Nonmetal
![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.](http://webcf.waybackmachine.org/web/20220326152224im_/http://upload.wikimedia.org/wikipedia/commons/thumb/7/71/Nonmetals_in_the_periodic_table.png/190px-Nonmetals_in_the_periodic_table.png)
Extract of periodic table showing how often each element is classified as a nonmetal:
14 effectively always[n 1]
3 frequently[n 2]
6 sometimes (metalloids)[n 3]
Nearby metals are shown in a gray font.[n 4]
There is no precise definition of a nonmetal; which elements are counted as such varies.
Hydrogen is usually in group 1 (per the below full table) but can be in group 17 (per the above extract).[n 5]
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In chemistry, a nonmetal is a chemical element that generally lacks a predominance of metallic properties; they are typically transparent or non-metallic in appearance, have low weight, and are poor thermal and electrical conductors. Solid nonmetals are brittle to crumbly, cannot be flattened into sheets or easily drawn into wires without shattering or breaking, and have low to no structural strength. The rest of the nonmetals are nearly all gases at room temperature; there is only one liquid nonmetal, bromine. Nonmetal atoms are moderately to highly electronegative; they 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, largely make up the Earth's crust, atmosphere, oceans and biosphere.
Most nonmetals have biological, technological or domestic roles or uses. Living organisms are composed almost entirely of the nonmetals hydrogen, oxygen, carbon, and nitrogen. Nearly all nonmetals have individual uses in medicine and pharmaceuticals; lasers and lighting; and household items.
While the term non-metallic dates from at least 1566, there is no widely-agreed precise definition of a nonmetal. Some elements have a marked mixture of metallic and nonmetallic properties; which of these borderline cases are counted as nonmetals can vary depending on the classification criteria used by each author. Fourteen elements are effectively always recognized as nonmetals and up to about nine more are frequently to sometimes added, as shown in the accompanying periodic table extract.
Definition and applicable elements[edit]
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A nonmetal is a chemical element having, among other properties, a relatively low density and moderate to high electronegativity. Broadly, they lack a preponderance of more metallic attributes such as luster, deformability, good thermal and electrical conductivity, and low electronegativity.[12] Since there is no rigorous definition of a nonmetal,[11][13][14] some variation may be encountered among sources as to which elements are classified as nonmetals. Such decisions depend on which property or properties are regarded as being most indicative of nonmetallic or metallic character.[15]
Generally, up to twenty-three elements may be regarded as nonmetals,[16] noting any such list is open to challenge.[1] The fourteen effectively always recognized as such 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.[1] Up to a further nine elements are frequently or sometimes considered 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.[1][3][5] Metalloids behave chemically like nonmetals but are sometimes treated as an intermediate class between the metals and the nonmetals when the criteria used to distinguish between metals and nonmetals are inconclusive.[17]
Since there are 118 known elements,[18] as of February 2022, the 23 nonmetals within the scope of this article are outnumbered by the metals several times.[19] 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.[21] The superheavy elements copernicium (Z = 112) and oganesson (118) may turn out to be nonmetals; their actual status has not yet been confirmed.[22]
General properties[edit]
Physical[edit]
of some nonmetallic elements
Outwardly, about half of nonmetallic elements are colorless or colored gases, with most of the rest being shiny solids; bromine, the only liquid, is usually topped by a layer of its vapours. They have relatively low densities, and the solids are brittle or crumbly with low mechanical and structural strength.[23] Nonmetals tend to have lower melting points and boiling points than metals, and to be good insulators of electricity and heat.[24]
Plasticity (malleability, ductility) and good thermal and electrical conductivity are ordinarily associated with the presence of free moving and uniformly distributed electrons in metals;[cite] with some exceptions, the electrons in nonmetals typically lack such mobility.[cite] Among the nonmetallic elements:
- plasticity occurs in limited circumstances only in carbon, phosphorus, sulfur, selenium, and the metalloid nonmetals;[n 8]
- good electrical conductivity and thermal conductivity is seen only for carbon, arsenic and antimony;[cite] and
- thermal conductivity values comparable to those of metals are otherwise evident only for boron, carbon (diamantine), silicon, phosphorus, and germanium;[30] such conductivity is transmitted though vibrations of the crystalline lattices of these elements.[cite]
Inwardly, the nonmetallic elements are composed of either polyatomic molecules with open-packed crystalline structures e.g. carbon and silicon; diatomic molecules e.g. hydrogen, a gas, and bromine, a liquid; or monatomic gases e.g. neon. They usually have smaller atomic radii than metals.[31]
The physical differences between metals and nonmetals arise from internal and external atomic forces. Internally, an atom's positive nuclear charge acts to hold its outer 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, outer electrons are expected to become itinerant (free to move between atoms) and metallic properties are predicted. Otherwise nonmetallic properties are anticipated.[32]
Chemical[edit]
Aspect | Metals | Nonmetals |
---|---|---|
Electronegativity | Lower than nonmetals, with some exceptions[34] |
Moderate to 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[35] |
In aqueous solution[36] |
Exist as cations | Exist as anions or oxyanions |
Nonmetals have moderate to high values of electronegativity[37] and, in chemical reactions, 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 is acidic or has acidic properties predominating.[n 9]
They tend to gain or share electrons when they react, unlike metals which tend to donate electrons. More specifically, and given the stability of the electron configurations of the noble gases (filled outer shells), 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).[40]
Quantitatively, nonmetals mostly have higher ionization energies, higher electron affinities, higher electronegativity values, and higher standard reduction potentials than metals. In general, the higher these values the more nonmetallic is the element in question.[41]
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 core increases.[42] There is an associated reduction in atomic radius[43] as the increasing nuclear charge draws the outer electrons closer to the core.[44] In metals, the effect of the nuclear charge is generally weaker than for 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.[45]
The number of compounds formed by nonmetals is vast.[46] The first ten places in a "top 20" table of elements most frequently encountered in 895,501,834 compounds, as listed in the Chemical Abstracts Service register for November 2, 2021, were occupied by nonmetals. Hydrogen, carbon, oxygen and nitrogen were 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.[47] 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;[48] phosphorus sesquisulfide (P4S3), in strike anywhere matches; and teflon ((C
2F
4)n).[49]
Complications[edit]
![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.](http://webcf.waybackmachine.org/web/20220326152224im_/http://upload.wikimedia.org/wikipedia/commons/thumb/8/81/EN_values_of_chalcogens.png/180px-EN_values_of_chalcogens.png)
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; and extend to secondary periodicity or non-uniform periodic trends going down most of the p-block groups;[50] and unusual oxidation states in the heavier nonmetals.
First row anomaly[edit]
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.[51] 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.[52] 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."[53]
For hydrogen and helium, and from boron to neon, since the 1s and 2p subshells have no inner analogues (i.e., there is no zeroth shell and no 1p subshell) and therefore experience no electron repulsion effects, they have relatively small radii, unlike the 3p, 4p and 5p subshells of heavier elements.[54] 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.[55]
While it would normally be expected that hydrogen and helium, on electron configuration grounds, would be located atop the strongly metallic 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.[56]
Secondary periodicity[edit]
Immediately after the first row of the transition metals, the 3d electrons in the 4th row of elements, i.e., in 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.[57] 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.[58]
Unusual oxidation states[edit]
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).[59]
Subclasses [edit]
![N, S and I are shown as moderately strong oxidizing agents; O, F, Cl and Br are relatively strong oxidizing agents.](http://webcf.waybackmachine.org/web/20220326152224im_/http://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/XPT_extract.png/220px-XPT_extract.png)
† moderately strong oxidising agent ‡ strong oxidising agent[n 12]
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 has noble gases, halogens, and other nonmetals, and splits the elements commonly recognized as metalloids between the "other metals" and the "other nonmetals";[71] whereas the Royal Society of Chemistry periodic table uses a different color for each of its eight main groups, and nonmetals can be found in seven of these.[72]
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[73] (the term used here) or stable halogens;[74]
- 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,[75] sometimes considered to be nonmetals and sometimes not.[n 13]
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[77] or as a sub-class of nonmetals.[78] Others count some of them as metals, for example arsenic and antimony, due to their similarities to heavy metals.[79][n 14] 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.[81]
Noble gases[edit]
![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](http://webcf.waybackmachine.org/web/20220326152224im_/http://upload.wikimedia.org/wikipedia/commons/thumb/0/0d/Argon_ice_1.jpg/220px-Argon_ice_1.jpg)
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.[82]
They have very similar properties, all 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.[83] That is why they are all gases under standard conditions, even those with atomic masses larger than many normally solid elements.[84]
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,[85] with most of these occurring via oxygen or fluorine combining with either krypton, xenon or radon.[86]
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.[87] As a further example, xenon, in the +8 oxidation state, forms a pale yellow explosive oxide, XeO4, while osmium, another noble metal, forms a yellow strongly oxidizing oxide, OsO4. There are parallels too in the formulas of the oxyfluorides: XeO2F4 and OsO2F4, and XeO3F2 and OsO3F2.[88]
About 1015 tonnes of noble gases are present in the Earth's atmosphere.[89] Helium is additionally found in natural gas to the extent of as much as 7%.[90] Radon further diffuses out of rocks, where it is formed during the natural decay sequence of uranium and thorium.[91] 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."[92]
Nonmetal halogens[edit]
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.[93]
Physically, fluorine and chlorine are pale yellow and yellowish green gases; bromine is a reddish-brown liquid (usually topped by a layer of is fumes); and iodine, under white light, is a metallic-looking[60] solid. Electrically, the first three are insulators while iodine is a semiconductor (along its planes).[94]
Chemically, they have high ionization energies, electron affinities, and electronegativity values, and are mostly relatively strong oxidizing agents.[95] Manifestations of this status include their intrinsically corrosive nature.[96] All four exhibit a tendency to form predominately ionic compounds with metals[97] whereas the remaining nonmetals, bar oxygen, tend to form predominately covalent compounds with metals.[n 15] The reactive and strongly electronegative nature of the nonmetal halogens represents the epitome of nonmetallic character.[101]
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.[102]
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.[103]
Unclassified nonmetals[edit]
![A small glass jar filled with small dull grey concave buttons. The pieces of selenium look like tiny mushrooms without their stems.](http://webcf.waybackmachine.org/web/20220326152224im_/http://upload.wikimedia.org/wikipedia/commons/thumb/d/d3/Selenium_black_%28cropped%29.jpg/220px-Selenium_black_%28cropped%29.jpg)
After the nonmetallic elements are classified as either noble gases, halogens or metalloids (following), the remaining seven nonmetals are hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur and selenium. In their most stable allotropes, 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[105] and a semiconductor in a direction perpendicular to its planes;[106] phosphorus and selenium are semiconductors;[107] and hydrogen, nitrogen, oxygen, and sulfur are insulators.[n 16]
They are generally regarded as being too diverse to merit a collective examination,[109] and have been referred to as other nonmetals,[110] or more plainly as nonmetals, located between metalloids and halogens.[111] Consequently, their chemistry tends to be taught disparately, according to their four respective periodic table groups,[112] 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 17]
Hydrogen, in particular, behaves in some respects like a metal and in others like a nonmetal.[114] Like a metal it can (first) lose its single electron;[115] it can stand in for alkali metals in typical alkali metal structures;[116] and is capable of forming alloy-like hydrides, featuring metallic bonding, with some transition metals.[117] 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.[118] It does this by way of forming a covalent or ionic bond[117] or, if it has lost its electron, attaching itself to a lone pair of electrons.[119]
Some or all of these nonmetals nevertheless have several shared properties. Most of them, being less reactive than the halogens[120] can occur naturally in the environment.[121] They have prominent biological[122][123] and geochemical roles.[109] While their physical and chemical character is "moderately non-metallic", on a net basis,[109] all of them have corrosive aspects. Hydrogen can corrode metals. Carbon corrosion can occur in fuel cells.[124] 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.[125] Untreated selenium in soils can give rise to corrosive hydrogen selenide gas.[126] When combined with metals, the unclassified nonmetals can form hard (interstitial or refractory) compounds,[127] on account of their relatively small atomic radii and sufficiently low ionization energy values.[109] They show a tendency to bond to themselves, especially in solid-state compounds.[128][109] Diagonal periodic table relationships among these nonmetals echo similar relationships among the metalloids.[129][130]
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".[131]
Unclassified nonmetals typically occur in elemental forms (oxygen, sulfur) or are found in association with either of these two elements:[132]
- Hydrogen occurs in the world's oceans as a component of water, and in natural gas as a component of methane and hydrogen sulfide.[133]
- Carbon occurs in limestone, dolomite, and marble, as carbonates.[134] Less well known is carbon as graphite, which mainly occurs in metamorphic silicate rocks[135] as a result of the compression and heating of sedimentary carbon compounds.[136]
- 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.[137]
- 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.[138]
- Selenium occurs in metal sulfide ores, where it partially replaces the sulfur; elemental selenium is occasionally found.[139]
Metalloids[edit]
![a cluster of bright cherry-red crystals](http://webcf.waybackmachine.org/web/20220326152224im_/http://upload.wikimedia.org/wikipedia/commons/thumb/5/52/Realgar-229713.jpg/220px-Realgar-229713.jpg)
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 periodic tables.[3]
They are brittle and poor to good conductors of electricity and heat. Boron, silicon, germanium and tellurium are semiconductors. Arsenic and antimony have the electronic structures of semimetals although both have less stable semiconducting allotropes.[3]
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.[3]
In periodic table terms, and to the left of the weakly nonmetallic metalloids, are an indeterminate set of weakly metallic metals (such as tin, lead and bismuth)[140] sometimes referred to as post-transition metals.[141] 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'.[142]
The metalloids tend to be found in forms combined with oxygen or sulfur or, in the case of tellurium, gold or silver.[132] 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]
Allotropes[edit]
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.[144]
Among the nonmetal halogens, and unclassified nonmetals:
- Iodine is known in a semiconducting amorphous form.[145]
- Graphite, the standard state of carbon, is a fairly good electrical conductor. The diamond allotrope of carbon is clearly nonmetallic, being translucent and, as an insulator, an extremely poor electrical conductor.[146] Carbon is further known in several other allotropic forms, including semiconducting buckminsterfullerene,[147] and amorphous[148] and paracrystalline (mixed amorphous and crystalline)[149] varieties.
- Nitrogen can form gaseous tetranitrogen (N4), an unstable polyatomic molecule with a lifetime of about one microsecond.[150]
- 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.[151]
- 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.[152] Phosphorus also exists as diphosphorus (P2), an unstable diatomic allotrope.[153]
- Sulfur has more allotropes than any other element.[154] Amorphous sulfur, a metastable mixture of such allotropes, is noted for its elasticity.[155]
- Selenium has several nonmetallic allotropes, all of which are much less electrically conducting than its standard state of gray "metallic" selenium.[156]
All the elements most commonly recognized as metalloids form allotropes:
- Boron is known in several crystalline and amorphous forms.[157]
- Silicon was most recently known only in its crystalline (diamond-like) and amorphous forms; the synthesis of an orthorhombic allotrope, Si24, was reported in 2014.[158]
- 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.[159]
- Arsenic and antimony form several well-known allotropes (yellow, grey, and black).[160]
- Tellurium is known in its crystalline and amorphous forms.[161]
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 18] starting with phosphorus at 1.7 GPa, have been observed to form metallic allotropes.[163][n 19] 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.[165]
Prevalence and access[edit]
Abundance[edit]
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.[167] Oxygen is thought to be the next most abundant element, at ca. 0.1%.[168] Less than five per cent of the universe is believed to be made of ordinary matter, represented by stars, planets and living beings. The balance is made of dark energy and dark matter, both of which are currently poorly understood.[169]
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[edit]
Nonmetals, and metalloids, are extracted in their raw forms from:[121]
- 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[edit]
As at January 2022, while non-radioactive nonmetals are relatively inexpensive[n 20] there are some exceptions. Boron, germanium, arsenic, and bromine can cost from about US$3 to $11 per gram (cf. silver at about $0.75 per gram). Prices can fall dramatically if bulk quantities are involved.[171] Phosphorus in its most stable black form, "can cost up to $1,000 per gram"[172] (ca. 15 times the cost of gold) whereas ordinary white phosphorus is available at $30 per 100 grams.[171] Researchers hope to be able to reduce the cost of black phosphorus to as low as $1 per gram.[172] 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.[173]
[edit]
Nearly all nonmetals have varying uses in household items; lasers and lighting; and medicine and pharmaceuticals. Nitrogen, for example, is found in some garden treatments; lasers; and diabetes medicines. Germanium, arsenic, and radon each have uses in one or two of these fields but not all three.[121] Aside from the noble gases most of the remaining nonmetals have, or have had, uses in agrochemicals and dyestuffs.[121] To the extent that metalloids show metallic character, they have speciality uses extending to (for example) oxide glasses, alloying components, and semiconductors.[174]
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.[121]
History, background, and taxonomy[edit]
Discovery[edit]
![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](http://webcf.waybackmachine.org/web/20220326152224im_/http://upload.wikimedia.org/wikipedia/commons/thumb/9/9c/Joseph_Wright_of_Derby_The_Alchemist.jpg/220px-Joseph_Wright_of_Derby_The_Alchemist.jpg)
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 21] Radon was the most recently discovered nonmetal, being discovered only at the end of the 19th century.[121]
Chemistry- or physics-based techniques used in the isolation efforts were spectroscopy, fractional distillation, radiation detection, electrolysis, ore acidification, combustion, displacement reactions, 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.[176]
The nonmetal halogens were obtained from their halides via electrolysis, adding an acid, or displacement. Some chemists died as a result of their experiments trying to isolate fluorine.[177]
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;[178] sulfur occurred naturally as a free element; and selenium[n 22] was detected as a residue in sulfuric acid.[180]
Most of the elements commonly recognized as metalloids were isolated by heating their oxides (boron, silicon, arsenic, tellurium) or a sulfide (germanium).[121] Antimony was known in its native form as well as being isolable by heating its sulfide.[181]
Origin of the concept[edit]
The distinction between metals and nonmetals arose, in a convoluted manner, from a crude recognition of natural 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.[182] "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).[183]
Use of the term[edit]
The term nonmetallic dates from as far back as 1566. In a medical treatise published that year, Loys de L’Aunay (a French doctor) mentioned the properties of plant substances from metallic and "non-metallic" lands. [184]
In early chemistry, Wilhelm Homberg (a German natural philosopher) referred to "non-metallic" sulfur in Des Essais de Chimie (1708).[185] He questioned the five-fold division of all matter into sulfur, mercury, salt, water and earth, as postulated by Étienne de Clave (1641) in New Philosophical Light of True Principles and Elements of Nature.[186] Homberg's approach represented "an important move toward the modern concept of an element".[187]
Lavoisier, in his "revolutionary"[188] 1789 work Traité élémentaire de chimie, published the first modern list of chemical elements in which he distinguished between gases, metals, nonmetals, and earths (heating resistant oxides).[189] 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."[190]
Suggested distinguishing criteria[edit]
|
Electron related
|
In 1809, Humphry Davy's discovery of sodium and potassium "annihilated"[211] the line of demarcation between metals and nonmetals. Before then metals had been distinguished on the basis of their ponderousness or relatively high densities.[212] Sodium and potassium, on the other hand, floated on water and yet were clearly metals on the basis of their chemical behaviour.[213]
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 of such properties, by type and date order.
Probably the most well-known property is that the electrical conductivity of a metal increases when temperature falls whereas that of a non-metal rises.[203] However this scheme does not work for plutonium, carbon, arsenic and antimony. Plutonium, which is a metal, increases its electrical conductivity when heated in the temperature range of around –175 to +125 °C.[214] Carbon, despite being widely regarded as a nonmetal, likewise increases its conductivity when heated.[215] Arsenic and antimony are sometimes classified as nonmetals yet act similarly to carbon.[216]
Emsley noted that, "No single property ... can be used to classify all the elements as either metals or nonmetals."[217] Kneen et al. suggested that the nonmetals could be discerned once a [single] criterion for metallicity had been chosen, adding that, "many arbitrary classifications are possible, most of which, if chosen reasonably, would be similar but not necessarily identical."[15] Jones, in contrast, observed that "classes are usually defined by more than two attributes".[218]
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. More specifically, he observed that all gaseous or nonconducting elements are nonmetals; solid nonmetals metals are hard and brittle or soft and crumbly whereas metals are usually malleable and ductile; and nonmetal oxides are acidic.[219]
Once a basis for distinguishing between the "two great classes of elements"[220] is established, the nonmetals are found to be those lacking the properties of metals,[221] to greater or lesser degrees.[222] Some authors further divide the elements into metals, metalloids, and nonmetals although anything not a metal is, on categorisation grounds, a nonmetal.[223]
Development of subclasses[edit]
A basic taxonomy of nonmetals was set out in 1844, by Alphonse Dupasquier, a French doctor, pharmacist and chemist.[224] To facilitate the study of nonmetals, he wrote:[225]
- They will be divided into four groups or sections, as in the following:
- Organogens O, N, H, C
- Sulphuroids S, Se, P
- Chloroides F, Cl, Br, I
- Boroids B, Si.
An echo of Dupasquier's fourfold classification is seen in the modern subclasses. The organogens and sulphuroids represent the set of unclassified nonmetals. Varying configurations of these seven nonmetals have been referred to as, for example, basic nonmetals.[226] biogens;[227] central nonmetals;[228] CHNOPS;[229] essential elements;[230] "nonmetals";[231][n 24] orphan nonmetals;[232] or redox nonmetals;[233] The chloroide nonmetals came to be independently referred to as halogens.[234] The boroid nonmetals expanded into the metalloids, starting from as early as 1864.[235] The noble gases, as a discrete grouping, were counted among the nonmetals from as early as 1900.[236]
Comparison[edit]
Some properties of metals, and of metalloids, unclassified nonmetals, nonmetal halogens, and noble gases are summarized in the table.[n 25] Physical properties apply to elements in their most stable forms in ambient conditions, and are listed in loose order of ease of determination. Chemical properties are listed from general to descriptive, and then to specific. The dashed line around the metalloids denotes that, depending on the author, the elements involved may or may not be recognized as a distinct class or subclass of elements. Metals are included as a reference point.
Most properties show a left-to-right progression in metallic to nonmetallic character or average values. The periodic table can thus be indicatively divided into metals and nonmetals, with more or less distinct gradations seen among the nonmetals.[237]
Some cross-subclass properties | |||||
Physical property | Metals | Metalloids | Unclassified nonmetals | Nonmetal halogens | Noble gases |
---|---|---|---|---|---|
Alkali, alkaline earth, lanthanide, actinide, transition and post-transition metals | Boron, silicon, germanium, arsenic, antimony (Sb), tellurium | Hydrogen, carbon, nitrogen, phosphorus, oxygen, sulfur, selenium | Fluorine, chlorine, bromine, iodine | Helium, neon, argon, krypton, xenon, radon | |
Form and heft[238] |
|
|
|
|
|
Appearance | lustrous[24] | lustrous[239] | colorless[244] | ||
Elasticity | mostly malleable and ductile[24] (Hg is liquid) | brittle[239] | iodine is brittle[251] | not applicable | |
Electrical conductivity | good[n 27] |
|
|
|
poor[n 31] |
Electronic structure[162] | metallic (Bi is a semimetal) | semimetal (As, Sb) or semiconductor |
|
semiconductor (I) or insulator | insulator |
Chemical property | Metals | Metalloids | Unclassified nonmetals | Nonmetal halogens | Noble gases |
Alkali, alkaline earth, lanthanide, actinide, transition and post-transition metals | Boron, silicon, germanium, arsenic, antimony (Sb), tellurium | Hydrogen, carbon, nitrogen, phosphorus, oxygen, sulfur, selenium | Fluorine, chlorine, bromine, iodine | Helium, neon, argon, krypton, xenon, radon | |
General chemical behavior |
|
weakly nonmetallic[n 32] | moderately nonmetallic[257] | strongly nonmetallic[258] | |
Oxides | |||||
Compounds with metals | alloys[24] or intermetallic compounds[276] | tend to form alloys or intermetallic compounds[277] | mainly ionic[97] | simple compounds in ambient conditions not known[n 35] | |
Ionization energy (kJ mol−1)† (data page) |
|
|
|
|
|
Electronegativity (Pauling scale)[n 36]† (data page) |
|
|
|
|
|
† The labels low, moderate, high, and very high are arbitrarily based on the value spans listed in the table ‡ Hydrogen can also form alloy-like hydrides[281] |
See also[edit]
- CHON (carbon, hydrogen, oxygen, nitrogen)
- List of nonmetal monographs
- Metallization pressure
- Period 1 elements (hydrogen, helium)
- Properties of nonmetals (and metalloids) by group
Notes[edit]
- ^ H; N; O, S; F, Cl, Br, I; He, Ne, Ar, Kr, Xe, Rn[1]
- ^ C; P; Se.[2] 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.[3]
- ^ B; Si, Ge; As, Sb; Te[4][5]
- ^ Al, Ga, In, Tl; Sn, Pb; Bi; Po; At
- ^ Hydrogen has historically been placed over one or more of lithium, boron,[6] carbon, or fluorine;[7] or over no group at all; or over all main groups simultaneously, and therefore may or may not be adjacent to other nonmetals.[8]
- ^ 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;[10] the density values used for At and Fr are theoretical estimates.
- ^ The first of these two sources says that chemists no longer use the term metalloid and that the elements involved are nonmetals. The second source says that the elements of the periodic table are "at all times” divided into metals, metalloids, and nonmetals.
- ^ For example, C as exfoliated (expanded) graphite,[25] and as metre-long carbon nanotube wire;[26] P as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature);[27] S as plastic sulfur;[28] and Se as selenium wires, drawn from the molten form[29]
- ^ Acids are formed by boron, phosphorus, selenium, arsenic, iodine;[38] oxides by carbon, silicon, germanium, sulfur, antimony, and tellurium.[39]
- ^ 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.
- ^ Noble gases: He, Ne, Ar, Kr, Xe, Rn; Nonmetal halogens: F, Cl, Br, I; Unclassified nonmetals: H, C, N, P, O, S, Se; Metalloids: B, Si, Ge, As, Sb, Te. Nearby metals are Al, Ga, In, Tl; Sn, Pb; Bi; Po; and At.
- ^ 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.[60] The remaining reactive nonmetallic elements have giant covalent structures, but for H which is a diatomic gas.[61]
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,[62] and a pedestrian oxidizing agent unless combined with a more active non-metal like O or F.[63]
S reacts in the cold with alkalic and post-transition metals, and Cu, Ag and Hg,[64] 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.[65]
Iodine is sufficiently corrosive to cause lesions resembling thermal burns, if handled without suitable protection,[66] and tincture of iodine will smoothly dissolve Au.[67] 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."[68] 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.[69] 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.[70] - ^ 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".[76]
- ^ 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".[80]
- ^ Metal oxides are usually ionic.[98] On the other hand, oxides of metals with high oxidation states are usually either polymeric or covalent.[99] A polymeric oxide has a linked structure composed of multiple repeating units.[100]
- ^ 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.[108]
- ^ 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.[113]
- ^ B; Si, Ge; N, P; O, S, Se, Te; nonmetal halogens; and the noble gases[162]
- ^ As at 2020, high pressure studies and experiments were said to represent, "a very active and vigorous research field".[164]
- ^ Costs for most nonmetals ranged from US$0.0002 per gram for bulk oxygen to $1.90 per gram for bulk fluorine.[171]
- ^ 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".[175]
- ^ Berzelius, who discovered selenium, thought it had the properties of a metal, combined with those of sulfur.[179]
- ^ The Goldhammer-Herzfeld ratio is roughly equal to the cube of the atomic radius divided by the molar volume.[194] 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.[195]
- ^ The quote marks are not found in the source; they are used here to make it clear that the source employs the word nonmetals as a formal term for the subset of chemical elements in question, rather than applying to nonmetals generally.
- ^ See also Properties of metals, metalloids and nonmetals, which treats metalloids as a class of their own
- ^ Carbon as exfoliated (expanded) graphite,[246] and as meter-long carbon nanotube wire;[247] phosphorus as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature);[248] sulfur as plastic sulfur;[249] and selenium as selenium wires[250]
- ^ Metals have electrical conductivity values of from 6.9×103 S•cm−1 for manganese to 6.3×105 for silver.[252]
- ^ Metalloids have electrical conductivity values of from 1.5×10−6 S•cm−1 for boron to 3.9×104 for arsenic.[253]
- ^ Unclassified nonmetals have electrical conductivity values of from ca. 1×10−18 S•cm−1 for the elemental gases to 3±4 in graphite.[254]
- ^ The nonmetal halogens have electrical conductivity values of from ca. 1×10−18 S•cm−1 for F and Cl to 1.7×10−8 S•cm−1 for iodine.[254][94]
- ^ The elemental gases have electrical conductivity values of ca. 1×10−18 S•cm−1.[254]
- ^ They always give "compounds less acidic in character than the corresponding compounds of the [typical] nonmetals"[239]
- ^ Arsenic trioxide reacts with sulfur trioxide, forming arsenic "sulfate" As2(SO4)3.[265]
- ^ CO and N2O are "formally the anhydrides of formic and hyponitrous acid, respectively: CO + H2O → H2CO2 (HCOOH, formic acid); N2O + H2O → H2N2O2 (hyponitrous acid)".[270]
- ^ 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.[279]
- ^ Values for the noble gases are from Allen and Huheey.[280]
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- ^ Jones 2010, p. 169
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- ^ Brady & Senese 2009, p. 52
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- ^ Parameswaran at al. 2020, p. 210
- ^ Knight 2002, p. 148
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- ^ Tregarthen 2003, p. 10
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- ^ Kneen, Rogers & Simpson 1972, p. 439
- ^ Kneen, Rogers & Simpson 1972, p. 465
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- ^ Mackin 2014, p. 80
- ^ Johnson 1966, pp. 105–108
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- ^ Wiberg 2001, p. 750
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External links[edit]
Media related to Nonmetals at Wikimedia Commons