A periodic table extract highlighting nonmetals

always/usually considered nonmetals[1][2][3]

metalloids, sometimes considered nonmetals[a]

status as nonmetal or metal unconfirmed[5]

In the context of the periodic table a nonmetal is a chemical element that mostly lacks distinctive metallic properties. They range from colorless gases like hydrogen to shiny crystals like iodine. Physically, they are usually lighter (less dense) than elements that form metals and are often poor conductors of heat and electricity. Chemically, nonmetals have relatively high electronegativity or usually attract electrons in a chemical bond with another element, 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 two lightest nonmetals, hydrogen and helium, together make up about 98% of the mass of the observable universe. Five nonmetallic elements—hydrogen, carbon, nitrogen, oxygen, and silicon—make up the bulk of Earth's atmosphere, biosphere, crust and oceans.

Industrial uses of nonmetals include in electronics, energy storage, agriculture, and chemical production.

Most nonmetallic elements were identified in the 18th and 19th centuries. While a distinction between metals and other minerals had existed since antiquity, a basic classification of chemical elements as metallic or nonmetallic emerged only in the late 18th century. Since then about twenty properties have been suggested as criteria for distinguishing nonmetals from metals.

Unless otherwise noted, this article describes the stable form of an element at standard temperature and pressure (STP).^[b]

Nonmetallic chemical elements are often described as lacking properties common to metals, namely shininess, pliability, good thermal and electrical conductivity, and a general capacity to form basic oxides.^[8][9] There is no widely accepted precise definition;^[10] any list of nonmetals is open to debate and revision.^[1] The elements included depend on the properties regarded as most representative of nonmetallic or metallic character.

Fourteen elements are almost always recognized as nonmetals:^[1][2]

Three more are commonly classed as nonmetals, but some sources list them as "metalloids",^[3] a term which refers to elements regarded as intermediate between metals and nonmetals:^[11]

One or more of the six elements most commonly recognized as metalloids are sometimes instead counted as nonmetals:

About 15–20% of the 118 known elements^[12] are thus classified as nonmetals.^[c]

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 yellow chunks

Liquid bromine at room temperature

Metallic appearance of iodine under white light

Liquefied xenon

Nonmetals vary greatly in appearance, being colorless, colored or shiny. For the colorless nonmetals (hydrogen, nitrogen, oxygen, and the noble gases), no absorption of light happens in the visible part of the spectrum, and all visible light is transmitted.^[15] 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".^[16][d] The shininess of boron, graphite (carbon), silicon, black phosphorus, germanium, arsenic, selenium, antimony, tellurium, and iodine^[e] is a result of varying degrees of metallic conduction where the electrons can reflect incoming visible light.^[19]

About half of nonmetallic elements are gases under standard temperature and pressure; most of the rest are solids. Bromine, the only liquid, is usually topped by a layer of its reddish-brown fumes. The gaseous and liquid nonmetals have very low densities, melting and boiling points, and are poor conductors of heat and electricity.^[20] The solid nonmetals have low densities and low mechanical strength (being either hard and brittle, or soft and crumbly),^[21] and a wide range of electrical conductivity.^[f]

This diversity in form stems from variability in internal structures and bonding arrangements. Covalent 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, although the molecules themselves have strong covalent bonds.^[25] In contrast, nonmetals that form extended structures, such as long chains of selenium atoms,^[26] sheets of carbon atoms in graphite,^[27] or three-dimensional lattices of silicon atoms^[28] have higher melting and boiling points, and are all solids, as it takes more energy to overcome their stronger bonding.^{[29][dubious – discuss]} Nonmetals closer to the left or bottom of the periodic table (and so closer to the metals) often have metallic interactions between their molecules, chains, or layers; this occurs in boron,^[30] carbon,^[31] phosphorus,^[32] arsenic,^[33] selenium,^[34] antimony,^[35] tellurium^[36] and iodine.^[37]

Covalently bonded nonmetals often share only the electrons required to achieve a noble gas electron configuration.^[43] 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.^[44]

Good electrical conductivity occurs when there is metallic bonding,^[45] however the electrons in nonmetals are often not metallic.^[45] Good electrical and thermal conductivity associated with metallic electrons is seen in carbon (as graphite, along its planes), arsenic, and antimony.^[g] Good thermal conductivity occurs in boron, silicon, phosphorus, and germanium;^[22] such conductivity is transmitted though vibrations of the crystalline lattices of these elements.^[46] Moderate electrical conductivity is observed in the semiconductors^[47] boron, silicon, phosphorus, germanium, selenium, tellurium, and iodine.

Many of the nonmetallic elements are hard and brittle,^[21] where dislocations cannot readily move so they tend to undergo brittle fracture rather than deforming.^[48] Some do deform such as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature),^[49] in plastic sulfur,^[50] and in selenium which can be drawn into wires from its molten state.^[51] Graphite is a standard solid lubricant where dislocations move very easily in the basal planes.^[52]

Three allotropes of carbon

a transparent electrical insulator

a brownish semiconductor

a blackish conductor

Diamond, buckminsterfullerene, and graphite

Over half of the nonmetallic elements exhibit a range of less stable allotropic forms, each with distinct physical properties.^[53] For example, carbon, the most stable form of which is graphite, can manifest as diamond, buckminsterfullerene,^[54] amorphous^[55] and paracrystalline^[56] variations. Allotropes also occur for nitrogen, oxygen, phosphorus, sulfur, selenium and iodine.^[57]

Nonmetals have relatively high values of electronegativity, and their oxides are 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^[63]) or neutral (like nitrous oxide, N₂O^[64][h]), but never basic.

Nonmetals tend to gain electrons during chemical reactions, in contrast to metals which tend to donate electrons. This behavior is related to the stability of electron configurations in the noble gases, which have complete outer shells as summarized by the duet and octet rules of thumb, more correctly explained in terms of valence bond theory.^[67]

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.^[68] For example, the chemically very active nonmetals fluorine, chlorine, bromine, and iodine have an average electronegativity of 3.19—a figure^[i] higher than that of any metallic element.

The chemical distinctions between metals and nonmetals is connected to 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 (number of protons in the atomic nucleus) increases.^[69] There is a corresponding reduction in atomic radius^[70] as the increased nuclear charge draws the outer electrons closer to the nuclear core.^[71] In chemical bonding, nonmetals tend to gain electrons due to their higher nuclear charge, resulting in negatively charged ions.^[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 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.

Starting with hydrogen, the first row anomaly primarily arises from the electron configurations of the elements concerned. Hydrogen is 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. 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 less electron-electron exchange interactions, unlike the 3p, 4p, and 5p subshells of heavier elements.^{[83][dubious – discuss]} As a result, ionization energies and electronegativities among these elements are higher than the 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 almost always placed above neon, in group 18, rather than above beryllium in group 2.^[85]

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.^[86][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.

The Soviet chemist Shchukarev [ru] gives two more tangible examples:^[88]

"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."

Roman numerals such as III, V and VIII denote oxidation states

Some nonmetallic elements exhibit oxidation states that deviate from those predicted by the octet rule, which typically results in an oxidation state of –3 in group 15, –2 in group 16, –1 in group 17, and 0 in group 18. Examples include 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 HN(V)O₃ 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(VIII) 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.^[89]

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,^[89] which are not commonly found in elements from later periods.

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,^[91] Humphrey^[92] 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);^[62]
• The electrical conductivity of graphite exceeds that of some metals;^[n]
• Selenium can be drawn into a wire;^[51]
• Radon is the most metallic of the noble gases and begins to show some cationic behavior, which is unusual for a nonmetal;^[96] and
• In extreme conditions, 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, sometimes being brittle, having a high electronegativity, and forming only anions in aqueous solution,^[98] and predominately acidic oxides.^[9][99]
• Gold, the "king of metals" 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.^[100] Gold has a large enough nuclear potential that the electrons have to be considered with relativistic effects included which changes some of the properties.^[101]

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 is linked to a small energy gap between their filled and empty molecular orbitals, which are the regions in a molecule where electrons reside and where they can be available for chemical reactions. In such compounds, this 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.^[102]

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".^[103] On the other hand, seven of twelve color categories on the Royal Society of Chemistry periodic table include nonmetals.^[104][p]

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

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

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.^[4] Some categorize certain metalloids as metals (e.g., arsenic and antimony due to their similarities to heavy metals).^[128][v] Metalloids resemble the elements universally considered "nonmetals" in having relatively low densities, high electronegativity, and similar chemical behavior.^[124][w]

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.^[105]

These elements exhibit similar properties, characterized by their colorlessness, odorlessness, and nonflammability. Due to their closed outer electron shells, noble gases possess weak interatomic forces of attraction, leading to exceptionally low melting and boiling points.^[129] As a consequence, they all exist as gases under standard conditions, even those with atomic masses surpassing many typically solid elements.^[130]

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,^[131] with most of these compounds involving the combination of oxygen or fluorine with either krypton, xenon, or radon.^[132]

Highly reactive sodium metal (Na, left) combines with corrosive halogen nonmetal chlorine gas (Cl, right) to form stable, unreactive table salt (NaCl, center).

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".^[133]

Chemically, the halogen nonmetals exhibit high ionization energies, electron affinities, and electronegativity values, and are mostly relatively strong oxidizing agents.^[134] These characteristics contribute to their corrosive nature.^[135] All four elements tend to form primarily ionic compounds with metals,^[136] in contrast to the remaining nonmetals (except for oxygen) which tend to form primarily covalent compounds with metals.^[x] The highly reactive and strongly electronegative nature of the halogen nonmetals epitomizes nonmetallic character.^[140]

Hydrogen behaves in some respects like a metallic element and in others like a nonmetal.^[142] Like a metallic element it can, for example, form a solvated cation in aqueous solution;^[143] 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 complexes^[144][145] as a nonmetal.^[146] It attains this configuration by forming a covalent or ionic bond^[147] or, if it has initially given up its electron, by attaching itself to a lone pair of electrons.^[148]

Some or all of these nonmetals share several properties. Being generally less reactive than the halogens,^[149] most of them can occur naturally in the environment.^[150] They have significant roles in biology^[151] and geochemistry.^[152] Collectively, their physical and chemical characteristics can be described as "moderately non-metallic".^[152] Sometimes they have corrosive aspects. Carbon corrosion can occur in fuel cells.^[153] Untreated selenium in soils can lead to the formation of corrosive hydrogen selenide gas.^[154] Very different, when combined with metals, the unclassified nonmetals can form interstitial or refractory compounds^[155] due to their relatively small atomic radii and sufficiently low ionization energies.^[152] They also exhibit a tendency to bond to themselves, particularly in solid compounds.^[156] Additionally, diagonal periodic table relationships among these nonmetals mirror similar relationships among the metalloids.^[157]

The abundance of elements in the universe results from nuclear physics processes like nucleosynthesis and radioactive decay.

The volatile noble gas nonmetal elements are less abundant in the atmosphere than expected based their overall abundance due to cosmic nucleosynthesis. Mechanisms to explain this difference is an important aspect of planetary science.^[162] Even within that challenge, the nonmetal element Xe is unexpectedly depleted. A possible explanation comes from theoretical models of the high pressures in the Earth's core suggest there may be around 10¹³ tons of xenon, in the form of stable XeFe₃ and XeNi₃ intermetallic compounds.^[163]

Five nonmetals—hydrogen, carbon, nitrogen, oxygen, and silicon—form the bulk of the directly observable structure of the Earth: about 73% of the crust, 93% of the biomass, 96% of the hydrosphere, and over 99% of the atmosphere, as shown in the accompanying table. Silicon and oxygen form highly stable tetrahedral structures, known as silicates. Here, "the powerful bond that unites the oxygen and silicon ions is the cement that holds the Earth's crust together."^[164]

In the biomass, the relative abundance of the first four nonmetals (and phosphorus, sulfur, and selenium marginally) is attributed to a combination of relatively small atomic size, and sufficient spare electrons. These two properties enable them to bind to one another and "some other elements, to produce a molecular soup sufficient to build a self-replicating system."^[165]

Nine of the 23 nonmetallic elements are gases, or form compounds that are gases, and are extracted from natural gas or liquid air. These elements include hydrogen, helium, nitrogen, oxygen, neon, sulfur, argon, krypton, and xenon. For example, nitrogen and oxygen are extracted from air through fractional distillation of liquid air. This method capitalizes on their different boiling points to separate them efficiently.^[166] Sulfur was extracted using the Frasch process, which involved injecting superheated water into underground deposits to melt the sulfur, which is then pumped to the surface. This technique leveraged sulfur's low melting point relative to other geological materials. It is now obtained by reacting the hydrogen sulfide in natural gas, with oxygen. Water is formed, leaving the sulfur behind.^[167]

Nonmetallic elements are extracted from the following sources:^[150]

Uses of nonmetals and non-metallic elements are broadly categorized as domestic, industrial, attenuative (lubricative, retarding, insulating or cooling), and agricultural

Many have domestic and industrial applications in household accoutrements;^[169][z] medicine and pharmaceuticals;^[171] and lasers and lighting.^[172] They are components of mineral acids;^[173] and prevalent in plug-in hybrid vehicles;^[174] and smartphones.^[175]

A significant number have attenuative and agricultural applications. They are used in lubricants;^[176] and flame retardants and fire extinguishers.^[177] They can serve as inert air replacements;^[178] and are used in cryogenics and refrigerants.^[179] Their significance extends to agriculture, through their use in fertilizers.^[180]

Additionally, a smaller number of nonmetals or nonmetallic elements find specialized uses in explosives;^[181] and welding gases.^[182]

• 
  Nitric acid (here colored due to the presence of nitrogen dioxide) is often used in the explosives industry^[183]
• 
  A high-voltage circuit-breaker employing sulfur hexafluoride (SF₆) as its inert (air replacement) interrupting medium^[184]
• 
  A COIL (chemical oxygen iodine laser) system mounted on a Boeing 747 variant known as the YAL-1 Airborne Laser
• 
  Cylinders containing argon gas for use in extinguishing fire without damaging computer server equipment

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".^[aa] 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".^[185]

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.^[186]

The term "nonmetallic" dates 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.^[187]

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.^[188]

French nobleman and chemist Antoine Lavoisier (1743–1794), with a page of the English translation of his 1789 Traité élémentaire de chimie,^[189] 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

Just as the ancients distinguished metals from other minerals, similar distinctions developed as the modern idea of chemical elements emerged in the late 1700s. French chemist Antoine Lavoisier published the first modern list of chemical elements in his revolutionary^[190] 1789 Traité élémentaire de chimie. The 33 elements known to Lavoisier were categorized into four distinct groups, including gases, metallic substances, nonmetallic substances that form acids when oxidized,^[191] and earths (heat-resistant oxides).^[192] 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.^[193]

In 1802 the term "metalloids" was introduced for elements with the physical properties of metals but the chemical properties of non-metals.^[194] However, in 1811, the Swedish chemist Berzelius used the term "metalloids"^[195] to describe all nonmetallic elements, noting their ability to form negatively charged ions with oxygen in aqueous solutions.^[196][197] Thus in 1864, the "Manual of Metalloids" divided all elements into either metals or metalloids, with the latter group including elements now called nonmetals.^[198]: 31 Reviews of the book indicated that the term "metalloids" was still endorsed by leading authorities,^[199] but there were reservations about its appropriateness. While Berzelius' terminology gained significant acceptance,^[200] it later faced criticism from some who found it counterintuitive,^[197] misapplied,^[201] or even invalid.^[202][203] The idea of designating elements like arsenic as metalloids had been considered.^[199] By as early as 1866, some authors began preferring the term "nonmetal" over "metalloid" to describe nonmetallic elements.^[204] In 1875, Kemshead^[205] 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.

In 1844, Alphonse Dupasquier [fr], a French doctor, pharmacist, and chemist,^[206] established a basic taxonomy of nonmetals to aid in their study. He wrote:^[207]

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

Organogens—oxygen, nitrogen, hydrogen, carbon

Sulphuroids—sulfur, selenium, phosphorus

Chloroides—fluorine, chlorine, bromine, iodine

Boroids—boron, silicon.

Dupasquier's quartet parallels the modern nonmetal types. The organogens and sulphuroids are akin to the unclassified nonmetals. The chloroides were later called halogens.^[208] The boroids eventually evolved into the metalloids, with this classification beginning from as early as 1864.^[199] The then unknown noble gases were recognized as a distinct nonmetal group after being discovered in the late 1800s.^[209]

His taxonomy was noted for its natural basis.^[210][ab] That said, it was a significant departure from other contemporary classifications, since it grouped together oxygen, nitrogen, hydrogen, and carbon.^[212]

In 1828 and 1859, the French chemist Dumas classified nonmetals as (1) hydrogen; (2) fluorine to iodine; (3) oxygen to sulfur; (4) nitrogen to arsenic; and (5) carbon, boron and silicon,^[213] 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.

Much of the early analyses were phenomenological, and a variety of physical, chemical, and atomic properties have been suggested for distinguishing metals from nonmetals (or other bodies); a comprehensive early set of characteristics was stated by Rev Thaddeus Mason Harrisn in the 1803 Minor Encyclopedia .^[214]

METAL, in natural history and chemistry, the name of a class of simple bodies; of which it is observed, that they posses; a lustre; that they are opaque; that they arc fusible, or may be melted; that their specific gravity is greater than that of any other bodies yet discovered; that they are better conductors of electricity, than any other body; that they are malleable, or capable of being extended and flattened by the hammer; and that they are ductile or tenacious, that is, capable of being drawn out into threads or wires.

Some criteria did not last long; for instance in 1809, the British chemist and inventor Humphry Davy isolated sodium and potassium,^[231] their low densities contrasted with their metallic appearance, so the density property was tenuous although these metals was firmly established by their chemical properties.^[232]

Johnson^[233] has a similar approach to Mason, distinguishing between metals and nonmetals on the basis of their physical states, electrical conductivity, mechanical properties, and the acid-base nature of their oxides:

1. gaseous elements are nonmetals (hydrogen, nitrogen, oxygen, fluorine, chlorine and the noble gases);
2. liquids (mercury, bromine) are either metallic or nonmetallic: mercury, as a good conductor, is a metal; bromine, with its poor conductivity, is a nonmetal;
3. 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 boron, silicon and germanium, which are semiconductors and therefore not metals; and

c. soft and crumbly elements include carbon, phosphorus, sulfur, arsenic, antimony,^[ag] tellurium and iodine, which have acidic oxides indicative of nonmetallic character.^[ah]

Density and electronegativity in the periodic table[ai] H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po Rn Ra                                                                                                                                                 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Ac Th Pa U Np Pu Am Cm Bk Cf Es Electronegativity (EN): < 1.9 ≥ 1.9 (revised Pauling) Density (D):  < 7 g/cm3  D < 7 and EN ≥ 1.9 for all nonmetallic elements ≥ 7 g/cm3  D ≥ 7 or EN < 1.9 (or both) for all metals

Several authors^[238] 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^[239] 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.

There is not full agreement about the use of phenomenological properties. Emsley^[240] pointed out the complexity of this task, asserting that no single property alone can unequivocally assign elements to either the metal or nonmetal category. Some authors divide elements into metals, metalloids, and nonmetals, but Oderberg^[241] disagrees, arguing that by the principles of categorization, anything not classified as a metal should be considered a nonmetal.

Kneen and colleagues^[242] 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. The describe electrical conductivity as the key property, arguing that this is the most common approach.

One of the most commonly recognized properties used 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.^[243] However, plutonium, carbon, arsenic, and antimony appear to defy the norm. When plutonium (a metal) is heated within a temperature range of −175 to +125 °C its conductivity increases.^[244] Similarly, despite its common classification as a nonmetallic element, carbon (as graphite) is a semimetal which when heated experiences a decrease in electrical conductivity.^[245] Arsenic and antimony, which are occasionally classified as nonmetallic elements are also semimetals, and show behavior similar to carbon.^{[246][dubious – discuss]}

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 at standard temperature and pressure. The dashed lines around the columns for metalloids signify that the treatment of these elements as a distinct type can vary depending on the author, or classification scheme in use.

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

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

† Hydrogen can also form alloy-like hydrides^[145]
‡ The labels low, moderate, high, and very high are arbitrarily based on the value spans listed in the table

• CHON (carbon, hydrogen, oxygen, nitrogen)
• List of nonmetal monographs
• Metallization pressure
• Nonmetal (astrophysics)
• Period 1 elements (hydrogen & helium)
• Properties of nonmetals (and metalloids) by group