Nonmetal

Sulfur as a yellow powder. When melted and cooled quickly it changes into rubbery ribbons of plastic sulfur, an allotropic form.^[1]

Germanium looks metallic, conducts electricity poorly and behaves chemically like a nonmetal.^[2]

About 25 ml of bromine, a liquid at room temperature, and an essential trace element.^[3]

A partially filled ampoule of liquefied xenon, set inside an acrylic cube. Xenon is otherwise a colorless gas at room temperature.

In chemistry, a nonmetal is an element that usually gains electrons when reacting with a metal, and which forms an acid if combined with oxygen and hydrogen. At room temperature, about half are colorless or pale yellow to pale green gases, while one (bromine) is a dark red liquid. The rest are silvery-gray (barring sulfur, which is yellow) solids and either hard and brittle or soft and crumbly. In contrast to most metals, nonmetals tend to be poor conductors of heat and electricity, with no structural uses.^[4]

Bromine and the elemental gases are universally recognised as nonmetals, but no standard definition distinguishes nonmetals and metals.^[5] Consequently the number of elements recognised as nonmetals generally ranges from fourteen to twenty-three, depending on the classification criteria.^[6]

Although five times more elements are metals than nonmetals, two nonmetals—hydrogen and helium—make up about 99% of the observable universe by mass.^[7] Another nonmetal, oxygen, makes up almost half of the Earth's crust, oceans, and atmosphere.^[8]

Most nonmetals have biological, technological or domestic roles or uses. Living organisms are composed almost entirely of the nonmetals hydrogen, oxygen, carbon, and nitrogen.^[9] Near-universal uses for nonmetals are in medicine and pharmaceuticals;^[10] lasers and lighting;^[11] and household accoutrements.^[12]

Definition and applicable elements[]

Periodic table extract showing the frequency with which authors class elements as nonmetals:
 14  effectively always^{[n 1]}
 3  usually^{[n 2]}
 6  sometimes^[13]

  nearby metals

---

While H is here shown above F in group 17^[14] it is usually shown in group 1, above Li.^{[n 3]}

There is no rigorous definition of a nonmetal.^[17] Broadly, any element lacking a preponderance of metallic properties such as luster, deformability, and good electrical conductivity, can be regarded as a nonmetal.^[18] Nevertheless some variation may be encountered among authors as to which elements are regarded as nonmetals, especially where the metals meet the nonmetals in periodic table terms.^[19] This lack of consistency occurs due to the absence of a universally agreed criterion or set of criteria for distinguishing between metals and nonmetals. Classification decisions of individual authors then become subject to which property or properties they regard as being most indicative of metallic or nonmetallic character.^[6]

The fourteen elements effectively always recognized as nonmetals are hydrogen, oxygen, nitrogen, and sulfur; the corrosive halogens fluorine, chlorine, bromine, and iodine; and the noble gases helium, neon, argon, krypton, xenon, and radon. Up to a further nine elements can be counted as nonmetals, including carbon, phosphorus, and selenium; and the elements otherwise commonly recognized as metalloids namely boron; silicon and germanium; arsenic and antimony; and tellurium, bringing the total up to twenty-three nonmetals.^[14]

Astatine, the fifth halogen, is often ignored on account of its rarity and intense radioactivity;^[20] the theoretical and experimental evidence is indirect, but strongly suggests that it is a metal. The superheavy elements copernicium (Z = 112) and oganesson (118) may turn out to be nonmetals; their actual status is currently not known.

Since there are 118 known elements,^[21] as of October 2021, the nonmetals are outnumbered by the metals several times.

• 

  Boron, shown here in the form of its β-rhombohedral phase (its most thermodynamically stable form)^[22]

• 

  Carbon (as graphite). Delocalized valence electrons within the layers of graphite give it a metallic appearance.^[23]

• 

  Liquid oxygen (LOX) boiling. The higher density of LOX increases the likelihood of excitation interactions between two O₂ molecules and a photon that result in a blue color.^[24]

• 

  A test tube of pale yellow liquid fluorine in a cryogenic bath. Unlike its next door neighbor oxygen, which is colorless as a gas, fluorine retains its yellow coloration in gaseous form.^[25]

Origin of the concept, distinguishing criteria, and use of the term[]

Origin of the concept[]

Various kinds of matter are shown in the upper half of this image. The lower half is an example delineating between metals and nonmetals based on electrical conductivity, and reactions with nitric acid.^[26][27][28]

The distinction between metals and nonmetals arose, in a convoluted manner, from a crude recognition of natural kinds of matter. Thus:

• matter could be divided into pure substances and mixtures;
• pure substances eventually could be distinguished as compounds and elements;
• ”metallic" elements—whether solid, liquid or gaseous—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).^[29]

Distinguishing criteria[]

Any one or more of a range of properties have been used in attempts to refine the distinction between metals and nonmetals, including:

• Physical—fusibility, malleability and ductility;^[30] opacity;^[31] bulk coordination number;^[32] minimum excitation potential;^[33] ;^[34] physical state;^[35] critical temperature;^[36] ;^[37] enthalpy of vaporization;^[38] ;^[39] temperature coefficient of resistivity;^[40] ;^[41] packing efficiency;^[42] 3D electrical conductivity;^[43] electrical conductivity at absolute zero;^[44] thermal conductivity;^[45]

• Chemical—cation formation;^[46] acid-base character of oxides;^[47] sulfate formation;^[48] oxide solubility in acids;^[49] or

• Electrical—electron configuration^[50] and band structure.^[44]

Johnson^[35] noted that physical properties can best indicate the metallic or nonmetallic properties of an element, with the proviso that other properties will be needed in a number of ambiguous cases. Kneen at al.^[6] added that:

"It is merely necessary to establish and apply a criterion of metallicity…many arbitrary classifications are possible, most of which, if chosen reasonably, would be similar, but not necessarily identical…the relevance of the criterion can only be judged by the usefulness of the related classification."

Once a basis for distinguishing between the "two great classes of elements"^[51] is established, the nonmetals are found to be those lacking the properties of metals,^[52] to greater or lesser degrees.^[53]

Use of the term[]

The term "nonmetallic" dates from as far back as 1708 when Wilhelm Homberg mentioned "non-metallic sulfur" in his .^[54] He had refuted the five-fold division of matter into sulfur, mercury, salt, water and earth, previously in vogue, as postulated by (1641) in .^[55] Homberg's approach represented "an important move toward the modern concept of an element".^[56] Subsequently, the first modern list of chemical elements was given by Lavoisier in his "revolutionary"^[57] 1789 work Traité élémentaire de chimie in which he distinguished between simple metallic and nonmetallic substances. In its first seventeen years, Lavoisier's work was republished in twenty-three editions and six languages, and carried his "new chemistry" across Europe and America.^[58]

General properties[]

Physical[]

Metals tend to have closer-packed structures. Nonmetals are more open-packed. Values for gaseous nonmetals refer to the liquid state at the boiling point^{[n 4]}

Physically, nonmetals in their most stable forms exist as either polyatomic solids (carbon, for example) with open-packed crystalline structures; diatomic molecules such as hydrogen (a gas) and bromine (a liquid); or monatomic gases (such as neon). They usually have small atomic radii. Metals, in contrast, are nearly all solid and close-packed, and mostly have larger atomic radii.^[61] Other than sulfur, solid nonmetals have a submetallic appearance and are brittle, as opposed to metals, which are lustrous, and generally ductile or malleable. Nonmetals usually have lower densities than metals; are mostly poorer conductors of heat and electricity; and tend to have significantly lower melting points and boiling points.^[62]

The physical differences between metals and nonmetals arise from internal and external atomic forces. Internally, an atom's nuclear charge acts to hold its valence electrons in place. Externally, the same electrons are subject to attractive forces from the nuclear charges in nearby atoms. When the external forces are greater than, or equal to, the internal force, valence electrons are expected to become itinerant (free to move between atoms) and metallic properties are predicted. Otherwise nonmetallic properties are anticipated.^[63]

Chemical[]

Chemically, nonmetals mostly have higher ionization energies, higher electron affinities, higher electronegativity values, and higher standard reduction potentials than metals. Here, and in general, the higher an element's ionization energy, electron affinity, electronegativity, or standard reduction potentials, the more nonmetallic that element is.^[64]

In chemical reactions, nonmetals tend to gain or share electrons unlike metals which tend to donate electrons. More specifically, and given the stability of the noble gases, nonmetals generally gain a number of electrons sufficient 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 encapsulated by the duet and octet rules of thumb (and for metals there is a less rigorously followed 18-electron rule). A key attribute of nonmetals is that they never form basic oxides; their oxides are generally acidic.^[65] Moreover, solid nonmetals (including metalloids) react with nitric acid to form an oxide (carbon, silicon, sulfur, antimony, and tellurium) or an acid (boron, phosphorus, germanium, selenium, arsenic, iodine).^[28]

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 valence electrons. From left to right across each period of the periodic table the nuclear charge increases as the number of protons in the core increases.^[68] There is an associated reduction in atomic radius^[69] as the increasing nuclear charge draws the valence electrons closer to the core.^[70] In metals, the nuclear charge is generally weaker than that of nonmetallic elements. In chemical 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.^[71]

The number of compounds formed by nonmetals is vast.^[72] 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 2nd, 2021, were occupied by nonmetals. Hydrogen, carbon, oxygen and nitrogen were 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.^[73] 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;^[74] phosphorus sesquisulfide (P₄S₃), in strike anywhere matches; and teflon (C
₂F
₄)_n.^[75]

Complications[]

Complicating the chemistry of the nonmetals are the anomalies seen in the first row of each periodic table block, particularly in hydrogen, (boron), carbon, nitrogen, oxygen and fluorine; secondary periodicity or non-uniform periodic trends going down most of the p-block groups;^[76] and unusual valence states in the heavier nonmetals. In this regard, Zuckerman and Nachod opined that:

“The marvellous variety and infinite subtlety of the nonmetallic elements, their compounds, structures and reactions, is not sufficiently acknowledged in the current teaching of chemistry.”^[77]

Periodic table highlighting the first row of each block. Helium (He), as a noble gas, is normally shown over neon with the rest of the noble gases.

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.^[78] It can lose its single valence electron in aqueous solution, leaving behind a bare proton with tremendous polarizing power. This subsequently attaches itself to the lone electron pair of an oxygen atom in a water molecule, thereby forming the basis of acid-base chemistry.^[79] 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."^[80]

For hydrogen and helium, and from boron to neon, since the 1s and 2p subshells have no inner analogues and experience no electron repulsion effects they have relatively small radii, unlike the 3p, 4p and 5p subshells of heavier elements.^[81] 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 triple or double bonds.^[82]

Electronegativity values of the group 16 chalcogen elements showing a W-shaped alternation or going down the group

Secondary periodicity. Immediately after the first row of the transition metals, the 3d electrons in the 4th row of periodic table elements, i.e. in gallium (a metal), germanium, arsenic, selenium, and bromine, are not as effective at shielding the increased nuclear charge. 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.^[83] The alternation is further compounded by the appearance of fourteen f-block metals between barium and hafnium.^[84]

Unusual valence 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 valences 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).^[85]

Subclasses []

Modern periodic table extract showing nonmetal subclasses.

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† moderately strong oxidising agent ‡ strong oxidising agent^{[n 5]}

Historical[]

A basic taxonomy of nonmetals was set out in 1844, by Dupasquier, a French doctor, pharmacist and chemist.^[96] To facilitate the study of nonmetals, he wrote, "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."

Dupasquier's organogens and sulphuroids correspond to the set of unclassified nonmetals. Eventually thereafter:

• the chloroide nonmetals came to be independently referred to as halogens;^[97]
• the boroid nonmetals came to expand into the metalloids, starting from as early as 1864;^[98]
• varying configurations of the orgaonogen and the sulphuroid nonmetals have been referred to as e.g. basic nonmetals;^[99] biogens;^[100] central nonmetals;^[101] CHNOPS;^[102] essential elements;^[103] "nonmetals";^[104] orphan nonmetals;^[105] or redox nonmetals;^[106]
• the noble gases, as a discrete grouping, were counted among the nonmetals as early as 1900.^[107]

Current[]

Approaches to classifying nonmetals may involve from as few as two subclasses to up to six or seven. For example, the Encyclopedia Britannica periodic table has noble gases, halogens, and other nonmetals, and splits the elements commonly recognized as metalloids between the "other metals" and the "other nonmetals";^[108] the Royal Society of Chemistry periodic table shows the nonmetallic elements as occupying seven groups.^[109]

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^[110] (the term used here) or stable halogens;^[111]
• 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,^[112] sometimes considered to be nonmetals and sometimes not.^{[n 6]}

Since the metalloids occupy frontier territory, where metals meet nonmetals, their treatment varies from author to author. Some consider them separate from both metals and the nonmetals; some regard them as nonmetals^[114] or as a sub-class of nonmetals;^[115] others count some of them as metals, for example arsenic and antimony, due to their similarities with heavy metals.^{[116][n 7]} 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) 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.^[126]

Noble gases[]

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.^[127]

They have very similar properties, all being colorless, odorless, and nonflammable. With their closed valence shells the noble gases have feeble interatomic forces of attraction resulting in very low melting and boiling points.^[128] That is why they are all gases under standard conditions, even those with atomic masses larger than many normally solid elements.^[129]

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,^[130] with most of these occurring via oxygen or fluorine combining with either krypton, xenon or radon.^[131]

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.^[132] 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; and there are parallels in the formulas of the oxyfluorides: XeO₂F₄ and OsO₂F₄, and XeO₃F₂ and OsO₃F₂.^[133]

Nonmetal halogens[]

A conventional periodic table showing the positions of the alkali metals (A) and the nonmetal halogens (H).

While the nonmetal halogens are corrosive and markedly reactive elements, they can be found in such innocuous compounds as ordinary table salt NaCl. Their remarkable chemical activity as nonmetals can be contrasted with the equally remarkable chemical activity of the alkali metals such as sodium and potassium, located at the far left of the periodic table.^[134]

Physically, fluorine and chlorine are pale yellow and yellowish green gases; bromine is a reddish-brown liquid; and iodine is a metallic-looking (under white light)^[13] solid. Electrically, the first three are insulators while iodine is a semiconductor (along its planes).^[135]

Chemically, they have high ionization energies, electron affinities, and electronegativity values, and are mostly relatively strong oxidizing agents.^[136] Manifestations of this status include their intrinsically corrosive nature.^[137] All four exhibit a tendency to form predominately ionic compounds with metals^[138] whereas the remaining nonmetals, bar oxygen, tend to form predominately covalent compounds with metals.^{[n 8]} The reactive and strongly electronegative nature of the nonmetal halogens represents the epitome of nonmetallic character.^[142]

In periodic table terms, the counterparts of the highly nonmetallic halogens, in group 17 are the highly reactive metals, such as sodium and potassium, in group 1. Curiously most of the alkali metals are known to form –1 anions (something that rarely occurs among nonmetals) as if in imitation of the nonmetal halogens.^[143]

Unclassified nonmetals[]

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. 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 and a semiconductor in a direction perpendicular to its planes;^[144] phosphorus and selenium are semiconductors;^[145] and hydrogen, nitrogen, oxygen, and sulfur are insulators.^{[n 9]}

They are generally regarded as being too diverse to merit a collective examination,^[147][74] and have been referred to as other nonmetals,^[148] or more plainly as nonmetals, located between metalloids and halogens.^[149] Consequently, their chemistry tends to be taught disparately, according to their four respective periodic table groups,^[147] for example: hydrogen in group 1; the group 14 carbon nonmetals (carbon, and possibly silicon and germanium); the group 15 pnictogen nonmetals (nitrogen, phosphorus, and possibly arsenic and antimony); and the group 16 chalcogen nonmetals (oxygen, sulfur, selenium, and possibly tellurium). Other subdivisions are possible according to the individual preferences of authors.^{[n 10]}

Hydrogen, in particular, behaves in some respects like a metal and in others like a nonmetal.^[151] Like a metal it can (first) lose its single valence electron;^[152] it can stand in for alkali metals in typical alkali metal structures;^[153] and is capable of forming alloy-like hydrides, featuring metallic bonding, with some transition metals.^[154] On the other hand, it is an insulating diatomic gas, like a typical nonmetal, and in chemical reactions more generally, it has a tendency to attain the electron configuration of helium.^[155] It does this by way of forming a covalent or ionic bond^[154] or, if its has lost its valence electron, attaching itself to a lone pair of electrons.^[156]

Some or all of these nonmetals nevertheless have several shared properties. Their physical and chemical character is "moderately non-metallic", on a net basis.^[74] Being less reactive than the halogens,^[157] most of them, except for phosphorus, can occur naturally in the environment.^[158] They have prominent biological^[159][160] and geochemical roles.^[74] When combined with halogens, unclassified nonmetals form (polar) covalent bonds.^[161] When combined with metals they can form hard (interstitial or refractory) compounds,^[162] in light of their relatively small atomic radii and sufficiently low ionization energy values.^[74] Unlike the halogens, unclassified nonmetals show a tendency to catenate, especially in solid-state compounds.^[163][74] Diagonal relationships among these nonmetalsecho similar relationships among the metalloids.^[164][165]

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".^[166]

Metalloids[]

The six elements more commonly recognized as metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium, with each having a metallic appearance. They are called metalloids mainly in light of their appearance.^[167] 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 periodic tables.^[13]

They are brittle and only fair conductors of electricity and heat. Boron, silicon, germanium, tellurium are semiconductors. Arsenic and antimony have the electronic band structures of semimetals although both have less stable semiconducting allotropes.^[13]

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.^[13]

Like hydrogen among the unclassified nonmetals, boron is chemically similar to metals in some respects.^{[168][n 11]} It has fewer electrons than orbitals available for bonding. Analogies with transition metals occur in the formation of complexes,^[170] and adducts (for example, BH₃ + CO →BH₃CO and, similarly, Fe(CO)₄ + CO →Fe(CO)₅),^{[n 12]} as well as in the geometric and electronic structures of cluster species such as [B₆H₆]²⁻ and [Ru₆(CO)₁₈]²⁻.^[172]

To the left of the weakly nonmetallic metalloids, in periodic table terms, are found an indeterminate set of weakly metallic metals (such as tin, lead and bismuth)^[173] sometimes referred to as post-transition metals.^[174] Dingle explains the situation this way:^[175]

"…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'."

Comparison[]

Properties of metals and those of the (sub)classes of metalloids, unclassified nonmetals, nonmetal halogens, and noble gases are summarized in the table. 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 specific, and then to descriptive. 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.^[224]

Allotropes[]

Most nonmetallic elements exist in allotropic forms. Carbon, for example, occurs as graphite and as diamond. Such allotropes may exhibit physical properties that are more metallic or less nonmetallic.^[225]

Among the nonmetal halogens, and unclassified nonmetals:

• Iodine is known in a semiconducting amorphous form.^[226]
• 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.^[227] Carbon is further known in several other allotropic forms, including semiconducting buckminsterfullerene (C₆₀).^[228]
• Nitrogen can form gaseous tetranitrogen (N₄), an unstable polyatomic molecule with a lifetime of about one microsecond.^[229]
• Oxygen is a diatomic molecule in its standard state; it also exists as ozone (O₃), an unstable nonmetallic allotrope with a half-life of around half an hour.^[230]
• Phosphorus, uniquely, exists in several allotropic forms that are more stable than that of 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.^[231] Phosphorus also exists as diphosphorus (P₂), an unstable diatomic allotrope.^[232]
• Sulfur has more allotropes than any other element.^[233] Amorphous sulfur, a metastable mixture of such allotropes, is noted for its elasticity.^[234]
• Selenium has several nonmetallic allotropes, all of which are much less electrically conducting than its standard state of gray "metallic" selenium.^[235]

All the elements most commonly recognized as metalloids form allotropes. Boron is known in several crystalline and amorphous forms. The discovery of a quasi-spherical allotropic molecule, borospherene (B₄₀), was announced in 2014. Silicon was most recently known only in its crystalline and amorphous forms. The synthesis of an orthorhombic allotrope, Si₂₄, was subsequently reported in 2014.^[236] At a pressure of ca. 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.^[237] Arsenic and antimony form several well-known allotropes (yellow, grey, and black). Tellurium is known in its crystalline and amorphous forms.^[238]

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.^{[239][n 25]} Single layer two-dimensional forms of nonmetals include borophene (boron), graphene (carbon), silicene (silicon), phosphorene (phosphorus), germanene (germanium), (arsenic), antimonene (antimony) and (tellurium), collectively referred to as "xenes".^[241]

Abundance, occurrence, extraction and cost[]

Abundance[]

Hydrogen and helium are estimated to make up approximately 99% of all ordinary matter in the universe and over 99.9% of its atoms.^[7] Oxygen is thought to the next most abundant element, at ca. 0.1%.^[242] Less than five percent 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.^[243]

Hydrogen, carbon, nitrogen, and oxygen constitute the great bulk of the Earth's atmosphere, oceans, crust, and biosphere; the remaining nonmetals have abundances of 0.5% or less. In comparison, 35% of the crust is made up of the metals sodium, magnesium, aluminium, potassium and iron; together with a metalloid, silicon. All other metals and metalloids have abundances within the crust, oceans or biosphere of 0.2% or less.^[244][245]

Occurrence[]

Noble gases[]

About 10¹⁵ tonnes of noble gases are present in the Earth's atmosphere.^[246] Helium is additionally found in natural gas to the extent of as much as 7%.^[247] Radon further diffuses out of rocks, where it is formed during the natural decay sequence of uranium and thorium.^[248] In 2014, it was reported that the Earth's core may contain ca. 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."^[249]

Nonmetal halogens[]

The nonmetal halogens are found in salt-related minerals. Fluorine occurs in fluorite, this being 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 to radiation from the presence of tiny amounts of uranium.^[250]

Unclassified nonmetals[]

Carbon as diamond, here shown in native form. Diamantine carbon is thermodynamically less stable than graphitic carbon.^[251]

Unclassified nonmetals occur typically occur in elemental forms (oxygen, sulfur) or are found in association with either of these two elements:^[252]

• Hydrogen occurs in the world's oceans as a component of water, and in natural gas as a component of methane and hydrogen sulfide.^[253]
• Carbon occurs in limestone, dolomite, and marble, as carbonates.^[254] Less well known is carbon as graphite, which mainly occurs in metamorphic silicate rocks^[255] as a result of the compression and heating of sedimentary carbon compounds.
• 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.
• Elemental sulfur can be found near hot springs and volcanic regions in many parts of the world; sulfur minerals are widespread, usually as sulfides or oxygen-sulfur sulfates.
• Selenium occurs in metal sulfide ores, where it partially replaces the sulfur;^[256] elemental selenium is occasionally found.

Metalloids[]

The metalloids tend to be found in forms combined with oxygen or sulfur or, in the case of tellurium, gold or silver.^[252] 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.^[257]

Extraction[]

Nonmetals, and metalloids, are extracted in their raw forms from:^[158]

• 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[]

While non-radioactive nonmetals are relatively inexpensive, there are some exceptions. As of July 2021, boron, germanium, arsenic, and bromine can cost from $3–10 US per gram (cf. silver at about $1 per gram). Prices can fall dramatically if bulk quantities are involved.^[258] Black phosphorus is produced only in gram quantities by boutique suppliers—a single crystal produced via chemical vapor transport can cost up to $1,000 US per gram (ca. seventeen times the cost of gold); in contrast, red phosphorus costs about 50 cents a gram or $227 a pound.^[259] 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 ca. $86,000,000 per gram (with no indication of a discount for bulk quantities).^[260]

Shared uses[]

Near universal uses for nonmetals are for household accoutrements; lasers and lighting; and medicine and pharmaceuticals. One or two of germanium, arsenic, and or radon will be absent. To the extent that metalloids show metallic character, they have speciality uses extending to (for example) oxide glasses, alloying components, and semiconductors.^[261]

Further shared uses of different subsets of the nonmetals encompass their presence in, or specific uses in the fields of air replacements; cryogenics and refrigerants; fertilizers; flame retardants or fire extinguishers; mineral acids; plug-in hybrid vehicles; welding gases; and smart phones.

Discovery[]

The majority of 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 first (and so far only) element not discovered on Earth^{[n 26]} while radon was the last entrant to the nonmetal club, being discovered only at the end of the 19th century.^[158]

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

• 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; and radioactive radon was observed emanating from compounds of thorium, three years after Henri Becquerel's discovery of radiation in 1896.^[263]

• The nonmetal halogens were obtained from their halides, either via electrolysis; adding an acid; or displacement. Some chemists died as a result of their experiments trying to isolate fluorine.^[264]

• Among 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;^[265] sulfur occurred naturally; and selenium^{[n 27]} was detected as a residue in sulfuric acid.^[267]

• Most of the elements commonly recognized as metalloids were isolated by heating their oxides (boron, silicon, arsenic, tellurium or a sulfide (germanium).^[158] Antimony was known in its native form as well as being isolable by heating its sulfide.^[268]

See also[]

• CHON
• Metallization pressure
• Properties of nonmetals (and metalloids) by group

Notes[]

References[]

Citations[]

Bibliography[]

Monographs[]

Note: Those marked with a ▲ classify the following 14 elements as nonmetals: H, N; O, S; the stable halogens; and the noble gases

• Steudel R 2020, Chemistry of the Non-metals: Syntheses - Structures - Bonding - Applications, in collaboration with D Scheschkewitz, Berlin, Walter de Gruyter, doi:10.1515/9783110578065. ▲

  Twenty-three nonmetals, including B, Si, Ge, As, Se, Te, and At but not Sb (nor Po). The nonmetals are identified on the basis of their electrical conductivity at absolute zero putatively being close to zero, rather than finite as in the case of metals. That does not work for As however, which has the electronic structure of a semimetal (like Sb).

• Halka M & Nordstrom B 2010, "Nonmetals", Facts on File, New York, ISBN 978-0-8160-7367-2

  A reading level 9+ book covering H, C, N, O, P, S, Se. Complementary books by the same authors examine (a) the post-transition metals (Al, Ga, In, Tl, Sn, Pb and Bi) and metalloids (B, Si, Ge, As, Sb, Te and Po); and (b) the halogens and noble gases.

• Woolins JD 1988, Non-Metal Rings, Cages and Clusters, John Wiley & Sons, Chichester, ISBN 978-0-471-91592-8.

  A more advanced text that covers H; B; C, Si, Ge; N, P, As, Sb; O, S, Se and Te.

• Steudel R 1977, Chemistry of the Non-metals: With an Introduction to Atomic Structure and Chemical Bonding, English edition by FC Nachod & JJ Zuckerman, Berlin, Walter de Gruyter, ISBN 978-3-11-004882-7. ▲

  Twenty-three nonmetals, including B, Si, Ge, As, Se, Te, and Po.

• Powell P & Timms PL 1974, The Chemistry of the Non-metals, Chapman & Hall, London, ISBN 978-0-470-69570-8. ▲

  Twenty-two nonmetals including B, Si, Ge, As and Te. Tin and antimony are shown as being intermediate between metals and nonmetals; they are later shown as either metals or nonmetals. Astatine is counted as a metal.

• Emsley J 1971, The Inorganic Chemistry of the Non-metals, Methuen Educational, London, ISBN 978-0-423-86120-4. ▲

  Twenty nonmetals. H is placed over F; B and Si are counted as nonmetals; Ge, As, Sb and Te are counted as metalloids.

• Johnson RC 1966, Introductory Descriptive Chemistry: Selected Nonmetals, their Properties, and Behavior, WA Benjamin, New York. ▲

  Eighteen nonmetals. H is shown floating over B and C. Silicon, Ge, As, Sb, Te, Po and At are shown as semimetals. At is later shown as a nonmetal (p. 133).

• Jolly WL 1966, The Chemistry of the Non-metals, Prentice-Hall, Englewood Cliffs, New Jersey. ▲

  Twenty-four nonmetals, including B, Si, Ge, As, Sb, Te and At. H is placed over F.

• Sherwin E & Weston GJ 1966, Chemistry of the Non-metallic Elements, Pergamon Press, Oxford. ▲

  Twenty-three nonmetals. H is shown over Li and F; Germanium, As, Se, and Te are later referred to as metalloids; Sb is shown as a nonmetal but later referred to as a metal. They write, "Whilst these heavier elements [Se and Te] look metallic they show the chemical properties of non-metals and therefore come into the category of "metalloids" (p. 64).

• Phillips CSG & Williams RJP 1965, Inorganic Chemistry, vol. 1, Principles and non-metals, Oxford University Press, Clarendon. ▲

  Twenty-three nonmetals, excluding Sb, including At. An advanced work for its time, presenting inorganic chemistry as the difficult and complex subject it was, with many novel insights.

• Yost DM & Russell Jr, H 1946 Systematic Inorganic Chemistry of the Fifth-and-Sixth-Group Nonmetallic Elements, Prentice-Hall, New York, accessed August 8, 2021.

  Includes tellurium as a nonmetallic element.

• Bailey GH 1918, The Tutorial Chemistry, Part 1: The Non-Metals, 4th ed., W Briggs (ed.), University Tutorial Press, London.

  Fourteen nonmetals (excl. the noble gases), including B, Si, Se, and Te. The author writes that arsenic and antimony resemble metals in their luster and conductivity of heat and electricity but that in their chemical properties they resemble the non-metals, since they form acidic oxides and insoluble in dilute mineral acids; "such elements are called metalloids" (p. 530).

• Appleton JH 1897, The Chemistry of the Non-metals: An Elementary Text-Book for Schools and Colleges, Snow & Farnham Printers, Providence, Rhode Island

  Eighteen nonmetals: He, Ar; F, Cl, Br, I; O, S, Se, Te; N, P, As, Sb; C, Si; B; H. Neon, germanium, krypton and xenon are listed as new or doubtful elements. For Sb, Appleton writes:

"Antimony is sometimes classed as a metal, sometimes as a non-metal. In case of several other elements the question of classification is difficult—indeed, the classification is one of convenience, in a sense, more than one of absolute scientific certainty. In some of its relations, especially its physical properties, antimony resembles the well-defined metals—in its chemical relations, it falls into the group containing boron, nitrogen, phosphorus, arsenic, well-defined non-metals." (p. 166).

• Appleton JH 1888, Beginners' Hand-book of Chemistry: The Subject Developed by Facts and Principles Drawn Chiefly from the Non-metals, Chautauqua Press, New York.

  Fifteen nonmetals including B, Si, As, Sb and Se (the six noble gases were not then known; Ge had only been discovered in 1886). Te is shown in a list of the chemical elements but not mentioned elsewhere.

• Gmelin L 1849, Handbook of Chemistry, vol. 2, Non-metallic elements, H Watts (trans.), Cavendish Society, London.

  Twelve nonmetals (then called "metalloids"): H, B, C, N, O, F, P, S, Cl, Se, Br, I.

External links[]

• Media related to Nonmetals at Wikimedia Commons