Nonmetal: Difference between revisions – Wikipedia

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Chemical element that mostly lacks the characteristics of a metal

Nonmetals in their periodic table context

  usually/always counted as a nonmetal

  sometimes counted as a nonmetal

  status as nonmetal or metal unconfirmed[n 1]

A nonmetal is a chemical element that mostly lacks distinctive metallic properties. Seventeen elements are widely recognized as nonmetals. This article also covers six borderline elements (the metalloids), some or all of which are sometimes considered nonmetals.
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 iodine. Nonmetals are often poor conductors of heat and electricity, and are brittle or crumbly as solids. Their oxides tend to be acidic.
The two lightest nonmetals, hydrogen and helium, together make up about 98% of the mass of the observable universe. Five nonmetallic elements—hydrogen, carbon, nitrogen, oxygen, and silicon—make up the overwhelming majority of Earth’s oceans, atmosphere, biosphere, and crust.
The great variability of nonmetal properties enable specific elements to have unique uses, for example, in electronics, energy storage, agriculture, and chemical production. Hydrogen, oxygen, carbon, and nitrogen are essential building blocks for life.
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 about two dozen properties have been suggested as single criteria for distinguishing nonmetals from metals.

Definition and applicable elements[edit]
Properties discussed in this article are those of the most stable form[n 2] of elements in ambient conditions unless otherwise stated.
Like carbon, arsenic (here sealed in a container to prevent tarnishing) vaporizes rather than melts when heated. The lemon-yellow vapor smells like garlic.[9] The chemistry of arsenic is predominately nonmetallic.[10]
Nonmetallic chemical elements generally have low density and high electronegativity. They lack most properties commonly associated with metals: shininess, malleability, ductility, and good thermal and electrical conductivity. When combined with oxygen, nonmetals tend to form acidic oxides (while metals usually form basic oxides).[11]
There is no widely-accepted precise definition of a nonmetal;[12] any list of such is open to debate and revision.[13] Which elements are included depends on the properties regarded as most representative of nonmetallic or metallic character.[n 3]

Fourteen elements are almost always recognized as nonmetals:[13][14]

Three more are commonly classed as nonmetals, but some sources list them as “metalloids”,[15] a term which refers to elements regarded as intermediate between metals and nonmetals:[16]
The six elements most commonly recognized metalloids have relatively low densities and predominantly nonmetallic chemistry; they are included in this article for comparison:

All together, about a fifth of the 118 known elements,[17] are classified as nonmetals.[18]

General properties[edit]
Physical properties of nonmetals[edit]

Metallic appearance of iodine under white lightLiquefied xenon
About half of nonmetallic elements are gases; most of the rest are shiny solids. Bromine, the only liquid, is so volatile that it is usually topped by a layer of its fumes; sulfur is the only colored solid nonmetal.[n 4] The gaseous and liquid nonmetals have very low densities, melting and boiling points, and are poor conductors of heat and electricity.[21] The solid elements have low densities and low mechanical and structural strength (being brittle or crumbly),[22] but a wide range of electrical conductivity.[n 5]
This variability in form stems from variability in 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.[26] In contrast, nonmetals that form giant structures, such as chains of up to 1,000 selenium atoms,[27] sheets of carbon atoms in graphite,[28] or three-dimensional lattices of silicon atoms[29] have higher melting and boiling points, and are all solids, as it takes more energy to overcome their stronger covalent bonds.[30] Nonmetals closer to the left or bottom of the periodic table (and so closer to the metals) often have some weak metallic interactions between their molecules, chains, or layers; this occurs in boron,[31] carbon,[32] phosphorus,[33] arsenic,[34] selenium,[35] antimony,[36] tellurium[37] and iodine.[38]
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.[39] 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”.[40][n 6] 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.[42]

Some general physicaldifferences between metals and nonmetals[21]

Aspect
Metals
Nonmetals

Appearanceand form

Shiny if freshly preparedor fractured; few colored;[43]all but one solid[44]

Shiny, colored ortransparent;[45] all butone solid or gaseous[44]

Density

Often higher

Often lower

Elasticity

Mostly malleableand ductile

Brittle if solid

Electricalconductivity[46]

Good

Poor to good

Electronicstructure[47]

Metallic or semimetallic

Semimetallic,semiconductor,or insulator

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.[48] In contrast, nonmetals share only the electrons required to achieve a noble gas electron configuration.[49] 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.[50]
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,[51] the electrons in nonmetals typically lack such mobility.[52] Among nonmetallic elements, good electrical and thermal conductivity is seen only in carbon (as graphite, along its planes), arsenic, and antimony.[n 7] Good thermal conductivity otherwise occurs only in boron, silicon, phosphorus, and germanium;[23] such conductivity is transmitted though vibrations of the crystalline lattices of these elements.[53] Moderate electrical conductivity is observed in boron, silicon, phosphorus, germanium, selenium, tellurium, and iodine.[n 8] Plasticity occurs under limited circumstances in carbon, as seen in exfoliated (expanded) graphite[55][56] and carbon nanotube wire,[57] in white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature),[58] in plastic sulfur,[59] and in selenium which can be drawn into wires from its molten state.[60]
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.[61]

Allotropes[edit]

Most nonmetals have two or more allotropes. Carbon allotropes include diamond, an electrical insulator and buckminsterfullerene, a semiconductor.
Over half of nonmetallic elements exhibit a range of less stable allotropic forms, each with distinct physical properties.[62] For example, carbon, the most stable form of which is graphite, can manifest as diamond, buckminsterfullerene,[63] and amorphous[64] and paracrystalline (mixed amorphous and crystalline)[65] variations. Allotropes also occur for nitrogen, oxygen, phosphorus, sulfur, selenium, the metalloids, and iodine.[66]

Chemical properties of nonmetals[edit]

Red fuming nitric acid: A nitrogen-rich compound, incorporating nitrogen dioxide (NO2), an acidic oxide used in the production of nitric acid

Nonmetals have relatively high values of electronegativity, and their oxides are therefore usually acidic. Exceptions may occur if a nonmetal is not very electronegative, or if its oxidation state is low, or both. These non-acidic oxides of nonmetals may be amphoteric (like water, H2O[72]) or neutral (like nitrous oxide, N2O[73][n 9]), but never basic (as is common with metals).
Nonmetals 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 electrode potentials than metals. Generally, the higher these values are (including electronegativity) the more nonmetallic the element tends to be.[76] For example, the chemically very active nonmetals fluorine, chlorine, bromine, and iodine have an average electronegativity of 3.19—a figure[n 10] higher than that of any individual metal. On the other hand, the 2.05 average[n 11] of the chemically weak metalloid nonmetals falls within the 0.70 to 2.54 range of metals.[71]
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.[77] Consequently, there is a corresponding reduction in atomic radius[78] as the heightened nuclear charge draws the outer electrons closer to the nucleus core.[79] 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.[80]
The number of compounds formed by nonmetals is vast.[81] 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.[82] A few examples of nonmetal compounds are: boric acid (H3BO3), used in ceramic glazes;[83] selenocysteine (C3H7NO2Se), the 21st amino acid of life;[84] phosphorus sesquisulfide (P4S3), found in strike anywhere matches;[85] and teflon ((C2F4)n), used to create non-stick coatings for pans and other cookware.[86]

Complications[edit]
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; multiple bond formation; and property overlaps with metals.

First row anomaly[edit]

Condensed periodic table highlighting the first row of each block

Period

s-block

1

H 1

He2

p-block

2

Li3

Be4

B 5

C 6

N 7

O 8

F 9

Ne10

3

Na11

Mg12

d-block

Al13

Si14

P 15

S 16

Cl17

Ar18

4

K 19

Ca20

Sc-Zn21-30

Ga31

Ge32

As33

Se34

Br35

Kr36

5

Rb37

Sr38

f-block

Y-Cd39-48

In49

Sn50

Sb51

Te52

I 53

Xe54

6

Cs55

Ba56

La-Yb57-70

Lu-Hg71-80

Tl81

Pb82

Bi83

Po84

At85

Rn86

7

Fr87

Ra88

Ac-No89-102

Lr-Cn103-112

Nh113

Fl114

Mc115

Lv116

Ts117

Og118

Group

(1)

(2)

(3-12)

(13)

(14)

(15)

(16)

(17)

(18)

The first-row anomaly strength by block is s>>p>d>f.[87]

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.[88] 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.[89] 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.”[90]
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.[91] 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.[92]
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.[93]
A relatively recent development involves certain compounds of heavier p-block elements, such as silicon, phosphorus, germanium, arsenic and antimony, exhibiting behaviors typically associated with transition metal complexes. This phenomenon is linked to a small energy gap between their filled and empty molecular orbitals, which are the regions in a molecule where electrons reside and where they can be available for chemical reactions. In such compounds, this closer energy alignment allows for unusual reactivity with small molecules like hydrogen (H2), ammonia (NH3), and ethylene (C2H4), a characteristic previously observed primarily in transition metal compounds. These reactions may open new avenues in catalytic applications.[94]

Secondary periodicity[edit]
Electronegativity values of the group 16 chalcogen elements showing a W-shaped alternation or secondary periodicity going down the group
An alternation in certain periodic trends, sometimes referred to as secondary periodicity, becomes evident when descending groups 13 to 15, and to a lesser extent, groups 16 and 17.[95][n 12] 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.[97]
The Soviet chemist Shchukarev [ru] gives two more tangible examples:[98]

“The toxicity of some arsenic compounds, and the absence of this property in analogous compounds of phosphorus [P] and antimony [Sb]; and the ability of selenic acid [H2SeO4] to bring metallic gold [Au] into solution, and the absence of this property in sufuric [H2SO4] and [H2TeO4] acids.”
Higher oxidation states[edit]
Some nonmetallic elements are able to exhibit oxidation states other than would be indicated by the octet rule, which ordinarily results in valency falling with group number that is –3, –2, –1, or 0. Such states occur in, for example, ammonia (NH3), hydrogen sulfide (H2S), hydrogen fluoride (HF), and elemental xenon (Xe). On the other hand, the maximum possible oxidation state increases from +5 in group 15, to +8 in group 18. The +5 oxidation state is found in period 2 onwards, for example in nitric acid (HNO3) and phosphorus pentafluoride (PCl5). Higher oxidation states in later groups occur only from period 3 onwards, for example, in sulfur hexafluoride (SF6), iodine heptafluoride (IF7), and xenon tetroxide (XeO4). For the heavier nonmetals, their larger atomic radii and lower electronegativity values enable higher bulk coordination numbers that better tolerate higher positive charges.[99]

Multiple bond formation[edit]
A further difference between period 2 elements and others, particularly carbon, nitrogen, and oxygen, lies in their propensity to form multiple bonds. The compounds formed by these elements often exhibit unique stoichiometries and structures, which are not commonly found in elements from later periods, such as the various nitrogen oxides.[99]

Property overlaps[edit]
Boron (here in its less stable amorphous form) shares some similarities with metals[n 13]
Molecular structure of pentazenium, a homopolyatomic cation of nitrogen with the formula N+5 and structure N−N−N−N−N.[101]
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,[102] Humphrey[103] 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:

silicon has an electronegativity (1.9) comparable with metals such as cobalt (1.88), copper (1.9), nickel (1.91) and silver (1.93);[71]
the electrical conductivity of graphite exceeds that of some metals;[n 14]
selenium can be drawn into a wire;[60]
radon is the most metallic of the noble gases and begins to show some cationic behavior, which is unusual for a nonmetal;[106] and
just over half of nonmetallic elements can form homopolyatomic cations;[n 15]
Examples of nonmetal-like properties occurring in metals are:

Tungsten displays some nonmetallic properties, being brittle, having a high electronegativity, forming only anions in aqueous solution,[108] and predominately acidic oxides.[109][110] These are characteristics more aligned with nonmetals. Even so, tungsten is still classified as a metal, illustrating the spectrum of behaviors elements can exhibit within their classifications.
Gold, the “king of metals” demonstrates several nonmetallic behaviors. It has the highest electrode potential among metals, suggesting a preference for gaining rather than losing electrons. Gold’s ionization energy is one of the highest among metals, and its electron affinity and electronegativity are high, with the latter exceeding that of some nonmetals. It forms the Au– auride anion and exhibits a tendency to bond to itself, behaviors which are unexpected for metals. In aurides (MAu, where M = Li–Cs), gold’s behavior is similar to halogens, bridging the traditional metal-nonmetal divide.[111]

Different alternative nonmetal classification schemes range from as few as two subtypes to as many as 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”.[112] On the other hand, seven of twelve color categories on the Royal Society of Chemistry periodic table include nonmetals.[113][n 16]

Starting on the right side of the periodic table, three types of nonmetals can be recognized:

  
the relatively inert noble gases;[114]

  
the notably reactive halogen nonmetals;[115] and

  
the mixed reactivity “unclassified nonmetals”, a set with no widely used collective name.[n 18]

The elements in a fourth set are sometimes recognized as nonmetals:

  
the generally unreactive[n 20] metalloids,[132] sometimes considered a third category distinct from metals and nonmetals.

The boundaries between these types are not sharp.[n 21] Carbon, phosphorus, selenium, and iodine border the metalloids and show some metallic character, as does hydrogen.
The greatest discrepancy between authors occurs in metalloid “frontier territory”.[134] Some consider metalloids distinct from both metals and nonmetals, while others classify them as nonmetals.[135] Some categorize certain metalloids as metals (e.g., arsenic and antimony due to their similarities to heavy metals).[136][n 22] Metalloids resemble the elements universally considered “nonmetals” in having relatively low densities, high electronegativity, and similar chemical behavior.[132][n 23]
For context, the metallic side of the periodic table also ranges widely in reactivity.[n 24] Highly reactive metals fill most of the s- and f-blocks on the left,[n 25] bleeding into the early part of the d-block. Thereafter, reactivity generally decreases closer to the p-block, whose metals are not particularly reactive.[n 26] The very unreactive noble metals, such as platinum and gold, are clustered in an island within the d-block.[142]

Noble gases[edit]

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.[114]
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.[143] As a consequence, they all exist as gases under standard conditions, even those with atomic masses surpassing many typically solid elements.[144]
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,[145] with most of these compounds involving the combination of oxygen or fluorine with either krypton, xenon, or radon.[146]

Halogen nonmetals[edit]

Highly reactive sodium metal (Na, left) combines with corrosive halogen nonmetal chlorine gas (Cl, center) to form stable, unreactive table salt (NaCl, right).
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); and food supplements (KI). The term “halogen” itself means “salt former”.[147]
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[148] solid. Electrically, the first three elements function as insulators while iodine behaves as a semiconductor (along its planes).[149]
Chemically, the halogen nonmetals exhibit high ionization energies, electron affinities, and electronegativity values, and are mostly relatively strong oxidizing agents.[150] These characteristics contribute to their corrosive nature.[151] All four elements tend to form primarily ionic compounds with metals,[152] in contrast to the remaining nonmetals (except for oxygen) which tend to form primarily covalent compounds with metals.[n 27] The highly reactive and strongly electronegative nature of the halogen nonmetals epitomizes nonmetallic character.[156]

Unclassified nonmetals[edit]

Selenium conducts electricity around 1,000 times better when light falls on it, a property used in light-sensing applications.[157]
After classifying the nonmetallic elements into noble gases and halogens, but before encountering the metalloids, there are seven nonmetals: 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 is a yellow solid (S). Electrically, graphitic carbon behaves as a semimetal along its planes[158] and a semiconductor perpendicular to its planes;[159] phosphorus and selenium are semiconductors;[160] while hydrogen, nitrogen, oxygen, and sulfur are insulators.[n 28]
These elements, often considered too diverse to merit a collective name,[162] have been referred to as other nonmetals,[163] or simply as nonmetals.[164] As a result, their chemistry is typically taught disparately, according to their respective periodic table groups:[165] 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 29]
Hydrogen, in particular, behaves in some respects like a metal and in others like a nonmetal.[167] Like a metal it can, for example, form a solvated cation in aqueous solution;[168] 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;[169] and it can form alloy-like hydrides with some transition metals.[170] 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.[171] It attains this configuration by forming a covalent or ionic bond[172] or, if it has initially given up its electron, by attaching itself to a lone pair of electrons.[173]
Some or all of these nonmetals share several properties. Being generally less reactive than the halogens,[174] most of them can occur naturally in the environment.[175] They have significant roles in biology[176] and geochemistry.[162] Collectively, their physical and chemical characteristics can be described as “moderately non-metallic”.[162] However, they all have corrosive aspects. Hydrogen can corrode metals. Carbon corrosion can occur in fuel cells.[177] 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.[178] Untreated selenium in soils can lead to the formation of corrosive hydrogen selenide gas.[179] When combined with metals, the unclassified nonmetals can form high-hardness (interstitial or refractory) compounds[180] due to their relatively small atomic radii and sufficiently low ionization energies.[162] They also exhibit a tendency to bond to themselves, particularly in solid compounds.[181] Additionally, diagonal periodic table relationships among these nonmetals mirror similar relationships among the metalloids.[182]
Unclassified nonmetals are typically found in elemental forms or in association with other elements:[183]

Metalloids[edit]

The six elements more commonly recognized as metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium, all of which have a metallic appearance. (Other elements appearing less commonly on lists of metalloids include carbon, aluminium, selenium and polonium. These have both metallic and nonmetallic properties, but one or the other predominates.) In the periodic table, metalloids 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.[15]
Metalloids 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 band structure of semimetals, although both have less stable semiconducting allotropes: arsenic as arsenolamprite, an extremely rare natually occurring form;[184] and antimony in its synthetic thin-film amorphous form.[15][185]
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.[15]

Abundance, sources, and uses[edit]
Abundance of nonmetallic elements[edit]

Approximate composition (by weight) ofprimary components and next most abundant

Universe[186]
H 70.5%, He 27.5%
O 1%

Atmosphere[187]
N 78%, O 21%
Ar 0.5%

Hydrosphere[187]
O 66.2%, H 33.2%
Cl 0.3%

Biomass[188]
O 63%, C 20%, H 10%
N 3.0%

Crust[187]
O 61%, Si 20%
H 2.9%

Hydrogen and helium dominate the observable universe, making up an estimated 98% of all ordinary matter by mass.[n 30] Oxygen, the next most abundant element, accounts for about 1%.[190]
Five nonmetals—hydrogen, carbon, nitrogen, oxygen, and silicon—form the vast majority of the directly observable structure of the earth: about 84% of the crust, 96% of the biomass, and over 99% of the atmosphere and hydrosphere, as shown in the accompanying table.[191]

Sources of nonmetallic elements[edit]

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 from a variety of raw materials.[175]

   From
mining byproducts:
germanium (from zinc ores); arsenic (copper and lead ores); selenium and tellurium (copper ores); and radon (uranium-bearing ores).

   From liquid air:
nitrogen, oxygen, neon, argon, krypton, and xenon.[n 33]

   From seawater brine:
chlorine, bromine, and iodine.

Uses of nonmetallic elements[edit]

Nearly all nonmetals have uses in:[196][197]Household goods, lighting and lasers, and medicine and pharmaceuticals

Most nonmetals have uses in:[196][198]Agrochemicals, dyestuffs and smart phones

Some nonmetals have uses in or as:[196][199]Alloys, cryogenics and refrigerants, explosives, fire retardants, fuel cells, inert air replacements, insulation (thermal & electric), mineral acids, nuclear control rods, photography, plastics, plug-in hybrid vehicles, solar cells, water treatment, welding gases, and vulcanization

Metalloids have uses in:[200]Alloys, ceramics, oxide glasses, solar cells, and semiconductors

The great variety of physical and chemical properties of nonmetals[201] enable a wide range of natural and technological uses as shown in the accompanying table. In living organisms, hydrogen, oxygen, carbon, and nitrogen serve as the foundational building blocks of life.[202] Some key technological uses of nonmetallic elements are in lighting and lasers, medicine and pharmaceuticals, and ceramics and plastics.
Some specific uses of later-discovered or rarer nonmetallic elements include:

Germanium, thought to be a metal up until the 1930s,[208] 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.[209]

Radon, the rarest noble gas,[212] 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.[213] Radon has now been replaced by sources of 137Cs, 192Ir, and 103Pd.[214]

History, background, and taxonomy[edit]
Discovery[edit]

The Alchemist Discovering Phosphorus (1771) by Joseph Wright. The alchemist is Hennig Brand; the glow emanates from the combustion of phosphorus inside the flask.
Although most nonmetallic elements were identified during the 18th and 19th centuries, a few were recognized much earlier. Carbon, sulfur, and antimony were known in antiquity. Arsenic was discovered in the Middle Ages (credited to Albertus Magnus) and phosphorus in 1669 (isolated from urine by Hennig Brand). Helium, identified in 1868, is the only element not initially discovered on Earth itself.[n 34] The most recently identified nonmetal is radon, detected at the end of the 19th century.[175]
Nonmetals were first isolated using a range of chemical and physical techniques, including spectroscopy, fractional distillation, radiation detection, electrolysis, ore acidification, displacement reactions, combustion, and controlled heating processes. Some nonmetals occur naturally 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.[216]
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[217] lost their lives in their pursuit of isolating fluorine.[218]
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.[219] Sulfur occurred naturally as a free element, simplifying its isolation. Selenium,[n 35] was first identified as a residue in sulfuric acid.[221]
Most metalloids were first isolated by heating their oxides (boron, silicon, arsenic, tellurium) or a sulfide (germanium).[175] Antimony, first obtained by heating its sulfide, stibnite, was later discovered in native form.[222]
Origin and use of the term[edit]
Greek philosopher and polymath Aristotle (384–322 BCE) categorized substances found within the Earth into two distinct groups: metals and “fossiles”.French nobleman and chemist Antoine Lavoisier (1743–1794) divided the elements into gases, metallic substances, nonmetallic substances, and earths.
An extract from the English translation of Lavoisier’s Traité élémentaire de chimie (1789),[223] 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 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 36] 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”.[224]
Until the Middle Ages the classification of minerals remained largely unchanged, albeit with varying terminology. In the fourteenth century, the English alchemist 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. The second category, labeled “minor minerals”, encompassed substances like salts, atramenta (iron sulfate), alums, vitriol, arsenic, orpiment, sulfur, and similar substances that were not metallic bodies.[225]
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.[226]
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.[227]
The pivotal moment in the systematic classification of chemical elements, distinguishing between metallic and nonmetallic substances, came in 1789 with the work of Antoine Lavoisier, a French chemist. He published the first modern list of chemical elements in his revolutionary[228] Traité élémentaire de chimie. The elements were categorized into distinct groups, including gases, metallic substances, nonmetallic substances, and earths (heat-resistant oxides).[229] 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.[230]
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”[231] to describe nonmetallic elements, noting their ability to form negatively charged ions with oxygen in aqueous solutions.[232][233] While Berzelius’ terminology gained significant acceptance,[234] it later faced criticism from some who found it counterintuitive,[233] misapplied,[235] or even invalid.[236][237] In 1864, reports indicated that the term “metalloids” was still endorsed by leading authorities,[238] but there were reservations about its appropriateness. The idea of designating elements like arsenic as metalloids had been considered.[238] By as early as 1866, some authors began preferring the term “nonmetal” over “metalloid” to describe nonmetallic elements.[239] In 1875, Kemshead[240] 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[edit]

In 1809, the British chemist and inventor Humphry Davy made a groundbreaking discovery that reshaped the understanding of metals and nonmetals.[263] When he isolated sodium and potassium, their low densities but metallic appearance challenged the conventional wisdom that metals were dense substances.[264] Sodium and potassium, on the contrary, floated on water.[n 38] Nevertheless, their classification as metals was firmly established by their distinct chemical properties.[267]
As early as 1811, attempts were made 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 year of mention and type.
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.[253] 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.[268] Similarly, despite its common classification as a nonmetal, when carbon (as graphite) is heated it experiences a decrease in electrical conductivity.[269] Arsenic and antimony, which are occasionally classified as nonmetals, show behavior similar to carbon, highlighting the complexity of the distinction between metals and nonmetals.[270]
Kneen and colleagues[271] 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.
Emsley[272] 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[273] emphasized that classification systems typically rely on more than two attributes to define distinct types.
Johnson[274] distinguished between metals and nonmetals on the basis of their physical states, electrical conductivity, mechanical properties, and the acid-base nature of their oxides:

gaseous elements are nonmetals (H, N, O, F, Cl and the noble gases);
liquids (Hg, Br) are either metallic or nonmetallic: Hg, as a good conductor, is a metal; Br, with its poor conductivity, is a nonmetal;
solids are either ductile and malleable, hard and brittle, or soft and crumbly:
a. ductile and malleable elements are metals;
b. hard and brittle elements include B, Si and Ge, which are semiconductors and therefore not metals; and
c. soft and crumbly elements include C, P, S, As, Sb,[n 39] Te and I, which have acidic oxides indicative of nonmetallic character.[n 40]

Several authors[279] have noted that nonmetals generally have low densities and high electronegativity. The accompanying table, using a threshold of 7 g/cm3 for density and 1.9 for electronegativity (revised Pauling), shows that all nonmetals have low density and high electronegativity. In contrast, all metals have either high density or low electronegativity (or both). Goldwhite and Spielman[280] 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 1.97 compared to 1.66 for the metals having densities of more than 7 gm/cm3.
Some authors divide elements into metals, metalloids, and nonmetals, but Oderberg[281] disagrees, arguing that by the principles of categorization, anything not classified as a metal should be considered a nonmetal.

Development of types[edit]
Bust of Dupasquier (1793–1848) in the Monument aux Grands Hommes de la Martinière [fr] in Lyon, France.
In 1844, Alphonse Dupasquier [fr], a French doctor, pharmacist, and chemist,[282] established a basic taxonomy of nonmetals to aid their study. He wrote:[283]

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 four “sections” parallel the modern nonmetal types. The organogens and sulphuroids are akin to the unclassified nonmetals. The chloroides were later called halogens.[284] The boroids eventually evolved into the metalloids, with this classification beginning as early as 1864.[238] The then-unknown noble gases were recognized as a distinct nonmetal group after being discovered in the late 1800s.[285]
Dupasquier’s taxonomy was commended for its natural basis, contrasting it with the artificial systems of that period.[286][n 42] That said, it represented a significant departure from other contemporary classifications, since it grouped together oxygen, nitrogen, hydrogen, and carbon.[288]
In 1828 and 1859, Dumas classified nonmetals as (1) hydrogen; (2) fluorine to iodine; (3) oxygen to sulfur; (4) nitrogen to arsenic; and (5) carbon, boron and silicon;[289] anticipating the vertical groupings of Mendeleev’s 1871 periodic table. Dumas’ five classes fall in modern day groups 1, 17, 16, 15 and 14-plus-13 respectively.

Classification of metalloids[edit]
Germanium, first thought to be a poorly conducting metal due to the presence of impuritiesBoron and silicon were recognized early on as nonmetals[n 43] but germanium, arsenic, antimony and tellurium have a more complicated history. Germanium, first regarded as a poorly conducting metal due to the presence of impurities, came to be understood as semiconductor in the 1930s with developments in physics.[208] It susbequently came to be regarded as a metalloid. While Mendeleev counted arsenic and antimony as metals in 1897,[291] arsenic had earlier been considered more suited to being counted as a metalloid.[238] Tellurium likely acquired an “ium” suffix due to its metallic appearance,[292] but Mendeleev said it represented a transition between metals and nonmetals, reflecting an evolving understanding of these elements.[293]
With their metallic appearance and nonmetallic chemistry recognized very early[294] metalloids came to be regarded as intermediate elements. In his classic and influential 1947 textbook[295] General chemistry: An introduction to descriptive chemistry and modern chemical theory, Pauling described these six and polonium as “elements with intermediate properties.”[296] He said they were in the center of his electronegativity scale, with values close to 2.[n 44] The emergence of the semiconductor industry and solid-state electronics in the 1950s and 1960s highlighted the semiconducting properties of germanium and silicon (and boron and tellurium), reinforcing the idea that metalloids were “in-between” or “half-way” elements.[298] Writing in 1982, Goldsmith[299] observed that, “The newest approach is to emphasize aspects of their physical and/or chemical nature such as electronegativity, crystallinity, overall electronic nature and the role of certain metalloids as semiconductors.”

Comparison of selected properties[edit]
The tables in this section describe ambient condition properties of five types of elements (noble gases, halogen nonmetals, unclassified nonmetals, metalloids, and, for comparison, metals), based on their most stable forms.
The aim is to show that most properties display a left-to-right progression in metallic-to-nonmetallic character or average values.[300][301] Some overlap occurs as outlier elements of each type exhibit less-distinct, hybrid-like, or atypical properties.[302][n 45] 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.[304]
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 properties of nonmetals by type[edit]

Physical properties are presented in loose order of ease of their determination.

Property

Element type

Metals

Metalloids

Unc. nonmetals

Halogen nonmetals

Noble gases

General physical appearance

lustrous[21]

lustrous[305]

◇ lustrous: C, P, Se[306]
◇ colorless: H, N, O[307]
◇ colored: S[308]

◇ colored: F, Cl, Br[309]
◇ lustrous: I[15]

colorless[310]

Form and density[311]

solid (Hg liquid)

solid

solid or gas

solid or gas (Br liquid)

gas

often high density such as Fe, Pb, W

low to moderately high density

low density

low density

low density

some light metals including Be, Mg, Al

all lighter than Fe

H, N lighter than air[312]

He, Ne lighter than air[313]

Elasticity

mostly malleable and ductile[21]

brittle[305]

C, black P, S, Se brittle[n 46]

iodine is brittle[316]

not applicable

Electrical conductivity

good[n 47]

◇ moderate: B, Si, Ge, Te
◇ good: As, Sb[n 48]

◇ poor: H, N, O, S
◇ moderate: P, Se
◇ good: C[n 49]

◇ poor: F, Cl, Br
◇ moderate: I[n 50]

poor[n 51]

Electronic structure[47]

metallic (Be, Sr, α-Sn, Yb, Bi are semimetals)

semimetal (As, Sb) or semiconductor

◇ semimetal: C
◇ semiconductor: P, Se
◇ insulator: H, N, O, S

semiconductor (I) or insulator

insulator

Chemical properties of nonmetals by type[edit]

Chemical properties are listed from general characteristics to more specific details.

Property

Element type

Metals

Metalloids

Unc. nonmetals

Halogen nonmetals

Noble gases

General chemical behavior

weakly nonmetallic[n 52]

moderately nonmetallic[301]

strongly nonmetallic[321]

◇ inert to nonmetallic[322]
◇ Rn shows some cationic behavior[323]

Oxides

basic; some amphoteric or acidic[109]

amphoteric or weakly acidic[324][n 53]

acidic[n 54] or neutral[n 55]

acidic[n 56]

metastable XeO3 is acidic;[329] stable XeO4 strongly so[330]

few glass formers[n 57]

all glass formers[332]

some glass formers[n 58]

no glass formers reported

no glass formers reported

ionic, polymeric, layer, chain, and molecular structures[334]

polymeric in structure[335]

◇ mostly molecular[335]
◇ C, P, S, Se have 1+ polymeric form

◇ mostly molecular
◇ iodine has 1+ polymeric form, I2O5[336]

◇ mostly molecular
◇ XeO2 is polymeric[337]

Compounds with metals

alloys[21] or intermetallic compounds[338]

tend to form alloys or intermetallic compounds[339]

◇ salt-like to covalent: H†, C, N, P, S, Se[16]
◇ mainly ionic: O[340]

mainly ionic[152]

simple compounds in ambient conditions not known[n 59]

Ionization energy (kJ mol−1)[70] ‡

low to high

moderate

moderate to high

high

high to very high

376 to 1,007

762 to 947

941 to 1,402

1,008 to 1,681

1,037 to 2,372

average 643

average 833

average 1,152

average 1,270

average 1,589

Electronegativity (Pauling)[n 60][71] ‡

low to high

moderate

moderate to high

high

high (Rn) to very high

0.7 to 2.54

1.9 to 2.18

2.19 to 3.44

2.66 to 3.98

ca. 2.43 to 4.7

average 1.5

average 2.05

average 2.65

average 3.19

average 3.3

† Hydrogen can also form alloy-like hydrides[170]‡ The labels low, moderate, high, and very high are arbitrarily based on the value spans listed in the table

See also[edit]

^ Astatine (At) is frequently ignored in the literature due to its rarity and extreme radioactivity.[1] As a halogen it has usually been presumed to be a nonmetal.[2] Chemically, studies on trace quantities of At, which are not necessarily reliable,[3] have demonstrated characteristics of both metals and nonmetals.[4] Alternatively, given the near-metallic character of its lighter congener iodine (a 2D semiconductor with a metallic appearance, showing evidence of delocalized electrons),[5] a succession of authors have suggested At may be a metal.[6] A 2013 study based on relativistic chemistry concluded that it would be a monatomic metal with a close-packed crystalline structure,[7] but this has not been experimentally verified. In a like manner, any of copernicium (Cn), flerovium (Fl), and oganesson (Og) may be nonmetals.[8] These four elements are not further considered in this article.

^ The primary form referenced in this article are:

All other nonmetallic elements have but one allotrope.

^ Metallic or nonmetallic character has often been taken to be indicated by a single property rather than two or more.

^ Solid iodine has a silvery metallic appearance under white light at room temperature.[19] It sublimes at ordinary and higher temperatures, passing from solid to gas; its vapours are violet-colored.[20]

^ The solid nonmetals have electrical conductivity values ranging from 10−18 S•cm−1 for sulfur[23] to 3 × 104 in graphite[24] or 3.9 × 104 for arsenic;[25] cf. 0.69 × 104 for manganese to 63 × 104 for silver, both metals.[23] 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[41]

^ Thermal conductivity values for metals range from 6.3 W m−1 K−1 for neptunium to 429 for silver; cf. antimony 24.3, arsenic 50, and carbon 2000.[23] Electrical conductivity values of metals range from 0.69 S•cm−1 × 104 for manganese to 63 × 104 for silver; cf. carbon 3 × 104,[24] arsenic 3.9 × 104 and antimony 2.3 × 104.[23]

^ These elements being semiconductors.[54]

^ 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−);[74] and in water, NO reacts with oxygen to form nitrous acid HNO2 (4NO + O2 + 2H2O → 4HNO2).[75]

^ F−I: 3.98 + 3.16 + 2.96 + 2.66 = 12.76/4 = 3.19

^ B−Te: 2.04 + 1.9 + 2.01 + 2.18 + 2.05 + 2.1 = 12.28/6 = 2.04

^ The net result is an even-odd difference between periods (except in the s-block): 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.[96]

^ Greenwood[100] commented that: “The extent to which metallic elements mimic boron (in having fewer electrons than orbitals available for bonding) has been a fruitful cohering concept in the development of metalloborane chemistry … Indeed, metals have been referred to as “honorary boron atoms” or even as “flexiboron atoms”. The converse of this relationship is clearly also valid …”

^ For example, the conductivity of graphite is 3 × 104 S•cm−1[104] whereas that of manganese is 6.9 × 103 S•cm−1[105]

^ 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.[107].

^ Of the twelve categories in the Royal Society periodic table, five only show up with the metal filter, three only with the nonmetal filter, and four with both filters. Interestingly, the six elements marked as metalloids (B, Si, Ge, As, Sb, and Te) show under both filters. Six other elements (113–120: Nh, Fl, Mc, Lv, Ts, and Og), whose status is unknown, also show up under both filters but are not included in any of the twelve color categories.

^ 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,[116] bioelements,[117] central nonmetals,[118] CHNOPS,[119] essential elements,[120] “non-metals”,[121][n 17] orphan nonmetals,[122] or redox nonmetals[123]
The descriptive phrase unclassified nonmetals is used here for convenience.

^ Arsenic is stable in dry air. Extended exposure in moist air results in the formation of a black surface coating. “Arsenic is not readily attacked by water, alkaline solutions or non-oxidizing acids”.[127] It can occasionally be found in nature in an uncombined form[128] It has a positive standard reduction potential (As → As3+ + 3e = +0.30 V), corresponding to a classification of semi-noble metal.[129]

^ “Crystalline boron is relatively inert.” Silicon “is generally highly unreactive.”[124] “Germanium is a relatively inert semimetal.”[125] “Pure arsenic is also relatively inert.”[126][n 19] “Metallic antimony is … inert at room temperature.”[130] “Compared to S and Se, Te has relatively low chemical reactivity.”[131]

^ Such boundary fuzziness and overlaps often occur in classification schemes.[133]

^ 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”.[133]

^ For a related comparison of the properties of metals, metalloids, and nonmetals, see Rudakiya & Patel (2021), p. 36

^ Thus, Weller at al.[137] write, “Those [elements] classified as metallic range from the highly reactive sodium and barium to the noble metals, such as gold and platinum. The nonmetals… encompass… the aggressive, highly-oxidizing fluorine and the unreactive gases such as helium.” On a related note, Beiser[138] adds, “Across each period is a more or less steady transition from an active metal through less active metals and weakly active non-metals to highly active nonmetals and finally to an inert gas.”

^ In a full-width periodic table the f-block is located between the s- and d-blocks.

^ For a p-block metal, aluminium can be quite reactive if its thin and transparent protective surface coating of Al2O3 is removed.[139] Aluminium is adjacent to the highly reactive s-block metal magnesium, as period 3 lacks f- or d-block elements. Magnesium too has “a very adherent thin film of oxide which protects the underlying metal from attack.”[140] Thallium, a p-block metal, is unaffected by water or alkalis but is attacked by acids, and is slowly oxidized in room temperature air.[141]

^ Metal oxides are usually ionic.[153] On the other hand, oxides of metals with high oxidation states are usually either polymeric or covalent.[154] A polymeric oxide has a linked structure composed of multiple repeating units.[155]

^ 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[161]

^ For example, Wulfsberg divides the nonmetals, based on their Pauling electronegativity, into very electronegative nonmetals (over 2.8: N, O, F, Cl, and Br) and electronegative nonmetals (1.9–2.8: H, B, C, Si, P, S, Ge, As, Se, Sb, Te, I, and Xe). He susbequently compares the two types on the basis of their standard reduction potentials. The remaining noble gases (He, Ne, Ar, Kr and Rn) are not allocated as they lack standard reduction potentials and, on this basis, cannot be compared to the other very electronegative and electronegative nonmetals. However, on the basis of their listed electronegativity values (p. 37), He, Ne, Ar and Kr would very electronegative nonmetals and Rn would be an electronegative nonmetal. The nonmetals B, Si, Ge, As, Se, Sb, and Te are additionally recognized by him as metalloids.[166]

^ Ordinary baryonic 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.[189]

^ Exceptionally, a study reported in 2012 noted the presence of 0.04% native fluorine (F2) by weight in antozonite, attributing these inclusions to radiation from tiny amounts of uranium.[192]

^ Xe is expected to be metallic at the pressures encountered in the Earth’s core[194]

^ About 1015 tonnes of noble gases are present in the Earth’s atmosphere.[193] In the Earth’s core there may be around 1013 tons of xenon, in the form of stable XeFe3 and XeNi3 intermetallic compounds.[n 32] This could explain why “studies of the Earth’s atmosphere have shown that more than 90% of the expected amount of Xe is depleted.”[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”.[215]

^ Berzelius, who discovered selenium, thought it had the properties of a metal, combined with the properties of sulfur[220]

^ 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.[244] 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.

^ It was thus proposed to refer to them as metalloids, meaning “resembling metals in form or appearance”.[265] 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[266] 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[275]

^ Johnson counted B as a nonmetal and Si, Ge, As, Sb, Te, Po and At as “semimetals” i.e. metalloids

^ (a) Up to element 99 (einsteinium) except for 85 and 87 (astatine and francium), with the values taken from Aylward and Findlay.[276]

(b) A survey of definitions of the term “heavy metal” reported density criteria ranging from above 3.5 g/cm3 to above 7 g/cm3.[277]

(c) Vernon specified a minimum electronegativity of 1.9 for the metalloids, on the revised Pauling scale[15]

(d) Electronegativity values for the noble gases are from Rahm, Zeng and Hoffmann[278]

^ A natural classification was based on “all the characters of the substances to be classified as opposed to the ‘artificial classifications’ based on one single character” such as the affinity of metals for oxygen. “A natural classification in chemistry would consider the most numerous and most essential analogies.”[287]

^ Both were initially isolated in their impure or amorphous forms; the pure crystalline, metallic-looking forms were isolated later.[290]

^ Pauling’s electronegativity scale ran from 0.7 to 4, giving a 2.35 midpoint. The electronegativity values of his metalloids spanned 1.9 for Si to 2.1 for Te. The unclassified nonmetals spanned 2.1 for H to 3.5 for O.[297]

^ 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)[303]

^ All four have less stable non-brittle forms:[314] carbon as exfoliated (expanded) graphite,[55][315] and as carbon nanotube wire;[57] phosphorus as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature);[58] sulfur as plastic sulfur;[59] and selenium as selenium wires[60]

^ Metals have electrical conductivity values of from 6.9×103 S•cm−1 for manganese to 6.3×105 for silver[317]

^ Metalloids have electrical conductivity values of from 1.5×10−6 S•cm−1 for boron to 3.9×104 for arsenic[318]

^ Unclassified nonmetals have electrical conductivity values of from ca. 1×10−18 S•cm−1 for the elemental gases to 3×104 in graphite[104]

^ 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[104][149]

^ The elemental gases have electrical conductivity values of ca. 1×10−18 S•cm−1[104]

^ Metalloids always give “compounds less acidic in character than the corresponding compounds of the [typical] nonmetals”[305]

^ Arsenic trioxide reacts with sulfur trioxide, forming arsenic “sulfate” As2(SO4)3[325]

^ NO2, N2O5, SO3, SeO3 are strongly acidic[326]

^ 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)”[327]

^ ClO2, Cl2O7, I2O5 are strongly acidic[328]

^ Metals that form glasses are: V; Mo, W; Al, In, Tl; Sn, Pb; Bi[331]

^ Unclassified nonmetals that form glasses are P, S, Se;[331] CO2 forms a glass at 40 GPa[333]

^ 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[341]

^ Values for the noble gases are from Rahm, Zeng and Hoffmann[278]

References[edit]
Citations[edit]

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^ Wiberg 2001, p. 1279

^ Encyclopaedia Britannica 2021

^ Royal Society of Chemistry 2021

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^ Kernion 2019, p. 191; Cao et al. 2021, pp. 20–21; Hussain et al. 2023; also called “nonmetal halogens”: Chambers & Holliday 1982, pp. 273–274; Bohlmann 1992, p. 213; Jentzsch 2015, p. 247 or “stable halogens”: Vassilakis, Kalemos & Mavridis 2014, p. 1; Hanley & Koga 2018, p. 24; Kaiho 2017, ch. 2, p. 1

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^ Fraústo da Silva & Williams 2001, p. 500: H, C, N, O, S, Se

^ Graves 2022

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^ Zhao, Tu & Chan 2021

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^ Burke 2020, p. 262; Csele 2016; Imberti & Sadler 2020, p. 8

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^ Bache 1832, pp. 248–276

^ Renouf 1901, pp. 268

^ Bertomeu-Sánchez et al. 2002, p. 249

^ Bertomeu-Sánchez et al. 2002, p. 249

^ Hoefer 1845, p. 85

^ Dumas 1828; Dumas 1859

^ Emsley 2011, pp. 80, 485

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^ Emsley 2011, p. 530

^ Mendeléeff 1897, p. 274

^ Newth 1894, pp. 7–8; Friend 1914, p.9: “Usually, the metalloids possess the form or appearance of metals, but are more closely allied to the non-metals in their chemical behaviour.”

^ [[#Lundgren2000|Lundgren & Bensaude-Vincent 2000, p. 409]; Greenberg 2007, p. 562

^ Pauling 1947, pp. 65, 160

^ Pauling 1947, p. 160

^ Chedd 1969

^ Goldsmith 1982

^ Vernon 2020, pp. 217–225

^ a b Welcher 2009, p. 3–32: “The elements change from … metalloids, to moderately active nonmetals, to very active nonmetals, and to a noble gas.”

^ Vernon 2020, pp. 224

^ MacKay, MacKay & Henderson 2002, pp. 195–196

^ Bynum, Browne & Porter 1981, p. 318

^ a b c Rochow 1966, p. 4

^ Wiberg 2001, p. 780; Emsley 2011, p. 397; Rochow 1966, pp. 23, 84

^ Kneen, Rogers & Simpson 1972, pp. 321, 404, 436

^ Kneen, Rogers & Simpson 1972, p. 439

^ Kneen, Rogers & Simpson 1972, p. 465

^ Kneen, Rogers & Simpson 1972, p. 308

^ Tregarthen 2003, p. 10

^ Lewis 1993, pp. 28, 827

^ Lewis 1993, pp. 28, 813

^ Wiberg 2001, pp. 505, 681, 781; Glinka 1958, p. 355

^ Godfrin & Lauter 1995, pp. 216‒218

^ Wiberg 2001, p. 416

^ Desai, James & Ho 1984, p. 1160; Matula 1979, p. 1260

^ Schaefer 1968, p. 76; Carapella 1968, pp. 29‒32

^ Kneen, Rogers & Simpson 1972, p. 264

^ Rayner-Canham 2018, p. 203

^ Mackin 2014, p. 80

^ Johnson 1966, pp. 105–108

^ Stein 1969, pp. 5396‒5397; Pitzer 1975, pp. 760‒761

^ Rochow 1966, p. 4; Atkins et al. 2006, pp. 8, 122–123

^ Wiberg 2001, p. 750

^ Sanderson 1967, p. 172; Mingos 2019, p. 27

^ House 2008, p. 441

^ Mingos 2019, p. 27; Sanderson 1967, p. 172

^ Wiberg 2001, p. 399

^ Kläning & Appelman 1988, p. 3760

^ a b Rao 2002, p. 22

^ Sidorov 1960, pp. 599‒603

^ McMillan 2006, p. 823

^ Wells 1984, p. 534

^ a b Puddephatt & Monaghan 1989, p. 59

^ King 1995, p. 182

^ Ritter 2011, p. 10

^ Yamaguchi & Shirai 1996, p. 3

^ Vernon 2020, p. 223

^ Woodward et al. 1999, p. 134

^ Dalton 2019

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