User:Sandbh/Nonmetal2

 

A nonmetal is a chemical element generally characterized by low density and high electronegativity. They lack a preponderance of metallic properties, such as luster or shininess, malleability and ductility, good thermal and electrical conductivity, and the tendency to produce basic rather than acidic oxides when combined with oxygen.^[7] There is no universally agreed definition of a nonmetal^[8] resulting in variations among sources as to which elements are counted as such. The criteria applied depend on the properties viewed as most representative of nonmetallic or metallic character.^{[9][n 3]}

While Steudel, in his 2020 textbook Chemistry of the Non-metals^{[10][n 4]} listed twenty-three elements as nonmetals, any such list is open to debate and revision.^[11] Among these elements, fourteen are commonly recognized as nonmetals namely hydrogen, oxygen, nitrogen, and sulfur, as well as the highly reactive halogens: fluorine, chlorine, bromine, and iodine.^[11] The noble gases, including helium, neon, argon, krypton, xenon, and radon, are also definitively classified as nonmetals, as supported by Hawley's Condensed Chemical Dictionary.^[11]

The classification of carbon, phosphorus, and selenium is not necessarily clear cut; while they are commonly deemed nonmetals, some sources have labelled them as metalloids.Cite error: A <ref> tag is missing the closing </ref> (see the help page). Nonetheless, their predominantly nonmetallic chemistry, characterized by weak acidity, also supports their classification as nonmetals.^[12]

Of the 118 known chemical elements,^[13] roughly 20% are classified as nonmetals.^[14] Astatine, the fifth halogen, is often ignored on account of its rarity and intense radioactivity;^[15] theory and experimental evidence suggest it is a metal.^{[4][n 5]} The superheavy elements copernicium (element 112), flerovium (114), and oganesson (118) may turn out to be nonmetals. As of August 2023 their status has not been confirmed.^[18]

Properties noted in this section refer to the elements in their most stable forms in ambient conditions unless otherwise mentioned

Variety in color and form
of some nonmetallic elements

 

Boron in its β-rhombohedral phase

 

Metallic appearance of carbon as graphite

 

Blue color of liquid oxygen

 

Pale yellow liquid fluorine in a cryogenic bath

 

Sulfur as a yellow powder

 

Liquid bromine at room temperature

 

Metallic appearance of iodine under white light

 

Liquefied 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 6]} The fluid nonmetals have very low densities, melting and boiling points, and are poor conductors of heat and electricity.^[21] The solid elements have low densities, are brittle or crumbly with low mechanical and structural strength,^[22] and are poor to good conductors of electricity.^{[n 7]}

The diverse forms of nonmetals can be attributed to their varied internal structures and bonding arrangements. Nonmetals existing as discrete atoms like xenon, or as 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 atoms (selenium, for example),^[27] sheets (carbon as graphite, for example),^[28] or three-dimensional lattices (silicon, for example)^[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 side of the periodic table, or further down a column, often have some weak metallic interactions between their molecules, chains, or layers, consistent with their proximity to the metals; this occurs in boron,^[31] carbon,^[32] phosphorus,^[33] arsenic,^[34] selenium,^[35] antimony,^[36] tellurium^[37] and iodine.^[38]

Nonmetallic elements are either shiny, colored, or colorless. The shiny appearance 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. In the case of chlorine, for example, Elliot writes that its "familiar yellow-green colour ... is due to a broad region of absorption in the violet and blue regions of the spectrum".^{[40][n 8]} 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]

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, arsenic, and antimony.^{[n 9]} 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.^[45] Moderate electrical conductivity is observed in boron, silicon, phosphorus, germanium, selenium, tellurium, and iodine.^{[n 10]} Plasticity occurs under limited circumstances in carbon, as seen in exfoliated (expanded) graphite^[47][48] and carbon nanotube wire,^[49] in phosphorus as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature),^[50] in sulfur, when it exists as 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 free to move between atoms, and metallic properties are predicted. Otherwise nonmetallic properties are expected.^[53]

 

Many nonmetallic elements exhibit a range of allotropic forms, each with distinct physical properties that may vary between metallic and nonmetallic.^[58] For example, carbon, a versatile nonmetal, can manifest as graphite, diamond, and other forms, with graphite displaying relatively good electrical conductivity, while diamond is transparent and an extremely poor conductor of electricity.^[59] Carbon further exists in several allotropic structures, including buckminsterfullerene,^[60] and amorphous^[61] and paracrystalline (mixed amorphous and crystalline)^[62] variations. Iodine among the halogen nonmetals,^[63] as well as the other unclassified nonmetals, and metalloids also have allotropic variations. Allotropic forms of the noble gases have not been observed.

Nonmetals possess relatively high values of electronegativity^[64] and tend to form acidic compounds. For example, solid nonmetals (including metalloids) react with nitric acid to produce either an acid, or an oxide with predominately acidic properties.^{[n 11]}

They tend to gain or share electrons during chemical reactions, in contrast to metals which tend to donate electrons. This behavior is closely related to the stability of electron configurations in the noble gases, which have complete outer shells. Nonmetals generally gain enough electrons to attain the electron configuration of the following noble gas, while metals tend to lose electrons, achieving the electron configuration of the preceding noble gas. These tendencies in nonmetallic elements are succinctly summarized by the duet and octet rules of thumb. Metals, on the other hand, follow a less rigorously predictive 18-electron rule.^[67]

Furthermore, nonmetals typically exhibit higher ionization energies, electron affinities, and standard reduction potentials than metals. General, the higher these values are (including electronegativity) the more nonmetallic the element tends to be.^[68]

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.^[69] Consequently, there is a corresponding reduction in atomic radius^[70] as the heightened nuclear charge draws the outer electrons closer to the nucleus core.^[71] 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.^[72]

The number of compounds formed by nonmetals is vast.^[73] 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.^[74] A few examples of nonmetal compounds are: boric acid (H
₃BO
₃), used in ceramic glazes;^[75] selenocysteine (C
₃H
₇NO
₂Se), the 21st amino acid of life;^[76] phosphorus sesquisulfide (P₄S₃), found in strike anywhere matches;^[77] and teflon ((C
₂F
₄)_n), used to create non-stick coatings for pans and other cookware.^[78]

 

 

Adding complexity to the chemistry of the nonmetals are the anomalies observed in the first row of each periodic table block. These anomalies are especially prominent in hydrogen, boron (whether as a nonmetal or metalloid), carbon, nitrogen, oxygen, and fluorine. In later rows these anomalies manifest as secondary periodicity or non-uniform periodic trends within most of the p-block groups,^[79] and unusual oxidation states in the heavier nonmetals.

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.^[80] 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.^[81] 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."^[82]

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.^[83] 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.^[84]

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.^[85]

A noticeable alternation in certain periodic trends becomes evident when descending groups 13 to 15, and to a lesser extent, groups 16 to 17.^{[86][n 13]} 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.^[88]

The larger atomic radii of the heavier group 15–18 nonmetals enable higher bulk coordination numbers, and result in lower electronegativity values that better tolerate higher positive charges. The elements involved are thereby able to exhibit oxidation states other than the lowest for their group (that is, 3, 2, 1, or 0), for example in phosphorus pentachloride (PCl₅), sulfur hexafluoride (SF₆), iodine heptafluoride (IF₇), and xenon difluoride (XeF₂).^[89]

 

The classification of nonmetals can vary, with approaches ranging from as few as two types to as many as six or seven. For instance, the periodic table in the Encyclopædia Britannica recognizes noble gases, halogens, and other nonmetals, and splits the elements commonly recognized as metalloids between "other metals" and "other nonmetals".^[101] On the other hand, the Royal Society of Chemistry periodic table uses different colors for each of its eight main groups, with nonmetals represented in seven of these.^[102]

When traversing the periodic table from right to left, three or four types of nonmetals are more or less commonly discerned:

• the relatively inert noble gases;^[103]
• a set of chemically strong halogen elements—fluorine, chlorine, bromine and iodine—sometimes referred to as nonmetal halogens^[104] or halogen nonmetals^[105] (as used here) or stable halogens;^[106]
• a set of unclassified nonmetals, encompassing elements like hydrogen, carbon, nitrogen, and oxygen, for which there is no widely recognized collective name;^{[n 16]} and
• the chemically weak nonmetallic metalloids^[115] which are sometimes considered nonmetals and sometimes not.^{[n 17]}

Since metalloids occupy a transition region or "frontier territory",^[117] where metals meet nonmetals, their classification varies among authors. Some consider them distinct from both metals and nonmetals, while others classify them as nonmetals^[118] or as a sub-class of nonmetals.^[119] There are also authors who categorize certain metalloids as metals, such as arsenic and antimony, due to their similarities to heavy metals.^{[120][n 18]} In this context metalloids are here treated as nonmetals, based on their chemical behavior,^[115] for the sake of comparative analysis.

Aside from the metalloids, some boundary fuzziness and overlapping (as occurs with classification schemes generally),^[121] can be discerned among the other types of nonmetals. Carbon, phosphorus, selenium, and iodine border the metalloids and show some metallic character, as does hydrogen. Among the noble gases, radon is the most metallic and begins to show some cationic behavior, which is unusual for a nonmetal.^[122]

 

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.^[103]

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.^[123] As a consequence, they all exist as gases under standard conditions, even those with atomic masses surpassing many typically solid elements.^[124]

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,^[125] with a most of these compounds involving the combination of oxygen or fluorine with either krypton, xenon, or radon.^[126]

In terms of the periodic table, an analogy can be drawn between the noble gases and noble metals such as platinum and gold, as they share a similar reluctance to combine with other elements.^[127] As a further example, xenon, in the +8 oxidation state, forms a pale yellow explosive oxide,XeO₄, while osmium, another noble metal, forms a yellow, strongly oxidizing oxide,^[128] OsO₄. Additionally, there are parallels in the formulas of the oxyfluorides: XeO₂F₄ and OsO₂F₄, and XeO₃F₂ and OsO₃F₂.^[129]

About 10¹⁵ tonnes of noble gases are present in the Earth's atmosphere.^[130] Additionally, natural gas can contain as much as 7% helium.^[131] Radon diffuses out of rocks, where it forms during the natural decay sequence of uranium and thorium.^[132] In the Earth's core there may be around 10¹³ tons of xenon, in the form of stable XeFe₃ and XeNi₃ intermetallic 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."^[133]

 

Although the halogen nonmetals are notably reactive and corrosive elements, they can also be found in everyday compounds like toothpaste (NaF); common table salt (NaCl); swimming pool disinfectant (NaBr); or food supplements (KI). The term "halogen" itself means "salt former".^[135]

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^[90] solid. Electrically, the first three elements function as insulators while iodine behaves as a semiconductor (along its planes).^[136]

Chemically, the halogen nonmetals exhibit high ionization energies, electron affinities, and electronegativity values, and are mostly relatively strong oxidizing agents.^[137] These characteristics contribute to their corrosive nature.^[138] All four elements tend to form primarily ionic compounds with metals,^[139] in contrast to the remaining nonmetals (except for oxygen) which tend to form primarily covalent compounds with metals.^{[n 19]} The highly reactive and strongly electronegative nature of the halogen nonmetals epitomizes nonmetallic character.^[143]

In terms of the periodic table, the highly nonmetallic halogens in group 17 find their counterparts in the highly reactive alkali metals, such as sodium and potassium, in group 1.^[144] Further, and much like the halogen nonmetals, most of the alkali metals are known to form –1 anions, a characteristic seldom observed among metals.^[145]

The halogen nonmetals are commonly found in salt-related minerals. Fluorine, for instance, is present in fluorite (CaF₂), a mineral found widely. Chlorine, bromine, and iodine are typically found in brines. Exceptionally, a study reported in 2012 noted the presence of 0.04% native fluorine (F
₂) by weight in antozonite, attributing these inclusions to radiation from tiny amounts of uranium.^[146]

 

The six elements more commonly recognized as metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium, all of which have a metallic appearance. On a standard periodic table, they occupy a diagonal region within the p-block extending from boron at the upper left to tellurium at the lower right, along the dividing line between metals and nonmetals shown on some tables.^[1]

They are brittle and poor-to-good conductors of heat and electricity. Specifically, boron, silicon, germanium, and tellurium are semiconductors. Arsenic and antimony have the electronic structures of semimetals, although both have less stable semiconducting forms.^[1]

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.^[1]

In the periodic table, to the left of the weakly nonmetallic metalloids are an indeterminate set of weakly metallic metals including tin, lead and bismuth)^[148] sometimes referred to as post-transition metals.^[149] Dingle explains the situation this way:

... with 'no-doubt' metals on the far left of the table, and no-doubt non-metals on the far right ... the gap between the two extremes is bridged first by the poor (post-transition) metals, and then by the metalloids—which, perhaps by the same token, might collectively be renamed the 'poor non-metals'.^[150]

The metalloids are commonly found combined with oxygen, sulfur, or, in the case of tellurium, gold or silver.^[151] Boron is typically found in boron-oxygen borate minerals, including in volcanic spring waters. Silicon is present in silicon-oxygen mineral silica (sand). Germanium, arsenic, and antimony are mainly found as components of sulfide ores. Tellurium is often found in telluride minerals of gold or silver. In some instances, native forms of arsenic, antimony, and tellurium have been reported.^[152]

 

After classifying the nonmetallic elements into noble gases, halogens, and metalloids, the remaining seven nonmetals are hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur, and selenium.

In their most stable forms, three of these are colorless gases (H, N, O); three have a metal-like appearance (C, P, Se); and one appears yellow (S). Electrically, graphitic carbon behaves as a semimetal along its planes^[154] and a semiconductor perpendicular to its planes;^[155] phosphorus and selenium are semiconductors;^[156] while hydrogen, nitrogen, oxygen, and sulfur are insulators.^{[n 20]}

These elements are often considered too diverse to merit a collective classification,^[158] and have been referred to as other nonmetals,^[159] or simply as nonmetals, located between the metalloids and the halogens.^[160] As a result, their chemistry is typically taught disparately, according to their respective periodic table groups:^[161] 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 21]}

Hydrogen, in particular, behaves in some respects like a metal and in others like a nonmetal.^[163] Like a metal, it can initially lose its single electron,^[164] it can substitute for alkali metals in typical alkali metal structures,^[165] and it can form alloy-like hydrides, featuring metallic bonding, with certain transition metals.^[166] Conversely, it behaves as an insulating diatomic gas, akin to a typical nonmetal, and during chemical reactions, it tends to ultimately attain the electron configuration of helium.^[167] It accomplishes this by forming a covalent or ionic bonds^[168] or, if it has lost its electron, by attaching itself to a lone pair of electrons.^[169]

Some or all of these nonmetals share several properties. Being less reactive than the halogens,^[170] they can occur naturally in the environment.^[171] They have significant roles in biology^[172] and geochemistry.^[173] Collectively, their physical and chemical characteristics can be described as "moderately non-metallic".^[173] However, they all have corrosive aspects. Hydrogen can corrode metals. Carbon corrosion can occur in fuel cells.^[174] 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.^[175] Untreated selenium in soils can lead to the formation of corrosive hydrogen selenide gas.^[176] When combined with metals, the unclassified nonmetals can form high-hardness (interstitial or refractory) compounds^[177] due to their relatively small atomic radii and sufficiently low ionization energies.^[173] They also exhibit a tendency to bond to themselves, particularly in solid compounds.^[178] Additionally, diagonal periodic table relationships among these nonmetals mirror similar relationships among the metalloids.^[179]

In terms of the periodic table, there is a geographic analogy between the unclassified nonmetals and transition metals. The unclassified nonmetals are positioned between the strongly nonmetallic halogens on the right and the weakly nonmetallic metalloids on the left. Similarly, the transition metals occupy a region between the "virulent and violent" metals on the left side of the periodic table, and the "calm and contented" metals on the right. They effectively serve as a "transitional bridge" connecting these two regions.^[180]

Unclassified nonmetals are typically found in elemental forms or in association with other elements:^[151]

• Hydrogen is present in the world's oceans as a component of water and occurs in natural gas as a component of methane and hydrogen sulfide.^[181]
• Carbon can be found in limestone, dolomite, and marble, as carbonates.^[182] Additionally, carbon exists as graphite, primarily occurring in metamorphic silicate rocks,^[183] resulting from the compression and heating of sedimentary carbon compounds.^[184]
• Oxygen is found in the atmosphere; in the oceans as a component of water; and in the Earth's crust as oxide minerals.^[185]
• Phosphorus minerals are widespread, typically appearing as phosphorus-oxygen phosphates.^[186]
• Elemental sulfur can be found in or near hot springs and volcanic regions around the world, while sulfur minerals are common and are often found as sulfides or oxygen-sulfur sulfates.^[187]
• Selenium is found in metal sulfide ores, where it may partially replace sulfur. In rare instances, elemental selenium can also be found.^[188]

Hydrogen and helium dominate the universe, comprising an estimated 99% of all ordinary matter and over 99.9% of all atoms.^[190] Oxygen, the next most abundant element, constitutes around 0.1% of the universe's composition.^[191] Less than 5% of the universe consists of ordinary matter, which includes stars, planets, and living entities, while the majority remains enigmatic, comprising dark energy and dark matter, both yet poorly understood.^[192]

Five nonmetals—hydrogen, carbon, nitrogen, oxygen, and silicon—play crucial roles in shaping Earth's composition. These elements form the fundamental building blocks of the Earth's crust, atmosphere, hydrosphere, and biomass. Their relative proportions, as outlined in the accompanying table, underscore their fundamental significance in the terrestrial environment.

 

Nonmetals, and metalloids, are extracted in their raw forms from:^[171]

• brine—chlorine, bromine, iodine;
• liquid air—nitrogen, oxygen, neon, argon, krypton, xenon;
• minerals—boron (borate minerals); carbon (coal; diamond; graphite); fluorine (fluorite); silicon (silica); phosphorus (phosphates); antimony (stibnite, tetrahedrite); iodine (in sodium iodate and sodium iodide);
• natural gas—hydrogen, helium, sulfur; and
• ores, as processing byproducts—germanium (zinc ores); arsenic (copper and lead ores); selenium, tellurium (copper ores); and radon (uranium-bearing ores).

Day to day costs will vary depending on purity, quantity,^{[n 22]} market conditions, and supplier surcharges.^[197]

Based on the available literature as of April 2023, the cited costs of most nonmetals are less than the US$0.74 per gram cost of silver.^[198] The exceptions are boron, phosphorus, germanium, xenon, and radon (notionally):

• Boron costs around $25 per gram for 99.7% pure polycrystalline chunks with a particle size of about 1 cm.^[199] Earlier, in 1997, boron was quoted at $280 per gram for polycrystalline 4-to-6-mm-diameter rods of 99.999% purity,^[200] about 10 times the then $28.35 per gram cost of gold.^[201]
• In 2020, phosphorus in its most-stable black form could "cost up to $1,000 per gram",^[202] more than 15 times the cost of gold, whereas ordinary red phosphorus, in 2017, was priced at about $3.40 per kilogram.^[203] Researchers hoped to be able to reduce the cost of black phosphorus to as low as $1 per gram.^[202]
• Germanium and xenon cost about $1.30 and $7.60 per gram.^[204]
• Up to 2013, radon was available from the National Institute of Standards and Technology for $1,636 per 0.2 ml unit of issue, equivalent to about $86,000,000 per gram, with no indication of a discount for bulk quantities.^[205]

 

Nonmetallic elements display unique properties that enable a wide range of applications, often unachievable solely with metallic elements. A remarkable aspect of nonmetals is their ubiquitous presence in living organisms, where hydrogen, oxygen, carbon, and nitrogen serve as the foundational building blocks of life. Additionally, these elements play pivotal roles in various industries, encompassing cutting-edge electronics, advanced energy storage systems, agriculture, and large-scale chemical production. A few examples include:

• Carbon fibers are characterized by their high strength and low weight, rendering them well-suited for use in aerospace, sports equipment, and the automotive manufacturing industry. Graphene, consisting of a single layer of carbon atoms, exhibits remarkable electrical and thermal conductivity, making it invaluable for electronic devices, energy storage, and the development of composite materials.^[207]

• Nitrogen plays a vital role in fertilizer production, contributing to enhanced crop growth and increased agricultural productivity. Its low-temperature characteristics find utility in cryogenic applications, including the preservation of biological samples and the freezing of food.^[208]

• Oxygen is indispensable for human respiration in life support systems, and it is employed in medical contexts to aid patients with respiratory challenges. It also supports combustion and finds application in various industrial processes, including metal smelting and waste incineration.^[209]

• Silicon serves as the cornerstone of the electronics industry, particularly in the production of computer chips, solar cells, and various electronic components. Silicon dioxide (silica) plays a crucial role in manufacturing glass, ceramics, and optical fibers, facilitating applications in windows, lenses, and communication networks.^[210]

• Sulfur is an essential component in the production of sulfuric acid, one of the most extensively utilized industrial chemicals. Additionally, it plays a key role in the synthesis of various organic compounds. Sulfur compounds are critically involved in the vulcanization process, enhancing the strength and elasticity of rubber products.^[211]

Various subsets of nonmetals find common applications in fields such as (inert) air replacements, dyestuffs, flame retardants or extinguishers, household items, lasers and lighting, mineral acids, plug-in hybrid vehicles, and welding gases.^[171][212] Metalloids, to the extent that they exhibit metallic properties, have specialized applications that include oxide glasses and alloying components, among others.^[213]

Most nonmetallic elements were identified during the 18th and 19th centuries. However, a few nonmetals were recognized in ancient times and later historical periods. Carbon, sulfur, and antimony were among the early nonmetals known to humanity. The discovery of arsenic can be traced back to the Middle Ages, credited to the work of Albertus Magnus. A significant moment in the history of nonmetal discovery occurred in 1669 when Hennig Brand successfully isolated phosphorus from urine. Helium, identified in 1868, holds a unique distinction as the only element not initially discovered on Earth itself.^{[n 23]} Radon is the most recently uncovered nonmetal, with its detection occurring at the end of the 19th century.^[171]

The isolation of nonmetallic elements depended on a range of chemical and physical techniques. These methods encompassed spectroscopy, fractional distillation, radiation detection, electrolysis, ore acidification, displacement reactions, combustion, and controlled heating processes. While some nonmetals were naturally occurring as free elements, others required intricate extraction procedures:

• The noble gases, renowned for their low reactivity, were first identified through unusual and mundane methods. 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.^[215]
• 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 lost their lives in their pursuit of isolating fluorine.^[216]
• 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.^[217] Sulfur occurred naturally as a free element, simplifying its isolation. Selenium,^{[n 24]} was first identified as a residue in sulfuric acid.^[219]
• Metalloids were commonly isolated by heating of their oxides ((boron, silicon, arsenic, tellurium) or a sulfide (germanium).^[171] Antimony, in an unusual twist, was found in its native form and could also be obtained through heating of its sulfide compound.^[220]

 

Although a distinction had existed between metals and other mineral substances since ancient times, it was only towards the end of the 18th century that a basic classification of chemical elements as either metallic or nonmetallic substances began to emerge. It would take another nine decades before the term "nonmetal" was widely adopted.

Around the year 340 BCE, in Book III of his treatise Meteorology, the ancient Greek philosopher Aristotle categorized substances found within the Earth into two distinct groups: metals and "fossiles".^{[n 25]} 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".^[222]

Up until the Middle Ages the classification of minerals remained largely unchanged, albeit with varying terminology. In the fourteenth century, an English alchemist named Richardus Anglicus expanded upon the classification of minerals in his work Correctorium Alchemiae. In this text, he proposed the existence of two primary types of minerals. The first category, which he referred to as "major minerals," included well-known metals such as gold, silver, copper, tin, lead, and iron. On the other hand, the second category, labeled as "minor minerals," encompassed substances like salts, atramenta (iron sulfate), alums, vitriol, arsenic, orpiment, sulfur, and similar substances that were not metallic bodies.^[223]

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.^[224]

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.^[225]

The pivotal moment in the systematic classification of chemical elements, distinguishing between metallic and nonmetallic substances, came in 1789 with the groundbreaking work of Antoine Lavoisier. Lavoisier, a French chemist, published the first modern list of chemical elements in his revolutionary^[226] work Traité élémentaire de chimie. In this work he categorized elements into distinct groups, including gases, metallic substances, nonmetallic substances, and earths (heat-resistant oxides).^[227] 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.^[228]

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"^[229][230] to describe nonmetallic elements, noting their ability to form negatively charged ions with oxygen in aqueous solutions.^[231][232] While Berzelius' terminology gained significant acceptance,^[229] it later faced criticism from some who found it counterintuitive,^[232] misapplied,^[233] or even invalid.^[234][235] In 1864, reports indicated that the term "metalloids" was still endorsed by leading authorities,^[236] but there were reservations about its appropriateness. The idea of designating elements like arsenic as metalloids had been considered.^[236] By as early as 1866, some authors began preferring the term "nonmetal" over "metalloid" to describe nonmetallic elements.^[237] In 1875, Kemshead 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.^[238]

In 1809, the British chemist and inventor Humphry Davy made a groundbreaking discovery that reshaped the understanding of metals and nonmetals.^[261] His isolation of sodium and potassium represented a significant departure from the conventional method of classifying metals solely based on their ponderousness or high densities.^[262] Sodium and potassium, on the contrary, floated on water. Nevertheless, their classification as metals was firmly established by their distinct chemical properties.^[263]

As early as 1811, attempts were initiated to enhance the differentiation between metals and nonmetals by examining a range of properties, including physical, chemical, and electron-related characteristics. The table provided here outlines 22 such properties, listed by type and the year of their mention.

One of the most commonly recognized (though somewhat unreliable) properties used in this context is the change in electrical conductivity with temperature. Typically, metals exhibit an increase in electrical conductivity as temperature decreases, while nonmetals display the opposite trend.^[251] However, there are exceptions that challenge this generalization. For example, plutonium, carbon, arsenic, and antimony defy the norm. Plutonium's electrical conductivity increases when heated within a specific temperature range of −175 to +125 °C.^[264] Similarly, despite its common classification as a nonmetal, carbon also experiences an increase in electrical conductivity when subjected to heating.^[265] Arsenic and antimony, which are occasionally classified as nonmetals, exhibit behavior similar to carbon, highlighting the complexity of the distinction between metals and nonmetals.^[266]

Kneen and colleagues 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.^[9]

However, Emsley pointed out the complexity of this task, asserting that no single property alone can unequivocally assign elements to either the metal or nonmetal category.^[267] Furthermore, Jones emphasized that classification systems typically rely on more than two attributes to define distinct types.^[268]

An approach to distinguishing between metallic and nonmetallic properties was suggested by Johnson, emphasizing the significance of physical properties, while acknowledging the potential need for other properties in certain ambiguous cases. His observations highlighted several key distinctions:^[273]

1. Physical state—Elements that exist as gases or are nonconductors are typically classified as nonmetals.
2. Solid nonmetals—Solid nonmetals exhibit characteristics such as hardness and brittleness or softness and crumbliness, setting them apart from metals that are generally malleable and ductile.
3. Chemical behavior—Nonmetal oxides tend to be acidic, providing another useful criterion for identifying nonmetals.

Hein and Arena observed that nonmetals generally have low densities and high electronegativity,^[64] which is consistent with the data presented in the table. Nonmetallic elements are predominantly located in the top left quadrant of this table, where both density and electronegativity values are relatively high. In contrast, the other three quadrants are primarily occupied by metals.

While some authors choose to further subdivide elements into metals, metalloids, and nonmetals, Oderberg disagrees with this approach. He argues that according to the principles of categorization, anything not classified as a metal should be considered a nonmetal.^[274]

 

In 1844, Alphonse Dupasquier [fr], a French doctor, pharmacist, and chemist,^[275] established a basic taxonomy of nonmetals to aid in the study of these elements. He wrote:^[276]

They will be divided into four groups or sections, as in the following:

Organogens O, N, H, C

Sulphuroids S, Se, P

Chloroides F, Cl, Br, I

Boroids B, Si.

Dupasquier's fourfold classification has echoes in the modern types of nonmetals. The organogens and sulphuroids are akin to the unclassified nonmetals. The chloroide nonmetals were later recognized independently as halogens.^[277] The boroid nonmetals eventually evolved into the metalloids, with this classification beginning as early as 1864.^[236] The noble gases were also identified as a distinct group among the nonmetals, dating back to as early as 1900.^[278]

The two tables in this section present a selection of physical and chemical properties of metals,^{[n 28]} metalloids, unclassified nonmetals, halogen nonmetals, and noble gases, based on the elements' most stable forms in ambient conditions.

The purpose is to show the left-to-right progression of metallic-to-nonmetallic character across the periodic table.^[279]

The dashed lines encircling 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 are presented in a loose order of ease of their determination.

Chemical properties start with general characteristics and proceed to more specific details.

† Hydrogen can also form alloy-like hydrides^[166]
‡ The labels low, moderate, high, and very high are arbitrarily based on the value spans listed in the table