allotropic forms, including semiconducting buckminsterfullerene,[165] and amorphous[166] and paracrystalline (mixed amorphous and crystalline)[167] varieties.
  • Nitrogen can form gaseous tetranitrogen (N4), an unstable polyatomic molecule with a lifetime of about one microsecond.[168]
  • Oxygen is a diatomic molecule in its standard state; it also exists as ozone (O3), an unstable nonmetallic allotrope with an "indoors" half-life of around half an hour, compared to about three days in ambient air at 20 °C.[169]
  • Phosphorus, uniquely, exists in several allotropic forms that are more stable than its standard state as white phosphorus (P4). The white, red and black allotropes are probably the best known; the first is an insulator; the latter two are semiconductors.[170] Phosphorus also exists as diphosphorus (P2), an unstable diatomic allotrope.[171]
  • Sulfur has more allotropes than any other element.[172] Amorphous sulfur, a metastable mixture of such allotropes, is noted for its elasticity.[173]
  • Selenium has several nonmetallic allotropes, all of which are much less electrically conducting than its standard state of gray "metallic" selenium.[174]

All the elements most commonly recognized as metalloids form allotropes:

  • Boron is known in several crystalline and amorphous forms.[175]
  • Silicon can form crystalline (diamond-like); amorphous; and orthorhombic Si24 allotropes.[176]
  • At a pressure of about 10–11 GPa, germanium transforms to a metallic phase with the same tetragonal structure as tin. When decompressed—and depending on the speed of pressure release—metallic germanium forms a series of allotropes that are metastable in ambient conditions.[177]
  • Arsenic and antimony form several well-known allotropes (yellow, grey, and black).[178]
  • Tellurium is known in crystalline and amorphous forms.[179]

Other allotropic forms of nonmetallic elements are known, either under pressure or in monolayers. Under sufficiently high pressures, at least half of the nonmetallic elements that are semiconductors or insulators,[n 20] starting with phosphorus at 1.7 GPa, have been observed to form metallic allotropes.[181][n 21] Single layer two-dimensional forms of nonmetals include borophene (boron), graphene (carbon), silicene (silicon), phosphorene (phosphorus), germanene (germanium), arsenene (arsenic), antimonene (antimony) and tellurene (tellurium), collectively referred to as xenes.[183]

Prevalence and access

Abundance
Approximate nonmetal composition
of the Earth and its biomass, by weight[184]
DomainMain componentsNext most
abundant
CrustO 61%, Si 20%H 2.9%
AtmosphereN 78%, O 21%Ar 0.5%
HydrosphereO 66.2%, H 33.2%Cl 0.3%
BiomassO 63%, C 20%, H 10%N 3.0%

Hydrogen and helium are estimated to make up approximately 99% of all ordinary matter in the universe and over 99.9% of its atoms.[185] Oxygen is thought to be the next most abundant element, at about 0.1%.[186] Less than five per cent of the universe is believed to be made of ordinary matter, represented by stars, planets and living beings. The balance is made of dark energy and dark matter, both of which are currently poorly understood.[187]

Five nonmetals namely hydrogen, carbon, nitrogen, oxygen and silicon constitute the bulk of the Earth's crust, atmosphere, hydrosphere and biomass, in the quantities shown in the table.

Extraction
Germanium occurs in some zinc-copper-lead ore bodies, in quantities sufficient to justify extraction.[188] In 2021, the 99.999% pure form was priced at US$1200 per kilogram.[189]

Nonmetals, and metalloids, are extracted in their raw forms from:[139]

  • brine—chlorine, bromine, iodine;
  • liquid air—nitrogen, oxygen, neon, argon, krypton, xenon;
  • minerals—boron (borate minerals); carbon (coal; diamond; graphite); fluorine (fluorite); silicon (silica); phosphorus (phosphates); antimony (stibnite, tetrahedrite); iodine (in sodium iodate and sodium iodide);
  • natural gas—hydrogen, helium, sulfur; and
  • ores, as processing byproducts—germanium (zinc ores); arsenic (copper and lead ores); selenium, tellurium (copper ores); and radon (uranium-bearing ores).

Cost

Day to day costs will vary depending on purity, quantity, market conditions, and supplier surcharges.[190]

Based on the available literature as at August 2022, while the cited costs of most nonmetals are less than the $US0.80 per gram cost of silver,[191] boron, phosphorus, germanium, xenon, and radon (notionally) are exceptions:

  • Boron costs around $25 per gram for 99.7% pure polycrystalline chunks with a particle size of about 1 cm.[192] Earlier, in 1997, boron was quoted at $280 per gram for polycrystalline 4 to 6 mm diameter rods of 99.999% purity,[193] about ten times the then $28.35 per gram cost of gold.[194]
  • In 2020 phosphorus in its most stable black form could "cost up to $1,000 per gram",[195] more than 15 times the cost of gold, whereas ordinary red phosphorus, in 2017, was priced at about $3.40 per kilogram.[196] Researchers hoped to be able to reduce the cost of black phosphorus to as low as $1 per gram.[195]
  • Germanium and xenon cost about $1.20 and $7.60 per gram.[197]
  • Up to 2013, radon was available from the National Institute of Standards and Technology for $1,636 per 0.2 ml unit of issue, equivalent to about $86,000,000 per gram, with no indication of a discount for bulk quantities.[198]

Shared uses

Nearly all nonmetals have varying uses in household items; lighting and lasers; and medicine and pharmaceuticals. Nitrogen, for example, is found in some garden treatments; lasers; and diabetes medicines. Germanium, arsenic, and radon each have uses in one or two of these fields but not all three.[139] Aside from the noble gases most of the remaining nonmetals have, or have had, uses in agrochemicals and dyestuffs.[139] To the extent that metalloids show metallic character, they have speciality uses extending to (for example) oxide glasses, alloying components, and semiconductors.[199]

Further shared uses of different subsets of the nonmetals occur in or as air replacements; cryogenics and refrigerants; fertilizers; flame retardants or fire extinguishers; mineral acids; plug-in hybrid vehicles; welding gases; and smart phones.[139]

History, background, and taxonomy

Discovery
The Alchemist Discovering Phosphorus (1771) by Joseph Wright. The alchemist is Hennig Brand; the glow emanates from the combustion of phosphorus inside the flask.

Most nonmetals were discovered in the 18th and 19th centuries. Before then carbon, sulfur and antimony were known in antiquity; arsenic was discovered during the Middle Ages (by Albertus Magnus); and Hennig Brand isolated phosphorus from urine in 1669. Helium (1868) holds the distinction of being the only element not first discovered on Earth.[n 22] Radon is the most recently discovered nonmetal, being found only at the end of the 19th century.[139]

Chemistry- or physics-based techniques used in the isolation efforts were spectroscopy, fractional distillation, radiation detection, electrolysis, ore acidification, displacement reactions, combustion and heating; a few nonmetals occurred naturally as free elements

Of the noble gases, helium was detected via its yellow line in the coronal spectrum of the sun, and later by observing the bubbles escaping from uranite UO2 dissolved in acid. Neon through xenon were obtained via fractional distillation of air. Radon was first observed emanating from compounds of thorium, three years after Henri Becquerel's discovery of radiation in 1896.[201]

The nonmetal halogens were obtained from their halides via either electrolysis, adding an acid, or displacement. Some chemists died as a result of their experiments trying to isolate fluorine.[202]

Among the unclassified nonmetals, carbon was known (or produced) as charcoal, soot, graphite and diamond; nitrogen was observed in air from which oxygen had been removed; oxygen was obtained by heating mercurous oxide; phosphorus was liberated by heating ammonium sodium hydrogen phosphate (Na(NH4)HPO4), as found in urine;[203] sulfur occurred naturally as a free element; and selenium[n 23] was detected as a residue in sulfuric acid.[205]

Most of the elements commonly recognized as metalloids were isolated by heating their oxides (boron, silicon, arsenic, tellurium) or a sulfide (germanium).[139] Antimony was known in its native form as well as being attainable by heating its sulfide.[206]

Origin of the concept

The distinction between metals and nonmetals arose, in a convoluted manner, from a crude recognition of different kinds of matter namely pure substances, mixtures, compounds and elements. Thus, matter could be divided into pure substances (such as salt, bicarb of soda, or sulfur) and mixtures (aqua regia, gunpowder, or bronze, for example) and pure substances eventually could be distinguished as compounds and elements.[207] "Metallic" elements then seemed to have broadly distinguishable attributes that other elements did not, such as their ability to conduct heat or for their "earths" (oxides) to form basic solutions in water, for example as occurred with quicklime (CaO).[208]

Use of the term

The term nonmetallic dates from as far back as 1566. In a medical treatise published that year, Loys de L’Aunay (a French doctor) mentioned the properties of plant substances from metallic and "non-metallic" lands.[209]

In early chemistry, Wilhelm Homberg (a German natural philosopher) referred to "non-metallic" sulfur in Des Essais de Chimie (1708).[210] He questioned the five-fold division of all matter into sulfur, mercury, salt, water and earth, as postulated by Étienne de Clave (1641) in New Philosophical Light of True Principles and Elements of Nature.[211] Homberg's approach represented "an important move toward the modern concept of an element".[212]

Lavoisier, in his "revolutionary"[213] 1789 work Traité élémentaire de chimie, published the first modern list of chemical elements in which he distinguished between gases, metals, nonmetals, and earths (heat resistant oxides).[214] In its first seventeen years, Lavoisier's work was republished in twenty-three editions in six languages, and "carried ... [his] new chemistry all over Europe and America."[215]

Suggested distinguishing criteria
Some single properties used to distinguish between
metals and nonmetals listed by type and date of source
Physical
Chemical
Electron related

In 1809, Humphry Davy's discovery of sodium and potassium "annihilated"[236] the line of demarcation between metals and nonmetals. Before then metals had been distinguished on the basis of their ponderousness or relatively high densities.[237] Sodium and potassium, on the other hand, floated on water and yet were clearly metals on the basis of their chemical behaviour.[238]

From as early as 1811, different properties—physical, chemical, and electron related—have been used in attempts to refine the distinction between metals and nonmetals. The accompanying table sets out 22 such properties, by type and date order.

Probably the most well-known property is that the electrical conductivity of a metal increases when temperature falls whereas that of a non-metal rises.[228] However this scheme does not work for plutonium, carbon, arsenic and antimony. Plutonium, which is a metal, increases its electrical conductivity when heated in the temperature range of around –175 to +125 °C.[239] Carbon, despite being widely regarded as a nonmetal, likewise increases its conductivity when heated.[240] Arsenic and antimony are sometimes classified as nonmetals yet act similarly to carbon.[241]

Kneen et al. suggested that the nonmetals could be discerned once a [single] criterion for metallicity had been chosen, adding that, "many arbitrary classifications are possible, most of which, if chosen reasonably, would be similar but not necessarily identical."[12] Emsley noted that, "No single property ... can be used to classify all the elements as either metals or nonmetals."[242] Jones added that "classes are usually defined by more than two attributes".[243]

The first 99 elements sorted by
density and electronegativity (EN)[n 25]
Density
EN< 7 gm/cm3> 7 gm/cm3
≥ 1.9Noble gases
F, Cl, Br, I
H, C, N, P, O, S, Se
B, Si, Ge, As, Sb, Te		Ni, Mo, W, Tc, Re,
Platinum group metals,
Coinage metals, Hg, Sn,
Bi, Po, At
< 1.9 Groups 1 and 2
Sc, Y, La
Ce, Pr, Eu, Yb
Ti, V; Al, Ga		Nd, Pm, Sm, Gd, Tb, Dy
Ho, Er, Tm, Lu; Actinides;
Cr, Mn, Fe, Co, Zn,
Hf, Nb, Ta; In, Tl, Pb

Johnson suggested that physical properties can best indicate the metallic or nonmetallic properties of an element, with the proviso that other properties will be needed in ambiguous cases. He observed that all gaseous or nonconducting elements are nonmetals; solid nonmetals metals are hard and brittle or soft and crumbly whereas metals are usually malleable and ductile; and nonmetal oxides are acidic.[249]

According to Hein and Arena, nonmetals have low densities and relatively high electronegativity;[250] the accompanying table bears this out. Nonmetallic elements occupy the top left quadrant, where densities are low and electronegativity values are relatively high. The other three quadrants are occupied by metals. Some authors further divide the elements into metals, metalloids, and nonmetals although Odberg argues that anything not a metal is, on categorisation grounds, a nonmetal.[251]

Development of subclasses

A basic taxonomy of nonmetals was set out in 1844, by Alphonse Dupasquier, a French doctor, pharmacist and chemist.[252] To facilitate the study of nonmetals, he wrote:[253]

They will be divided into four groups or sections, as in the following:
Organogens O, N, H, C
Sulphuroids S, Se, P
Chloroides F, Cl, Br, I
Boroids B, Si.

An echo of Dupasquier's fourfold classification is seen in the modern subclasses. The organogens and sulphuroids represent the set of unclassified nonmetals. Varying configurations of these seven nonmetals have been referred to as, for example, basic nonmetals;[254] biogens;[255] central nonmetals;[256] CHNOPS;[257] essential elements;[258] "nonmetals";[259][n 26] orphan nonmetals;[260] or redox nonmetals.[261] The chloroide nonmetals came to be independently referred to as halogens.[262] The boroid nonmetals expanded into the metalloids, starting from as early as 1864.[263] The noble gases, as a discrete grouping, were counted among the nonmetals from as early as 1900.[264]

Comparison

Some properties of metals, and of metalloids, unclassified nonmetals, nonmetal halogens, and noble gases are summarized in the table.[n 27] Physical properties apply to elements in their most stable forms in ambient conditions, and are listed in loose order of ease of determination. Chemical properties are listed from general to descriptive, and then to specific. The dashed line around the metalloids denotes that, depending on the author, the elements involved may or may not be recognized as a distinct class or subclass of elements. Metals are included as a reference point.

Most properties show a left-to-right progression in metallic to nonmetallic character or average values. The periodic table can thus be indicatively divided into metals and nonmetals, with more or less distinct gradations seen among the nonmetals.[265]

Some cross-subclass properties
Physical property Metals
alkali, alkaline earth, lanthanide, actinide, transition, post-transition		 Metalloids
boron, silicon, germanium, arsenic, antimony, tellurium		 Unclassified nonmetals
hydrogen, carbon, nitrogen, phosphorus, oxygen, sulfur, selenium		 Nonmetal halogens
fluorine, chlorine, bromine, iodine		 Noble gases
helium, neon, argon, krypton, xenon, radon
Form and heft[266]
  • ◇ solid
  • ◇ often high density such as Fe, Pb, W
  • ◇ some light metals including Be, Mg, Al
  • ◇ solid
  • ◇ low to higher density
  • ◇ all lighter than Fe
  • ◇ solid or gas
  • ◇ low density
  • ◇ H, N lighter than air[267]
  • ◇ solid, liquid or gas
  • ◇ low density
  • ◇ gas
  • ◇ low density
  • ◇ He, Ne lighter than air[268]
Appearance lustrous[20] lustrous[269]
  • ◇ lustrous: C, P, Se[270]
  • ◇ colorless: H, N, O[271]
  • ◇ colored: S[272]
  • ◇ colored: F, Cl, Br[273]
  • ◇ lustrous: I[2]
 colorless[274]
Elasticity mostly malleable and ductile[20] (Hg is liquid) brittle[269] C, black P, S, Se brittle; all four have less stable non-brittle forms[275][n 28] iodine is brittle[277] not applicable
Electrical conductivity good[n 29]
  • ◇ moderate: B, Si, Ge, Te
  • ◇ good: As, Sb[n 30]
  • ◇ poor: H, N, O, S
  • ◇ moderate: P, Se
  • ◇ good: C[n 31]
  • ◇ poor: F, Cl, Br
  • ◇ moderate: I[n 32]
 poor[n 33]
Electronic structure[180] metallic (Bi is a semimetal) semimetal (As, Sb) or semiconductor
  • ◇ semimetal: C
  • ◇ semiconductor: P, Se
  • ◇ insulator: H, N, O, S
 semiconductor (I) or insulator insulator
Chemical property Metals
alkali, alkaline earth, lanthanide, actinide, transition, post-transition		 Metalloids
boron, silicon, germanium, arsenic, antimony, tellurium		 Unclassified nonmetals
hydrogen, carbon, nitrogen, phosphorus, oxygen, sulfur, selenium		 Nonmetal halogens
fluorine, chlorine, bromine, iodine		 Noble gases
helium, neon, argon, krypton, xenon, radon
General chemical behavior
weakly nonmetallic[n 34] moderately nonmetallic[283] strongly nonmetallic[284]
  • ◇ inert to nonmetallic[285]
  • ◇ Rn shows some cationic behavior[286]
Oxides
  • ◇ basic; some amphoteric or acidic[287]
  • ◇ V; Mo, W; Al, In, Tl; Sn, Pb; Bi are glass formers[288]
  • ◇ ionic, polymeric, layer, chain, and molecular structures[289]
  • ◇ acidic (NO
    2    , N
    2    O
    5    , SO
    3    , SeO
    3     strongly so)[294][295] or neutral (H2O, CO, NO, N2O)[n 36]
  • ◇ P, S, Se are glass formers;[288] CO2 forms a glass at 40 GPa[297]
  • ◇ mostly molecular[293]
  • ◇ C, P, S, Se have at least one polymeric form
  • ◇ acidic; ClO
    2    , Cl
    2    O
    7    , I
    2    O
    5     strongly so[295][294]
  • ◇ no glass formers reported
  • ◇ molecular[293]
  • ◇ iodine has at least one polymeric form, I2O5[298]
  • ◇ metastable XeO3 is acidic;[299] stable XeO4 strongly so[300]
  • ◇ no glass formers reported
  • ◇ molecular[293]
  • ◇ XeO2 is polymeric[301]
Compounds with metals alloys[20] or intermetallic compounds[302] tend to form alloys or intermetallic compounds[303]
  • ◇ salt-like to covalent: H†, C, N, P, S, Se[4]
  • ◇ mainly ionic: O[304]
 mainly ionic[114] simple compounds in ambient conditions not known[n 37]
Ionization energy (kJ mol−1)‡
(data page)
  • ◇ low to high
  • ◇ 376 to 1,007
  • ◇ average 643
  • ◇ moderate
  • ◇ 762 to 947
  • ◇ average 833
  • ◇ moderate to high
  • ◇ 941 to 1,402
  • ◇ average 1,152
  • ◇ high
  • ◇ 1,008 to 1,681
  • ◇ average 1,270
  • ◇ high to very high
  • ◇ 1,037 to 2,372
  • ◇ average 1,589
Electronegativity (Pauling)[n 38]‡
(data page)
  • ◇ low to high
  • ◇ 0.79 to 2.54
  • ◇ average 1.5
  • ◇ moderate
  • ◇ 1.9 to 2.18
  • ◇ average 2.05
  • ◇ moderate to high
  • ◇ 2.19 to 3.44
  • ◇ average 2.65
  • ◇ high
  • ◇ 2.66 to 3.98
  • ◇ average 3.19
  • ◇ high (Rn) to very high
  • ◇ ca. 2.43 to 4.7
  • ◇ average 3.3
† Hydrogen can also form alloy-like hydrides[307]
‡ The labels low, moderate, high, and very high are arbitrarily based on the value spans listed in the table

See also
  • CHON (carbon, hydrogen, oxygen, nitrogen)
  • List of nonmetal monographs
  • Metallization pressure
  • Period 1 elements (hydrogen, helium)
  • Properties of nonmetals (and metalloids) by group

Notes
  1. H; N; O, S; F, Cl, Br, I; He, Ne, Ar, Kr, Xe, Rn[1]
  2. C; P; Se.[1] On the other hand, these three elements were counted as metalloids in a survey of 194 lists of metalloids, 16, 10, and 46 times respectively.[2]
  3. B; Si, Ge; As, Sb; Te[3][4]
  4. Al, Ga, In, Tl; Sn, Pb; Bi; Po; At
  5. Hydrogen has historically been placed over one or more of lithium, boron,[5] carbon, or fluorine;[6] or over no group at all; or over all main groups simultaneously, and therefore may or may not be adjacent to other nonmetals.[7]
  6. The solid nonmetals have thermal conductivity values of from 0.27 W m–1 K–1 for sulfur to 2,000 for carbon cf. 6.3 for neptunium to 429 for silver, metals both;[22] electrical conductivity values range from 10−18 S•cm−1 for sulfur[22] to 3 × 104 in graphite[23] or 3.9 × 104 for arsenic[24] cf. 0.69 × 104 for manganese to 63 × 104 for silver, metals both.[22]
  7. The absorbed light may be converted to heat or re-emitted in all directions so that the emission spectrum is thousands of times weaker than the incident light radiation.[37]
  8. Thermal conductivity values for metals range from 6.3 W m–1 K–1 for neptunium to 429 for silver; cf. antimony 24.3, arsenic 50, and carbon 2000;[22] electrical conductivity values of metals range from 0.69 S•cm−1 × 104 for manganese to 63 × 104 for silver; cf. carbon 3 × 104,[23] arsenic 3.9 × 104 and antimony 2.3 × 104[22]
  9. These elements being semiconductors[42]
  10. For example, C as exfoliated (expanded) graphite[43] and as carbon nanotube wire;[44] P as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature);[45] S as plastic sulfur;[46] and Se as selenium wires, drawn from the molten form[47]
  11. Acids are formed by boron, phosphorus, selenium, arsenic, iodine;[54] oxides by carbon, silicon, germanium, sulfur, antimony, and tellurium.[55]
  12. These elements are hydrogen and helium in the s-block; boron to neon in the p-block; scandium to zinc in the d-block; and lanthanum to ytterbium in the f-block.
  13. Noble gases: He, Ne, Ar, Kr, Xe, Rn; Nonmetal halogens: F, Cl, Br, I; Unclassified nonmetals: H, C, N, P, O, S, Se; Metalloids: B, Si, Ge, As, Sb, Te. Nearby metals are Al, Ga, In, Tl; Sn, Pb; Bi; Po; and At.
  14. The seven nonmetals marked with single or double daggers each have a lackluster appearance and discrete molecular structures, but for I which has a metallic appearance under white light.[76] The remaining reactive nonmetallic elements have giant covalent structures, but for H which is a diatomic gas.[77]
    The single dagger nonmetals N, S and iodine are somewhat hobbled as "strong" nonmetals.
    While N has a high electronegativity, it is a reluctant anion former,[78] and a pedestrian oxidizing agent unless combined with a more active non-metal like O or F.[79]
    S reacts in the cold with alkalic and post-transition metals, and Cu, Ag and Hg,[80] but otherwise has low values of ionization energy, electron affinity, and electronegativity compared to the averages of the others; it is regarded as being not a particularly good oxidizing agent.[81]
    Iodine is sufficiently corrosive to cause lesions resembling thermal burns, if handled without suitable protection,[82] and tincture of iodine will smoothly dissolve Au.[83] That said, while "F, Cl and Br will all oxidize Fe2+ (aq) to Fe3+(aq) ... iodine ... is such a [relatively] weak oxidizing agent that it cannot remove electrons from Fe(II) ions to form Fe(III) ions."[84] Thus, for the reaction X2 + 2e− → 2X−(aq) the reduction potentials are F +2.87 V; Cl +1.36; Br +1.09; I +0.54. Here Fe3+ + e− → Fe3+ +0.77.[85] Thus F2, Cl2 and Br2 will oxidize Fe2+ to Fe3+ but Fe3+ will oxidize I− to I2. Iodine has previously been referred to as a moderately strong oxidizing agent.[86]
  15. Tshitoyan et al. (2019) conducted a machine-based analysis of the proximity of names of the elements based on 3.3 million abstracts published between 1922 and 2018 in more than 1,000 journals. The resulting map shows that "chemically similar elements are seen to cluster together and the overall distribution exhibits a topology reminiscent of the periodic table itself".[92]
  16. Jones takes a philosophical or pragmatic view to these questions. He writes: "Though classification is an essential feature of all branches of science, there are always hard cases at the boundaries. The boundary of a class is rarely sharp ... Scientists should not lose sleep over the hard cases. As long as a classification system is beneficial to economy of description, to structuring knowledge and to our understanding, and hard cases constitute a small minority, then keep it. If the system becomes less than useful, then scrap it and replace it with a system based on different shared characteristics".[97]
  17. Metal oxides are usually ionic.[115] On the other hand, oxides of metals with high oxidation states are usually either polymeric or covalent.[116] A polymeric oxide has a linked structure composed of multiple repeating units.[117]
  18. Sulfur, an insulator, and selenium, a semiconductor are each photoconductors—their electrical conductivities increase by up to six orders of magnitude when exposed to light.[126]
  19. For example, Wulfsberg divides the nonmetals, including B, Si, Ge, As, Sb, Te, Xe, into very electronegative nonmetals (Pauling electronegativity over 2.8) and electronegative nonmetals (1.9 to 2.8). This results in N and O being very electronegative nonmetals, along with the halogens; and H, C, P, S and Se being electronegative nonmetals. Se is further recognized as a semiconducting metalloid.[131]
  20. B; Si, Ge; N, P; O, S, Se, Te; nonmetal halogens; and the noble gases[180]
  21. In 2020, high pressure studies and experiments were said to represent "a very active and vigorous research field".[182]
  22. How helium acquired the -ium suffix is explained in the following passage by its discoverer, William Lockyer: "I took upon myself the responsibility of coining the word helium ... I did not know whether the substance ... was a metal like calcium or a gas like hydrogen, but I did know that it behaved like hydrogen [being found in the sun] and that hydrogen, as Dumas had stated, behaved as a metal".[200]
  23. Berzelius, who discovered selenium, thought it had the properties of a metal, combined with those of sulfur.[204]
  24. The Goldhammer-Herzfeld ratio is roughly equal to the cube of the atomic radius divided by the molar volume.[219] More specifically, it is the ratio of the force holding an individual atom's outer electrons in place with the forces on the same electrons from interactions between the atoms in the solid or liquid element. When the interatomic forces are greater than, or equal to, the atomic force, outer electron itinerancy is indicated and metallic behaviour is predicted. Otherwise nonmetallic behaviour is anticipated.[220]
  25. (a) The values are from Aylward and Findlay.[244]
    (b) Weighable amounts of the extremely radioactive elements At (element 85), Fr (87), and elements with an atomic number higher than Es (99), have not been prepared.[245]
    (c) The density values used for At and Fr are theoretical estimates.[246][247]
    (d) Bjerrum classified "heavy metals" as those metals with densities above 7 g/cm3.[248]
    (e) Vernon specified a minimum electronegativity of 1.9 for the metalloids, on the revised Pauling scale.[2]
  26. The quote marks are not found in the source; they are used here to make it clear that the source employs the word nonmetals as a formal term for the subset of chemical elements in question, rather than applying to nonmetals generally.
  27. See also Properties of metals, metalloids and nonmetals, which treats metalloids as a class of their own
  28. Carbon as exfoliated (expanded) graphite,[276] and as carbon nanotube wire;[44] phosphorus as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature);[45] sulfur as plastic sulfur;[46] and selenium as selenium wires[47]
  29. Metals have electrical conductivity values of from 6.9×103 S•cm−1 for manganese to 6.3×105 for silver.[278]
  30. Metalloids have electrical conductivity values of from 1.5×10−6 S•cm−1 for boron to 3.9×104 for arsenic.[279]
  31. Unclassified nonmetals have electrical conductivity values of from ca. 1×10−18 S•cm−1 for the elemental gases to 3±4 in graphite.[280]
  32. The nonmetal halogens have electrical conductivity values of from ca. 1×10−18 S•cm−1 for F and Cl to 1.7×10−8 S•cm−1 for iodine.[280][111]
  33. The elemental gases have electrical conductivity values of ca. 1×10−18 S•cm−1.[280]
  34. They always give "compounds less acidic in character than the corresponding compounds of the [typical] nonmetals"[269]
  35. Arsenic trioxide reacts with sulfur trioxide, forming arsenic "sulfate" As2(SO4)3.[291]
  36. CO and N2O are "formally the anhydrides of formic and hyponitrous acid, respectively: CO + H2O → H2CO2 (HCOOH, formic acid); N2O + H2O → H2N2O2 (hyponitrous acid)".[296]
  37. Disodium helide (Na2He) is a compound of helium and sodium that is stable at high pressures above 113 GPa. Argon forms an alloy with nickel, at 140 GPa and close to 1,500 K however at this pressure argon is no longer a noble gas.[305]
  38. Values for the noble gases are from Rahm, Zeng and Hoffmann.[306]

References

Citations
  1. Larrañaga, Lewis & Lewis 2016, p. 988
  2. Vernon 2013
  3. Hérold 2006, pp. 149–50
  4. Vernon 2020, p. 220
  5. Luchinskii & Trifonov 1981, pp. 200–220
  6. Jolly 1966, inside cover
  7. Rayner-Canham 2020, p. 212
  8. Glinka 1958, p. 77; Oxtoby, Gillis & Butler 2015, p. I.23
  9. Godovikov & Nenasheva 2020, p. 4
  10. Sanderson 1957, p. 229
  11. Morely & Muir 1892, p. 241
  12. Kneen, Rogers & Simpson 1972, pp. 218–219
  13. Steudel 2020, p. 43
  14. Hill, Holman & Hulme 2017, p. 182: Atomic conductance is the electrical conductivity of one mole of a substance. It is equal to electrical conductivity divided by molar volume.
  15. IUPAC Periodic Table of the Elements
  16. Johnson 2007, p. 13
  17. Bodner & Pardue 1993, p. 354; Cherim 1971, p. 98
  18. Restrepo et al. 2006, p. 411; Thornton & Burdette 2010, p. 86; Hermann, Hoffmann & Ashcroft 2013, pp. 11604‒1‒11604‒5
  19. Mewes et al. 2019; Smits et al. 2020; Florez et al. 2022
  20. Kneen, Rogers & Simpson 1972, pp. 261–264
  21. Phillips 1973, p. 7
  22. Aylward & Findlay 2008, pp. 6–12
  23. Jenkins & Kawamura 1976, p. 88
  24. Carapella 1968, p. 30
  25. Zumdahl & DeCoste 2010, pp. 455, 456, 469, A40
  26. Still 2016, p. 120
  27. Siekierski & Burgess 2002, p. 86
  28. Charlier, Gonze & Michenaud 1994
  29. Taniguchi et al. 1984, p. 867: "... black phosphorus ... [is] characterized by the wide valence bands with rather delocalized nature."; Morita 1986, p. 230; Carmalt & Norman 1998, p. 7: "Phosphorus ... should therefore be expected to have some metalloid properties."; Du et al. 2010. Interlayer interactions in black phosphorus, which are attributed to van der Waals-Keesom forces, are thought to contribute to the smaller band gap of the bulk material (calculated 0.19 eV; observed 0.3 eV) as opposed to the larger band gap of a single layer (calculated ~0.75 eV).
  30. Wiberg 2001, pp. 742
  31. Evans 1966, pp. 124–25
  32. Wiberg 2001, pp. 758
  33. Stuke 1974, p. 178; Donohue 1982, pp. 386–87; Cotton et al. 1999, p. 501
  34. Steudel 1977, p. 240: "... considerable orbital overlap must exist, to form intermolecular, many-center ... [sigma] bonds, spread through the layer and populated with delocalized electrons, reflected in the properties of iodine (lustre, color, moderate electrical conductivity)."; Segal 1989, p. 481: "Iodine exhibits some metallic properties ..."
  35. Wiberg 2001, p. 416; Wiberg is here referring to iodine.
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