Nonmetal

In chemistry, a nonmetal is a chemical element that usually gains one or more electrons when reacting with a metal and forms an acid when combined with oxygen and hydrogen. At room temperature about half are gases, one (bromine) is a liquid, and the rest are solids. Most solid nonmetals are shiny, whereas bromine is colored, and the remaining (gaseous) nonmetals are colored or colorless. The solids are either hard and brittle or soft and crumbly and, in contrast to most metals, tend to be poor conductors of heat and electricity with no structural uses (as is the case for nonmetals generally).

yellow powdery chunks
Sulfur as a yellow powder. When melted and cooled quickly it changes into rubbery ribbons of plastic sulfur, an allotropic form.[1]
a jar of small metal donut-like shapes
Germanium looks metallic, conducts electricity poorly and behaves chemically like a nonmetal.[2]
a small capped jar a quarter filled with a very dark liquid
About 25 ml of bromine, a liquid at room temperature, and an essential trace element.[3]
a partially filled ampoule containing a colorless liquid
A partially filled ampoule of liquefied xenon, set inside an acrylic cube. Xenon is otherwise a colorless gas at room temperature.

There is no universal agreement on which elements are nonmetals; the numbers generally range from fourteen to twenty-three, depending on the criterion or criteria of interest.

Different kinds of nonmetallic elements include, for example, (i) noble gases; (ii) halogens; (iii) elements such as silicon, which are sometimes instead called metalloids; and (iv) several remaining nonmetals, such as hydrogen and selenium. The latter have no widely recognised collective name and are hereafter informally referred to as "unclassified nonmetals". Metalloids have a predominately (weak) nonmetallic chemistry. The unclassified nonmetals are moderately nonmetallic, on a net basis. Halogens, such as bromine, are characterized by stronger nonmetallic properties and a tendency to form predominantly ionic compounds with metals. Noble gases such as xenon are distinguished by their reluctance to form compounds.

The distinction between different kinds of nonmetals is not absolute. Boundary overlaps, including with the metalloids, occur as outlying elements among each of the kinds of nonmetals show or begin to show less-distinct, hybrid-like, or atypical properties.

Although five times more elements are metals than nonmetals, two of the nonmetals—hydrogen and heliummake up about 99% of the observable universe by mass.[4] Another nonmetal, oxygen, makes up almost half of the Earth's crust, oceans, and atmosphere.[5]

Nonmetals largely exhibit a breadth of roles in sustaining life. Living organisms are composed almost entirely of the nonmetals hydrogen, oxygen, carbon, and nitrogen.[6] A near-universal use for nonmetals is in medicine and pharmaceuticals.

Definition and applicable elementsEdit

There is no rigorous definition of a nonmetal.[7] Broadly, any element lacking a preponderance of metallic properties such as luster, deformability, and good electrical conductivity,[n 1] can be regarded as a nonmetal.[11] Some variation may be encountered among authors as to which elements are regarded as nonmetals.

The fourteen elements effectively always recognized as nonmetals are hydrogen, oxygen, nitrogen, and sulfur; the corrosive halogens fluorine, chlorine, bromine, and iodine; and the noble gases helium, neon, argon, krypton, xenon, and radon.[n 2] Up to a further nine elements can be counted as nonmetals, including carbon, phosphorus, and selenium; and the elements otherwise commonly recognized as metalloids namely boron; silicon and germanium; arsenic and antimony; and tellurium, bringing the total up to twenty-three nonmetals.[14]

Astatine, the fifth halogen, is often ignored on account of its rarity and intense radioactivity;[15] it is here regarded as a metal.[n 3] The superheavy elements copernicium (Z = 112) and oganesson (118) may turn out to be nonmetals; their actual status is not known.[n 4]

Since there are 118 known elements,[35] as of September 2021, the nonmetals are outnumbered by the metals several times.

Origin and use of the termEdit

 
A basic taxonomy of matter showing the hierarchical location of nonmetals.[40][41] Classification arrangements below the level of solids can vary depending on the properties on interest. Some authors divide the elements into metals, metalloids, and nonmetals (although, on ontological grounds, anything not a metal is a nonmetal).[42]

The distinction between metals and nonmetals arose, in a convoluted manner, from a crude recognition of natural kinds[n 5] of matter. Thus, matter could be divided into pure substances and mixtures; pure substances eventually could be distinguished as compounds and elements; and "metallic" elements seemed to have broadly distinguishable attributes that other elements did not, such as their ability to conduct heat or for their "earths" (oxides) to form basic solutions in water, quicklime (CaO) for example.[44] Use of the word nonmetal can be traced to as far back as Lavoisier's 1789 work Traité élémentaire de chimie in which he distinguished between simple metallic and nonmetallic substances.[n 6]

Honing the conceptEdit

Any one or more of a range of properties have been used to hone the distinction between metals and nonmetals, including:

Johnson[51] noted that physical properties can best indicate the metallic or nonmetallic properties of an element, with the proviso that other properties will be needed in a number of ambiguous cases.

Kneen at al.[67] added that:

"It is merely necessary to establish and apply a criterion of metallicity…many arbitrary classifications are possible, most of which, if chosen reasonably, would be similar, but not necessarily identical…the relevance of the criterion can only be judged by the usefulness of the related classification."

Once a basis for distinguishing between the "two great classes of elements"[68] is established, the nonmetals are found to be those lacking the properties of metals,[69] to greater or lesser degrees.[70]

PropertiesEdit

PhysicalEdit

% packing efficiencies of non-gaseous nonmetals
 (with nearby metals for comparison)[71]
Group
13 14 15 16 17[n 7]
 B  38  C  17
 Al 74  Si 34  S  19
 Ga 39  Ge 34  As 38  Se 24  Br 15
 In 68  Sn 53  Sb 41  Te 36  I  24
 Tl 74  Pb 74  Bi 43  Po 53   At 74
Most metals, such as those in a gray cell,[n 8] have close-packed centro-symmetrical structures featuring metallic bonding and a packing efficiency of at least 68%.[n 9] Nonmetals, and some nearby metals (Ga, Sn, Bi, Po) have more open-packed directional structures featuring either covalent or partial covalent bonding and, subsequently, lower packing densities.

Physically, nonmetals in their most stable forms exist as either polyatomic solids (carbon, for example) with open-packed forms; diatomic molecules such as hydrogen (a gas) and bromine (a liquid); or monatomic gases (such as neon). They usually have small atomic radii. Metals, in contrast, are nearly all solid and close-packed, and mostly have larger atomic radii.[75] Other than sulfur, solid nonmetals have a submetallic appearance and are brittle, as opposed to metals, which are lustrous, and generally ductile or malleable. Nonmetals usually have lower densities than metals; are mostly poorer conductors of heat and electricity; and tend to have significantly lower melting points and boiling points.[76]

The physical differences between metals and nonmetals arise from internal and external atomic forces. Internally, an atom's nuclear charge acts to hold its valence electrons in place. Externally, the same electrons are subject to attractive forces from the nuclear charges in nearby atoms. When the external forces are greater than, or equal to, the internal force, valence electrons are expected to become itinerant and metallic properties are predicted. Otherwise nonmetallic properties are anticipated.[77]

ChemicalEdit

Chemically, nonmetals mostly have higher ionization energies, higher electron affinities,[n 10] higher electronegativity values, and higher standard reduction potentials than metals. Here, and in general, the higher an element's ionization energy, electron affinity, electronegativity, or standard reduction potentials, the more nonmetallic that element is.[79]

In chemical reactions, nonmetals tend to gain or share electrons unlike metals which tend to donate electrons. More specifically, and given the stability of the noble gases, nonmetals generally gain a number of electrons sufficient to give them the electron configuration of the following noble gas whereas metals tend to lose electrons sufficient to leave them with the electron configuration of the preceding noble gas.[n 11] For nonmetallic elements this tendency is encapsulated by the duet and octet rules of thumb (and for metals there is a less rigorously followed 18-electron rule). A key attribute of nonmetals is that they never form basic oxides; their oxides are generally acidic.[81] Moreover, solid nonmetals (including metalloids) react with nitric acid to form an oxide (carbon, silicon, sulfur, antimony, and tellurium) or an acid (boron, phosphorus, germanium, selenium, arsenic, iodine).[41]

Some typical chemistry-based
differences between nonmetals and metals[82]
Aspect Nonmetals Metals
In aqueous
solution[83]
Exist as anions
or oxyanions[n 12]
Exist as cations
Oxidation
states
Negative or positive Positive
Bonding Covalent
between nonmetals
Metallic between metals
(via alloy formation)
Ionic between nonmetals and metals
Oxides Acidic Basic in lower oxides;
increasingly acidic
in higher oxides

The chemical differences between metals and nonmetals largely arise from the attractive force between the positive nuclear charge of an individual atom and its negatively charged valence electrons. From left to right across each period of the periodic table the nuclear charge increases as the number of protons in the core increases.[85] There is an associated reduction in atomic radius[86] as the increasing nuclear charge draws the valence electrons closer to the core.[87] In metals, the nuclear charge is generally weaker than that of nonmetallic elements. In chemical bonding, metals therefore tend to lose electrons, and form positively charged or polarized atoms or ions whereas nonmetals tend to gain those same electrons due to their stronger nuclear charge, and form negatively charged ions or polarized atoms.[88]

The number of compounds formed by nonmetals is vast.[89] The first nine places in a "top 20" table of elements most frequently encountered in 8,427,300 compounds, as listed in the Chemical Abstracts Service register for July 1987, were occupied by nonmetals. Hydrogen, carbon, oxygen and nitrogen were found in the majority (greater than 64%) of compounds. Silicon, a metalloid, was in 10th place. The highest rated metal, with an occurrence frequency of 2.3%, was iron, in 11th place.[90] Examples of nonmetal compounds are: boric acid H
3
BO
3
, used in ceramic glazes; selenocysteine; C
3
H
7
NO
2
Se
, the 21st amino acid of life;[91] phosphorus sesquisulfide (P4S3), in strike anywhere matches; and teflon (C
2
F
4
)n.[92]

ComplicationsEdit

Complicating the chemistry of the nonmetals are the anomalies seen in the first row of each periodic table block, particularly in hydrogen, (boron), carbon, nitrogen, oxygen and fluorine; secondary periodicity or non-uniform periodic trends going down most of the p-block groups;[93] and unusual valence states in the heavier nonmetals.

 
Periodic table highlighting the first row of each block. Helium, shown here over beryllium, in group 2, on electron configuration grounds, is normally located above neon in group 18 since the resulting physiochemical trend lines going down the group are smoother.

First row anomaly. The first row anomaly largely arises from the electron configurations of the elements concerned. Hydrogen is noted for the different ways it forms bonds. It most commonly forms covalent bonds.[94] It can lose its single valence electron in aqueous solution, leaving behind a bare proton with tremendous polarizing power. This subsequently attaches itself to the lone electron pair of an oxygen atom in a water molecule, thereby forming the basis of acid-base chemistry.[95] A hydrogen atom in a molecule can form a second, 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."[96]

From boron to neon, since the 2p subshell has no inner analogue and experiences no electron repulsion effects it has a relatively small radius, unlike the 3p, 4p and 5p subshells of heavier elements.[97][n 13] Ionization energies and electronegativities among these elements are consequently higher than would otherwise be expected, having regard to periodic trends. The small atomic radii of carbon, nitrogen, and oxygen facilitate the formation of triple or double bonds.[98]

 
Electronegativity values of the group 16 chalcogen elements showing a W-shaped alternation or secondary periodicity going down the group

Secondary periodicity. Immediately after the first row of the transition metals, the 3d electrons in the 4th row of periodic table elements, i.e. in gallium (a metal), germanium, arsenic, selenium, and bromine, are not as effective at shielding the increased nuclear charge. The net result, especially for the group 13–15 elements, is that there is an alternation in some periodic trends going down groups 13 to 17.[99][n 14]

Unusual valence states. The larger atomic radii of the heavier group 15–18 nonmetals enable higher bulk coordination numbers, and result in lower electronegativity values that better tolerate higher positive charges. The elements involved are thereby able to exhibit valences other than the lowest for their group (that is, 3, 2, 1, or 0) for example in phosphorus pentachloride (PCl5), sulfur hexafluoride (SF6), iodine heptafluoride (IF7), and xenon difluoride (XeF2).[101]

Subclasses Edit

 
Periodic table extract, exploded to show the frequency that authors list elements as nonmetals:
  • ① Included by nearly all authors (×14)
  • ② Also included by most authors (3)
  • ③ Also included by some authors (6)

While hydrogen (H) is usually placed at the top of group 1, to the far left of the extract, it is sometimes instead placed over F as is the case here.[n 15]
The cross-cutting thick borderline encloses non-metals noted for their moderate to high strengths as oxidizing agents and which, with the exception of iodine, have a lackluster appearance.[n 16]
Nearby metals are shown for context.
The dashed step-like line running to either side of the six metalloids denotes that elements to the lower left of the line generally display increasing metallic behaviour and that elements to the upper right display increasing nonmetallic behaviour. Such a line, which can appear in varying configurations, is sometimes called a "dividing line between metals and nonmetals". The line is fuzzy as there is no universally accepted distinction between metals and nonmetals.[114][115]

Approaches to classifying nonmetals may involve from as few as two subclasses to up to six or seven. For example, the Encyclopedia Britannica periodic table has noble gases, halogens, and other nonmetals, and splits the elements commonly recognized as metalloids between the "other metals" and the "other nonmetals";[116] the Royal Society of Chemistry periodic table shows the nonmetallic elements as occupying seven groups.[117]

From right to left in periodic table terms, three or four kinds of nonmetals are more or less commonly discerned.[n 17] These are:

  • the relatively inert noble gases;
  • a set of chemically strong halogen elements—fluorine, chlorine, bromine and iodine—sometimes referred to as nonmetal halogens[130] (the term used here) or stable halogens;[131]
  • a set of unclassified nonmetals, including elements such as hydrogen, carbon, nitrogen, and oxygen, with no widely recognized collective name; and
  • the chemically weak nonmetallic metalloids,[132] sometimes considered to be nonmetals and sometimes not.[n 18]

Since the metalloids occupy frontier territory, where metals meet nonmetals, their treatment varies from author to author. Some consider them separate from both metals and the nonmetals; some regard them as nonmetals[134] or as a sub-class of nonmetals;[135] others count some of them as metals, for example, arsenic and antimony due to their similarities with heavy metals.[136][n 19] Metalloids are here treated as nonmetals in light of their chemical behavior, and for comparative purposes.

Aside from the metalloids, some boundary fuzziness and overlapping (as occurs with classification schemes generally) can be discerned among the other nonmetal subclasses. Carbon, phosphorus, selenium, iodine border the metalloids and show some metallic character, as does hydrogen. Among the noble gases, radon is the most metallic and begins to show some cationic behavior, which is unusual for a nonmetal.[146]

Noble gasesEdit

Some property spans and average values
for the subclasses of nonmetallic elements[147]
Property Metalloid Unclassified
nonmetal
Nonmetal
halogen
Noble
gas
Atomic radii (Å), periods 2 to 4[n 20]
Span 2.05 to 2.31 1.9 to 2.24 1.63 to 2.19 1.56 to 2.12
Average 2.25 2.04 1.96 1.88
Ionization energy (kJ mol−1)
Span 768 to 953 947 to 1,320 1,015 to 1,687 1,037 to 2,372
Average 855 1,158 1,276 1,590
Electron affinity (kJ mol−1)
Span 27 to 190 −0.07 to 200 295 to 349 −120 to −50
Average 108 134 324 −79
Electronegativity (Allred-Rochow)
Span 1.9 to 2.18 2.19 to 3.44 2.66 to 3.98 2.1 to 5.2
Average 2.05 2.65 3.19 3.38
Standard reduction potential (V)
Span −0.91 to 0.93 0.00 to 2.08 0.53 to 2.87 2.12 to 2.26
Average −0.09 0.55 1.48 2.26
Average 4.77 6.76 8.5 8.6
Goldhammer-Herzfeld criterion ratio (unit less)
Span 0.87–1.09 0.07–0.95 0.1–0.77 0.02–0.16
Average 0.99 0.50[n 21] 0.39 0.16
Average values of atomic radius, ionization energy, electron affinity, electronegativity,[n 22] and standard reduction potential generally show a left to right increase consistent with increased nonmetallic character.
Electron affinity values collapse at the noble gases due to their filled outer orbitals. Electron affinity can be defined as, "the energy required to remove the electron of a gaseous anion of −1 charge to produce a gaseous atom of that element e.g. Cl(g) → e = 348.8 kJ mol−1"; the zeroth ionization energy, in other words.[154]
The standard reduction potentials are for stable species in water, at pH 0, within the range −3 to 3 V.[155] The values in the noble gas column are for xenon only.

The Goldhammer-Herzfeld ratio [156] is an approximate (non-relativistic) measure of how metallic an element is, metals having values ≥ 1. It quantifies the explanation given for the differences between metals and nonmetals set out at the end of the Properties section.[n 23]

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 in light of their characteristically very low chemical reactivity.[157]

They have very similar properties, all being colorless, odorless, and nonflammable. With their closed valence shells the noble gases have feeble interatomic forces of attraction resulting in very low melting and boiling points.[158] That is why they are all gases under standard conditions, even those with atomic masses larger than many normally solid elements.[159]

Chemically, the noble gases have relatively high ionization energies, nil or negative electron affinities, and relatively high electronegativities. Compounds of the noble gases number in the hundreds although the list continues to grow,[160] with most of these occurring via oxygen or fluorine combining with either krypton, xenon or radon.[161]

In periodic table terms, an analogy can be drawn between the noble gases and noble metals such as platinum and gold, with the latter being similarly reluctant to enter into chemical combination.[162]

Nonmetal halogensEdit

While the nonmetal halogens are corrosive and markedly reactive elements, they can be found in such innocuous compounds as ordinary table salt NaCl. Their remarkable chemical activity as nonmetals can be contrasted with the equally remarkable chemical activity of the alkali metals such as sodium and potassium, located at the far left of the periodic table.[n 24][164]

Physically, fluorine and chlorine are pale yellow and yellowish green gases; bromine is a reddish-brown liquid; and iodine is a silvery metallic solid.[n 25] Electrically, the first three are insulators while iodine is a semiconductor (along its planes).[165]

Chemically, they have high ionization energies, electron affinities, and electronegativity values, and are mostly relatively strong oxidizing agents.[166] Manifestations of this status include their intrinsically corrosive nature.[167] All four exhibit a tendency to form predominately ionic compounds with metals[168] whereas the remaining nonmetals, bar oxygen, tend to form predominately covalent compounds with metals.[n 26] The reactive and strongly electronegative nature of the nonmetal halogens represents the epitome of nonmetallic character.[172]

In periodic table terms, the counterparts of the highly nonmetallic halogens, in group 17 are the highly reactive alkali metals, such as sodium and potassium, in group 1.[173][n 27]

Unclassified nonmetalsEdit

After the nonmetallic elements are classified as either noble gases, halogens or metalloids (following), the remaining seven nonmetals are hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur and selenium. Three are colorless gases (H, N, O); three have a metal-like appearance (C, P, Se);[n 28] and one is yellow (S). Electrically, graphitic carbon is a semimetal (along its planes);[n 29] phosphorus and selenium are semiconductors;[178] and hydrogen, nitrogen, oxygen, and sulfur are insulators.[n 30]

They are generally regarded as being too diverse to merit a collective examination,[180][91] and have been referred to as other nonmetals,[181] or more plainly as nonmetals, located between metalloids and halogens.[182] Consequently, their chemistry tends to be taught disparately, according to their four respective periodic table groups,[180] for example: hydrogen in group 1; the group 14 carbon nonmetals (carbon, and possibly silicon and germanium); the group 15 pnictogen nonmetals (nitrogen, phosphorus, and possibly arsenic and antimony); and the group 16 chalcogen nonmetals (oxygen, sulfur, selenium, and possibly tellurium). Other subdivisions are possible according to the individual preferences of authors.[n 31]

Hydrogen, in particular, behaves in some respects like a metal and in others like a nonmetal.[184] Like a metal it can (first) lose its single valence electron;[185] it can stand in for alkali metals in typical alkali metal structures;[186] and is capable of forming alloy-like hydrides, featuring metallic bonding, with some transition metals.[187] On the other hand, it is an insulating diatomic gas, like a typical nonmetal, and in chemical reactions more generally, it has a tendency to attain the electron configuration of helium.[188] It does this by way of forming a covalent or ionic bond[187] or, if its has lost its valence electron, attaching itself to a lone pair of electrons.[189]

Some or all of these nonmetals nevertheless have several shared properties. Their physical and chemical character is "moderately non-metallic", on a net basis.[91] Being less reactive than the halogens,[190] most of them, except for phosphorus, can occur naturally in the environment.[16] They have prominent biological[191][192] and geochemical aspects.[91] When combined with halogens, unclassified nonmetals form (polar) covalent bonds.[193] When combined with metals they can form hard (interstitial or refractory) compounds,[194] in light of their relatively small atomic radii and sufficiently low ionization energy values.[91] Unlike the halogens, unclassified nonmetals show a tendency to catenate, especially in solid-state compounds.[195][91] Diagonal relationships among these nonmetals echo similar relationships among the metalloids.[196][n 32]

In periodic table terms, a geographic analogy is seen between the unclassified nonmetals and transition metals. The unclassified nonmetals occupy territory between the strongly nonmetallic halogens on the right and the weakly nonmetallic metalloids on the left. The transition metals occupy territory, "between the 'virulent and violent' metals on the left of the periodic table, and the 'calm and contented' metals to the right...[and]...form "a transitional bridge between the two".[203]

MetalloidsEdit

The six elements more commonly recognized as metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium, with each having a metal appearance.[n 33] On a standard periodic table, they occupy a diagonal area in the p-block extending from boron at the upper left to tellurium at lower right, along the dividing line between metals and nonmetals shown on some periodic tables.[12]

They are brittle and only fair conductors of electricity and heat. Boron, silicon, germanium, tellurium are semiconductors. Arsenic and antimony have the electronic band structures of semimetals although both have less stable semiconducting allotropes.[12]

Chemically the metalloids generally behave like (weak) nonmetals. Among the nonmetallic elements they tend to have the lowest ionization energies, electron affinities, and electronegativity values; and are relatively weak oxidizing agents. They further demonstrate a tendency to form alloys with metals.[12]

Like hydrogen among the unclassified nonmetals, boron is chemically similar to metals in some respects.[205][n 34] It has fewer electrons than orbitals available for bonding. Analogies with transition metals occur in the formation of complexes,[207] and adducts (for example, BH3 + CO →BH3CO and, similarly, Fe(CO)4 + CO →Fe(CO)5),[n 35] as well as in the geometric and electronic structures of cluster species such as [B6H6]2− and [Ru6(CO)18]2−.[209]

To the left of the weakly nonmetallic metalloids, in periodic table terms, are found an indeterminate set of weakly metallic metals (such as tin, lead and bismuth)[210] sometimes referred to as post-transition metals.[211]

ComparisonEdit

Properties of metals and those of the (sub)classes of metalloids, unclassified nonmetals, nonmetal halogens, and noble gases are summarized in the following two tables. Physical properties apply to elements in their most stable forms in ambient conditions, unless otherwise specified, and are listed in loose order of ease of determination. Chemical properties are listed from general to specific, and then to descriptive. The dashed line around the metalloids denotes that, depending on the author, the elements involved may or may not be recognized as a distinct class or subclass of elements. Metals are included as a reference point.

PhysicalEdit

Some cross-subclass physical properties
Physical property Metals Metalloids Unclassified nonmetals Nonmetal halogens Noble gases
Alkali, alkaline earth, lanthanide, actinide, transition and post-transition metals Boron, silicon, germanium, arsenic, antimony (Sb), tellurium Hydrogen, carbon, nitrogen, phosphorus, oxygen, sulfur, selenium Fluorine, chlorine, bromine, iodine Helium, neon, argon, krypton, xenon, radon
Form[212] solid (Hg is liquid) solid solid: C, P, S, Se
gaseous: H, N, O
solid: I
liquid: Br
gaseous: F, Cl
gaseous
Appearance lustrous[76] semi-lustrous[213] semi-lustrous: C, P, Se[214]
colorless: H, N, O[215]
colored: S[216]
colored: F, Cl, Br[217]
semi-lustrous: I[12]
colorless[218]
Elasticity mostly malleable and ductile[76] (Hg is liquid) brittle[213] C, black P, S and Se are brittle[219]
the same four have less stable non-brittle forms[n 36]
iodine is brittle[225] not applicable
Structure mainly close-packed centrosymmetrical[75] polyatomic[226] polyatomic: C, P, S, Se[226]
diatomic: H, N, O[227]
diatomic[228] monatomic[229]
Bulk coordination number[230] mostly 8−12, or more 6, 4, 3, or 2 3, 2, or 1 1 0
Allotropes[231] common with temperature or pressure changes all form[232] known for C, P, O, S, Se iodine is known in amorphous form[233]

none reported
Electrical conductivity high[n 37] moderate: B, Si, Ge, Te
high: As, Sb[n 38]
low: H, N, O, S
moderate: P, Se
high: C[n 39]
low: F, Cl, Br
moderate: I[n 40]
low[n 41]
Volatility[n 42] low
Hg is the most volatile in its class
low
As is lowest in class
low: C, P, S, Se
high: H, N, O
high higher
Electronic structure[239] metallic (Bi is a semimetal) semimetal (As, Sb) or semiconductor semimetal (C), semiconductor (P, Se) or insulator (H, N, O, S) semiconductor (I) or insulator insulator
Outer electrons[240] 1–8 valence 3–6 4–6 (H has 1) 7 8 (He has 2)
Crystal structure[241][n 43] mainly cubic or hexagonal rhombohedral: B, As, Sb
cubic: Si, Ge
hexagonal: Te
cubic: P, O
hexagonal: H, C, N, Se
orthorhombic: S
cubic: F
orthorhombic: Cl, Br, I
cubic: Ne, Ar, Kr, Xe, Rn
hexagonal: He

ChemicalEdit

Some cross-subclass chemical properties
Chemical property Metals Metalloids Unclassified nonmetals Nonmetal halogens Noble gases
Alkali, alkaline earth, lanthanide, actinide, transition and post-transition metals Boron, silicon, germanium, arsenic, antimony (Sb), tellurium Hydrogen, carbon, nitrogen, phosphorus, oxygen, sulfur, selenium Fluorine, chlorine, bromine, iodine Helium, neon, argon, krypton, xenon, radon
General chemical behavior strong to weakly metallic[248]
noble metals are disinclined to react[249]
weakly nonmetallic[n 45] moderately nonmetallic[n 46] strongly nonmetallic[252] inert to nonmetallic[253]
Rn shows some cationic behavior[254]
Ionization energy relatively low
higher for noble metals
ionization energy for Hg and possibly Rg, Ds, Cn[n 47] exceed those for some nonmetals
electronegativity values of noble metals exceed that of P
moderate moderate to high high high to very high
Electron affinity moderate moderate: H, C, O, P (N is c. zero)
higher: S, Se
high zero or less
Electronegativity[n 48] moderate:
Si < Ge ≈ B ≈ Sb < Te < As
moderate (P) to high:
P < Se ≈ C < S < N < O
high:
I < Br < Cl < F
moderate (Rn) to very high
Standard reduction
potential
moderate moderate to high high high for Xe
Non-zero oxidation states[257] largely positive
negative anionic states known for most alkali and alkaline earth metals; Pt, Au[258]
negative and positive known for all negative states known for all, but for H this is an unstable state
positive known for all but only exceptionally for F[259] and O
from −5 for B to +7 for Cl, Br, I, and At
only positive oxidation states known, and only for heavier noble gases
from +2 for Kr, Xe, and Rn to +8 for Xe
Catenation tendency[260] known for group 8‒11 transition metals;[261] and Hg, Ga, In,[262] Sn and Bi[263] significant: B, Si; Te
less so: Ge, As, Sb
predominant: C
significant: P, S, Se
less so: H, N, O
known in cationic (Cl, Br, I) and anionic forms[264] not known; no catenation reported
Biological interactions (human life)[265][266][n 49] 17% of naturally occurring metals are essential in major or trace quantities
Most heavier metals, including Cr, Cd, Hg and Pb, are known for their toxicity[268]
33% (two of six) are essential trace elements: B, Si[n 50][270]
As is noted for its toxicity[268]
100% are essential: H, C, N, O form the basis for life; P and S are major elements;[n 51] Se occurs in selenocysteine, the 21st amino acid of life, as a trace element[91]
O, P and Se are potentially toxic[n 52]
100% are essential: Cl as a major constituent; F, Br, I as trace elements
corrosive in their elemental forms[167]
0% essential
He is used in respiratory medicine and diving gas mixtures;[273] Ar has been used in human studies, while Xe has several medical uses;[274] Rn was formerly used to treat tumours[275]
Compounds with metals alloys[76] or intermetallic compounds[276] tend to form alloys or intermetallic compounds[277] salt-like to covalent: H†, C, N, P, S, Se[278]
mainly ionic: O[279]
mainly ionic: F, Cl, Br, I[168] simple compounds in ambient conditions not known[n 53]
Oxides ionic, polymeric, layer, chain, and molecular structures[281]
V; Mo, W; Al, In, Tl; Sn, Pb; Bi are glass formers[282]
basic; some amphoteric or acidic[283]
polymeric in structure[284]
B, Si, Ge, As, Sb, Te are glass formers[285]
amphoteric or weakly acidic[250][286][n 54]
mostly molecular[284]
C, P, S, Se are known in at least one polymeric form
P, S, Se are glass formers;[282] CO2 forms a glass at 40 GPa[288]
acidic (NO
2
, N
2
O
5
, SO
3
, and SeO
3
strongly so)[289][290] or neutral (H2O, CO, NO, N2O)[n 55]
molecular[284]
iodine is known in at least one polymeric form, I2O5[292]
no glass formers reported
acidic; ClO
2
, Cl
2
O
7
, and I
2
O
5
strongly so[290][289]
molecular[284]

XeO2 is polymeric[293]
no glass formers reported
metastable XeO3 is acidic;[294] stable XeO4 strongly so[295]
Reaction with conc. nitric acid most form nitrates[296] B forms boric acid; As forms arsenic acid[297]
Si, Ge, Sb, and Te form oxides[298]
C, S form oxides[299][300]
P, Se form phosphoric acid, and selenic acid[301]
F forms nitroxyfluoride NO3F[302]
I forms iodic acid[300]
nil; no reaction reported
Reaction with conc. sulfuric acid most form sulfates[n 56][n 57] B forms boric acid[307]
Si does not react[308]
Ge, Sb form sulfates[309]
As forms an oxide[310]
Te forms the sulfoxide TeSO3[311]
C, S form oxides[312]
P (red) forms phosphoric acid[313]
Se forms the sulfoxide SeSO3[314]
iodine can form a polymeric yellow sulfate (IO)2SO4 in the presence of I2O5[315] nil; no reaction reported
† Hydrogen can also form alloy-like hydrides[316]
‡ The labels moderate, high, higher, and very high are based on the value spans listed in the table "Property spans and average values for the subclasses of nonmetallic elements"

AllotropesEdit

Most nonmetallic elements exist in allotropic forms. Carbon, for example, occurs as graphite and as diamond. Such allotropes may exhibit physical properties that are more metallic or less nonmetallic.[317]

Among the nonmetal halogens, and unclassified nonmetals:

  • Iodine is known in a semiconducting amorphous form.[318]
  • Graphite, the standard state of carbon, is a fairly good electrical conductor. The diamond allotrope of carbon is clearly nonmetallic, being translucent, and an extremely poor electrical conductor.[319] Carbon is further known in several other allotropic forms, including semiconducting buckminsterfullerene (C60).[320]
  • Nitrogen can form gaseous tetranitrogen (N4), an unstable polyatomic molecule with a lifetime of about one microsecond.[321]
  • Oxygen is a diatomic molecule in its standard state; it also exists as ozone (O3), an unstable nonmetallic allotrope with a half-life of around half an hour.[322]
  • Phosphorus, uniquely, exists in several allotropic forms that are more stable than that of its standard state as white phosphorus (P4). The white, red and black allotropes are probably the best known; the first is an insulator; the latter two are semiconductors.[323] Phosphorus also exists as diphosphorus (P2), an unstable diatomic allotrope.[324]
  • Sulfur has more allotropes than any other element.[325] Amorphous sulfur, a metastable mixture of such allotropes, is noted for its elasticity.[326]
  • Selenium has several nonmetallic allotropes, all of which are much less electrically conducting than its standard state of gray "metallic" selenium.[327][n 58]

All the elements most commonly recognized as metalloids form allotropes. Boron is known in several crystalline and amorphous forms. The discovery of a quasi-spherical allotropic molecule, borospherene (B40), was announced in 2014. Silicon was most recently known only in its crystalline and amorphous forms. The synthesis of an orthorhombic allotrope, Si24, was subsequently reported in 2014.[329] At a pressure of c. 10–11 GPa, germanium transforms to a metallic phase with the same tetragonal structure as tin; when decompressed—and depending on the speed of pressure release—metallic germanium forms a series of allotropes that are metastable in ambient conditions.[330] Arsenic and antimony form several well-known allotropes (yellow, grey, and black). Tellurium is known in its crystalline and amorphous forms.[331]

Other allotropic forms of nonmetallic elements are known, either under pressure or in monolayers. Under sufficiently high pressures, at least half of the nonmetallic elements that are semiconductors or insulators,[n 59] starting with phosphorus at 1.7 GPa, have been observed to form metallic allotropes.[332][n 60] Single layer two-dimensional forms of nonmetals include borophene (boron), graphene (carbon), silicene (silicon), phosphorene (phosphorus), germanene (germanium), arsenene (arsenic), antimonene (antimony) and tellurene (tellurium), collectively referred to as "xenes".[334]

Abundance, occurrence, extraction and costEdit

AbundanceEdit

Hydrogen and helium are estimated to make up approximately 99% of all ordinary matter in the universe and over 99.9% of its atoms.[4] Oxygen is thought to the next most abundant element, at c. 0.1%.[335] Less than five percent of the universe is believed to be made of ordinary matter, represented by stars, planets and living beings. The balance is made of dark energy and dark matter, both of which are currently poorly understood.[336]

Hydrogen, carbon, nitrogen, and oxygen constitute the great bulk of the Earth's atmosphere, oceans, crust, and biosphere; the remaining nonmetals have abundances of 0.5% or less. In comparison, 35% of the crust is made up of the metals sodium, magnesium, aluminium, potassium and iron; together with a metalloid, silicon. All other metals and metalloids have abundances within the crust, oceans or biosphere of 0.2% or less.[337][338]

OccurrenceEdit

Noble gasesEdit

About 1015 tonnes of noble gases are present in the Earth's atmosphere.[339] Helium is additionally found in natural gas to the extent of as much as 7%.[340] Radon further diffuses out of rocks, where it is formed during the natural decay sequence of uranium and thorium.[341] In 2014, it was reported that the Earth's core may contain c. 1013 tons of xenon, in the form of stable XeFe3 and XeNi3 intermetallic compounds. This may explain why "studies of the Earth's atmosphere have shown that more than 90% of the expected amount of Xe is depleted."[342]

Nonmetal halogensEdit

The nonmetal halogens are found in salt-related minerals. Fluorine occurs in fluorite, this being a widespread mineral. Chlorine, bromine and iodine are found in brines. Exceptionally, a 2012 study reported the presence of 0.04% native fluorine (F
2
) by weight in antozonite, attributing these inclusions to radiation from the presence of tiny amounts of uranium.[343]

Unclassified nonmetalsEdit

 
Carbon as diamond, here shown in native form. Diamantine carbon is thermodynamically less stable than graphitic carbon.[344]

Unclassified nonmetals occur typically occur in elemental forms (oxygen, sulfur) or are found in association with either of these two elements.

  • Hydrogen occurs in the world's oceans as a component of water, and in natural gas as a component of methane and hydrogen sulfide.[345]
  • Carbon, as graphite, mainly occurs in metamorphic silicate rocks[346] as a result of the compression and heating of sedimentary carbon compounds.
  • Oxygen is found in the atmosphere; in the oceans as a component of water; and in the crust as oxide minerals.
  • Phosphorus minerals are widespread, usually as phosphorus-oxygen phosphates.
  • Elemental sulfur can be found near hot springs and volcanic regions in many parts of the world; sulfur minerals are widespread, usually as sulfides or oxygen-sulfur sulfates.
  • Selenium occurs in metal sulfide ores, where it partially replaces the sulfur;[347] elemental selenium is occasionally found.[348]

MetalloidsEdit

The metalloids tend to be found in forms combined with oxygen or sulfur or, in the case of tellurium, gold or silver. Boron is found in boron-oxygen borate minerals including in volcanic spring waters. Silicon occurs in the silicon-oxygen mineral silica (sand). Germanium, arsenic and antimony are mainly found as components of sulfide ores. Tellurium occurs in telluride minerals of gold or silver. Native forms of arsenic, antimony and tellurium have been reported.[349]

ExtractionEdit

Nonmetals, and metalloids, are extracted in their raw forms from:[16]

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

CostEdit

While non-radioactive nonmetals are relatively inexpensive, there are some exceptions. As of July 2021, boron, germanium, arsenic, and bromine can cost from $3–10 US per gram (cf. silver at about $1 per gram). Prices can fall dramatically if bulk quantities are involved.[350] Black phosphorus is produced only in gram quantities by boutique suppliers—a single crystal produced via chemical vapor transport can cost up to $1,000 US per gram (ca. seventeen times the cost of gold); in contrast, red phosphorus costs about 50 cents a gram or $227 a pound.[351] Up to 2013, radon was available from the National Institute of Standards and Technology for $1,636 per 0.2 ml unit of issue, equivalent to c. $86,000,000 per gram (with no indication of a discount for bulk quantities).[352]

Shared usesEdit

Shared uses of nonmetallic elements
Field Elements
air replacements (inert)[353] H, He, C, N, O, F (in SF6), P, S and Ar
cryogenics and refrigerants[354] H, He, N, O, F, Ne, S, Cl and Ar
fertilizers[355] H, B, C, N, O, Si, P, S
flame retardants[356] or fire extinguishers[357] H, B, C (including graphite), N, O, F, Si, P, S, Cl, Ar, Br and Sb
household accoutrements[358][n 61] H (primary constituent of water); He (party balloons); B (in detergents); C (in pencils, as graphite); N (beer widgets); O (as peroxide, in detergents);[360] F (as fluoride, in toothpaste); Ne (lighting); Si (in glassware); P (matches); S (garden treatments); Cl (bleach constituent); Ar (insulated windows); Ge (in wide-angle camera lenses); Se (glass; solar cells); Br (as bromide, for purification of spa water); Kr (energy saving fluorescent lamps);[361] Sb (in batteries); Te (in ceramics, solar panels, rewrite-able DVDs); I (in antiseptic solutions); Xe (in plasma TV display cells, a technology subsequently made redundant by low cost LED and OLED displays)[362]
lasers and lighting[363] H, He, C (in carbon dioxide lasers, CO2); N, O (in a chemical oxygen iodine laser);[364] F (in a hydrogen fluoride laser, HF); Ne, Si, P, S (in a sulfur lamp);[365] Cl, Ar, Ge, As, Se, Br, Kr, Te, I and Xe
medicine and pharmaceuticals[366] H, He, B, C, N, O, F, Si, P, S, Cl, Ar, As, Se, Br, Kr, Sb, Te, I, Xe and Rn
mineral acids[367] H, B, C, N, O, F, Si, P, S, Cl, Ge, As, Sb, Br, Te, I and Xe
plug-in hybrid vehicles[368] H, He, B, C, N, O, F, Si, P, S, Cl, Ar, Br, Sb, Te and I
welding gases[369] H, He, C (in CO2), N, O, Ar
smart phones[370] H, He, B, C, N, O, F, Si, P, S, Cl, Ge, As, Se, Br, Sb

Near universal uses for nonmetals are for household accoutrements; lasers and lighting; and medicine and pharmaceuticals. One or two of germanium, arsenic, and or radon will be absent. In a like manner, most metals have structural uses. To the extent that metalloids show metallic character, they have speciality uses extending to (for example) oxide glasses, alloying components, and semiconductors.[371]

Further shared uses of different subsets of the nonmetals encompass their presence in, or specific uses in the fields of air replacements (inert); cryogenics and refrigerants; fertilizers; flame retardants or fire extinguishers; mineral acids; plug-in hybrid vehicles; welding gases; and smart phones.

DiscoveryEdit

 
The Alchemist Discovering Phosphorus (1771) by Joseph Wright. The alchemist is Hennig Brand; the glow emanates from the combustion of phosphorus inside the flask.

Most nonmetallic elements were not discovered until after Hennig Brand isolated phosphorus from urine in 1669. Before then, carbon, sulfur and antimony were known in antiquity, and arsenic was discovered during the Middle Ages (by Albertus Magnus). The remainder were isolated in the 18th and 19th centuries. Helium (1868), was the first element not discovered on Earth.[n 62] Radon was discovered at the end of the 19th century.[16]

Arsenic, phosphorus and nonmetallic elements subsequently discovered were isolated using one or more of the tools of chemists or physicists namely spectroscopy; fractional distillation; radiation detection; electrolysis; adding acid to an ore; combustion; displacement reactions; or heating:

  • Of the noble gases, helium was detected via its yellow line in the coronal spectrum of the sun, and later by observing the bubbles escaping from uranite UO2 dissolved in acid; neon through xenon were obtained via fractional distillation of air; and radioactive radon was observed emanating from compounds of thorium, four years after the discovery of radiation, in 1895, by Henri Becquerel.
  • The nonmetal halogens were obtained from their halides, either via electrolysis; adding an acid; or displacement. Some chemists died as a result of their experiments trying to isolate fluorine.

See alsoEdit

NotesEdit

  1. ^ It is usually considered characteristic of nonmetals that they have a negative temperature coefficient of resistivity, in which electrical resistance falls with rising temperature.[8] The converse nearly always holds true for metals: their resistivity increases with rising temperature. Plutonium is an exception. Its electrical resistivity falls when heated in the temperature range of around –175 to +125 °C.[9] The divalent metals barium, europium and ytterbium, in liquid form, likewise exhibit a negative temperature coefficient of resistivity.[10]
  2. ^ The elements commonly recognised as metalloids are boron, silicon, germanium, arsenic, antimony and tellurium.[12] Rochow observed that it is sometimes desirable to think of carbon, phosphorus and selenium as metalloids.[13] These three elements have been referred to as near metalloids.[12] The remaining nonmetallic elements are hydrogen, oxygen, nitrogen, sulfur, the four nonmetal halogens, and the noble gases.
  3. ^ The bulk properties of astatine remain unknown as a visible quantity of it would immediately self-vaporize from the heat generated by its radioactivity.[16] It remains to be seen if, with sufficient cooling, a macroscopic quantity could be deposited as a thin film.[17]

    Qualitative and quantitive assessments of the status of astatine, including having regard to relativistic effects, have been consistent with it being a metal:

    1940: Astatine was judged to be a metal when it was first synthesized.[18] That assessment was consistent with some metallic character seen in iodine,[19] its lighter halogen congener.
    1972: Batsanov calculated astatine would have a band gap of 0.7 eV (but see the 2013 entry).[20]
    1983: Edwards and Sienko speculated that, on the basis of the non-relativistic Goldhammer-Herzfeld criterion for metallicity, astatine was probably a metalloid.[21] As the ratio is based on classical arguments[22] it does not accommodate the finding that polonium (cf. 2006 entry following) adopts a metallic (rather than covalent) crystalline structure, on relativistic grounds.[23] Even so it offers a first order rationalization for the occurrence of metallic character amongst the elements.[24]
    2002: Siekierski and Burgess presumed astatine would be a metal in the context of some of the properties of iodine.[25]
    2006: Restrepo et al.,[26] on the basis of a comparative study of 128 known and interpolated physiochemical, geochemical and chemical properties of 72 of the elements, reported that astatine appeared to share more in common with polonium (a metal) than it did with the established halogens and that, "At should not be considered as a halogen." In so doing they echoed the 1940 observation that, "The chemical properties of the unknown substance are very close to those of polonium."[18]
    2010: Thornton and Burdette observed that "Since elements in heavier periods often resemble their n+1 and n-1 neighbors more than their lighter congeners, eka-iodine [astatine]...was expected to be radioactive and metallic like polonium."[27]
    2013: Hermann, Hoffmann, and Ashcroft predicted At would be an fcc metal, once all relativistic effects are taken into account, and that it would have a band gap of 0.68 eV (cf Batsanov) if only some of these effects were taken into account.[17] As at 24 August 2021, they had been cited 38 times.
  4. ^ For copernicium, calculations and predictions made in 2007; 2017, 2018; and 2019 have suggested it may be either a (nonmetallic) semiconductor; a noble metal; or a (nonmetallic) liquid insulator.[28][29][30]

    Tennessine, as a heavier congener of astatine, is likewise expected to have metallic properties.[17][31]


    Oganesson, the period 7 congener of the noble gases, was originally predicted to be a noble gas[32] but may instead be a fairly reactive metallic-looking semiconducting solid with an anomalously low first ionization potential, and a positive electron affinity, due to relativistic effects.[33] On the other hand, if relativistic effects peak in period 7 at copernicium, oganesson may turn out to be a noble gas after all, albeit more reactive than either xenon or radon.[34]

  5. ^ A natural kind can be said to be a grouping that reflects divisions in the world, as understood at the time, rather than (so much) the interests and actions of humans. "The periodic table is considered by many authors to be a perfect illustration of how things in the world are divided into natural kinds." Since kinds are revealed by science, a science can revise which kinds it holds to exist: phlogiston was regarded as a kind until after Lavoisier's chemical revolution.[43]
  6. ^ Subſtances ſimples non-métalliques and métalliques, as Lavoisier put it.[45]
  7. ^ Bromine (15%): Packing efficiency is determined by dividing the volume of one mole of atoms by the applicable molar volume. The bond distance in solid bromine is 2.2836 Å and 2.27 ± 0.10 in the gas, giving an atomic radius r of ca. 1.14.[72] The volume of one bromine atom is 4/3πr3. The volume of one mole of bromine atoms is given by the volume of one atom multiplied by the Avogadro's number, that is, 6.0221409×1023.

    In comparison, liquid mercury has a packing efficiency of 58%.[73]

  8. ^ The figure for At is based on the expectation that it will be a monatomic metal with a close packed structure.[17]
  9. ^ Gallium, as a metal, has, "[an] odd structure [that] somewhat resembles covalently bonded Ga2 molecules within a metal lattice."[74]
  10. ^ Nitrogen and the noble gases have no or negative electron affinities.[78]
  11. ^ Exceptions can arise among the transition metals and the lanthanide and actinide metals, which have additional inner electron orbitals that may or may not participate in chemical bonding.[80]
  12. ^ including Xe and Rn to a limited extent.[84]
  13. ^ A similar effect is seen in the 1s elements, hydrogen and helium.[97]
  14. ^ The alternation in some properties is further compounded by the appearance of fourteen f-block metals between barium and hafnium.[100]
  15. ^ Hydrogen has historically been placed over one or more of lithium, boron,[102] carbon, or fluorine; or no group at all; or all main groups simultaneously, and therefore may or may not be proximal to the bulk of unclassified nonmetals.[103]
  16. ^ These seven "strong" non-metals (N; O, S; F, Cl, Br, I) have discrete molecular structures. But for H the remaining reactive nonmetallic elements have giant covalent structures.[104]

    N, S and iodine are somewhat hobbled as "strong" nonmetals.


    While N has a high electronegativity, it is a reluctant anion former,[105] and a pedestrian oxidizing agent unless combined with a more active non-metal like O or F.[106]


    S reacts in the cold with alkalic and post-transition metals, and Cu, Ag and Hg,[107] but otherwise has low values of ionization energy, electron affinity, and electronegativity compared to the averages of the others; it is regarded as being not a particularly good oxidizing agent.[108]
    Iodine is sufficiently corrosive to cause lesions resembling thermal burns, if handled without suitable protection,[109] and tincture of iodine will smoothly dissolve Au.[110] That said, while F, Cl and Br will all oxidize Fe2+ (aq) to Fe3+...iodine...is such a [relatively] weak oxidizing agent that it cannot remove electrons from Fe(II) ions to form Fe(III) ions."[111] Thus, for the reaction X2 + 2e → 2X(aq) the reduction potentials are F +2.87 V; Cl +1.36; Br +1.09; I +0.54. Here Fe3+ + e → Fe3+ +0.77.[112] Thus F2, Cl2 and Br2 will oxidize Fe2+ to Fe3+ but Fe2+ will oxidize I to I2. Iodine has previously been referred to as a moderately strong oxidizing agent.[113]
  17. ^ A basic taxonomy of nonmetals was set out in 1844, by Dupasquier, a French doctor, pharmacist and chemist.[118] To facilitate the study of nonmetals, he wrote, "they will be divided into four groups or sections, as in the following:"
    Organogens O, N, H, C
    Sulphuroids S, Se, P
    Chloroides F, Cl, Br, I
    Boroids B, Si
    Dupasquier's organogens and sulphuroids correspond to the set of unclassified nonmetals. Eventually thereafter:
    • the chloroide nonmetals came to be independently referred to as halogens;[119]
    • the boroid nonmetals came to expand into the metalloids, starting from as early as 1864;[120]
    • varying configurations of the orgaonogen and the sulphuroid nonmetals have been referred to as e.g. basic nonmetals;[121] biogens;[122] central nonmetals;[123] CHNOPS;[124] essential elements;[125] "nonmetals";[126] orphan nonmetals;[127] or redox nonmetals;[128]
    • the noble gases, as a discrete grouping, were counted among the nonmetals as early as 1900.[129]
  18. ^ Tshitoyan et al. (2019) conducted a machine-based analysis of the proximity of names of the elements based on 3.3 million abstracts published between 1922 and 2018 in more than 1,000 journals. The resulting map shows that "chemically similar elements are seen to cluster together and the overall distribution exhibits a topology reminiscent of the periodic table itself."[133] They labeled individual nonmetals as either metalloids; polyatomic nonmetals; diatomic nonmetals; halogens; or noble gases. Word proximity clusters for the metalloids, halogens, and noble gases are apparent. The remaining polyatomic (C, P, S, Se) and diatomic nonmetals (H, N, O) occupy territory between the metalloids and the nonmetal halogens.[133]
  19. ^ The elements involved may instead be classified on a case-by-case basis.[137] For example, germanium[138] and antimony[139] may be counted as metals or selenium may be admitted to the metalloid club.[140]

    The considerations of authors in making these decisions may or not be made explicit and may, at times, seem arbitrary.[141] A binary classification can facilitate the establishment of rules for determining bond types between metals and nonmetals.[142] Alternatively, classifying some elements as metalloids "emphasizes that properties change gradually rather than abruptly as one moves across or down the periodic table".[143] Oderberg[144] argues on ontological grounds that anything not a metal is therefore a nonmetal, and that this includes semi-metals (i.e. metalloids).


    Jones[145] takes a more philosophical or pragmatic view. 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."

  20. ^ Atomic radius is here defined as the average distance from the nucleus where the electron density falls to 0.001 electrons per bohr3.[148]
  21. ^ The values given in the source[149] for C, P and Se are those for diamond; white P; and Se8. Since the values scale with density,[149] the values used here are for a single layer of graphite (i.e. graphene) within which electron delocalization occurs in graphite;[150] black P, the most stable form,[151] and gray or metallic selenium, the most stable form.[152]
  22. ^ Electronegativity values for the noble gases are from Allen and Huheey[153]
  23. ^ As the ratio is based on classical arguments[22] it does not accommodate the finding that polonium, which has a value of ~0.95, adopts a metallic (rather than covalent) crystalline structure, on relativistic grounds.[23] Even so it offers a first order rationalization for the occurrence of metallic character amongst the elements.[24]
  24. ^
    32-column table showing
    the alkali metals (left) and
    the nonmetal halogens (right)
    Hydrogen Helium
    Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
    Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
    Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
    Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
    Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
    Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
    The 32-column form of periodic table is constructed by incorporating the f-block metals (green), which normally appear floating below the transition metals (blue), into the main body of the table. It has the advantage of showing all elements in their correct sequence, but it has the disadvantage of requiring more space.[163]
  25. ^ Solid iodine has a silvery metallic appearance under white light, at room temperature.[12]
  26. ^ Metal oxides are usually ionic.[169] On the other hand, high valence oxides of metals are usually either polymeric or covalent.[170] A polymeric oxide has a linked structure composed of multiple repeating units.[171]
  27. ^ Jones and Atkins refer to the chemically active metals of Groups 1 and 2.[174] Elsewhere, Ambrose refers to the lanthanides and actinides as "active metals".[175]
  28. ^ C as graphite; and P as black P, the most stable form[176]
  29. ^ Graphite is a semiconductor in a direction perpendicular to its planes.[177]
  30. ^ Sulfur, an insulator, and selenium, a semiconductor are each photoconductors—their electrical conductivities increase by up to six orders of magnitude when exposed to light.[179]
  31. ^ For example, Wulfsberg divides the nonmetals, including B, Si, Ge, As, Sb, Te, Xe, into very electronegative nonmetals (Pauling electronegativity over 2.8) and electronegative nonmetals (1.9 to 2.8). This results in N and O being very electronegative nonmetals, along with the halogens; and H, C, P, S and Se being electronegative nonmetals. Se is further recognized as a semiconducting metalloid.[183]
  32. ^ Diagonal relationships encompass hydrogen (possibly), carbon, nitrogen, (oxygen), phosphorus, sulfur, selenium, and (chlorine).

    For hydrogen, such relationships depend on its placement in the periodic table. Arguments have been advanced for alternatively housing hydrogen over either boron[102] or carbon.[197]


    Chemical similarities between hydrogen and carbon include, "comparable ionization energies, electron affinities and electronegativity values; half-filled valence shells; and correlations between the chemistry of H–H and C–H bonds."[198]


    Hydrogen and nitrogen are each, "relatively unreactive colourless diatomic gases, with comparably high ionization energies (1312.0 and 1402.3 kJ/mol), each having half-valence subshells, 1s and 2p respectively. Like the reactive azide (N
    3
    ) anion, inter-electron repulsions in the hydride (H) anion (with its single nuclear charge) make ionic hydrides highly reactive. Unusually for nonmetals, the two elements are known in cationic forms. In water the H+ "cation" exists as an H
    13
    O+
    6
    ion, with a delocalized proton in a central OHO group. Nitrogen forms a pentazenium (N+
    5
    ) cation; bulk quantities of the salt N+
    5
    SbF
    6
    can be prepared. Coincidentally, the ammonium (NH+
    4
    ) cation behaves in many respects as an alkali metal anion."[180]


    Carbon and phosphorus form an extensive series of organophosphorus compounds, so much so that a book with the title Phosphorus: The Carbon Copy was published in 1998.[199]


    Nitrogen and sulfur are able to form an extensive series of seemingly interchangeable sulfur nitrides.[200]


    "In terms of a less well-known diagonal relationship between...[oxygen and chlorine], chlorination reactions have many similarities to oxidation reactions. Such reactions tend not to be limited to thermodynamic equilibrium and often go to complete chlorination. They are often highly exothermic. Chlorine, like oxygen, forms flammable mixtures with organic compounds."[201]


    Phosphorus reacts with selenium to form a large number of compounds characterized by structural analogies derived from the white phosphorus (P4) tetrahedron.[202]

  33. ^ They are called metalloids mainly in light of their metal-like appearance.[204]
  34. ^ Greenwood[206] 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 ..."
  35. ^ The BH3 and Fe(CO4) species in these reactions are short-lived reaction intermediates.[208]
  36. ^ Carbon as exfoliated (expanded) graphite,[220] and as meter-long carbon nanotube wire;[221] phosphorus as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature);[222] sulfur as plastic sulfur;[223] and selenium as selenium wires[224]
  37. ^ Metals have electrical conductivity values of from 6.9 × 103 S•cm−1 for manganese to 6.3 × 105 for silver.[234]
  38. ^ Metalloids have electrical conductivity values of from 1.5 × 10−6 S•cm−1 for boron to 3.9 × 104 for arsenic.[235]
  39. ^ Unclassified nonmetals have electrical conductivity values of from c. ~10−18 S•cm−1 for the elemental gases to 3 × 104 in graphite.[236]
  40. ^ The nonmetal halogens have electrical conductivity values of from c. ~10−18 S•cm−1 for F and Cl to 1.7 × 10−8 S•cm−1 for iodine.[236][237]
  41. ^ The elemental gases have electrical conductivity values of c. ~10−18 S•cm−1[236]
  42. ^ Based on vapor pressures of the elements[238]
  43. ^ At point of solidification for mercury, bromine, and gases
  44. ^ The pale yellow appearance of white phosphorus is probably due to the presence of small amounts of red phosphorus[245]
  45. ^ They always give compounds less acidic in character than the corresponding compounds of the typical nonmetals[250]
  46. ^ "The elements change from...metalloids, to moderately active nonmetals, to very active nonmetals, and to a noble gas."[251]
  47. ^ Ionization energies for Ds, Rg and Cn are predictions[255]
  48. ^ While there is some variation between different electronegativity scales the Pauling scale, as refined by Allred, has become the standard.[256]
  49. ^ It needs to be borne in mind here that, "establishing evidence for the essentiality of elements is highly challenging, and often controversial."[267]
  50. ^ On the other hand, Prinessa and Sadler wrote, "As yet there is no convincing evidence that boron...is an essential element for [humans]."[269]
  51. ^ Cockell observes that C, N, O, P, S and H, "have just the right atomic size and the right number of spare electrons to allow for binding to [one another] and...some other elements, to produce a molecular soup sufficient to build a self-replicating system."[271]
  52. ^ Breathing too much oxygen will poison the brain and can lead to death; "as little as 100mg [of white phosphorus] may be a fatal dose for a human"; a 5mg dose of selenium will produce a highly toxic reaction.[272]
  53. ^ 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.[280]
  54. ^ Arsenic trioxide reacts with sulfur trioxide, forming arsenic "sulfate" As2(SO4)3;[287] see also the Sulfates row.
  55. ^ CO and NO are, "formally the anhydrides of formic and hyponitrous acid, respectively: CO + H2O → H2CO2 (HCOOH, formic acid); N2O + H2O → H2N2O2 (hyponitrous acid)."[291]
  56. ^ See, for example, the sulfates of the transition metals,[303] the lanthanides[304] and the actinides.[305]
  57. ^ Sulfates of osmium have not been characterized with any great degree of certainty.[306]
  58. ^ For amorphous selenium, the increase in conductivity is a thousand-fold; for "metallic" selenium the increase is from three to as much as two-hundredfold.[328]
  59. ^ B; Si, Ge; N, P; O, S, Se, Te; nonmetal halogens; and the noble gases[239]
  60. ^ As at 2020, high pressure studies and experiments were said to represent, "a very active and vigorous research field".[333]
  61. ^ Radon sometimes occurs as potentially hazardous indoor pollutant[359]
  62. ^ Helium acquired the "-ium" suffix as its discoverer, William Lockyer, wrote: "I took upon myself the responsibility of coining the word helium.... I did not know whether the substance ... was a metal like calcium or a gas like hydrogen, but I did know that it behaved like hydrogen [being found in the sun] and that hydrogen, as Dumas had stated, behaved as a metal."[374]
  63. ^ Berzelius, who discovered selenium, thought it had the properties of a metal, combined with those of sulfur.[375]
  64. ^ It is conjectured that Albert Magnus heated a combination of arsenic trioxide with either vegetable oil or charcoal.[376]
  65. ^ The tellurium oxide[377] was derived from a tellurium ore containing 92.6% Te, 7.2% Fe, and 0.2% Au.[378] Weeks[379] explains what happened:
    "After digesting the ore with aqua regia, he [Klaproth] filtered off the residue and diluted the filtrate slightly with water. When he made the solution alkaline with caustic potash, a white precipitate appeared, but this dissolved in excess alkali, leaving only a brown, flocculent deposit containing gold and hydrous ferric oxide. Klaproth removed this precipitate by filtration and added hydrochloric acid to the filtrate until it was exactly neutral. A copious precipitate appeared [tellurium dioxide]. After washing and drying it he stirred it up with oil and introduced the oil paste into a glass retort; which he gradually heated to redness. When he cooled the apparatus, he found metallic globules of tellurium in the receiver and retort."

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  336. ^ Ostriker & Steinhardt 2001, pp. 46‒53
  337. ^ Nelson 1987, p. 732
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  344. ^ Emsley 2011, p. 117
  345. ^ National Center for Biotechnology Information 2021
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  347. ^ Boyd 2011, p. 570
  348. ^ Cox 1997, pp. 130–132; Emsley 2011, passim
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  350. ^ Stewart n.d.
  351. ^ Boise State University
  352. ^ National Institute of Standards and Technology 2013
  353. ^ Reinhardt at al. 2015; Eagleson 1994, p. 1053: SF6
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MonographsEdit

  • Steudel R 2020, Chemistry of the Non-metals: Syntheses - Structures - Bonding - Applications, in collaboration with D Scheschkewitz, Berlin, Walter de Gruyter, doi:10.1515/9783110578065.
    Twenty-three nonmetals, including B, Si, Ge, As, Se, Te, and At but not Sb (nor Po). The nonmetals are identified on the basis of their electrical conductivity at absolute zero putatively being close to zero, rather than finite as in the case of metals. That does not work for As however, which has the electronic structure of a semimetal (like Sb).
  • Halka M & Nordstrom B 2010, "Nonmetals", Facts on File, New York, ISBN 978-0-8160-7367-2
    A reading level 9+ book covering H, C, N, O, P, S, Se. Complementary books by the same authors examine (a) the post-transition metals (Al, Ga, In, Tl, Sn, Pb and Bi) and metalloids (B, Si, Ge, As, Sb, Te and Po); and (b) the halogens and noble gases.
  • Woolins JD 1988, Non-Metal Rings, Cages and Clusters, John Wiley & Sons, Chichester, ISBN 978-0-471-91592-8.
    A more advanced text that covers H; B; C, Si, Ge; N, P, As, Sb; O, S, Se and Te.
  • Steudel R 1977, Chemistry of the Non-metals: With an Introduction to Atomic Structure and Chemical Bonding, English edition by FC Nachod & JJ Zuckerman, Berlin, Walter de Gruyter, ISBN 978-3-11-004882-7.
    Twenty-three nonmetals, including B, Si, Ge, As, Se, Te, and Po.
  • Powell P & Timms PL 1974, The Chemistry of the Non-metals, Chapman & Hall, London, ISBN 978-0-470-69570-8.
    Twenty-two nonmetals including B, Si, Ge, As and Te. Tin and antimony are shown as being intermediate between metals and nonmetals; they are later shown as either metals or nonmetals. Astatine is counted as a metal.
  • Emsley J 1971, The Inorganic Chemistry of the Non-metals, Methuen Educational, London, ISBN 978-0-423-86120-4.
    Twenty nonmetals. H is placed over F; B and Si are counted as nonmetals; Ge, As, Sb and Te are counted as metalloids.
  • Johnson RC 1966, Introductory Descriptive Chemistry: Selected Nonmetals, their Properties, and Behavior, WA Benjamin, New York.
    Eighteen nonmetals. H is shown floating over B and C. Silicon, Ge, As, Sb, Te, Po and At are shown as semimetals. At is later shown as a nonmetal (p. 133).
  • Jolly WL 1966, The Chemistry of the Non-metals, Prentice-Hall, Englewood Cliffs, New Jersey.
    Twenty-four nonmetals, including B, Si, Ge, As, Sb, Te and At. H is placed over F.
  • Sherwin E & Weston GJ 1966, Chemistry of the Non-metallic Elements, Pergamon Press, Oxford.
    Twenty-three nonmetals. H is shown over Li and F; Germanium, As, Se, and Te are later referred to as metalloids; Sb is shown as a nonmetal but later referred to as a metal. They write, "Whilst these heavier elements [Se and Te] look metallic they show the chemical properties of non-metals and therefore come into the category of "metalloids" (p. 64).
  • Phillips CSG & Williams RJP 1965, Inorganic Chemistry, vol. 1, Principles and non-metals, Oxford University Press, Clarendon.
    Twenty-three nonmetals, excluding Sb, including At. An advanced work for its time, presenting inorganic chemistry as the difficult and complex subject it was, with many novel insights.
  • Yost DM & Russell Jr, H 1946 Systematic Inorganic Chemistry of the Fifth-and-Sixth-Group Nonmetallic Elements, Prentice-Hall, New York, accessed August 8, 2021.
    Includes tellurium as a nonmetallic element.
  • Bailey GH 1918, The Tutorial Chemistry, Part 1: The Non-Metals, 4th ed., W Briggs (ed.), University Tutorial Press, London.
    Fourteen nonmetals (excl. the noble gases), including B, Si, Se, and Te. The author writes that arsenic and antimony resemble metals in their luster and conductivity of heat and electricity but that in their chemical properties they resemble the non-metals, since they form acidic oxides and insoluble in dilute mineral acids; "such elements are called metalloids" (p. 530).
  • Appleton JH 1897, The Chemistry of the Non-metals: An Elementary Text-Book for Schools and Colleges, Snow & Farnham Printers, Providence, Rhode Island
    Eighteen nonmetals: He, Ar; F, Cl, Br, I; O, S, Se, Te; N, P, As, Sb; C, Si; B; H. Neon, germanium, krypton and xenon are listed as new or doubtful elements. For Sb, Appleton writes:
"Antimony is sometimes classed as a metal, sometimes as a non-metal. In case of several other elements the question of classification is difficult—indeed, the classification is one of convenience, in a sense, more than one of absolute scientific certainty. In some of its relations, especially its physical properties, antimony resembles the well-defined metals—in its chemical relations, it falls into the group containing boron, nitrogen, phosphorus, arsenic, well-defined non-metals." (p. 166).

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