Nonmetals are
chemical elements that mostly lack distinctive
metallic properties. They range from colorless gases like
hydrogen to shiny crystals like
iodine. Physically, they are usually lighter (less dense) than metals; brittle or crumbly if solid: and often poor conductors of
heat and
electricity. Chemically, nonmetals have high
electronegativity (meaning they usually attract electrons in a chemical bond); and their oxides tend to be
acidic.
Seventeen elements are widely recognized as nonmetals. Additionally, some or all of six borderline elements (
metalloids) are sometimes counted as nonmetals.
The diverse properties of nonmetals enable a range of natural and technological uses. Hydrogen, oxygen, carbon, and nitrogen are essential building blocks for life. In technology, nonmetals are used in, for example,
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 over twenty properties have been suggested as criteria for distinguishing nonmetals from metals.
Definition and applicable elements
Unless otherwise noted, this article describes the most stable form of an element in ambient conditions.[a]
Nonmetallic
chemical elements are generally described as lacking properties common to metals, namely shininess, pliability, good thermal and electrical conductivity, and a general capacity to form basic oxides.[2][3] There is no widely-accepted precise definition;[4] any list of nonmetals is open to debate and revision.[5] The elements included depend on the properties regarded as most representative of nonmetallic or metallic character.
Fourteen elements are almost always recognized as nonmetals:[5][6]
Three more are commonly classed as nonmetals, but some sources list them as "
metalloids",[7] a term which refers to elements regarded as intermediate between metals and nonmetals:[8]
Nonmetals vary greatly in appearance, being colorless, colored or shiny.
For the colorless nonmetals (hydrogen, nitrogen, oxygen, and the noble gases), their electrons are held sufficiently strongly so that no absorption of light happens in the visible part of the spectrum, and all visible light is transmitted.[12]
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".[13][c] The shininess of boron, graphitic carbon, silicon, black phosphorus, germanium, arsenic, selenium, antimony, tellurium, and iodine[d] is a result of their structures featuring varying degrees of delocalized (free-moving) electrons that scatter incoming visible light.[16]
About half of nonmetallic elements are gases; most of the rest are solids. Bromine, the only liquid, is so volatile that it is usually topped by a layer of its fumes. The gaseous and liquid nonmetals have very low densities,
melting and
boiling points, and are poor conductors of heat and electricity.[17] The solid nonmetals have low densities and low mechanical and structural strength (being brittle or crumbly),[18] but a wide range of electrical conductivity.[e]
This diversity 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.[22] In contrast, nonmetals that form giant structures, such as chains of up to 1,000 selenium atoms,[23] sheets of carbon atoms in graphite,[24] or three-dimensional lattices of silicon atoms[25] have higher melting and boiling points, and are all solids, as it takes more energy to overcome their stronger
covalent bonds.[26] 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,[27] carbon,[28] phosphorus,[29] arsenic,[30] selenium,[31] antimony,[32] tellurium[33] and iodine.[34]
Some general physical differences between metals and nonmetals[17]
Aspect
Metals
Nonmetals
Appearance and form
Shiny if freshly prepared or fractured; few colored;[35] all but one solid[36]
Shiny, colored or transparent;[37] all but one solid or gaseous[36]
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.[40] In contrast, nonmetals share only the electrons required to achieve a noble gas electron configuration.[41] 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. 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.[42]
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,[43] the electrons in nonmetals typically lack such mobility.[44] Among nonmetallic elements, good electrical and thermal conductivity is seen only in carbon (as graphite, along its planes), arsenic, and antimony.[f] Good thermal conductivity otherwise occurs only in boron, silicon, phosphorus, and germanium;[19] such conductivity is transmitted though vibrations of the crystalline lattices of these elements.[45] Moderate electrical conductivity is observed in the semiconductors[46] boron, silicon, phosphorus, germanium, selenium, tellurium, and iodine. Plasticity occurs under limited circumstances in carbon, as seen in exfoliated (expanded) graphite[47][48] and carbon nanotube wire,[49] in
white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature),[50] in
plastic sulfur,[51] and in selenium which can be drawn into wires from its molten state.[52]
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.[53]
Most nonmetals have two or more allotropes. Carbon allotropes include
diamond, an electrical insulator;
buckminsterfullerene, a semiconductor; and
graphite, a conductor.
Over half of the nonmetallic elements exhibit a range of less stable allotropic forms, each with distinct physical properties.[54] For example, carbon, the most stable form of which is
graphite, can manifest as
diamond,
buckminsterfullerene,[55] and
amorphous[56] and
paracrystalline (mixed amorphous and crystalline)[57] variations. Allotropes also occur for nitrogen, oxygen, phosphorus, sulfur, selenium, the six metalloids, and iodine.[58]
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[64]) or neutral (like nitrous oxide, N2O[65][g]), 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, in some cases 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.[68]
They 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.[69] For example, the chemically very active nonmetals fluorine, chlorine, bromine, and iodine have an average electronegativity of 3.19—a figure[h] higher than that of any individual metal. On the other hand, the 2.05 average of the chemically weak metalloid nonmetals[i] falls within the 0.70 to 2.54 range of metals.[63]
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.[70] Consequently, there is a corresponding reduction in atomic radius[71] as the heightened nuclear charge draws the outer electrons closer to the nucleus core.[72] 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
ions or polarized atoms, while nonmetals tend to gain these electrons due to their stronger nuclear charge, resulting in negatively charged ions or polarized atoms.[73]
The number of compounds formed by nonmetals is vast.[74] 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.[75] A few examples of nonmetal compounds are:
boric acid (H 3BO 3), used in
ceramic glazes;[76]selenocysteine (C 3H 7NO 2Se), the 21st
amino acid of life;[77]phosphorus sesquisulfide (P4S3), found in
strike anywhere matches;[78] and
teflon ((C 2F 4)n), used to create non-stick coatings for pans and other cookware.[79]
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; multiple bond formation; and property overlaps with metals.
First row anomaly
Condensed periodic table highlighting the first row of each block
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.[81] 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.[82] 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."[83]
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.[84] As 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.[85]
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.[86]
Secondary periodicity
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.[87][k] 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.[89]
The Soviet chemist
Shchukarev [
ru] gives two more tangible examples:[90]
"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 sulfuric
H2SO4 and
H2TeO4 acids."
Higher oxidation states
Some nonmetallic elements exhibit
oxidation states that deviate from those predicted by the octet rule, which typically results in a valency of –3 in group 15, –2 in group 16, –1 in group 17, and 0 in group 18. Examples of such states can include compounds like
ammonia (NH3),
hydrogen sulfide (H2S),
hydrogen fluoride (HF), and elemental xenon (Xe). Meanwhile, the maximum possible oxidation state increases from +5 in
group 15, to +8 in
group 18. The +5 oxidation state is observable from period 2 onward, in compounds such as
nitric acid (HNO3) and
phosphorus pentafluoride (PCl5).[l]Higher oxidation states in later groups emerge from period 3 onwards, as seen in
sulfur hexafluoride (SF6),
iodine heptafluoride (IF7), and
xenon tetroxide (XeO4). For heavier nonmetals, their larger atomic radii and lower electronegativity values enable the formation of compounds with higher oxidation numbers, supporting higher bulk coordination numbers.[91]
Multiple bond formation
Period 2 nonmetals, particularly carbon, nitrogen, and oxygen, show a propensity to form multiple bonds. The compounds formed by these elements often exhibit unique stoichiometries and structures, as seen in the various nitrogen oxides,[91] which are not commonly found in elements from later periods.
Property overlaps
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,[93] Humphrey[94] 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);[63]
the electrical conductivity of graphite exceeds that of some metals;[n]
radon is the most metallic of the noble gases and begins to show some
cationic behavior, which is unusual for a nonmetal;[98] and
just over half of nonmetallic elements can form homopolyatomic cations.[o]
Examples of nonmetal-like properties occurring in metals are:
Tungsten displays some nonmetallic properties, being brittle, having a high electronegativity, and forming only anions in aqueous solution,[100] and predominately acidic oxides.[3][101] These are characteristics more aligned with nonmetals. Even so, tungsten is 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 that of a halogen, thereby bridging the traditional metal-nonmetal divide.[102]
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 emptymolecular 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.[103]
Types
Nonmetal classification schemes vary widely, with some accommodating as few as two subtypes and others identifying up to seven. For example, 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".[104] On the other hand, seven of twelve color categories on the Royal Society of Chemistry periodic table include nonmetals.[105][p]
the mixed reactivity "unclassified nonmetals", a set with no widely used collective name.[r] The descriptive phrase unclassified nonmetals is used here for convenience.
The elements in a fourth set are sometimes recognized as nonmetals:
the generally unreactive[t] metalloids,[124] sometimes considered a third category distinct from metals and nonmetals.
While many of the early workers attempted to classify elements none of their classifications were satisfactory. They were divided into metals and nonmetals, but some were soon found to have properties of both. These were called metalloids. This only added to the confusion by making two indistinct divisions where one existed before.[125]
Whiteford & Coffin 1939, Essentials of College Chemistry
The boundaries between these types are not sharp.[u] 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".[127] Some consider metalloids distinct from both metals and nonmetals, while others classify them as nonmetals.[128] Some categorize certain metalloids as metals (e.g., arsenic and antimony due to their similarities to
heavy metals).[129][v] Metalloids resemble the elements universally considered "nonmetals" in having relatively low densities, high electronegativity, and similar chemical behavior.[124][w]
For context, the metallic side of the periodic table also ranges widely in reactivity.[x] Highly reactive metals fill most of the s- and f-blocks on the left,[y] bleeding into the early part of the d-block. Thereafter, reactivity generally decreases closer to the p-block, whose metals are not particularly reactive.[z] The very unreactive
noble metals, such as
platinum and
gold, are clustered in an island within the d-block.[135]
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.[106]
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]
Highly reactive
sodium metal (Na, left) combines with corrosive halogen nonmetal
chlorine gas (Cl, center) to form stable, unreactive
table salt (NaCl, right).
While 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".[140]
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 is a solid and under white light is metallic-looking.[141] Electrically, the first three elements function as
insulators while iodine behaves as a
semiconductor (along its planes).[142]
Chemically, the halogen nonmetals exhibit high ionization energies, electron affinities, and electronegativity values, and are mostly relatively strong
oxidizing agents.[143] These characteristics contribute to their corrosive nature.[144] All four elements tend to form primarily
ionic compounds with metals,[145] in contrast to the remaining nonmetals (except for oxygen) which tend to form primarily
covalent compounds with metals.[aa] The highly reactive and strongly electronegative nature of the halogen nonmetals epitomizes nonmetallic character.[149]
Unclassified nonmetals
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 are metallic looking solids (C, P, Se); and one is a yellow solid (S). Electrically, graphitic carbon behaves as a semimetal along its planes[151] and a semiconductor perpendicular to its planes;[152] phosphorus and selenium are semiconductors;[153] while hydrogen, nitrogen, oxygen, and sulfur are insulators.[ab]
These elements are often considered too diverse to merit a collective name,[155] and have been referred to as other nonmetals,[156] or simply as nonmetals.[157] As a result, their chemistry is typically taught disparately, according to their respective
periodic table groups:[158] 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.[ac]
Hydrogen, in particular, behaves in some respects like a metal and in others like a nonmetal.[160] Like a metal it can, for example, form a
solvated cation in
aqueous solution;[161] 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;[162] and it can form
alloy-like hydrides with some
transition metals.[163] 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.[164] It attains this configuration by forming a covalent or ionic bond[165] or, if it has initially given up its electron, by attaching itself to a lone pair of electrons.[166]
Some or all of these nonmetals share several properties. Being generally less reactive than the halogens,[167] most of them can occur naturally in the environment.[168] They have significant roles in
biology[169] and
geochemistry.[155] Collectively, their physical and chemical characteristics can be described as "moderately non-metallic".[155] However, they all have corrosive aspects. Hydrogen can
corrode metals. Carbon corrosion can occur in
fuel cells.[170]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.[171] Untreated selenium in soils can lead to the formation of corrosive
hydrogen selenide gas.[172] When combined with metals, the unclassified nonmetals can form high-
hardness (
interstitial or
refractory) compounds[173] due to their relatively small atomic radii and sufficiently low ionization energies.[155] They also exhibit a tendency to
bond to themselves, particularly in solid compounds.[174] Additionally,
diagonal periodic table relationships among these nonmetals mirror similar relationships among the metalloids.[175]
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.[7]
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 structures of
semimetals, although both have less stable semiconducting
allotropes: arsenic as
arsenolamprite, an extremely rare naturally occurring form;[176] and antimony in its synthetic
thin-filmamorphous form.[7][177]
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.[7]
Abundance, sources, and uses
Abundance of nonmetallic elements
Approximate composition (by weight) of primary components and next most abundant
Hydrogen and helium dominate the observable universe, making up an estimated 98% of all ordinary matter by mass.[ad] Oxygen, the next most abundant element, accounts for about 1%.[182]
Five nonmetals—hydrogen, carbon, nitrogen, oxygen, and silicon—form the bulk 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.[179][180]
The
Earth's mantle and
core, making up about 99% of the Earth's volume,[183] are estimated to be made up of oxygen (31% by weight) and silicon (16%), with the remainder largely composed of the metals iron (31%), magnesium (15%) and nickel (2%).[184][ae]
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 from a variety of raw materials:[168]
The great variety of physical and chemical properties of nonmetals[192] 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.[193] 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:
Boron, first produced in a pure form in 1909,[194] is used in the form of
high-strength fibers for aerospace components and certain sporting goods.[195] It is also added to steel alloys to improve
hardenability.[196]
Germanium, thought to be a metal up until the 1930s,[199] 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.[200]
Radon, the rarest noble gas,[203] 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.[204] In radiotherapy, radon has now been replaced by
137Cs,
192Ir, and
103Pd.[205]
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.[ag] The most recently identified nonmetal is radon, detected at the end of the 19th century.[168]
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.[207]
The isolation of the halogen nonmetals from their
halides involved techniques including electrolysis, acid addition, or displacement. These efforts were not without peril, as some chemists tragically[208] lost their lives in their pursuit of isolating fluorine.[209]
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.[210] Sulfur occurred naturally as a free element, simplifying its isolation. Selenium,[ah] was first identified as a residue in
sulfuric acid.[212]
Most metalloids were first isolated by heating their oxides (
boron, silicon,
arsenic,
tellurium) or a sulfide (
germanium).[168] Antimony, first obtained by heating its sulfide,
stibnite, was later discovered in native form.[213]
Origin and use of the term
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 metals and "fossiles".[ai] 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".[214]
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.[215]
The term "nonmetallic" has historical origins dating back to at least the 16th century. In his 1566 medical treatise, French physician
Loys de L'Aunay distinguished substances from plant sources based on whether they originated from metallic or non-metallic soils.[216]
Later, the French chemist
Nicolas Lémery discussed metallic 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.[217]
French nobleman and chemist
Antoine Lavoisier (1743–1794), with a page of the English translation of his 1789 Traité élémentaire de chimie,[218] listing the elemental gases oxygen, hydrogen and nitrogen (and erroneously including
light and
caloric); the nonmetallic substances sulfur, phosphorus, and carbon; and the
chloride,
fluoride and
borate ions
The pivotal moment in the systematic classification of chemical elements into 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[219]Traité élémentaire de chimie. The elements were categorized into distinct groups, including gases, metallic substances, nonmetallic substances, and
earths (heat-resistant oxides).[220] 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.[221]
The widespread adoption of the term "nonmetal" followed a complex process spanning nearly nine decades. In 1811, the Swedish chemist
Berzelius introduced the term "metalloids"[222] to describe nonmetallic elements, noting their ability to form
negatively charged ions with oxygen in
aqueous solutions.[223][224] While Berzelius' terminology gained significant acceptance,[225] it later faced criticism from some who found it counterintuitive,[224] misapplied,[226] or even invalid.[227][228] In 1864, reports indicated that the term "metalloids" was still endorsed by leading authorities,[229] but there were reservations about its appropriateness. The idea of designating elements like
arsenic as metalloids had been considered.[229] By as early as 1866, some authors began preferring the term "nonmetal" over "metalloid" to describe nonmetallic elements.[230] In 1875, Kemshead[231] 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
List of properties suggested for distinguishing metals from nonmetals
From the early 1800s, a variety of physical, chemical, and atomic properties have been suggested for distinguishing metals from nonmetals, as listed in the accompanying table. One of the earliest recorded properties from 1803 refers to (high) density and (good) electrical conductivity.
In 1809, the British chemist and inventor
Humphry Davy made a groundbreaking discovery that reshaped the understanding of metals and nonmetals.[255] When he isolated
sodium and
potassium, their low densities (floating on water!) contrasted with their metallic appearance, challenging the stereotype of metals as dense substances.[256][ap] Nevertheless, their classification as metals was firmly established by their distinct chemical properties.[258]
One of the most commonly recognized properties used in this context is the
temperature coefficient of resistivity, the effect of heating on electrical resistance and conductivity. As temperature rises, the conductivity of metals decreases while that of nonmetals increases.[248] 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.[259] Similarly, despite its common classification as a nonmetal, when carbon (as graphite) is heated it experiences a decrease in electrical conductivity.[260] Arsenic and antimony, which are occasionally classified as nonmetals, show behavior similar to carbon, highlighting the complexity of the distinction between metals and nonmetals.[261]
Kneen and colleagues[262] 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[263] 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[264] emphasized that classification systems typically rely on more than two attributes to define distinct types.
Johnson[265] 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,[aq] Te and I, which have acidic oxides indicative of nonmetallic character.[ar]
Periodic table with elements shaded by density and electronegativity[as]
Several authors[270] 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[271] 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[272] disagrees, arguing that by the principles of categorization, anything not classified as a metal should be considered a nonmetal.
Development of types
In 1844,
Alphonse Dupasquier [
fr], a French doctor, pharmacist, and chemist,[273] established a basic taxonomy of nonmetals to aid in their study. He wrote:[274]
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 quartet parallels the modern nonmetal types. The organogens and sulphuroids are akin to the unclassified nonmetals. The chloroides were later called halogens.[275] The boroids eventually evolved into the metalloids, with this classification beginning from as early as 1864.[229] The then unknown noble gases were recognized as a distinct nonmetal group after being discovered in the late 1800s.[276]
His taxonomy was commended for its natural basis, contrasting it with the artificial systems of that period.[277][at] That said, it represented a significant departure from other contemporary classifications, since it grouped together oxygen, nitrogen, hydrogen, and carbon.[279]
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,[280] thereby anticipating the vertical groupings of Mendeleev's 1871 periodic table. Dumas' five classes fall into modern groups
1,
17,
16,
15, and
14 to
13 respectively.
Classification of metalloids
Boron and silicon were recognized early on as nonmetals[au] but arsenic, antimony, tellurium, and germanium have a more complicated history. While the suitability of arsenic being counted as a metalloid had been considered in 1864,[229] Mendeleev, in 1897, counted it and antimony as metals.[282] Although tellurium likely acquired an "ium" suffix due to its metallic appearance,[283] Mendeleev said it represented a transition between metals and nonmetals.[284] The semiconductor germanium was first regarded as a poorly conducting metal due to the presence of impurities. The understanding of it as a semiconductor, and subsequently as a metalloid, emerged in the 1930s with the development of semiconductor physics.[199]
Since the 1940s, these six elements have been increasingly, but not universally, recognized as metalloids.[285] In 1947,
Linus Pauling included a reference to them in his classic[286] and influential[287] textbook General chemistry: An introduction to descriptive chemistry and modern chemical theory. He described boron, silicon, germanium, arsenic, antimony (and
polonium) as "elements with intermediate properties."[288] He said they were in the center of his electronegativity scale, with values close to 2.[av] 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.[290] Writing in 1982, Goldsmith[285] 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
The two tables in this section list some of the properties of five types of elements (noble gases, halogen nonmetals, unclassified nonmetals, metalloids and, for comparison, metals) based on their most stable forms 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.[291][292] Some overlap occurs as outlier elements of each type exhibit less-distinct, hybrid-like, or atypical properties.[293][aw] 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.[295]
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.
† Hydrogen can also form alloy-like hydrides[163]
‡ The labels low, moderate, high, and very high are arbitrarily based on the value spans listed in the table
^At higher temperatures and pressures the numbers of nonmetals can be called into question. For example, when germanium melts it changes from a semiconducting metalloid to a metallic conductor with an electrical conductivity similar to that of liquid mercury.[10] At a high enough pressure,
sodium (a metal) becomes a non-conducting
insulator.[11]
^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[14]
^Solid iodine has a silvery metallic appearance under white light at room temperature. It
sublimes at ordinary and higher temperatures, passing from solid to gas; its vapours are violet-colored.[15]
^The solid nonmetals have electrical conductivity values ranging from 10−18 S•cm−1 for sulfur[19] to 3 × 104 in graphite[20] or 3.9 × 104 for
arsenic;[21] cf. 0.69 × 104 for
manganese to 63 × 104 for
silver, both metals.[19] 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.
^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.[19] Electrical conductivity values of metals range from 0.69 S•cm−1 × 104 for
manganese to 63 × 104 for
silver; cf. carbon 3 × 104,[20] arsenic 3.9 × 104 and antimony 2.3 × 104.[19]
^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−);[66] and in water, NO reacts with oxygen to form nitrous acid HNO2 (4NO + O2 + 2H2O → 4HNO2).[67]
^Helium is shown above beryllium for electron configuration consistency purposes; as a noble gas it is usually placed above neon, in group 18.
^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.[88]
^Oxidation states, which denote hypothetical charges for conceptualizing electron distribution in chemical bonding, do not necessarily reflect the net charge of molecules or ions. This concept is illustrated by anions such as NO3−, where the nitrogen atom is considered to have an oxidation state of +5 due to the distribution of electrons. However, the net charge of the ion remains −1. Such observations underscore the role of oxidation states in describing electron loss or gain within bonding contexts, distinct from indicating the actual electrical charge, particularly in covalently bonded molecules.
^Greenwood[95] 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[96] whereas that of
manganese is 6.9 × 103 S•cm−1[97]
^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] This is unusual behaviour for nonmetals which are better known for their capacity to form negatively charged anions or polarized atoms, whereas metals are better known for their capacity to form positively charged cations or polarized atoms.
^ 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,[108] bioelements,[109] central nonmetals,[110] CHNOPS,[111] essential elements,[112] "non-metals",[113][q] orphan nonmetals,[114] or redox nonmetals[115]
^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”.[119] It can occasionally be found in nature in an uncombined form.[120] It has a positive standard reduction potential (As → As3+ + 3e = +0.30 V), corresponding to a classification of semi-noble metal.[121]
^"Crystalline boron is relatively inert." Silicon "is generally highly unreactive."[116] "Germanium is a relatively inert semimetal."[117] "Pure arsenic is also relatively inert."[118][s] "Metallic antimony is … inert at room temperature."[122] "Compared to S and Se, Te has relatively low chemical reactivity."[123]
^Such boundary fuzziness and overlaps often occur in classification schemes.[126]
^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".[126]
^Thus, Weller at al.[130] 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[131] 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.[132] 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."[133]Thallium, a p-block metal, is unaffected by water or alkalis but is attacked by acids, and is slowly oxidized in room temperature air.[134]
^Metal oxides are usually ionic.[146] On the other hand, oxides of metals with high oxidation states are usually either polymeric or covalent.[147] A polymeric oxide has a linked structure composed of multiple repeating units.[148]
^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[154]
^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.[159]
^ 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.[181]
^In the Earth's core there may be around 1013 tons of xenon, in the form of stable XeFe3 and XeNi3intermetallic compounds. This could explain why "studies of the Earth's atmosphere have shown that more than 90% of the expected amount of Xe is depleted."[185]
^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[186]
^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".[206]
^Berzelius, who discovered selenium, thought it had the properties of a metal, combined with the properties of sulfur[211]
^Not to be confused with the modern usage of
fossil to refer to the preserved remains, impression, or trace of any once-living thing
^"... their specific gravity is greater than that of any other bodies yet discovered;
they are better conductors of electricity, than any other body."
^The Goldhammer-Herzfeld ratio is roughly equal to the cube of the atomic radius divided by the
molar volume.[235] 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.
^ Sonorousness is making a ringing sound when struck.
^ Liquid range is the difference between melting point and boiling point.
^Configuration energy is the average energy of the valence electrons in a free atom.
^Atomic conductance is the electrical conductivity of one mole of a substance. It is equal to electrical conductivity divided by
molar volume.
^It was subsequently proposed, by Erman and Simon,[257] to refer to sodium and potassium as metalloids, meaning "resembling metals in form or appearance". Their 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 and Bache[255] 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[266]
^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;[267] (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;[268] (c) Vernon specified a minimum electronegativity of 1.9 for the metalloids, on the revised Pauling scale; and[7] (d) Electronegativity values for the noble gases are from Rahm, Zeng and Hoffmann.[269]
^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."[278]
^Both were initially isolated in their impure or amorphous forms; the pure crystalline, metallic-looking forms were isolated later.[281]
^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.[289]
^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)[294]
^All four have less stable non-brittle forms:[305] carbon as
exfoliated (expanded) graphite,[47][306] and as
carbon nanotube wire;[49] phosphorus as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature);[50] sulfur as plastic sulfur;[51] and selenium as selenium wires[52]
^Metals have electrical conductivity values of from 6.9×103 S•cm−1 for
manganese to 6.3×105 for
silver[308]
^Metalloids have electrical conductivity values of from 1.5×10−6 S•cm−1 for boron to 3.9×104 for
arsenic[309]
^Unclassified nonmetals have electrical conductivity values of from ca. 1×10−18 S•cm−1 for the elemental gases to 3×104 in graphite[96]
^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[96][142]
^The elemental gases have electrical conductivity values of ca. 1×10−18 S•cm−1[96]
^Metalloids always give "compounds less acidic in character than the corresponding compounds of the [typical] nonmetals"[296]
^Arsenic trioxide reacts with sulfur trioxide, forming arsenic "sulfate" As2(SO4)3[316]
^NO 2, N 2O 5, SO 3, SeO 3 are strongly acidic[317]
^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)"[318]
^Metals that form glasses are: V; Mo, W; Al, In, Tl; Sn, Pb; Bi[322]
^Unclassified nonmetals that form glasses are P, S, Se;[322]CO2 forms a glass at 40 GPa[324]
^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[332]
^Values for the noble gases are from Rahm, Zeng and Hoffmann[269]
^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 2020, 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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