Nonmetal

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 over thirty properties have been suggested as criteria for distinguishing nonmetals from metals.

Definition and applicable elements

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



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. There is no widely-accepted precise definition; any list of nonmetals is open to debate and revision. The elements included depend on the properties regarded as most representative of nonmetallic or metallic character.

Fourteen elements are almost always recognized as nonmetals: Hydrogen · Nitrogen · Oxygen · Sulfur Fluorine · Chlorine · Bromine · Iodine Helium · Neon · Argon · Krypton · Xenon · Radon

Three more are commonly classed as nonmetals, but some sources list them as "metalloids", a term which refers to elements regarded as intermediate between metals and nonmetals: Carbon · Phosphorus · Selenium

One or more of the six elements most commonly recognized as metalloids are sometimes instead counted as nonmetals: Boron · Silicon · Germanium · Arsenic · Antimony · Tellurium

About 15–20% of the 118 known elements are thus classified as nonmetals.

Physical
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. 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". The shininess of boron, graphite (carbon), silicon, black phosphorus, germanium, arsenic, selenium, antimony, tellurium, and iodine is a result of varying degrees of metallic conduction where the electrons can reflect incoming visible light.

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. The solid nonmetals have low densities and low mechanical strength (often being brittle or crumbly), and a wide range of electrical conductivity.

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. In contrast, nonmetals that form extended structures, such as chains of up to 1,000 selenium atoms, sheets of carbon atoms in graphite, or three-dimensional lattices of silicon atoms have higher melting and boiling points, and are all solids, as it takes more energy to overcome their stronger bonding. 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, carbon, phosphorus, arsenic, selenium, antimony, tellurium and iodine.

Covalently bonded nonmetals often share only the electrons required to achieve a noble gas electron configuration. 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.

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

Plasticity, which depends upon the movement of dislocations, occurs under limited circumstances in white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature), in plastic sulfur, and in selenium which can be drawn into wires from its molten state. Graphite is a standard solid lubricant where dislocations move very easily in the basal planes.

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

Chemical
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 ) or neutral (like nitrous oxide, N2O ), but never basic.

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. These tendencies in nonmetallic elements are succinctly summarized by the duet and octet rules of thumb.

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

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 (number of protons in the atomic nucleus) increases. There is a corresponding reduction in atomic radius as the increased nuclear charge draws the outer electrons closer to the nuclear core. In chemical bonding, nonmetals tend to gain electrons due to their higher nuclear charge, resulting in negatively charged ions.

The number of compounds formed by nonmetals is vast. 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. A few examples of nonmetal compounds are: boric acid, used in ceramic glazes; selenocysteine , the 21st amino acid of life; phosphorus sesquisulfide (P4S3), found in strike anywhere matches; and teflon n), used to create non-stick coatings for pans and other cookware.

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
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. 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. 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."

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. 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.

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.

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. 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 gives two more tangible examples:
 * "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

 * 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 a valency of –3 in group 15, –2 in group 16, –1 in group 17, and 0 in group 18. Examples of such typical states can include compounds like ammonia N(III)H3, hydrogen sulfide(II) H2S, hydrogen fluoride(I) HF, and elemental xenon(0) 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. 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.

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, 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, Humphrey 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);
 * The electrical conductivity of graphite exceeds that of some metals;
 * Selenium can be drawn into a wire;
 * Radon is the most metallic of the noble gases and begins to show some cationic behavior, which is unusual for a nonmetal; and
 * In extreme conditions, just over half of nonmetallic elements can form homopolyatomic cations.

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, and predominately acidic oxides.
 * 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. Gold has a large enough nuclear potential that the electrons have to be considered with relativistic effects included which changes some of the properties.

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

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". On the other hand, seven of twelve color categories on the Royal Society of Chemistry periodic table include nonmetals.

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

the relatively inert noble gases—helium, neon, argon, krypton, xenon, radon;

the notably reactive halogen nonmetals—fluorine, chlorine, bromine, iodine; and

the mixed reactivity "unclassified nonmetals", a set with no widely used collective name—hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur, selenium. The descriptive phrase unclassified nonmetals is used here for convenience.

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

the generally unreactive metalloids, sometimes considered a third category distinct from metals and nonmetals—boron, silicon, germanium, arsenic, antimony, tellurium.

The boundaries between these types are not sharp. 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". Some consider metalloids distinct from both metals and nonmetals, while others classify them as nonmetals. Some categorize certain metalloids as metals (e.g., arsenic and antimony due to their similarities to heavy metals). Metalloids resemble the elements universally considered "nonmetals" in having relatively low densities, high electronegativity, and similar chemical behavior.

Noble gases
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.

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

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, with most of these compounds involving the combination of oxygen or fluorine with either krypton, xenon, or radon.

Halogen nonmetals
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".

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

Unclassified nonmetals


Hydrogen behaves in some respects like a metallic element and in others like a nonmetal. Like a metallic element it can, for example, form a solvated cation in aqueous solution; it can substitute for alkali metals in compounds such as the chlorides (NaCl cf. HCl) and nitrates (KNO3 cf. HNO3), and in certain alkali .etal o i ein this way as a nonmetal. It attains this configuration by forming a covalent or ionic bond or, if it has initially given up its electron, by attaching itself to a lone pair of electrons.

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

Abundance
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. 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.

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."

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."

Extraction
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. 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.

Nonmetallic elements are extracted from the following sources:

Gases (3): hydrogen, from methane; helium, from natural gas; sulfur, from hydrogen sulfide in natural gas

Liquids (9): nitrogen, oxygen, neon, argon, krypton and xenon from liquid air; chlorine, bromine and iodine from brine

Solids (12): boron, from borates; carbon occurs naturally as graphite; silicon, from silica; phosphorus, from phosphates; iodine, from sodium iodate; radon, as a decay product from uranium ores; fluorine, from fluorite; germanium, arsenic, selenium, antimony and tellurium, from sulfides.

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

Many such elements have domestic and industrial applications. They have uses in household accoutrements; medicine and pharmaceuticals; and lasers and lighting. They are components of mineral acids; and prevalent in plug-in hybrid vehicles; and smartphones.

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

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

Background
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". 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".

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.

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.

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.

Origin and use of the term
Just as the ancients distinguished metals from other minerals, a similar dichotomy 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 1789 Traité élémentaire de chimie. The elements were categorized into distinct groups, including gases, metallic substances, nonmetallic substances, and earths (heat-resistant oxides). 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.

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

Suggested distinguishing criteria
From the mid-1700s, a variety of physical, chemical, and atomic properties have been suggested for distinguishing metals from nonmetals (or other bodies), as listed in the accompanying table. Some of the earliest recorded properties are the (high) density and (good) electrical conductivity of metals.

In 1809, the British chemist and inventor Humphry Davy made a groundbreaking discovery that reshaped the understanding of metals and nonmetals. 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. Nevertheless, their classification as metals was firmly established by their distinct chemical properties.

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. 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. Similarly, despite its common classification as a nonmetal, when carbon (as graphite) is heated it experiences a decrease in electrical conductivity. Arsenic and antimony, which are occasionally classified as nonmetals, show behavior similar to carbon, highlighting the complexity of the distinction between metals and nonmetals.

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.

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. Furthermore, Jones emphasized that classification systems typically rely on more than two attributes to define distinct types.

Johnson distinguished 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, tellurium and iodine, which have acidic oxides indicative of nonmetallic character.

Several authors 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 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 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, a French doctor, pharmacist, and chemist, established a basic taxonomy of nonmetals to aid in their study. He wrote:


 * 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. The boroids eventually evolved into the metalloids, with this classification beginning from as early as 1864. The then unknown noble gases were recognized as a distinct nonmetal group after being discovered in the late 1800s.

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

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, 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.

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

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

† Hydrogen can also form alloy-like hydrides

‡ The labels low, moderate, high, and very high are arbitrarily based on the value spans listed in the table