Chemistry:Nonmetal

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Short description: Chemical element that mostly lacks the characteristics of a metal
This page is about chemical elements. For nonmetallic substances, see materials science.
A periodic table showing 14 elements listed by nearly all authors as nonmetals (the noble gases plus fluorine, chlorine, bromine, iodine, nitrogen, oxygen, and sulfur); 3 elements listed by most authors as nonmetals (carbon, phosphorus and selenium); and 6 elements listed as nonmetals by some authors (boron, silicon, germanium, arsenic, antimony). Nearby metals are aluminium, gallium, indium, thallium, tin, lead, bismuth, polonium, and astatine.
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 (as in the full table below) but can be in group 17 (as in the extract above).[n 5]
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A nonmetal is a chemical element that, in the broadest sense of the term, has a relatively low density and high electronegativity; they range from colorless gases (like hydrogen) to shiny solids (like carbon, as graphite). They are usually poor conductors of heat and electricity, and brittle or crumbly when solid due to their electrons having low mobility. In contrast, metals are good conductors and most are easily flattened into sheets and drawn into wires since their electrons are generally free-moving. Nonmetal atoms tend to attract electrons in chemical reactions and to form acidic compounds.

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, make up most of the Earth's crust, atmosphere, oceans and biosphere.

The distinctive properties of nonmetallic elements allow for specific applications that often cannot be fulfilled by metallic elements alone. Living organisms are composed almost entirely of the nonmetals hydrogen, oxygen, carbon, and nitrogen. Nonmetallic elements are important to industries ranging from electronics and energy storage to agriculture and chemical production.

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, and which of these borderline cases are counted as nonmetals varies depending on the classification criteria used. Generally, from 14 to 23 elements are recognized as nonmetals.

Definition and applicable elements

A nonmetal is a chemical element that, in the broadest sense of the term, has a relatively low density and high electronegativity.[7] More generally they are deemed to lack a preponderance of metallic properties such as luster or shininess; the capacity to be flattened into a sheet or drawn into a wire; 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] 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.[10][n 6]

Although Steudel,[11][n 7] in 2020, recognised twenty-three elements as nonmetals, any such list is open to challenge.[1] The fourteen elements that are almost always recognized as nonmetals 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, as listed in Hawley’s Condensed Chemical Dictionary.[1] While carbon, phosphorus and selenium were included as nonmetals, it had earlier been reported that these three elements were instead sometimes counted as metalloids.[2] The elements commonly recognized as metalloids (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.[12] At other times they are counted as nonmetals in light of their predominately nonmetallic (weakly acidic) chemistry.[13]

Of the 118 known elements,[14] no more than about 20% are regarded as nonmetals.[15] The status of a few elements is less certain. Astatine, the fifth halogen, is often ignored on account of its rarity and intense radioactivity;[16] theory and experimental evidence suggest it is a metal.[17] The superheavy elements copernicium (element 112), flerovium (114), and oganesson (118) may turn out to be nonmetals. (As of April 2023) their status has not been confirmed.[18]

General properties

Properties noted in this section refer to the elements in their most stable forms in ambient conditions

Physical

Variety in color and form
of some nonmetallic elements
Several dozen small angular stone like shapes, grey with scattered silver flecks and highlights.
Boron in its β-rhombohedral phase
A shiny grey-black cuboid nugget with a rough surface.
Metallic appearance of carbon as graphite
A pale blue liquid in a clear beaker
Blue color of liquid oxygen
A glass tube, is inside a larger glass tube, has some clear yellow liquid in it
Pale yellow liquid fluorine in a cryogenic bath
Yellow powdery chunks
Sulfur as a yellow powder
A small capped jar a quarter filled with a very dark liquid
Liquid bromine at room temperature
Shiny violet-black colored crystalline shards.
Metallic appearance of iodine under white light
A partly filled ampoule containing a colorless liquid
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.[n 8] The fluid nonmetals have very low densities, melting points, and boiling points, and are poor conductors of heat and electricity.[21] The solid elements have low densities, are brittle or crumbly with low mechanical and structural strength,[22] and are poor to good conductors.[n 9]

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, the outer electrons are expected to become free to move between atoms, and metallic properties are predicted. Otherwise nonmetallic properties are expected.[26]

Those nonmetals existing as discrete atoms (xenon, for example) or molecules (oxygen, sulfur, and bromine, for example) have low melting and boiling points, and many are gases at room temperature, since they are held together by weak London dispersion forces acting between their atoms or molecules.[27] Nonmetals that form giant structures, such as chains of up to 1,000 atoms (selenium),[28] sheets (carbon as graphite, for example),[29] or three-dimensional lattices (silicon, for example),[30] have higher melting and boiling points, and are all solids, as it takes more energy to overcome their stronger covalent bonds.[31] 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,[32] carbon,[33] phosphorus,[34] arsenic,[35] selenium,[36] antimony,[37] tellurium[38] and iodine.[39]

Nonmetallic elements are either shiny, colored, or colorless. The shiny appearance of boron, graphitic carbon, silicon, black phosphorus, germanium, arsenic, selenium, antimony, tellurium, and iodine is a result of their structures featuring varying degrees of delocalised (free-moving) electrons that scatter incoming visible light.[40] The colored nonmetals (sulfur, fluorine, chlorine, bromine) absorb some colors (wavelengths) and transmit the complementary or opposite colors. In the case of chlorine, for example, Elliot writes that its "familiar yellow-green colour...is due to a broad region of absorption in the violet and blue regions of the spectrum".[41][n 10]

For the colorless nonmetals (hydrogen, nitrogen, oxygen, and the noble gases), their electrons are held sufficiently strongly such that no absorption happens in the visible part of the spectrum, and all visible light is transmitted.[43]

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[44] the electrons in nonmetals typically lack such mobility.[45] Among the nonmetallic elements, good electrical and thermal conductivity is seen only in carbon, arsenic, and antimony.[n 11] Good thermal conductivity otherwise occurs only in boron, silicon, phosphorus, and germanium;[23] such conductivity is transmitted though vibrations of the crystalline lattices of these elements.[46] Moderate electrical conductivity is evidenced in boron, silicon, phosphorus, germanium, selenium, tellurium, and iodine.[n 12] Plasticity occurs under limited circumstances in carbon, as exfoliated (expanded) graphite[48] and as carbon nanotube wire;[49] phosphorus as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature);[50] sulfur as plastic sulfur;[51] and selenium as selenium wires, drawn from the molten form.[52]

Chemical

Some chemistry-based typical
differences between metals and nonmetals[53]
Aspect Metals Nonmetals
Electronegativity Lower than nonmetals,
with some exceptions[54] 		Relatively 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[55]
In aqueous
solution[56] 		Exist as cations Exist as anions
or oxyanions

Nonmetals have relatively high values of electronegativity[7] 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 13]

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).[59]

Nonmetals further mostly have higher ionization energies, electron affinities, and standard reduction potentials than metals. In general, the higher these values are (including electronegativity) the more nonmetallic the element is.[60]

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.[61] There is an associated reduction in atomic radius[62] as the increasing nuclear charge draws the outer electrons closer to the core.[63] 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.[64]

The number of compounds formed by nonmetals is vast.[65] The first 10 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.[66] A few examples of nonmetal compounds are: boric acid (H3BO3), used in ceramic glazes;[67] selenocysteine (C3H7NO2Se), the 21st amino acid of life;[68] phosphorus sesquisulfide (P4S3), in strike anywhere matches;[69] and teflon ((C2F4)n),[70] as used in non-stick coatings for pans and other cookware.

Complications

H and He are in the first row of the s-block. B through Ne take up the first row of the p-block. Sc through Zn occupy the first row of the d-block. La to Yb make up the first row of the f block. The elements within scope of the article are hydrogen, helium, boron, carbon, nitrogen, oxygen, fluorine, neon, silicon, phosphorus, sulfur, chlorine, argon, germanium, arsenic, selenium, bromine, krypton, antimony, tellurium, iodine, xenon, and radon.
Periodic table highlighting the first row of each block.[n 14] 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.
A graph with a vertical electronegativity axis and a horizontal atomic number axis. The five elements plotted are O, S, Se, Te and Po. The electronegativity of Se looks too high, and causes a bump in what otherwise be a smooth curve.
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,[71] 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.[72] 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.[73] A hydrogen atom in a molecule can form a second, weaker, bond with an atom or group of atoms in another molecule. According to Cressey 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."[74]

Hydrogen and helium, and boron to neon, have unusually small atomic radii. This occurs because the 1s and 2p subshells have no inner analogues (that is, 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.[75] 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.[76]

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.[77]

Secondary periodicity

Immediately after the first row of d-block metals, scandium to zinc, the 3d electrons in the p-block elements—that is, 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.[78] 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.[79]

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).[80]

Subclasses

N, S and I are shown as moderately strong oxidizing agents; O, F, Cl and Br are relatively strong oxidizing agents.
Modern periodic table extract showing nonmetal subclasses. H is usually shown in group 1 but can instead be in group 17.[n 15]
† moderately strong oxidising agent ‡ strong oxidising agent[n 16]

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".[92] 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.[93]

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;[94]
  • a set of chemically strong halogen elements—fluorine, chlorine, bromine and iodine—sometimes referred to as nonmetal halogens[95] or halogen nonmetals[96] (the term used here) or stable halogens;[97]
  • a set of unclassified nonmetals, including elements such as hydrogen, carbon, nitrogen, and oxygen, with no widely recognized collective name;[n 18] and
  • the chemically weak nonmetallic metalloids[106] sometimes considered to be nonmetals and sometimes not.[n 19]

Since the metalloids occupy "frontier territory",[108] 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[109] or as a sub-class of nonmetals.[110] Other authors count some of them as metals, for example arsenic and antimony, due to their similarities to heavy metals.[111][n 20] Metalloids are here treated as nonmetals in light of their chemical behavior,[106] and for comparative purposes.

Aside from the metalloids, some boundary fuzziness and overlapping (as occurs with classification schemes generally),[112] can be discerned among the other nonmetal subclasses. Carbon, phosphorus, selenium, and 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.[113]

Noble gases

Main page: Chemistry:Noble gas
a glass tube, held upside down by some tongs, has a clear-looking ice-like plug in it which is slowly melting judging from the clear drops falling out of the open end of the tube
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.[94]

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.[114] That is why they are all gases under standard conditions, even those with atomic masses larger than many normally solid elements.[115]

Chemically, the noble gases have relatively high ionization energies, nil or negative electron affinities, and high to very high electronegativities. Compounds of the noble gases number in the hundreds, and the list continues to grow,[116] with most of these involving oxygen or fluorine combining with either krypton, xenon or radon.[117]

In periodic table terms, an analogy can be drawn between the noble gases and noble metals (such as platinum and gold) which are similarly reluctant to combine with other elements.[118] 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,[119] OsO4. There are parallels, too, in the formulas of the oxyfluorides: XeO2F4 and OsO2F4, and XeO3F2 and OsO3F2.[120]

About 1015 tonnes of noble gases are present in the Earth's atmosphere.[121] Additionally, natural gas is found to be as much as 7% Helium.[122] Radon diffuses out of rocks, where it is formed during the natural decay sequence of uranium and thorium.[123] 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."[124]

Halogen nonmetals

a cluster of purple crystals flanked to either side by some white crystals which themselves have a sprinkling of brown, translucent crystals on their sides
A cluster of purple fluorite CaF2, the most common fluorine mineral, between two quartzes. It is the main source of fluorine for commercial uses.[125]

While the halogen nonmetals 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".[126]

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[81] solid. Electrically, the first three are insulators while iodine is a semiconductor (along its planes).[127]

Chemically, they have high ionization energies, electron affinities, and electronegativity values, and are mostly relatively strong oxidizing agents.[128] Manifestations of this status include their corrosive nature.[129] All four exhibit a tendency to form predominately ionic compounds with metals[130] whereas the remaining nonmetals, bar oxygen, tend to form predominately covalent compounds with metals.[n 21] The reactive and strongly electronegative nature of the halogen nonmetals represents the epitome of nonmetallic character.[134]

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.[135] Most of the alkali metals, as if in imitation of the halogen nonmetals, are known to form –1 anions (something that rarely occurs among metals).[136]

The halogen nonmetals 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 (F2) by weight in antozonite, attributing these inclusions as a result of radiation from the presence of tiny amounts of uranium.[137]

Metalloids

Main page: Chemistry:Metalloid
a cluster of bright cherry-red crystals
A crystal of realgar, also known as "ruby sulphur" or "ruby of arsenic", an arsenic sulfide mineral As4S4. The two elements involved each have a predominately nonmetallic chemistry.[138]

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)[139] sometimes referred to as post-transition metals.[140] 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'.[141]

The metalloids tend to be found in forms combined with oxygen, sulfur, or (in the case of tellurium) gold or silver.[142] 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.[143]

Unclassified nonmetals

A small glass jar filled with small dull grey concave buttons. The pieces of selenium look like tiny mushrooms without their stems.
Selenium conducts electricity around 1,000 times better when light falls on it, a property used in light-sensing applications.[144]

After the nonmetallic elements are classified as either noble gases, halogens, or metalloids, 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[145] and a semiconductor in a direction perpendicular to its planes;[146] phosphorus and selenium are semiconductors;[147] and hydrogen, nitrogen, oxygen, and sulfur are insulators.[n 22]

These elements are generally regarded as being too diverse to merit a collective classification,[149] and have been referred to as other nonmetals,[150] or more plainly as nonmetals, located between the metalloids and the halogens.[151] Consequently, their chemistry tends to be taught disparately, according to their four respective periodic table groups.[152] 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 23]

Hydrogen, in particular, behaves in some respects like a metal and in others like a nonmetal.[154] Like a metal, it can (first) lose its single electron;[155] it can stand in for alkali metals in typical alkali metal structures;[156] and is capable of forming alloy-like hydrides, featuring metallic bonding, with some transition metals.[157] 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.[158] It does this by way of forming a covalent or ionic bond[157] or, if it has lost its electron, attaching itself to a lone pair of electrons.[159]

Some or all of these nonmetals nevertheless have several shared properties. Most of them, being less reactive than the halogens,[160] can occur naturally in the environment.[161] They have prominent biological[162] and geochemical roles.[149] While their physical and chemical character is "moderately non-metallic", on a net basis,[149] all of them have corrosive aspects. Hydrogen can corrode metals. Carbon corrosion can occur in fuel cells.[163] 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.[164] Untreated selenium in soils can give rise to corrosive hydrogen selenide gas.[165] When combined with metals, the unclassified nonmetals can form high hardness (interstitial or refractory) compounds,[166] on account of their relatively small atomic radii and sufficiently low ionization energies.[149] They show a tendency to bond to themselves, especially in solid compounds.[167] Diagonal periodic table relationships among these nonmetals echo similar relationships among the metalloids.[168]

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.[169]

Unclassified nonmetals typically occur in elemental forms (oxygen, sulfur) or are found in association with either of these two elements:[142]

  • Hydrogen occurs in the world's oceans as a component of water, and in natural gas as a component of methane and hydrogen sulfide.[170]
  • Carbon occurs in limestone, dolomite, and marble, as carbonates.[171] Less well known is carbon as graphite, which mainly occurs in metamorphic silicate rocks,[172] as a result of the compression and heating of sedimentary carbon compounds.[173]
  • Oxygen is found in the atmosphere; in the oceans as a component of water; and in the crust as oxide minerals.[174]
  • Phosphorus minerals are widespread, usually as phosphorus-oxygen phosphates.[175]
  • 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.[176]
  • Selenium occurs in metal sulfide ores, where it partially replaces the sulfur; elemental selenium is occasionally found.[177]

Allotropes

Main page: Physics:Allotropy
a haphazard aggregate of brownish crystals
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.[178]

Among the halogen nonmetals, and unclassified nonmetals:

  • Iodine is known in a semiconducting amorphous form.[179]
  • 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.[180] Carbon is known in several other allotropic forms, including semiconducting buckminsterfullerene,[181] and amorphous[182] and paracrystalline (mixed amorphous and crystalline)[183] varieties.
  • Nitrogen can form gaseous tetranitrogen (N4), an unstable polyatomic molecule with a lifetime of about one microsecond.[184]
  • 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.[185]
  • 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.[186] Phosphorus also exists as diphosphorus (P2), an unstable diatomic allotrope.[187]
  • Sulfur has more allotropes than any other element.[188] Amorphous sulfur, a metastable mixture of such allotropes, is noted for its elasticity.[189]
  • Selenium has several nonmetallic allotropes, all of which are much less electrically conducting than its standard state of gray "metallic" selenium.[190]

All the elements most commonly recognized as metalloids form allotropes:

  • Boron is known in several crystalline and amorphous forms.[191]
  • Silicon can form crystalline (diamond-like); amorphous; and orthorhombic Si24 allotropes.[192]
  • 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.[193]
  • Arsenic and antimony form several well-known allotropes (yellow, grey, and black).[194]
  • Tellurium is known in crystalline and amorphous forms.[195]

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 24] starting with phosphorus at 1.7 GPa, have been observed to form metallic allotropes.[197][n 25] 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.[199]

Prevalence and access

Abundance

Approximate nonmetal composition
of the Earth and its biomass, by weight[200]
Domain Main components Next most
abundant
Crust O 61%, Si 20% H 2.9%
Atmosphere N 78%, O 21% Ar 0.5%
Hydrosphere O 66.2%, H 33.2% Cl 0.3%
Biomass O 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.[201] Oxygen is thought to be the next most abundant element, at about 0.1%.[202] 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 hypothesized to be made of dark energy and dark matter, both of which are currently poorly understood.[203]

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

Greyish lustrous block with uneven cleaved surface.
Germanium occurs in some zinc-copper-lead ore bodies, in quantities sufficient to justify extraction.[204] In 2022, the 99.999% pure form was priced at US$1300 per kilogram.[205]

Nonmetals, and metalloids, are extracted in their raw forms from:[161]

  • 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,[n 26] market conditions, and supplier surcharges.[208]

Based on the available literature as of April 2023, the cited costs of most nonmetals are less than the $US0.74 per gram cost of silver.[209] The exceptions are boron, phosphorus, germanium, xenon, and radon (notionally):

  • Boron costs around $25 per gram for 99.7% pure polycrystalline chunks with a particle size of about 1 cm.[210] Earlier, in 1997, boron was quoted at $280 per gram for polycrystalline 4-to-6-mm-diameter rods of 99.999% purity,[211] about 10 times the then $28.35 per gram cost of gold.[212]
  • In 2020, phosphorus in its most-stable black form could "cost up to $1,000 per gram",[213] more than 15 times the cost of gold, whereas ordinary red phosphorus, in 2017, was priced at about $3.40 per kilogram.[214] Researchers hoped to be able to reduce the cost of black phosphorus to as low as $1 per gram.[213]
  • Germanium and xenon cost about $1.30 and $7.60 per gram.[215]
  • 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.[216]

Uses

The distinctive properties of nonmetallic elements allow for specific applications that often cannot be fulfilled by metallic elements alone. Living organisms are composed almost entirely of the nonmetals hydrogen, oxygen, carbon, and nitrogen. Nonmetallic elements are important to industries ranging from electronics and energy storage to agriculture and chemical production, for example:

  • Carbon fibers possess high strength and low weight, making them ideal for applications in aerospace, sports equipment, and Automotive industry . Graphene, a single layer of carbon atoms, has exceptional electrical and thermal conductivity, making it valuable for electronic devices, energy storage, and composite materialss.[217]
  • Nitrogen is a key component in the production of fertilizers, which enhance crop growth and agricultural productivity. It's low temperature properties make it useful for cryogenic applications, such as preserving biological samples and freezing food.[218]
  • In life support, oxygen is vital for human respiration, and it is used in medical settings to assist patients with respiratory difficulties. It supports combustion and is used in various industrial processes, such as metal smelting and waste incineration.[219]
  • In semiconductors, silicon is the backbone of the electronics industry. It is used to manufacture computer chips, solar cells, and various electronic components. Silicon dioxide (silica) is used in the production of glass, ceramics, and optical fibers, enabling applications in windows, lenses, and communication networks.[220]
  • Sulfur is used in the production of sulfuric acid, one of the most widely used industrial chemicals. It is also used in the synthesis of various organic compounds. Sulfur compounds are crucial in the vulcanization process, which imparts strength and elasticity to rubber products.[221]

Shared uses of different subsets of the nonmetals encompass their presence in, or specific uses in the fields of air replacements (inert); dyestuffs; flame retardants or extinguishers; household accoutrements; lasers and lighting; mineral acids; plug-in hybrid vehicles; and welding gases.[161][222] To the extent that metalloids show metallic character, they have speciality uses extending to (for example) oxide glasses and alloying components.[223]

History, background, and taxonomy

Discovery

Main page: Chemistry:Discovery of the nonmetals
a man kneels in one corner of a darkened room, before a glowing flask; some assistants are further behind him and discernible in the dark
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 27] Radon is the most recently discovered nonmetal, being found only at the end of the 19th century.[161]

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.[225]
  • The halogen nonmetals 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.[226]
  • 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;[227] sulfur occurred naturally as a free element; and selenium[n 28] was detected as a residue in sulfuric acid.[229]
  • Most of the elements commonly recognized as metalloids were isolated by heating their oxides (boron, silicon, arsenic, tellurium) or a sulfide (germanium).[161] Antimony was known in its native form, as well as being attainable by heating its sulfide.[230]

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.[231] "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).[232]

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) described the different properties of plant substances from metallic and "non-metallic" land.[233]

In early chemistry, Wilhelm Homberg (a German natural philosopher) referred to "non-metallic" sulfur in Des Essais de Chimie (1708).[234] He questioned the five-fold division of all matter into sulfur, mercury, salt, water, and earth, as postulated by Étienne de Clave [fr] (1641) in the New Philosophical Light of True Principles and Elements of Nature.[235] Homberg's approach represented "an important move toward the modern concept of an element".[236]

Lavoisier, in his "revolutionary"[237] 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).[238] 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."[239]

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"[262] the line of demarcation between metals and nonmetals. Before then, metals had been distinguished on the basis of their ponderousness or relatively high densities.[263] Sodium and potassium, on the other hand, floated on water and yet were clearly metals on the basis of their chemical behaviour.[264]

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, ordered by type and date of discovery.

Probably the most well-known property is that the electrical conductivity of a metal increases when temperature falls, whereas that of a nonmetal rises.[252] However, this does not follow for plutonium, carbon, arsenic, and antimony. The electrical conductivity of plutonium is increased when this metal is heated within a temperature range of –175 to +125 °C.[265] The electrical conductivity of carbon, despite being widely regarded as a nonmetal, is likewise increased when heated.[266] Arsenic and antimony are sometimes classified as nonmetals, yet act similarly to carbon.[267]

Kneen et al. suggested that the nonmetals could be distinguished 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."[10] Emsley noted that, "No single property ... can be used to classify all the elements as either metals or nonmetals."[268] Jones added that "classes are usually defined by more than two attributes".[269]

The first 99 elements sorted by
density and electronegativity (EN)[n 30]
EN
Density < 1.9 ≥ 1.9
< 7 gm/cm3 Groups 1 and 2
Sc, Y, La
Ce, Pr, Eu, Yb
Ti, Zr, V; Al, Ga		 Noble gases
F, Cl, Br, I
H, C, N, P, O, S, Se
B, Si, Ge, As, Sb, Te^
> 7 gm/cm3 Nd, Pm, Sm, Gd, Tb, Dy
Ho, Er, Tm, Lu; Ac–Es;
Hf, Nb, Ta; Cr, Mn, Fe,
Co, Zn, Cd, In, Tl, Pb		 Ni, Mo, W, Tc, Re,
Platinum group metals,
Coinage metals, Hg; Sn,
Bi, Po, At
^ italicized elements are commonly recognised by some authors as metalloids

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 are hard and brittle or soft and crumbly, whereas metals are usually malleable and ductile; and nonmetal oxides are acidic.[274]

According to Hein and Arena, nonmetals have relatively low densities and high electronegativity;[7] the accompanying table bears this out. Nonmetallic elements occupy the top left quadrant, where densities are relatively low and electronegativity values 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, by rules of categorisation, a nonmetal.[275]

Development of subclasses

A basic taxonomy of nonmetals was set out in 1844, by Alphonse Dupasquier, a French doctor, pharmacist and chemist.[276] To facilitate the study of nonmetals, he wrote:[277]

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. The chloroide nonmetals came to be independently referred to as halogens.[278] The boroid nonmetals were expanded into the metalloids, starting from as early as 1864.[279] The noble gases, as a discrete grouping, were counted among the nonmetals from as early as 1900.[280]

Comparison

Some properties of metals, metalloids, unclassified nonmetals, halogen nonmetals, and noble gases are summarized in the following table.[n 31] Physical properties apply to elements in their most stable forms under 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.[281]

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 		Halogen nonmetals
fluorine, chlorine, bromine, iodine 		Noble gases
helium, neon, argon, krypton, xenon, radon
Form and heft[282]
  • ◇ solid
  • ◇ often high density such as Fe, Pb, W
  • ◇ some light metals including Be, Mg, Al
  • ◇ solid
  • ◇ low to moderately high density
  • ◇ all lighter than Fe
  • ◇ solid or gas
  • ◇ low density
  • ◇ H, N lighter than air[283]
  • ◇ solid, liquid or gas
  • ◇ low density
  • ◇ gas
  • ◇ low density
  • ◇ He, Ne lighter than air[284]
Appearance lustrous[21] lustrous[285]
  • ◇ lustrous: C, P, Se[286]
  • ◇ colorless: H, N, O[287]
  • ◇ colored: S[288]
  • ◇ colored: F, Cl, Br[289]
  • ◇ lustrous: I[2]
 colorless[290]
Elasticity mostly malleable and ductile[21] (Hg is liquid) brittle[285] C, black P, S, Se brittle; all four have less stable non-brittle forms[291][n 32] iodine is brittle[293] not applicable
Electrical conductivity good[n 33]
  • ◇ moderate: B, Si, Ge, Te
  • ◇ good: As, Sb[n 34]
  • ◇ poor: H, N, O, S
  • ◇ moderate: P, Se
  • ◇ good: C[n 35]
  • ◇ poor: F, Cl, Br
  • ◇ moderate: I[n 36]
 poor[n 37]
Electronic structure[196] 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 		Halogen nonmetals
fluorine, chlorine, bromine, iodine 		Noble gases
helium, neon, argon, krypton, xenon, radon
General chemical behavior
weakly nonmetallic[n 38] moderately nonmetallic[299] strongly nonmetallic[300]
  • ◇ inert to nonmetallic[301]
  • ◇ Rn shows some cationic behavior[302]
Oxides
  • ◇ basic; some amphoteric or acidic[303]
  • ◇ V; Mo, W; Al, In, Tl; Sn, Pb; Bi are glass formers[304]
  • ◇ ionic, polymeric, layer, chain, and molecular structures[305]
  • ◇ amphoteric or weakly acidic[306][n 39]
  • ◇ B, Si, Ge, As, Sb, Te are glass formers[308]
  • ◇ polymeric in structure[309]
  • ◇ acidic (NO2, N2O5, SO3, SeO3 strongly so)[310] or neutral (H2O, CO, NO, N2O)[n 40]
  • ◇ P, S, Se are glass formers;[304] CO2 forms a glass at 40 GPa[312]
  • ◇ mostly molecular[309]
  • ◇ C, P, S, Se have at least one polymeric form
  • ◇ acidic; ClO2, Cl2O7, I2O5 strongly so[313]
  • ◇ no glass formers reported
  • ◇ molecular[309]
  • ◇ iodine has at least one polymeric form, I2O5[314]
  • ◇ metastable XeO3 is acidic;[315] stable XeO4 strongly so[316]
  • ◇ no glass formers reported
  • ◇ molecular[309]
  • ◇ XeO2 is polymeric[317]
Compounds with metals alloys[21] or intermetallic compounds[318] tend to form alloys or intermetallic compounds[319]
  • ◇ salt-like to covalent: H†, C, N, P, S, Se[320]
  • ◇ mainly ionic: O[321]
 mainly ionic[130] simple compounds in ambient conditions not known[n 41]
Ionization energy (kJ mol−1)‡
[323]
  • ◇ 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 42]‡
[325]
  • ◇ low to high
  • ◇ 0.7 to 2.54
  • ◇ average 1.5
  • ◇ moderately high
  • ◇ 1.9 to 2.18
  • ◇ average 2.05
  • ◇ moderately high 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[326]
‡ The labels low, moderate, high, and very high are arbitrarily based on the value spans listed in the table

See also

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. ↑ Al, Ga, In, Tl; Sn, Pb; Bi; Po; At.
  5. ↑ Hydrogen has historically been placed over one or more of lithium, boron,[4] carbon, or fluorine;[5] or over no group at all; or over all main groups simultaneously, and therefore may or may not be adjacent to other nonmetals.[6]
  6. ↑ Metallic or nonmetallic character is usually taken to be indicated by one property rather than two or more properties.
  7. ↑ Steudel's monograph is an updated translation of the fifth German edition of 2013, incorporating the literature up to Spring 2019.
  8. ↑ Solid iodine has a silvery metallic appearance under white light at room temperature.[19] It volatizes at ordinary and higher temperatures, passing from solid to gas; its vapours are violet-colored.[20]
  9. ↑ 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, both metals;[23] electrical conductivity values range from 10−18 S•cm−1 for sulfur[23] to 3 × 104 in graphite[24] or 3.9 × 104 for arsenic[25] cf. 0.69 × 104 for manganese to 63 × 104 for silver, metals both.[23]
  10. ↑ 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.[42]
  11. ↑ 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;[23] electrical conductivity values of metals range from 0.69 S•cm−1 × 104 for manganese to 63 × 104 for silver; cf. carbon 3 × 104,[24] arsenic 3.9 × 104 and antimony 2.3 × 104.[23]
  12. ↑ These elements being semiconductors.[47]
  13. ↑ Acids are formed by boron, phosphorus, selenium, arsenic, iodine;[57] oxides by carbon, silicon, germanium, sulfur, antimony, and tellurium.[58]
  14. ↑ 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.
  15. ↑ Noble gases: He, Ne, Ar, Kr, Xe, Rn; Halogen nonmetals: 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.
  16. ↑ 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.[81] The remaining reactive nonmetallic elements have giant covalent structures, but for H which is a diatomic gas.[82]

    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,[83] and a pedestrian oxidizing agent unless combined with a more active nonmetal like O or F.[84]

    S reacts in the cold with alkalic and post-transition metals, and Cu, Ag and Hg,[85] 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.[86]
    Iodine is sufficiently corrosive to cause lesions resembling thermal burns, if handled without suitable protection,[87] and tincture of iodine will smoothly dissolve Au.[88] 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."[89] 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.[90] 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.[91]
  17. ↑ The quote marks are not found in the source; they are used here to make it clear that the source employs the word non-metals as a formal term for the subset of chemical elements in question, rather than applying to nonmetals generally.
  18. ↑ Varying configurations of these nonmetals have been referred to as, for example, basic nonmetals,[98] bioelements,[99] central nonmetals,[100] CHNOPS,[101] essential elements,[102] "non-metals",[103][n 17] orphan nonmetals,[104] or redox nonmetals.[105]
  19. ↑ 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".[107]
  20. ↑ 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".[112]
  21. ↑ Metal oxides are usually ionic.[131] On the other hand, oxides of metals with high oxidation states are usually either polymeric or covalent.[132] A polymeric oxide has a linked structure composed of multiple repeating units.[133]
  22. ↑ 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.[148]
  23. ↑ 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.[153]
  24. ↑ B; Si, Ge; N, P; O, S, Se, Te; halogen nonmetals; and the noble gases.[196]
  25. ↑ In 2020, high-pressure studies and experiments were said to represent "a very active and vigorous research field".[198]
  26. ↑ For example, as at April 2023, the commercial price of silicon was $4 per pound or $0.0088 per gram.[206] On the other hand, the price quoted for a 335 gram sample of silicon for hobbyists and science enthusiasts was about $57, or 0.170 per gram, or about 20 times the commercial price.[207]
  27. ↑ 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".[224]
  28. ↑ Berzelius, who discovered selenium, thought it had the properties of a metal, combined with the properties of sulfur.[228]
  29. ↑ The Goldhammer-Herzfeld ratio is roughly equal to the cube of the atomic radius divided by the molar volume.[243] 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.[244]
  30. ↑ (a) The values are from Aylward and Findlay.[270]
    (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.[271]
    (c) The density values used for At and Fr are theoretical estimates.[272]
    (d) Bjerrum classified "heavy metals" as those metals with densities above 7 g/cm3.[273]
    (e) Vernon specified a minimum electronegativity of 1.9 for the metalloids, on the revised Pauling scale.[2]
  31. ↑ See also Properties of metals, metalloids and nonmetals, which treats metalloids as a class of their own.
  32. ↑ Carbon as exfoliated (expanded) graphite,[292] and as carbon nanotube wire;[49] phosphorus as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature);[50] sulfur as plastic sulfur;[51] and selenium as selenium wires.[52]
  33. ↑ Metals have electrical conductivity values of from 6.9×103 S•cm−1 for manganese to 6.3×105 for silver.[294]
  34. ↑ Metalloids have electrical conductivity values of from 1.5×10−6 S•cm−1 for boron to 3.9×104 for arsenic.[295]
  35. ↑ Unclassified nonmetals have electrical conductivity values of from ca. 1×10−18 S•cm−1 for the elemental gases to 3±4 in graphite.[296]
  36. ↑ The halogen nonmetals 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.[296][127]
  37. ↑ The elemental gases have electrical conductivity values of ca. 1×10−18 S•cm−1.[296]
  38. ↑ They always give "compounds less acidic in character than the corresponding compounds of the [typical] nonmetals".[285]
  39. ↑ Arsenic trioxide reacts with sulfur trioxide, forming arsenic "sulfate" As2(SO4)3.[307]
  40. ↑ CO and N2O are "formally the anhydrides of formic and hyponitrous acid, respectively: CO + H2O → H2CO2 (HCOOH, formic acid); N2O + H2O → H2N2O2 (hyponitrous acid)".[311]
  41. ↑ 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.[322]
  42. ↑ Values for the noble gases are from Rahm, Zeng and Hoffmann.[324]

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