This article is about the chemical elements that are not metals. For other meanings, seeNonmetal (disambiguation).
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In the context of the periodic table, anonmetal is achemical element that mostly lacks distinctivemetallic properties. They range from colorless gases likehydrogen to shiny crystals likeiodine. Physically, they are usually lighter (less dense) than elements that form metals and are often poor conductors ofheat andelectricity. Chemically, nonmetals have relatively highelectronegativity or usually attract electrons in a chemical bond with another element, and their oxides tend to beacidic.
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 andhelium, together account for about 98% of the mass of theobservable universe. Five nonmetallic elements—hydrogen, carbon,nitrogen,oxygen, andsilicon—form the bulk of Earth'satmosphere,biosphere,crust andoceans, although metallic elements are believed to be slightly more than half of the overall composition of the Earth.
Most nonmetallic elements were identified in the 18th and 19th centuries. While a distinction between metals and other minerals had existed since antiquity, a 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. In contemporary research usage it is common to use adistinction between metal and not-a-metal based upon the electronic structure of the solids; the elements carbon, arsenic and antimony are thensemimetals, a subclass of metals. The rest of the nonmetallic elements are insulators, some of which such as silicon and germanium can readily accommodatedopants that change the electrical conductivity leading tosemiconducting behavior.
Whilearsenic (here sealed in a container to preventtarnishing) has a shiny appearance and is a reasonable conductor of heat and electricity, it is soft and brittle and its chemistry is predominately nonmetallic.[6]
Nonmetallicchemical elements are often broadly defined as those that mostly lack properties commonly associated with metals—namely shininess, pliability, good thermal and electrical conductivity (due to theirband structure), and a general capacity to form basic oxides.[7][8] There is no widely accepted precise definition in terms of these properties;[9] any list of nonmetals is open to debate and revision.[1]
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 intermediate between metals and nonmetals:[10]
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.[14]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".[15][c] The shininess of boron, graphite (carbon), silicon, black phosphorus, germanium, arsenic, selenium, antimony, tellurium, and iodine[d] is a result of the electrons reflecting incoming visible light.[18]
About half of nonmetallic elements are gases understandard 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 andboiling points, and are poor conductors of heat and electricity.[19] The solid nonmetals have low densities and low mechanical strength (being either hard and brittle, or soft and crumbly),[20] and a wide range of electrical conductivity.[e]
This diversity stems from variability in crystallographic 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 weakLondon dispersion forces acting between their atoms or molecules, although the molecules themselves have strong covalent bonds.[24] In contrast, nonmetals that form extended structures, such as long chains of selenium atoms,[25] sheets of carbon atoms in graphite,[26] or three-dimensional lattices of silicon atoms[27] have higher melting and boiling points, and are all solids. Nonmetals closer to the left or bottom of the periodic table (and so closer to the metals) often havemetallic interactions between their molecules, chains, or layers; this occurs in boron,[28] carbon,[29] phosphorus,[30] arsenic,[31] selenium,[32] antimony,[33] tellurium[34] and iodine.[35]
Some general physical differences between elemental metals and nonmetals[19]
Aspect
Metals
Nonmetals
Appearance and form
Shiny if freshly prepared or fractured; few colored;[36] all but one solid[37]
Shiny, colored or transparent;[38] all but one solid or gaseous[37]
Covalently bonded nonmetals often share only the electrons required to achieve a noble gas electron configuration.[41] For example, nitrogen forms diatomic molecules featuring a triple bonds between each atom, both of which thereby attain the configuration of the noble gas neon. In contrast antimony has buckled layers in which each antimony atom is singly bonded with three other nearby atoms.[42]
Good electrical conductivity occurs when there ismetallic bonding,[43] however the electrons in some nonmetals are not metallic.[43] Good electrical and thermal conductivity associated with metallic electrons is seen in carbon (as graphite, along its planes), arsenic, and antimony.[f] Good thermal conductivity occurs in boron, silicon, phosphorus, and germanium;[21] such conductivity is transmitted though vibrations of the crystalline lattices (phonons of these elements.[44] Moderate electrical conductivity is observed in the semiconductors[45] boron, silicon, phosphorus, germanium, selenium, tellurium, and iodine.
Many of the nonmetallic elements are hard and brittle,[20] wheredislocations cannot readily move so they tend to undergobrittle fracture rather than deforming.[46] Some do deform such aswhite phosphorus (soft as wax, pliable and can be cut with a knife at room temperature),[47]plastic sulfur,[48] and selenium which can be drawn into wires from its molten state.[49] Graphite is a standardsolid lubricant where dislocations move very easily in the basal planes.[50]
Over half of the nonmetallic elements exhibit a range of less stable allotropic forms, each with distinct physical properties.[51] For example, carbon, the most stable form of which isgraphite, can manifest asdiamond,buckminsterfullerene,[52]amorphous[53] andparacrystalline[54] variations. Allotropes also occur for nitrogen, oxygen, phosphorus, sulfur, selenium and iodine.[55]
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 itsoxidation state is low, or both. These non-acidic oxides of nonmetals may beamphoteric (like water, H2O[61]) or neutral (likenitrous oxide, N2O[62][g]), but never basic.
They tend to gain electrons during chemical reactions, in contrast to metallic elements which tend to donate electrons. This behavior is related to the stability ofelectron configurations in the noble gases, which have complete outershells, empirically described by theduet andoctet rules of thumb, more correctly explained in terms ofvalence bond theory.[65]
The chemical differences between metals and nonmetals stem from variations in how strongly atoms attract and retain electrons. Across a period of the periodic table, the nuclear charge increases as more protons are added to the nucleus.[66] However, because the number of inner electron shells remains constant, theeffective nuclear charge experienced by the outermost electrons also increases, pulling them closer to the nucleus. This leads to a corresponding reduction in atomic radius,[67] and a greater tendency of these elements to gain electrons during chemical reactions, forming negatively charged ions.[68] Nonmetals, which occupy the right-hand side of the periodic table, exemplify this behavior.
Nonmetals typically exhibit higherionization energies,electron affinities, andstandard electrode potentials than metals. The higher these values are (including electronegativity) the more nonmetallic the element tends to be.[69] For example, the chemically very active nonmetals fluorine, chlorine, bromine, and iodine have an average electronegativity of 3.19—a figure[h] higher than that of any metallic element.
The number of compounds formed by nonmetals is vast.[70] The first 10 places in a "top 20" table of elements most frequently encountered in 895,501,834 compounds, as listed in theChemical 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.[71]
Adding complexity to the chemistry of the nonmetals are anomalies occurring in the first row of eachperiodic table block; non-uniform periodic trends; higher oxidation states; multiple bond formation; and property overlaps with metals.
Starting with hydrogen, thefirst-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 anaqueous solution, leaving behind a bare proton with high polarizing power.[73] Consequently, this proton can attach itself to the lone electron pair of an oxygen atom in a water molecule, laying the foundation foracid–base chemistry.[74] Moreover, a hydrogen atom in a molecule can form asecond, albeit weaker, bond with an atom or group of atoms in another molecule. Such bonding, "helps givesnowflakes their hexagonal symmetry, bindsDNA into adouble helix; shapes the three-dimensional forms ofproteins; and even raises water's boiling point high enough to make a decent cup of tea."[75]
Hydrogen and helium, as well as boron through neon, have small atomic radii. The ionization energies and electronegativities among these elements are higher than theperiodic trends would otherwise suggest.
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.[76]
Electronegativity values of the group 16chalcogen elements showing a W-shaped alternation or secondary periodicity going down the group
An alternation in certain periodic trends, sometimes referred to assecondary periodicity, becomes evident when descending groups 13 to 15, and to a lesser extent, groups 16 and 17.[77][j] Immediately after the first row ofd-block metals, from scandium to zinc, the 3d electrons in thep-block elements—specifically, gallium (a metal), germanium, arsenic, selenium, and bromine—prove less effective atshielding the increasing positive nuclear charge.
The Soviet chemistShchukarev [ru] gives two more tangible examples:[79]
"The toxicity of some arsenic compounds, and the absence of this property in analogous compounds of phosphorus [P] and antimony [Sb]; and the ability ofselenic 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 exhibitoxidation 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 includeammonia NH3,hydrogen sulfide H2S,hydrogen fluoride HF, and elemental xenon Xe. Meanwhile, the maximum possible oxidation state increases from +5 ingroup 15, to +8 ingroup 18. The +5 oxidation state is observable from period 2 onward, in compounds such asnitric acid HN(V)O3 andphosphorus pentafluoride PCl5.[k]Higher oxidation states in later groups emerge from period 3 onwards, as seen insulfur hexafluoride SF6,iodine heptafluoride IF7, andxenon(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 bulkcoordination numbers.[80]
Molecular structure ofpentazenium, a homopolyatomic cation of nitrogen with the formula N5+ and structure N−N−N−N−N.[81]
Period 2 nonmetals, particularly carbon, nitrogen, and oxygen, show a propensity to form multiple bonds. The compounds formed by these elements often exhibit uniquestoichiometries and structures, as seen in the various nitrogen oxides,[80] 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[82] (although not as yet more precisequantum chemistry) Humphrey[83] 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.
Boron (here in its less stable amorphous form) shares some similarities with metals[l]
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);[60]
The electrical conductivity of graphite exceeds that of some metals;[m]
Radon is the most metallic of the noble gases and begins to show somecationic behavior, which is unusual for a nonmetal;[87] and
In extreme conditions, just over half of nonmetallic elements can form homopolyatomic cations.[n]
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,[89] and predominately acidic oxides.[8][90]
Gold, the "king of metals" has the highestelectrode 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.[91] The reason for this is that gold has a large enough nuclear potential that the electrons have to be considered withrelativistic effects included, which changes some of the properties.[92]
A relatively recent development involves certain compounds of heavier p-block elements, such as silicon, phosphorus, germanium, arsenic and antimony, exhibiting behaviors typically associated withtransition metal complexes. This is linked to a small energy gap between theirfilled and emptymolecular orbitals, which are the regions in a molecule where electrons reside and where they can be available for chemical reactions. In such compounds, this allows for unusual reactivity with small molecules like hydrogen (H2),ammonia (NH3), andethylene (C2H4), a characteristic previously observed primarily in transition metal compounds. These reactions may open new avenues incatalytic applications.[93]
Nonmetal classification schemes vary widely, with some accommodating as few as two subtypes and others up to seven. For example, the periodic table in theEncyclopaedia Britannica recognizes noble gases, halogens, and other nonmetals, and splits the elements commonly recognized as metalloids between "other metals" and "other nonmetals".[94] On the other hand, seven of twelve color categories on the Royal Society of Chemistry periodic table include nonmetals.[95][o]
Starting on the right side of the periodic table, three types of nonmetals can be recognized:
the relatively unreactive noble gases—helium, neon, argon, krypton, xenon, radon;[96]
the reactive halogen nonmetals—fluorine, chlorine, bromine, iodine;[97] and
the mixed reactivity "unclassified nonmetals", a set with no widely used collective name—hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur, selenium.[q] The descriptive phraseunclassified nonmetals is used here for convenience.
The elements in a fourth set are sometimes recognized as nonmetals:
the generally unreactive[s] metalloids,[115] sometimes considered a third category distinct from metals and nonmetals—boron, silicon, germanium, arsenic, antimony, tellurium.
While many of the early workers attempted to classify elements none of their classifications were satisfactory. They were divided into metals and nonmetals, but some were soon found to have properties of both. These were called metalloids. This only added to the confusion by making two indistinct divisions where one existed before.[116]
Whiteford & Coffin 1939,Essentials of College Chemistry
The boundaries between these types are not sharp.[t] 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".[118] Some consider metalloids distinct from both metals and nonmetals, while others classify them as nonmetals.[119] Some categorize certain metalloids as metals (e.g., arsenic and antimony due to their similarities toheavy metals).[120][u] Metalloids resemble the elements universally considered "nonmetals" in having relatively low densities, high electronegativity, and similar chemical behavior.[115][v]
A small (about 2 cm long) piece of rapidly meltingargon ice
Six nonmetals are classified as noble gases: helium, neon, argon, krypton, xenon, and the radioactive radon. In conventional periodic tables they occupy the rightmost column. They are callednoble gases due to their exceptionally lowchemical reactivity.[96]
These elements exhibit similar properties, being colorlessness, odorless, and nonflammable. Due to their closed outer electron shells, noble gases possess weakinteratomic forces of attraction, leading to exceptionally low melting and boiling points.[121]
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,[122] with most of these compounds involving the combination of oxygen or fluorine with either krypton, xenon, or radon.[123]
Chemically, the halogen nonmetals have high ionization energies, electron affinities, and electronegativity values, and are relatively strongoxidizing agents.[124] All four elements tend to form primarilyionic compounds with metals,[125] in contrast to the remaining nonmetals (except for oxygen) which tend to form primarilycovalent compounds with metals.[w]
Hydrogen behaves in some respects like a metallic element and in others like a nonmetal.[130] Like a metallic element it can, for example, form asolvated cation inaqueous solution;[131] it can substitute foralkali metals in compounds such as the chlorides (NaCl cf.HCl) and nitrates (KNO3 cf.HNO3), and in certain alkali metalcomplexes[132][133] as a nonmetal.[134] It attains this configuration by forming a covalent or ionic bond[135] or by bonding as an ion to a lone pair of electrons.[136]
Some or all of these nonmetals share several properties. Being generally less reactive than the halogens,[137] most of them can occur naturally in the environment.[138] Collectively, their physical and chemical characteristics can be described as "moderately non-metallic".[139] When combined with metals, the unclassified nonmetals can forminterstitial orrefractory compounds.[140] They also exhibit a tendency tobond to themselves, particularly in solid compounds.[141] Additionally,diagonal periodic table relationships among these nonmetals mirror similar relationships among the metalloids.[142]
Five nonmetals—hydrogen, carbon, nitrogen, oxygen, and silicon—form the bulk of the directly observable structure of the Earth: about 73% of thecrust, 93% of thebiomass, 96% of thehydrosphere, and over 99% of theatmosphere, as shown in the accompanying table. Silicon and oxygen form stable tetrahedral structures, known assilicates. Here, "the powerful bond that unites the oxygen and silicon ions is the cement that holds the Earth's crust together."[150] However, they make up less than 50% of the total weight of the earth.[147]
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".[151]
Nine of the 23 nonmetallic elements are gases, or form compounds that are gases, and are extracted fromnatural gas orliquid air, including hydrogen, nitrogen, oxygen, sulfur, and most of the noble gases. For example, nitrogen and oxygen are extracted from liquid air throughfractional distillation[152] and sulfur from the hydrogen sulfide in natural gas by reacting it with oxygen to yield water and sulfur.[153] Three nonmetals are extracted from seawater; the rest of the nonmetals – and almost all metals – from mining solid ores.[citation needed]
Group (1, 13−18)
Period
13
14
15
16
1/17
18
(1−6)
H
He
1
B
C
N
O
F
Ne
2
Si
P
S
Cl
Ar
3
Ge
As
Se
Br
Kr
4
Sb
Te
I
Xe
5
Rn
6
Nonmetallic elements are extracted from these sources:[138]
Cylinders containing argon gas for use in extinguishing fire without damagingcomputer server equipment
Nonmetallic elements are present in combination with other elements in almost everything around us, from water to plastics and within metallic alloys. There are some specific uses of the elements themselves, although these are less common; extensive details can be found in the specific pages of the relevant elements. A few examples are:
Oxygen is a critical component of the air we breath. (While nitrogen is also present, it is less used from the air, mainly by certain bacteria.[158]) Oxygen gas and liquid is also heavily used for combustion inwelding and cutting torches and as a component ofrocket fuels.[159]
Silicon is the most widely used semiconductor. While ultra-pure silicon is an insulator, by selectively addingelectronic dopants it can be used as asemiconductor where thechemical potential of the electrons can be manipulated, this being exploited in a wide range ofelectronic devices.[160]
French nobleman and chemistAntoine Lavoisier (1743–1794), with a page of the English translation of his 1789Traité élémentaire de chimie,[165] listing the elemental gases oxygen, hydrogen and nitrogen (and erroneously includinglight andcaloric); the nonmetallic substances sulfur, phosphorus, and carbon; and thechloride,fluoride andborate ions
In the late 1700s, French chemistAntoine Lavoisier published the first modern list of chemical elements in his revolutionary[166] 1789Traité 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,[167] andearths (heat-resistant oxides).[168] 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.[169] Lavoisier's chemistry was "dualistic",: "salts" were combinations of "acid" and "base"; acids where combinations of oxygen and metals while bases where combinations of oxygen and nonmetals. This view prevailed despite increasing evidence that chemicals likechlorine andammonia contained no oxygen, in large part due the vigious if sometimes misguided defense by the Swedish chemistBerzelius.[167]: 165
In 1802 the term "metalloids" was introduced for elements with the physical properties of metals but the chemical properties of non-metals.[170] In 1811 Berzelius used the term "metalloids"[171] to describe all nonmetallic elements, noting their ability to formnegatively charged ions with oxygen inaqueous solutions.[172][173] Drawing on this, in 1864 the "Manual of Metalloids" divided all elements into either metals or metalloids, with the latter group including elements now called nonmetals.[174]: 31 Reviews of the book indicated that the term "metalloids" was still endorsed by leading authorities,[175] but there were reservations about its appropriateness. While Berzelius' terminology gained significant acceptance,[176] it later faced criticism from some who found it counterintuitive,[173] misapplied,[177] or even invalid.[178] The idea of designating elements likearsenic as metalloids had been considered.[175] The use of the term "metalloids" persisted in France as textbooks of chemistry appeared in the 1800s. During this period, "metals" as a chemical category were characterized by a single property, their affinity for oxygen, while "metalloids" were organized by comparison of many characteristic analogous to the approach ofnaturalists.[179]
In 1844,Alphonse Dupasquier [fr], a French doctor, pharmacist, and chemist,[180] established a basic taxonomy of nonmetals to aid in their study. He wrote:[181]
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.[182] The boroids eventually evolved into the metalloids, with this classification beginning from as early as 1864.[175] The then unknown noble gases were recognized as a distinct nonmetal group after being discovered in the late 1800s.[183] This taxonomy was noted as a "natural classification" of the substance considering all aspects rather than an single characteristic like oxygen affinity.[184] It was a significant departure from other contemporary classifications, since it grouped together oxygen, nitrogen, hydrogen, and carbon.[185]
In 1828 and 1859, the French chemistDumas classified nonmetals as (1) hydrogen; (2) fluorine to iodine; (3) oxygen to sulfur; (4) nitrogen to arsenic; and (5) carbon, boron and silicon,[186] thereby anticipating the vertical groupings of Mendeleev's 1871 periodic table. Dumas' five classes fall into modern groups1,17,16,15, and14 to13 respectively.
By as early as 1866, some authors began preferring the term "nonmetal" over "metalloid" to describe nonmetallic elements.[187] In 1875, Kemshead[188] 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.
The early terminologies were empirical categorizations based upon observables. As the 20th century started there were significant changes in understanding. The first was due to methods, mainlyx-ray crystallography, for determining how atoms are arranged in the various materials. As early as 1784René Just Haüy discovered that every face of a crystal could be described by simple stacking patterns of blocks of the same shape and size (law of decrements).[189] Haüy's study led to the idea that crystals are a regular three-dimensional array (aBravais lattice) of atoms andmolecules, with a singleunit cell repeated indefinitely, along with other developments in theearly days of physical crystallography. AfterMax von Laue demonstrated in 1912 that x-rays diffract,[190] fairly quicklyWilliam Lawrence Bragg and his fatherWilliam Henry Bragg were able to solve previously unknown structures.[191][192][193] Building on this, it became clear that most of the simple elemental metals hadclose packed structures. With this determined the concept ofdislocations originally developed byVito Volterra in 1907[194] became accepted, for instance being used to explain the ductility of metals byG. I. Taylor in 1934.[195]
From this the concept of nonmetals as "not-a-metal" originates. The original approach to describe metals and nonmetals was a band-structure withdelocalized electrons (i.e. spread out in space). A nonmetal has agap in theenergy levels of the electrons at theFermi level.[160]: Chpt 8 & 19 In contrast, a metal would have at least one partially occupied band at the Fermi level;[160] in a semiconductor or insulator there are no delocalized states at the Fermi level, see for instanceAshcroft and Mermin.[160] (Asemimetal is similar to a metal, with a slightly more complex band structure.) These definitions are equivalent to stating that metals conduct electricity atabsolute zero, as suggested byNevill Francis Mott,[199]: 257 and the equivalent definition at other temperatures is also commonly used as in textbooks such asChemistry of the Non-Metals byRalf Steudel[200] and work onmetal–insulator transitions.[201][202]
Originally[203][204] this band structure interpretation was based upon a single-electron approach with the Fermi level in the band gap as illustrated in the Figure, not including a complete picture of themany-body problem where bothexchange andcorrelation terms matter, as well asrelativistic effects such asspin-orbit coupling. For instance, the passivity of gold is typically associated with relativistic terms.[205] A key addition by Mott andRudolf Peierls was that these could not be ignored.[206] For instance,nickel oxide would be a metal if a single-electron approach was used, but in fact has quite a large band gap.[207] As of 2024 it is more common to use an approach based upondensity functional theory where the many-body terms are included.[208][209] Although accurate calculations remain a challenge, reasonable results are now available in many cases.[210][211]
It is common to nuance the early definition ofAlan Herries Wilson and Mott which was for a zero temperature. As discussed byPeter Edwards and colleagues,[212] as well asFumiko Yonezawa,[199]: 257–261 it is important to consider the temperatures at which both metals and nonmetals are used. Yonezawa provides a general definition for both general temperatures and conditions (such as standard temperature and pressure):[199]: 260
When a material conducts and at the same time the temperature coefficient of the electric conductivity of that material is not positive under a certain environmental condition, the material is metallic under that environmental condition. A material which does not satisfy these requirements is not metallic under that environmental condition.
The precise meaning of semiconductor needs a little care. In terms of the temperature dependence of their conductivity they are all classified as insulators; the pure forms areintrinsic semiconductors. When they are doped their conductivity continues to increase with temperature,[160] and can become substantial; hence the ambiguities with an empirical categorisation using conductivity described earlier. Indeed, some elements that are normally considered as insulators have been exploited as semiconductors. For instance diamond, which has the largest band gap of the elements that are solids under normal conditions,[213] has a number of semiconductor applications.[214][215]
Band structure definitions of metals and nonmetals are widely used in current research into materials, and apply both to single elements such as insulating boron[216] as well as compounds such asstrontium titanate.[217] The characteristics associated with metals and nonmetals in early work such as their appearance and mechanical properties are now understood to be consequences of how the atoms and electrons are arranged.
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.
† Hydrogen can also form alloy-like hydrides[133] ‡ The labelslow,moderate,high, andvery high are arbitrarily based on the value spans listed in the table
^At higher temperatures and pressures the numbers of nonmetals can be called into question. For example, when germanium melts it changes from a semiconductor to a metallic conductor with an electrical conductivity similar to that of liquid mercury.[12] At a high enough pressure,sodium (a metal) becomes a non-conductinginsulator.[13]
^The absorbed light may be converted to heat or re-emitted in all directions so that the emission spectrum is thousands of times weaker than the incident light radiation.[16]
^Solid iodine has a silvery metallic appearance under white light at room temperature. At ordinary and higher temperatures itsublimes from the solid phase directly into a violet-colored vapor.[17]
^The solid nonmetals have electrical conductivity values ranging from 10−18 S•cm−1 for sulfur[21] to 3 × 104 in graphite[22] or 3.9 × 104 forarsenic;[23] cf. 0.69 × 104 formanganese to 63 × 104 forsilver, both metals.[21] The conductivity of graphite and arsenic (both semimetals) exceed that of manganese.
^Thermal conductivity values for metals range from 6.3 W m−1 K−1 forneptunium to 429 forsilver; cf. antimony 24.3, arsenic 50, and carbon 2000.[21] Electrical conductivity values of metals range from 0.69 S•cm−1 × 104 formanganese to 63 × 104 forsilver; cf. carbon 3 × 104,[22] arsenic 3.9 × 104 and antimony 2.3 × 104.[21]
^WhileCO andNO are commonly referred to as being neutral, CO is a slightly acidic oxide, reacting with bases to produce formates (CO + OH− → HCOO−);[63] and in water, NO reacts with oxygen to form nitrous acid HNO2 (4NO + O2 + 2H2O → 4HNO2).[64]
^Electronegativity values of fluorine to iodine are: 3.98 + 3.16 + 2.96 + 2.66 = 12.76/4 3.19.
^Helium is shown above beryllium for electron configuration consistency purposes; as a noble gas it is usually placed above neon, in group 18.
^The net result is an even-odd difference between periods (except in thes-block): elements in even periods have smaller atomic radii and prefer to lose fewer electrons, while elements in odd periods (except the first) differ in the opposite direction. Many properties in the p-block then show a zigzag rather than a smooth trend along the group. For example, phosphorus and antimony in odd periods of group 15 readily reach the +5 oxidation state, whereas nitrogen, arsenic, and bismuth in even periods prefer to stay at +3.[78]
^Oxidation states do not reflect the actual net charge of atoms in molecules or ions, they represents the valence which refers more to how many bonds there are. For instance carbon typically has a valence of +4, but that only means that it forms three bonds. Electronegative elements such as fluorine are conventionally associated with negative valence, while electropositive ones have positive valence.
^Greenwood[84] commented that: "The extent to which metallic elements mimic boron (in having fewer electrons than orbitals available for bonding) has been a fruitful cohering concept in the development of metalloborane chemistry ... Indeed, metals have been referred to as "honorary boron atoms" or even as "flexiboron atoms". The converse of this relationship is clearly also valid."
^For example, the conductivity of graphite is 3 × 104 S•cm−1.[85] whereas that ofmanganese is 6.9 × 103 S•cm−1.[86]
^A homopolyatomic cation consists of two or more atoms of the same element bonded together and carrying a positive charge, for example, N5+, O2+ and Cl4+. This is unusual behavior for nonmetals since cation formation is normally associated with metals, and nonmetals are normally associated with anion formation. Homopolyatomic cations are further known for carbon, phosphorus, antimony, sulfur, selenium, tellurium, bromine, iodine and xenon.[88]
^ Of the twelve categories in the Royal Society periodic table, five only show up with the metal filter, three only with the nonmetal filter, and four with both filters. Interestingly, the six elements marked as metalloids (boron, silicon, germanium, arsenic, antimony, and tellurium) show under both filters. Six other elements (113–118: nihonium, flerovium, moscovium, livermorium, tennessine, and oganesson), whose status is unknown, also show up under both filters but are not included in any of the twelve color categories.
^The quote marks are not found in the source; they are used here to make it clear that the source employs the wordnon-metals as a formal term for the subset of chemical elements in question, rather than applying to nonmetals generally.
^Varying configurations of these nonmetals have been referred to as, for example, basic nonmetals,[98] bioelements,[99] central nonmetals,[100] CHNOPS,[101] essential elements,[102] "non-metals",[103][p] orphan nonmetals,[104] or redox nonmetals.[105]
^Arsenic is stable in dry air. Extended exposure in moist air results in the formation of a black surface coating. "Arsenic is not readily attacked by water, alkaline solutions or non-oxidizing acids".[110] It can occasionally be found in nature in an uncombined form.[111] It has a positive standard reduction potential (As → As3+ + 3e = +0.30 V), corresponding to a classification of semi-noble metal.[112]
^Boundary fuzziness and overlaps often occur in classification schemes.[117]
^Jones takes a philosophical or pragmatic view to these questions. He writes: "Though classification is an essential feature of all branches of science, there are always hard cases at the boundaries. The boundary of a class is rarely sharp... Scientists should not lose sleep over the hard cases. As long as a classification system is beneficial to economy of description, to structuring knowledge and to our understanding, and hard cases constitute a small minority, then keep it. If the system becomes less than useful, then scrap it and replace it with a system based on different shared characteristics."[117]
^Metal oxides are usually somewhat ionic, depending upon the metal element electropositivity.[126] On the other hand, oxides of metals with high oxidation states are often either polymeric or covalent.[127] A polymeric oxide has a linked structure composed of multiple repeating units.[128]
^All four have less stable non-brittle forms: carbon asexfoliated (expanded) graphite,[227][228] and ascarbon nanotube wire;[229] phosphorus as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature);[47] sulfur as plastic sulfur;[48] and selenium as selenium wires.[49]
^Metals have electrical conductivity values of from6.9×103 S•cm−1 formanganese to6.3×105 forsilver.[231]
^Metalloids have electrical conductivity values of from1.5×10−6 S•cm−1 for boron to3.9×104 forarsenic.[232]
^Unclassified nonmetals have electrical conductivity values of from ca.1×10−18 S•cm−1 for the elemental gases to3×104 in graphite.[85]
^Elemental gases have electrical conductivity values of ca.1×10−18 S•cm−1.[85]
^Metalloids always give "compounds less acidic in character than the corresponding compounds of the [typical] nonmetals."[218]
^Arsenic trioxide reacts with sulfur trioxide, formingarsenic "sulfate" As2(SO4)3.[241] This substance is covalent in nature rather than ionic;[242] it is also given as As2O3·3SO3.[243]
^Metals that form glasses are: vanadium; molybdenum, tungsten; alumnium, indium, thallium; tin, lead; and bismuth.[249]
^Unclassified nonmetals that form glasses are phosphorus, sulfur, selenium;[249]CO2 forms a glass at 40 GPa.[251]
^Disodium helide (Na2He) is a compound of helium and sodium that is stable at high pressures above 113 GPa. Argon forms an alloy with nickel, at 140 GPa and close to 1,500 K, however at this pressure argon is no longer a noble gas.[259]
^Values for the noble gases are from Rahm, Zeng and Hoffmann.[260]
^abSteudel 2020, p. 43: Steudel's monograph is an updated translation of the fifth German edition of 2013, incorporating the literature up to Spring 2019.
^Taniguchi et al. 1984, p. 867: "... black phosphorus ... [is] characterized by the wide valence bands with rather delocalized nature.";Carmalt & Norman 1998, p. 7: "Phosphorus ... should therefore be expected to have some metalloid properties.";Du et al. 2010: Interlayer interactions in black phosphorus, which are attributed to van der Waals-Keesom forces, are thought to contribute to the smaller band gap of the bulk material (calculated 0.19 eV; observed 0.3 eV) as opposed to the larger band gap of a single layer (calculated ~0.75 eV).
^Steudel 2020, p. 601: "... Considerable orbital overlap can be expected. Apparently, intermolecular multicenter bonds exist in crystalline iodine that extend throughout the layer and lead to the delocalization of electrons akin to that in metals. This explains certain physical properties of iodine: the dark color, the luster and a weak electric conductivity, which is 3400 times stronger within the layers then perpendicular to them. Crystalline iodine is thus a two-dimensional semiconductor.";Segal 1989, p. 481: "Iodine exhibits some metallic properties ..."
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