CnFlOgcopernicium, flerovium, and/or oganesson may turn out to be nonmetallic however their status has not been confirmed.
A nonmetal is a chemical element that mostly lacks metallic properties. Seventeen elements are generally considered nonmetals, though some authors recognize more or fewer depending on the properties considered most representative of metallic or nonmetallic character. Some borderline elements further complicate the situation.
Nonmetals tend to have low density and high electronegativity (the ability of an atom in a molecule to attract electrons to itself). They range from colorless gases like hydrogen to shiny solids like the graphite form of carbon. Nonmetals are often poor conductors of heat and electricity, and when solid tend to be brittle or crumbly. In contrast, metals are good conductors and most are pliable. While oxides of metals tend to be basic, those of nonmetals tend to be acidic.
The distinct properties of nonmetallic elements allow for specific uses that metals often cannot achieve. Elements like hydrogen, oxygen, carbon, and nitrogen are essential building blocks for life itself. Moreover, nonmetallic elements are integral to industries such as electronics, energy storage, agriculture, and chemical production.
Most nonmetallic elements were not identified until the 18th and 19th centuries. While a distinction between metals and other minerals had existed since antiquity, a basic classification of chemical elements as metallic or nonmetallic emerged only in the late 18th century. Since then nigh on two dozen properties have been suggested as single criteria for distinguishing nonmetals from metals.
Definition and applicable elements
Properties mentioned hereafter refer to the elements in their most stable forms in ambient conditions unless otherwise mentioned
Like carbon, arsenic (here sealed in a container to prevent tarnishing) vaporises rather than melts when heated. The vapor is lemon-yellow and smells like garlic.[2] The chemistry of arsenic is predominately nonmetallic in nature.[3]
There is no precise definition of a nonmetal;[6] any list of such is open to debate and revision.[7] Which elements are included depends on the properties regarded as most representative of nonmetallic or metallic character.[n 3]
These fourteen elements are effectively always recognized as nonmetals:[7][8]
The six elements most commonly recognized as metalloids, which are typically seen as intermediates between metals and nonmetals, are here counted as a type of nonmetal due to their relatively low densities and predominantly nonmetallic chemistry, and for comparative purposes:[10]
Of the 118 known elements,[11] roughly 20% are classified as nonmetals.[12] Opinions differ as to the status of astatine. Its rarity and extreme radioactivity has resulted in it being frequently ignored in the literature.[13] With no comprehensive understanding of its properties, its classification remains uncertain. As a halogen it has usually been presumed to be a nonmetal.[14] Chemically, studies on trace quantities of astatine, which are not necessarily reliable,[15] have demonstrated characteristics of both metals and nonmetals.[16] Alternatively, given the near-metallic character of its lighter congener iodine,[n 4] a succession of authors suggest astatine may be a metal.[18] A 2013 study based on relativistic chemistry concluded that it would be a monatomic metal with a close-packed crystalline structure,[19] but this has not been experimentally verified. Astatine is not considered further in this article due to uncertainty as to its behavior and status. The superheavy elements copernicium (element 112), flerovium (114), and oganesson (118) may or may not turn out to be nonmetals; their status has not been confirmed.[20]
General properties
Physical
Variety in color and form of some nonmetallic elements
About half of nonmetallic elements are gases; most of the rest are shiny solids. Bromine, the only liquid, is so volatile that it is usually topped by a layer of its fumes; sulfur is the only colored solid nonmetal.[n 5] The gaseous and liquid nonmetals have very low densities, melting and boiling points, and are poor conductors of heat and electricity.[23] The solid elements have low densities and low mechanical and structural strength (being brittle or crumbly),[24] but a wide range of electrical conductivity.[n 6]
These diverse forms are caused by varied internal structures and bonding arrangements. Nonmetals existing as discrete atoms like xenon, or as small molecules, such as oxygen, sulfur, and bromine, have low melting and boiling points; many are gases at room temperature, as they are held together by weak London dispersion forces acting between their atoms or molecules.[28] In contrast, nonmetals that form giant structures, such as chains of up to 1,000 atoms (e.g., selenium),[29] sheets (e.g., carbon as graphite),[30] or three-dimensional lattices (e.g., silicon)[31] have higher melting and boiling points, and are all solids, as it takes more energy to overcome their stronger covalent bonds.[32] Nonmetals closer to the left or bottom of the periodic table, often have some weak metallic interactions between their molecules, chains, or layers, consistent with their proximity to the metals; this occurs in boron,[33] carbon,[34] phosphorus,[35] arsenic,[36] selenium,[37] antimony,[38] tellurium[39] and iodine.[40]
The structures of nonmetallic elements differ from those of metals primarily due to variations in valence electrons and atomic size. Metals typically have fewer valence electrons than available orbitals, leading them to share electrons with many nearby atoms, resulting in centrosymmetrical crystalline structures.[41] In contrast, nonmetals share only the electrons required to achieve a noble gas electron configuration.[42] For example, nitrogen forms diatomic molecules featuring a triple bonds between each atom, both of which thereby attain the configuration of the noble gas neon; while antimony's larger atomic size prevents triple bonding, resulting in buckled layers in which each antimony atom is singly bonded with three other nearby atoms.[43]
Nonmetals vary greatly in appearance. The shininess of boron, graphitic carbon, silicon, black phosphorus, germanium, arsenic, selenium, antimony, tellurium, and iodine is a result of their structures featuring varying degrees of delocalized (free-moving) electrons that scatter incoming visible light.[44] The colored nonmetals (sulfur, fluorine, chlorine, bromine) absorb some colors (wavelengths) and transmit the complementary or opposite colors. For example, chlorine’s "familiar yellow-green colour ... is due to a broad region of absorption in the violet and blue regions of the spectrum".[45][n 7] For the colorless nonmetals (hydrogen, nitrogen, oxygen, and the noble gases), their electrons are held sufficiently strongly so that no absorption happens in the visible part of the spectrum, and all visible light is transmitted.[47]
The electrical and thermal conductivities of nonmetals, along with the brittle nature of solid nonmetals are likewise related to their internal arrangements. Whereas good conductivity and plasticity (malleability, ductility) are ordinarily associated with the presence of free-moving and evenly distributed electrons in metals,[48] the electrons in nonmetals typically lack such mobility.[49] Among nonmetallic elements, good electrical and thermal conductivity is seen only in carbon (as graphite, along its planes), arsenic, and antimony.[n 8] Good thermal conductivity otherwise occurs only in boron, silicon, phosphorus, and germanium;[25] such conductivity is transmitted though vibrations of the crystalline lattices of these elements.[50] Moderate electrical conductivity is observed in boron, silicon, phosphorus, germanium, selenium, tellurium, and iodine.[n 9] Plasticity occurs under limited circumstances in carbon, as seen in exfoliated (expanded) graphite[52][53] and carbon nanotube wire,[54] in white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature),[55] in plastic sulfur,[56] and in selenium which can be drawn into wires from its molten state.[57]
The physical differences between metals and nonmetals arise from internal and external atomic forces. Internally, the positive charge stemming from the protons in an atom's nucleus acts to hold the atom's outer electrons in place. Externally, the same electrons are subject to attractive forces from protons in neighboring atoms. When the external forces are greater than, or equal to, the internal force, the outer electrons are expected to become relatively free to move between atoms, and metallic properties are predicted. Otherwise nonmetallic properties are expected.[58]
Many nonmetallic elements exhibit a range of allotropic forms, each with distinct physical properties that may vary between metallic and nonmetallic.[63] For example, carbon, a versatile nonmetal, can manifest as graphite, diamond, and other forms, with graphite displaying relatively good electrical conductivity, while diamond is transparent and an extremely poor conductor of electricity.[64] Carbon further exists in several allotropic structures, including buckminsterfullerene,[65] and amorphous[66] and paracrystalline (mixed amorphous and crystalline)[67] variations.
Allotropes also occur for the other unclassified nonmetals, the metalloids, and iodine among the halogen nonmetals.[68]
Chemical
Nonmetals have relatively high values of electronegativity, and their oxides are therefore usually acidic. Exceptions occur when the oxidation state is low, the nonmetal is not very electronegative, or both: thus, water (H2O) is amphoteric[69] and nitrous oxide (N2O) is neutral.[70][n 10]
They tend to gain or share electrons during chemical reactions, in contrast to metals which tend to donate electrons. This behavior is closely related to the stability of electron configurations in the noble gases, which have complete outer shells. Nonmetals generally gain enough electrons to attain the electron configuration of the following noble gas, while metals tend to lose electrons, achieving the electron configuration of the preceding noble gas. These tendencies in nonmetallic elements are succinctly summarized by the duet and octet rules of thumb.
Furthermore, nonmetals typically exhibit higher ionization energies, electron affinities, and standard reduction potentials than metals. Generally, the higher these values are (including electronegativity) the more nonmetallic the element tends to be.[73] For example, the chemically very active nonmetals fluorine, chlorine, bromine, and iodine have an average electronegativity of 3.19—a figure[n 11] higher than that of any individual metal. On the other hand, the 2.05 average[n 12] of the chemically weak metalloid nonmetals falls within the 0.70 to 2.54 range of metals.[74]
The chemical distinctions between metals and nonmetals primarily stem 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 in tandem with the number of protons in the atomic nucleus.[75] Consequently, there is a corresponding reduction in atomic radius[76] as the heightened nuclear charge draws the outer electrons closer to the nucleus core.[77] In metals, the impact of the nuclear charge is generally weaker compared to nonmetallic elements. As a result, in chemical bonding, metals tend to lose electrons, leading to the formation of positively charged or polarized atoms or ions, while nonmetals tend to gain these electrons due to their stronger nuclear charge, resulting in negatively charged ions or polarized atoms.[78]
The number of compounds formed by nonmetals is vast.[79] The first 10 places in a "top 20" table of elements most frequently encountered in 895,501,834 compounds, as listed in the Chemical Abstracts Service register for November 2, 2021, were occupied by nonmetals. Hydrogen, carbon, oxygen, and nitrogen collectively appeared in most (80%) of compounds. Silicon, a metalloid, ranked 11th. The highest-rated metal, with an occurrence frequency of 0.14%, was iron, in 12th place.[80] A few examples of nonmetal compounds are: boric acid (H 3BO 3), used in ceramic glazes;[81]selenocysteine (C 3H 7NO 2Se), the 21st amino acid of life;[82]phosphorus sesquisulfide (P4S3), found in strike anywhere matches;[83] and teflon ((C 2F 4)n), used to create non-stick coatings for pans and other cookware.[84]
Complications
Adding complexity to the chemistry of the nonmetals are anomalies occurring in the first row of each periodic table block; non-uniform periodic trends; higher oxidation states; and property overlaps with metals.
First row anomaly
Periodic table highlighting the first row of each block.[n 13] Helium (He), as a noble gas, is normally shown over neon (Ne) with the rest of the noble gases. The elements within scope of this article are inside the thick black borders. The status of oganesson (Og, element 118) is not yet known.
Starting with hydrogen, the first row anomaly primarily arises from the electron configurations of the elements concerned. Hydrogen is particularly notable for its diverse bonding behaviors. It most commonly forms covalent bonds, but it can also lose its single electron in an aqueous solution, leaving behind a bare proton with tremendous polarizing power.[85] Consequently, this proton can attach itself to the lone electron pair of an oxygen atom in a water molecule, laying the foundation for acid-base chemistry.[86] Moreover, a hydrogen atom in a molecule can form a second, albeit weaker, bond with an atom or group of atoms in another molecule. As Cressey explains, 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."[87]
Hydrogen and helium, as well as boron through neon, have unusually small atomic radii. This phenomenon arises because the 1s and 2p subshells lack inner analogues (meaning there is no zero shell and no 1p subshell), and they therefore experience no electron repulsion effects, unlike the 3p, 4p, and 5p subshells of heavier elements.[88] A a result, ionization energies and electronegativities among these elements are higher than what periodic trends would otherwise suggest. The compact atomic radii of carbon, nitrogen, and oxygen facilitate the formation of double or triple bonds.[89]
While it would normally be expected, on electron configuration consistency grounds, that hydrogen and helium would be placed atop the s-block elements, the significant first row anomaly shown by these two elements justifies alternative placements. Hydrogen is occasionally positioned above fluorine, in group 17, rather than above lithium in group 1. Helium is commonly placed above neon, in group 18, rather than above beryllium in group 2.[90]
Secondary periodicity
Electronegativity values of the group 16 chalcogen elements showing a W-shaped alternation or secondary periodicity going down the group
A secondary periodicity alternation in certain periodic trends becomes evident when descending groups 13 to 15, and to a lesser extent, groups 16 and 17.[91][n 14] Immediately after the first row of d-block metals, from scandium to zinc, the 3d electrons in the p-block elements—specifically, gallium (a metal), germanium, arsenic, selenium, and bromine—prove less effective at shielding the increasing positive nuclear charge. This same effect is observed with the emergence of fourteen f-block metals located between barium and lutetium, ultimately leading to atomic radii that are smaller than expected for elements from hafnium (Hf) onward.[93]
While certain elements have traditionally been classified as nonmetals and others as metals, some overlapping of properties occurs. Writing early in the twentieth century, by which time the era of modern chemistry had been well-established,[95] Humphrey[96] observed that:
... these two groups, however, are not marked off perfectly sharply from each other; some nonmetals resemble metals in certain of their properties, and some metals approximate in some ways to the non-metals.
Examples of metal-like properties occurring in nonmetallic elements include:
the electrical conductivity of graphite exceeds that of some metals;[n 15]
just over half of nonmetallic elements can form homopolyatomic cations;[n 16] and
silicon has an electronegativity (1.9) comparable with metals such as cobalt (1.88), copper (1.9), nickel (1.91) and silver (1.93).[74]
Types
Periodic table excerpt highlighting the four types of nonmetals. Hydrogen is typically found in group 1, but occasionally placed in group 17. † moderately strong oxidizing agents ‡ strong oxidizing agents[n 17]
The classification of nonmetals can vary, with approaches ranging from as few as two types to as many as six or seven. For instance, the periodic table in the Encyclopaedia Britannica recognizes noble gases, halogens, and other nonmetals, and splits the elements commonly recognized as metalloids between "other metals" and "other nonmetals".[111] On the other hand, the Royal Society of Chemistry periodic table uses different colors for each of its eight main groups, with nonmetals represented in seven of these.[112]
Traversing the periodic table from right to left, three or four types of nonmetals can be discerned:
a set of chemically strong halogen elements—fluorine, chlorine, bromine and iodine—sometimes referred to as nonmetal halogens[114] or halogen nonmetals[115] (as used here) or stable halogens;[116]
a set of unclassified nonmetals, encompassing elements like hydrogen, carbon, nitrogen, and oxygen, for which there is no widely recognized collective name;[n 19] and
the chemically weak nonmetallic metalloids[125] which are sometimes considered nonmetals and sometimes considered a third category distinct from metals and nonmetals.[n 20]
In the periodic table, metalloids – so metallic they are often not considered nonmetals – come beside post-transition metals, the least metallic of metals. This follows a general left-to-right metallic-to-nonmetallic trend. Surrounding these are more reactive elements (transition metals and unclassified nonmetals); and further outside, the most reactive elements (alkali and alkaline earth metals and halogen nonmetals). The least reactive elements include noble gases on the far right and noble metals buried within transition metals.
The boundaries between these sets of nonmetals are not sharp.[n 21] Carbon, phosphorus, selenium, and iodine border the metalloids and show some metallic character, as does hydrogen. Among the noble gases, radon is the most metallic and begins to show some cationic behavior, which is unusual for a nonmetal.[128]
The greatest discrepancy between authors occurs in the metalloid "frontier territory".[129] Some consider metalloids distinct from both metals and nonmetals, while others classify them as nonmetals.[130] Some categorize certain metalloids as metals (e.g., arsenic and antimony due to their similarities to heavy metals).[131][ambiguous][n 22] This article includes metalloids for comparative purposes[n 23] and due to their relatively low densities, high electronegativity, and chemical behavior.[125]
A broadly comparable range of types occurs among the metals, from highly reactive to less reactive (even noble). On the left side of the periodic table, and below its main body, are highly to fairly reactive metals, such as sodium, calcium and uranium. Towards the middle of the periodic table are transition metals, such as scandium, iron and nickel, of high to low reactivity. To the right of the transition metals, (from group 13 onwards) are metals such as tin and lead, none of which are particularly reactive. A subset of the transition metals (including platinum and gold) are referred to as noble metals on account of their reluctance to engage in chemical activity.[132][n 24]
A small (about 2 cm long) piece of rapidly melting argon ice
Six nonmetals are classified as noble gases: helium, neon, argon, krypton, xenon, and the radioactive radon. In conventional periodic tables they occupy the rightmost column. They are called noble gases due to their exceptionally low chemical reactivity.[113]
These elements exhibit remarkably similar properties, characterized by their colorlessness, odorlessness, and nonflammability. Due to their closed outer electron shells, noble gases possess feeble interatomic forces of attraction, leading to exceptionally low melting and boiling points.[136] As a consequence, they all exist as gases under standard conditions, even those with atomic masses surpassing many typically solid elements.[137]
Chemically, the noble gases exhibit relatively high ionization energies, negligible or negative electron affinities, and high to very high electronegativities. The number of compounds formed by noble gases is in the hundreds and continues to expand,[138] with most of these compounds involving the combination of oxygen or fluorine with either krypton, xenon, or radon.[139]
About 1015 tonnes of noble gases are present in the Earth's atmosphere.[140] Additionally, natural gas can contain as much as 7% helium.[141] Radon diffuses out of rocks, where it forms during the natural decay sequence of uranium and thorium.[142] In the Earth's core there may be around 1013 tons of xenon, in the form of stable XeFe3 and XeNi3intermetallic compounds.[n 25] This could explain why "studies of the Earth's atmosphere have shown that more than 90% of the expected amount of Xe is depleted."[144]
sodium (Na), chlorine (Cl), and table salt (NaCl)Corrosive chlorine, a halogen nonmetal, combines with highly reactive sodium to form stable, unreactive table salt.
Although the halogen nonmetals are notably reactive and corrosive elements, they can also be found in everyday compounds like toothpaste (NaF); common table salt (NaCl); swimming pool disinfectant (NaBr); or food supplements (KI). The term "halogen" itself means "salt former".[145]
Physically, fluorine and chlorine exist as pale yellow and yellowish-green gases, respectively, while bromine is a reddish-brown liquid, typically covered by a layer of its fumes; iodine, when observed under white light, appears as a metallic-looking[100] solid. Electrically, the first three elements function as insulators while iodine behaves as a semiconductor (along its planes).[146]
Chemically, the halogen nonmetals exhibit high ionization energies, electron affinities, and electronegativity values, and are mostly relatively strong oxidizing agents.[147] These characteristics contribute to their corrosive nature.[148] All four elements tend to form primarily ionic compounds with metals,[149] in contrast to the remaining nonmetals (except for oxygen) which tend to form primarily covalent compounds with metals.[n 26] The highly reactive and strongly electronegative nature of the halogen nonmetals epitomizes nonmetallic character.[153]
The halogen nonmetals are commonly found in salt-related minerals. Fluorine, for instance, is present in fluorite (CaF2), a mineral found widely. Chlorine, bromine, and iodine are typically found in brines. Exceptionally, a study reported in 2012 noted the presence of 0.04% native fluorine (F 2) by weight in antozonite, attributing these inclusions to radiation from tiny amounts of uranium.[154]
A crystal of realgar, also known as "ruby sulphur" or "ruby of arsenic", an arsenic sulfide mineral As4S4. The two elements involved each have a predominately nonmetallic chemistry.[155]
The six elements more commonly recognized as metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium, all of which have a metallic appearance. (There are other elements less commonly recongised as metalloids, including carbon, aluminium, selenium and polonium. They have metallic and nonmetallic properties, but one or the other kind predominates.) On a standard periodic table, they occupy a diagonal region within the p-block extending from boron at the upper left to tellurium at the lower right, along the dividing line between metals and nonmetals shown on some tables.[9]
They are brittle and poor-to-good conductors of heat and electricity. Specifically, boron, silicon, germanium, and tellurium are semiconductors. Arsenic and antimony have the electronic structures of semimetals, although both have less stable semiconducting forms.[9]
Chemically, 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. Additionally, they tend to form alloys when combined with metals.[9]
The metalloids are commonly found combined with oxygen, sulfur, or, in the case of tellurium, gold or silver.[156] Boron is typically found in boron-oxygen borate minerals, including in volcanic spring waters. Silicon is present in the silicon-oxygen mineral silica (sand). Germanium, arsenic, and antimony are mainly found as components of sulfide ores. Tellurium is often found in telluride minerals of gold or silver. In some instances, native forms of arsenic, antimony, and tellurium have been reported.[157]
After classifying the nonmetallic elements into noble gases, halogens, and metalloids, the remaining seven nonmetals are hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur, and selenium.
In their most stable forms, three of these are colorless gases (H, N, O); three have a metal-like appearance (C, P, Se); and one appears yellow (S). Electrically, graphitic carbon behaves as a semimetal along its planes[159] and a semiconductor perpendicular to its planes;[160] phosphorus and selenium are semiconductors;[161] while hydrogen, nitrogen, oxygen, and sulfur are insulators.[n 27]
These elements are often considered too diverse to merit a collective classification,[163] and have been referred to as other nonmetals,[164] or simply as nonmetals, located between the metalloids and the halogens.[165] As a result, their chemistry is typically taught disparately, according to their respective periodic table groups:[166] hydrogen in group 1; the group 14 nonmetals (including carbon, and possibly silicon and germanium); the group 15 nonmetals (including nitrogen, phosphorus, and possibly arsenic and antimony); and the group 16 nonmetals (including oxygen, sulfur, selenium, and possibly tellurium). Authors may choose other subdivisions based on their preferences.[n 28]
Hydrogen, in particular, behaves in some respects like a metal and in others like a nonmetal.[168] Like a metal it can, for example, form a solvated cation in aqueous solution;[169] it can substitute for alkali metals in compounds such as the chlorides (NaCl cf. HCl) and nitrates (KNO3 cf. HNO3), and in certain alkali metal organometallic structures;[170] and it can form alloy-like hydrides with some transition metals.[171] Conversely, it is an insulating diatomic gas, akin to the nonmetals nitrogen, oxygen, fluorine and chlorine. In chemical reactions, it tends to ultimately attain the electron configuration of helium (the following noble gas) behaving in this way as a nonmetal.[172] It attains this configuration by forming a covalent or ionic bond[173] or, if it has initially given up its electron, by attaching itself to a lone pair of electrons.[174]
Some or all of these nonmetals share several properties. Being less reactive than the halogens,[175] they can occur naturally in the environment.[176] They have significant roles in biology[177] and geochemistry.[163] Collectively, their physical and chemical characteristics can be described as "moderately non-metallic".[163] However, they all have corrosive aspects. Hydrogen can corrode metals. Carbon corrosion can occur in fuel cells.[178]Acid rain is caused by dissolved nitrogen or sulfur. Oxygen causes iron to corrode via rust. White phosphorus, the most unstable form, ignites in air and leaves behind phosphoric acid residue.[179] Untreated selenium in soils can lead to the formation of corrosive hydrogen selenide gas.[180] When combined with metals, the unclassified nonmetals can form high-hardness (interstitial or refractory) compounds[181] due to their relatively small atomic radii and sufficiently low ionization energies.[163] They also exhibit a tendency to bond to themselves, particularly in solid compounds.[182] Additionally, diagonal periodic table relationships among these nonmetals mirror similar relationships among the metalloids.[183]
Unclassified nonmetals are typically found in elemental forms or in association with other elements:[156]
Carbon can be found in limestone, dolomite, and marble, as carbonates.[185] Additionally, carbon exists as graphite, primarily occurring in metamorphic silicate rocks,[186] resulting from the compression and heating of sedimentary carbon compounds.[187]
Oxygen is found in the atmosphere; in the oceans as a component of water; and in the Earth's crust as oxide minerals.[188]
Phosphorus minerals are widespread, typically appearing as phosphorus-oxygen phosphates.[189]
Elemental sulfur can be found in or near hot springs and volcanic regions around the world, while sulfur minerals are common and are often found as sulfides or oxygen-sulfur sulfates.[190]
Selenium is found in metal sulfide ores, where it may partially replace sulfur. In rare instances, elemental selenium can also be found.[191]
Hydrogen and helium dominate the universe, making up an estimated 98% of all ordinary matter by mass.[n 30] Oxygen, the next most abundant element, constitutes around 1% of the universe's composition.[196]
Five nonmetals—hydrogen, carbon, nitrogen, oxygen, and silicon—dominate the accessible structure of the earth, forming the vast majority of the Earth's crust, atmosphere, hydrosphere, and biomass as shown in the accompanying table.[197]
Sources of nonmetallic elements
Group (1, 13−18)
Period
13
14
15
16
1/17
18
(1−6)
H
He
1
B
C
N
O
F
Ne
2
Si
P
S
Cl
Ar
3
Ge
As
Se
Br
Kr
4
Sb
Te
I
Xe
5
Rn
6
Nonmetals and metalloids are extracted in their raw forms from:[176]
Nonmetallic elements have distinct properties[199] that enable a wide range of natural and technological uses. In living organisms, hydrogen, oxygen, carbon, and nitrogen serve as the foundational building blocks of life. Some key technological uses of nonmetallic elements are in lighting and lasers, medicine and pharmaceuticals, and ceramics and plastics. The accompanying table groups nonmetallic elements according to their uses.
Some specific uses of later-discovered or rarer nonmetallic elements include:
Boron, first produced in a pure form in 1909,[200] is used in the form of high-strength fibers for aerospace components and certain sporting goods.[201] It is also added to steel alloys to improve hardenability.[202]
Germanium, thought to be a metal up until the 1930s,[205] was historically used in electronics, particularly early transistors and diodes, and still has roles in specialized high-frequency electronics. It is also used in the production of infrared optical components for thermal imaging and spectroscopy.[206]
Radon, rarest of the noble gases,[209] was formerly used in radiography and radiation therapy. Usually, radium in either an aqueous solution or as a porous solid was stored in a glass vessel. The radium decayed to produce radon, which was pumped off, filtered, and compressed into a small tube every few days. The tube was then sealed and removed. It was a source of gamma rays which came from bismuth-214, one of radon’s decay products.[210] Radon has now been replaced by sources of 137Cs, 192Ir, and 103Pd.[211]
Most nonmetallic elements were identified during the 18th and 19th centuries. However, a few nonmetals were recognized in ancient times and later historical periods. Carbon, sulfur, and antimony were among the early nonmetals known to humanity. The discovery of arsenic can be traced back to the Middle Ages, credited to the work of Albertus Magnus. A significant moment in the history of nonmetal discovery occurred in 1669 when Hennig Brand successfully isolated phosphorus from urine. Helium, identified in 1868, holds a unique distinction as the only element not initially discovered on Earth itself.[n 31] Radon is the most recently identified nonmetal, with its detection occurring at the end of the 19th century.[176]
The isolation of nonmetallic elements depended on a range of chemical and physical techniques. These methods encompassed spectroscopy, fractional distillation, radiation detection, electrolysis, ore acidification, displacement reactions, combustion, and controlled heating processes. While some nonmetals were naturally occurring as free elements, others required intricate extraction procedures:
The noble gases, renowned for their low reactivity, were first identified via spectroscopy, air fractionation, and radioactive decay studies. Helium was initially detected by its distinctive yellow line in the solar corona spectrum. Subsequently, it was observed escaping as bubbles when uranite UO2 was dissolved in acid. Neon, argon, krypton, and xenon were obtained through the fractional distillation of air. The discovery of radon occurred three years after Henri Becquerel's pioneering research on radiation in 1896.[213]
The isolation of halogen nonmetals from their halides involved techniques including electrolysis, acid addition, or displacement. These efforts were not without peril, as some chemists tragically[214] lost their lives in their pursuit of isolating fluorine.[215]
The unclassified nonmetals have a diverse history. Hydrogen was discovered and first described in 1671 as the product of the reaction between iron filings and dilute acids. Carbon was found naturally in forms like charcoal, soot, graphite, and diamond. Nitrogen was discovered by examining air after carefully removing oxygen. Oxygen itself was obtained by heating mercurous oxide. Phosphorus was derived from the heating of ammonium sodium hydrogen phosphate (Na(NH4)HPO4), a compound found in urine.[216] Sulfur occurred naturally as a free element, simplifying its isolation. Selenium,[n 32] was first identified as a residue in sulfuric acid.[218]
Metalloids were commonly isolated by heating of their oxides (boron, silicon, arsenic, tellurium) or a sulfide (germanium).[176] Antimony was obtained primarily through the heating of its sulfide, stibnite; it was later discovered in native form.[219]
Origin and use of the term
An extract from the English translation of Lavoisier's Traité élémentaire de chimie (1789),[220] listing the elemental gases oxygen, hydrogen and nitrogen (and erroneously including light and caloric), and the nonmetallic substances sulfur, phosphorus, and carbon, and including the chloride, fluoride and borate ions
Although a distinction had existed between metals and other mineral substances since ancient times, it was only towards the end of the 18th century that a basic classification of chemical elements as either metallic or nonmetallic substances began to emerge. It would take another nine decades before the term "nonmetal" was widely adopted.
Around the year 340 BCE, in Book III of his treatise Meteorology, the ancient Greek philosopher Aristotle categorized substances found within the Earth into two distinct groups: metals and "fossiles".[n 33] The latter category included various minerals such as realgar, ochre, ruddle, sulfur, cinnabar, and other substances that he referred to as "stones which cannot be melted".[221]
Up until the Middle Ages the classification of minerals remained largely unchanged, albeit with varying terminology. In the fourteenth century, an English alchemist named Richardus Anglicus expanded upon the classification of minerals in his work Correctorium Alchemiae. In this text, he proposed the existence of two primary types of minerals. The first category, which he referred to as "major minerals", included well-known metals such as gold, silver, copper, tin, lead, and iron. On the other hand, the second category, labeled as "minor minerals", encompassed substances like salts, atramenta (iron sulfate), alums, vitriol, arsenic, orpiment, sulfur, and similar substances that were not metallic bodies.[222]
The term "nonmetallic" has historical origins dating back to at least the 16th century. In a 1566 medical treatise, the French physician Loys de L'Aunay discussed the distinct properties exhibited by substances derived from plant sources. In his writings, he made a significant comparison between the characteristics of materials originating from what he referred to as metallic soils and non-metallic soils.[223]
Later, the French chemist Nicolas Lémery discussed metallic minerals and nonmetallic minerals in his work Universal Treatise on Simple Drugs, Arranged Alphabetically published in 1699. In his writings, he contemplated whether the substance "cadmia" belonged to either the first category, akin to cobaltum (cobaltite), or the second category, exemplified by what was then known as calamine—a mixed ore containing zinc carbonate and silicate.[224]
The pivotal moment in the systematic classification of chemical elements, distinguishing between metallic and nonmetallic substances, came in 1789 with the groundbreaking work of Antoine Lavoisier. Lavoisier, a French chemist, published the first modern list of chemical elements in his revolutionary[225] work Traité élémentaire de chimie. In this work he categorized elements into distinct groups, including gases, metallic substances, nonmetallic substances, and earths (heat-resistant oxides).[226] Lavoisier's work gained widespread recognition and was republished in twenty-three editions across six languages within its first seventeen years, significantly advancing the understanding of chemistry in Europe and America.[227]
The eventual and widespread adoption of the term "nonmetal" followed a complex and lengthy developmental process that spanned nearly nine decades. In 1811, the Swedish chemist Berzelius introduced the term "metalloids"[228] to describe nonmetallic elements, noting their ability to form negatively charged ions with oxygen in aqueous solutions.[229][230] While Berzelius' terminology gained significant acceptance,[231] it later faced criticism from some who found it counterintuitive,[230] misapplied,[232] or even invalid.[233][234] In 1864, reports indicated that the term "metalloids" was still endorsed by leading authorities,[235] but there were reservations about its appropriateness. The idea of designating elements like arsenic as metalloids had been considered.[235] By as early as 1866, some authors began preferring the term "nonmetal" over "metalloid" to describe nonmetallic elements.[236] In 1875, Kemshead[237] observed that elements were categorized into two groups: non-metals (or metalloids) and metals. He noted that the term "non-metal", despite its compound nature, was more precise and had become universally accepted as the nomenclature of choice.
Suggested distinguishing criteria
Some single properties used to distinguish metals and nonmetals
In 1809, the British chemist and inventor Humphry Davy made a groundbreaking discovery that reshaped the understanding of metals and nonmetals.[259] His isolation of sodium and potassium represented a significant departure from the conventional method of classifying metals solely based on their ponderousness or high densities.[260] Sodium and potassium, on the contrary, floated on water.[n 35] Nevertheless, their classification as metals was firmly established by their distinct chemical properties.[263]
As early as 1811, attempts were initiated to enhance the differentiation between metals and nonmetals by examining a range of properties, including physical, chemical, and electron-related characteristics. The table provided here outlines 22 such properties, sorted by type and year of mention.
One of the most commonly recognized properties used in this context is the effect of heating on electrical conductivity. As temperature rises, the conductivity of metals decreases while that of nonmetals increases.[250] However, plutonium, carbon, arsenic, and antimony defy the norm. When plutonium (a metal) is heated within a temperature range of −175 to +125 °C its conductivity increases.[264] Similarly, despite its common classification as a nonmetal, when carbon (as graphite) is heated it experiences a decrease in electrical conductivity.[265] Arsenic and antimony, which are occasionally classified as nonmetals, show behavior similar to carbon, highlighting the complexity of the distinction between metals and nonmetals.[266]
Kneen and colleagues[267] proposed that the classification of nonmetals can be achieved by establishing a single criterion for metallicity. They acknowledged that various plausible classifications exist and emphasized that while these classifications may differ to some extent, they would generally agree on the categorization of nonmetals.
However, Emsley[268] pointed out the complexity of this task, asserting that no single property alone can unequivocally assign elements to either the metal or nonmetal category. Furthermore, Jones[269] emphasized that classification systems typically rely on more than two attributes to define distinct types.
Characteristics of nonmetals according to Johnson[270]
An approach to distinguishing between metallic and nonmetallic properties was suggested by Johnson,[270] emphasizing the significance of physical properties, while acknowledging the potential need for other properties in certain ambiguous cases. His observations highlighted several key distinctions:
Physical state—Elements that exist as gases or are nonconductors are typically classified as nonmetals.
Solid nonmetals—Solid nonmetals exhibit characteristics such as hardness and brittleness (B, Si, Ge) or softness and crumbliness, setting them apart from metals that are generally malleable and ductile.
Chemical behavior—Nonmetal oxides tend to be acidic, providing another useful criterion for identifying nonmetals.
Metals and nonmetals sorted by density and electronegativity (EN)[n 37]
EN
Density
< 1.9
≥ 1.9
< 7 gm/cm3
Groups 1 and 2 Sc, Y, La Ce, Pr, Eu, Yb Ti, Zr, V; Al, Ga
Noble gases F, Cl, Br, I H, C, N, P, O, S, Se B, Si, Ge, As, Sb, Te^
> 7 gm/cm3
Nd, Pm, Sm, Gd, Tb, Dy Ho, Er, Tm, Lu; Ac–Es; Hf, Nb, Ta; Cr, Mn, Fe, Co, Zn, Cd, In, Tl, Pb
^ italicized elements are commonly recognized by some authors as metalloids
Several authors[277] have noted that, in general, and among other properties, nonmetals have low densities and high electronegativity, which is consistent with the data presented in the table. Nonmetallic elements are predominantly located in the top right quadrant of this table, where density is low and electronegativity values are relatively high. In contrast, the other three quadrants are primarily occupied by metals. Goldwhite and Spielman[278] added that, "... lighter elements tend to be more electronegative than heavier ones." The average electronegativity for the elements in the table with densities less than 7 gm/cm3 (metals and nonmetals) is 2.09 compared to 1.68 for the metals having densities of more than 7 gm/cm3.
While some authors choose to further subdivide elements into metals, metalloids, and nonmetals, Oderberg[279] disagrees with this approach. He argues that according to the principles of categorization, anything not classified as a metal should be considered a nonmetal.
Development of types
Gaspard Alphonse Dupasquier (1793–1848), French doctor, pharmacist and chemist as depicted in the Monument aux Grands Hommes de la Martinière [fr] in Lyon, France. In 1844 he put forward a basic taxonomy of nonmetals.
In 1844, Alphonse Dupasquier [fr], a French doctor, pharmacist, and chemist,[280] established a basic taxonomy of nonmetals to aid in the study of these elements. He wrote:[281]
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 fourfold classification has echoes in the modern types of nonmetals. The organogens and sulphuroids are akin to the unclassified nonmetals. The chloroide nonmetals were later recognized independently as halogens.[282] The boroid nonmetals eventually evolved into the metalloids, with this classification beginning as early as 1864.[235] The noble gases were also identified as a distinct group among the nonmetals, dating back to as early as 1900.[283]
Comparison of selected properties
The two tables in this section list some physical and chemical properties of metals[n 38] and those of the three to four types of nonmetals, based on the most stable forms of the elements in ambient conditions.
The aim is to show that most properties display a left-to-right progression in metallic-to-nonmetallic character or average values.[284][285] Some overlapping of boundaries can occur as outlier elements of each type exhibit less-distinct, hybrid-like, or atypical properties.[286][n 39] These overlaps or transitional points, along with horizontal, diagonal, and vertical relationships between the elements, form part of the "great deal of information" summarized by the periodic table.[288]
The dashed lines around the columns for metalloids signify that the treatment of these elements as a distinct type can vary depending on the author, or classification scheme in use.
Physical
Physical properties are presented in loose order of ease of their determination.
† Hydrogen can also form alloy-like hydrides[171]
‡ The labels low, moderate, high, and very high are arbitrarily based on the value spans listed in the table
^A 2D semiconductor with a metallic appearance, showing evidence of delocalized electrons[17]
^Solid iodine has a silvery metallic appearance under white light at room temperature.[21] It sublimes at ordinary and higher temperatures, passing from solid to gas; its vapours are violet-colored.[22]
^The solid nonmetals have electrical conductivity values ranging from 10−18 S•cm−1 for sulfur[25] to 3 × 104 in graphite[26] or 3.9 × 104 for arsenic;[27] cf. 0.69 × 104 for manganese to 63 × 104 for silver, both metals.[25] The conductivity of graphite (a nonmetal) and arsenic (a metalloid nonmetal) exceeds that of manganese. Such overlaps show that it can be difficult to draw a clear line between metals and nonmetals.
^The absorbed light may be converted to heat or re-emitted in all directions so that the emission spectrum is thousands of times weaker than the incident light radiation[46]
^Thermal conductivity values for metals range from 6.3 W m−1 K−1 for neptunium to 429 for silver; cf. antimony 24.3, arsenic 50, and carbon 2000.[25] Electrical conductivity values of metals range from 0.69 S•cm−1 × 104 for manganese to 63 × 104 for silver; cf. carbon 3 × 104,[26] arsenic 3.9 × 104 and antimony 2.3 × 104.[25]
^While CO and NO are commonly referred to as being neutral, CO is a slightly acidic oxide, reacting with bases to produce formates (CO + OH− → HCOO−);[71] and in water, NO reacts with oxygen to form nitrous acid HNO2 (4NO + O2 + 2H2O → 4HNO2).[72]
^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
^The net result is an even-odd difference between periods (except in the s-block) that is sometimes known as secondary periodicity: elements in even periods have smaller atomic radii and prefer to lose fewer electrons, while elements in odd periods (except the first) differ in the opposite direction. Many properties in the p-block then show a zigzag rather than a smooth trend along the group. For example, phosphorus and antimony in odd periods of group 15 readily reach the +5 oxidation state, whereas nitrogen, arsenic, and bismuth in even periods prefer to stay at +3.[92]
^For example, the conductivity of graphite is 3 × 104 S•cm−1[97] whereas that of manganese is 6.9 × 103 S•cm−1[98]
^A homopolyatomic cation consists of two or more atoms of the same element bonded together and carrying a positive charge, for example, N5+, O2+ and Cl4+, Such ions are further known for C, P, Sb, S, Se, Te, Br, I and Xe[99].
^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.[100] The remaining reactive nonmetallic elements have giant covalent structures, but for H which is a diatomic gas.[101]
The single dagger nonmetals N, S and iodine are somewhat hobbled as to the strength of their nonmetallic character:
While N has a high electronegativity, it is a reluctant anion former,[102] and a pedestrian oxidizing agent unless combined with a more active nonmetal like O or F.[103]
S reacts in the cold with alkalic and post-transition metals, and Cu, Ag and Hg,[104] 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.[105]
Iodine is sufficiently corrosive to cause lesions resembling thermal burns, if handled without suitable protection,[106] and tincture of iodine will smoothly dissolve Au.[107] 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."[108] 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.[109] 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.[110]
^The quote marks are not found in the source; they are used here to make it clear that the source employs the word non-metals as a formal term for the subset of chemical elements in question, rather than applying to nonmetals generally
^Varying configurations of these nonmetals have been referred to as, for example, basic nonmetals,[117] bioelements,[118] central nonmetals,[119] CHNOPS,[120] essential elements,[121] "non-metals",[122][n 18] orphan nonmetals,[123] or redox nonmetals[124]
The descriptive phrase unclassified nonmetals is used here for convenience.
^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".[126]
^Such boundary fuzziness and overlap often occur in classification schemes.[127]
^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".[127]
^1. For aluminium, Whitten and Davis[133] write, "[It] is quite reactive , but a thin, transparent film of Al2O3 forms when Al comes into contact with air. This protects it from further oxidation For this reason it is even passive toward nitric acid (HNO3), a strong oxidizing agent. When the oxide coating is sanded off, Al reacts vigorously with HNO3." 2. For the transition metal manganese, Parish[134] writes: "Very reactive. Reacts with water." Russell & Lee[135] add, "Mn reacts with all mineral acids, and it even slowly dissociates pure water, liberating H2 gas as it forms Mn hydroxide."
^Xe is expected to be metallic at the pressures encountered in the Earth's core[143]
^Metal oxides are usually ionic.[150] On the other hand, oxides of metals with high oxidation states are usually either polymeric or covalent.[151] A polymeric oxide has a linked structure composed of multiple repeating units.[152]
^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[162]
^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.[167]
^ Ordinary matter – including the stars, planets, and all living creatures – constitutes less than 5% of the universe. The rest – dark energy and dark matter – is as yet poorly understood.[195]
^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".[212]
^Berzelius, who discovered selenium, thought it had the properties of a metal, combined with the properties of sulfur[217]
^Not to be confused with the modern usage of fossil to refer to the preserved remains, impression, or trace of any once-living thing
^The Goldhammer-Herzfeld ratio is roughly equal to the cube of the atomic radius divided by the molar volume.[241] 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.
^When Davy isolated sodium and potassium their low densities challenged the conventional wisdom that metals were ponderous substances. It was thus proposed to refer to them as metalloids, meaning "resembling metals in form or appearance".[261] This suggestion was ignored; the two new elements were admitted to the metal club in cognizance of their physical properties (opacity, luster, malleability, conductivity) and "their qualities of chemical combination".
Hare[262] observed that the line of demarcation between metals and nonmetals had been "annihilated" by the discovery of alkaline metals having a density less than that of water:
"Peculiar brilliance and opacity were in the next place appealed to as a means of discrimination; and likewise that superiority in the power of conducting heat and electricity ... Yet so difficult has it been to draw the line between metallic…and non-metallic ... that bodies which are by some authors placed in one class, are by others included in the other. Thus selenium, silicon, and zirconion [sic] have by some chemists been comprised among the metals, by others among non-metallic bodies." ...
^While antimony trioxide is usually listed as being amphoteric its very weak acid properties dominate over those of a very weak base[271]
^(a) Up to element 99 (Es), with the values taken from Aylward and Findlay.[272] (b) Weighable amounts of the extremely radioactive elements At (element 85), Fr (87), and elements with an atomic number higher than Es (99), have not been prepared.[273] (c) The density values used for At and Fr are theoretical estimates.[274] (d) A survey of definitions of the term "heavy metal" reported density criteria ranging from above 3.5 g/cm3 to above 7 g/cm3.[275] (e) Vernon specified a minimum electronegativity of 1.9 for the metalloids, on the revised Pauling scale[9] (f) Electronegativity values for the noble gases are from Rahm, Zeng and Hoffmann[276]
^A similar phenomenon applies more generally to certain Groups of the periodic table where, for example, the noble gases in Group 18 act as bridge between the nonmetals of the p-block and the metals of the s-block (Groups 1 and 2)[287]
^All four have less stable non-brittle forms:[298] carbon as exfoliated (expanded) graphite,[52][299] and as carbon nanotube wire;[54] phosphorus as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature);[55] sulfur as plastic sulfur;[56] and selenium as selenium wires[57]
^Metals have electrical conductivity values of from 6.9×103 S•cm−1 for manganese to 6.3×105 for silver[301]
^Metalloids have electrical conductivity values of from 1.5×10−6 S•cm−1 for boron to 3.9×104 for arsenic[302]
^Unclassified nonmetals have electrical conductivity values of from ca. 1×10−18 S•cm−1 for the elemental gases to 3×104 in graphite[97]
^The halogen nonmetals have electrical conductivity values of from ca. 1×10−18 S•cm−1 for F and Cl to 1.7×10−8 S•cm−1 for iodine[97][146]
^The elemental gases have electrical conductivity values of ca. 1×10−18 S•cm−1[97]
^Metalloids always give "compounds less acidic in character than the corresponding compounds of the [typical] nonmetals"[292]
^Arsenic trioxide reacts with sulfur trioxide, forming arsenic "sulfate" As2(SO4)3[311]
^NO 2, N 2O 5, SO 3, SeO 3 are strongly acidic[312]
^H2O, CO, NO, N2O are neutral oxides; CO and N2O are "formally the anhydrides of formic and hyponitrous acid, respectively viz. CO + H2O → H2CO2 (HCOOH, formic acid); N2O + H2O → H2N2O2 (hyponitrous acid)"[313]
^Metals that form glasses are: V; Mo, W; Al, In, Tl; Sn, Pb; Bi[317]
^Unclassified nonmetals that form glasses are P, S, Se;[317]CO2 forms a glass at 40 GPa[319]
^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[328]
^Values for the noble gases are from Rahm, Zeng and Hoffmann[276]
^Steudel 2020, p. 43: Steudel's monograph is an updated translation of the fifth German edition of 2013, incorporating the literature up to Spring 2019.
^Taniguchi et al. 1984, p. 867: "... black phosphorus ... [is] characterized by the wide valence bands with rather delocalized nature."; Morita 1986, p. 230; Carmalt & Norman 1998, p. 7: "Phosphorus ... should therefore be expected to have some metalloid properties."; Du et al. 2010. Interlayer interactions in black phosphorus, which are attributed to van der Waals-Keesom forces, are thought to contribute to the smaller band gap of the bulk material (calculated 0.19 eV; observed 0.3 eV) as opposed to the larger band gap of a single layer (calculated ~0.75 eV).
^Steudel 2000, p. 601: "... Considerable orbital overlap can be expected. Apparently, intermolecular multicenter bonds exist in crystalline iodine that extend throughout the layer and lead to the delocalization of electrons akin to that in metals. This explains certain physical properties of iodine: the dark color, the luster and a weak electric conductivity, which is 3400 times stronger within the layers then perpendicular to them. Crystalline iodine is thus a two-dimensional semiconductor."; Segal 1989, p. 481: "Iodine exhibits some metallic properties ..."
^Crawford 1968, p. 540; Benner, Ricardo & Carrigan 2018, pp. 167–168: "The stability of the carbon-carbon bond... has made it the first choice element to scaffold biomolecules. Hydrogen is needed for many reasons; at the very least, it terminates C-C chains. Heteroatoms (atoms that are neither carbon nor hydrogen) determine the reactivity of carbon-scaffolded biomolecules. In... life, these are oxygen, nitrogen and, to a lesser extent, sulfur, phosphorus, selenium, and an occasional halogen."
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