wikipedia's featured article - 2014-10-04 - metalloid

5/19/2018 Wikipedia's Featured Article - 2014-10-04 - Metalloid

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Metalloid

A metalloid is achemical elementthat has propertiesin between, or is a mixture of, those of metals andnonmetals. There is no standard definition of a metal-loid, nor is there complete agreement as to which ele-ments are appropriately classified as such. Despite thislack of specificity, the term remains in use in the litera-ture ofchemistry.

The six commonly recognised metalloids are boron,silicon, germanium, arsenic, antimony,and tellurium. El-ements less commonly recognised as metalloids include

carbon, aluminium, selenium, polonium, and astatine.On a standard periodic table all of these elements maybe found in a diagonal region of the p-block, extendingfrom boron at one end, to astatine at the other. Some pe-riodic tables include adividing line between metals andnonmetalsand the metalloids may be found close to thisline.

Typical metalloids have a metallic appearance, but theyare brittle and only fairconductorsof electricity. Chem-ically, they mostly behave as nonmetals. They can formalloys with metals. Mostof their other physical and chem-ical properties are intermediate in nature. Metalloids usu-ally are too brittle to have any structural uses. They andtheir compounds are used in alloys, biological agents,catalysts, flame retardants, glasses, optical storageandoptoelectronics,pyrotechnics,semiconductors, and elec-tronics.

The electrical properties of silicon and germanium en-abled the establishment of thesemiconductor industryinthe 1950s and the development ofsolid-state electronicsfrom the early 1960s.[2]

The termmetalloidoriginally referred to nonmetals. Itsmore recent meaning, as a category of elements with

intermediate or hybrid properties, became widespreadin 194060. Metalloids sometimes are called semimet-als, a practice that has been discouraged,[3] as the termsemimetal has a different meaning in physics than inchemistry. In physics it more specifically refers to theelectronic band structureof a substance.

Broadly, the recognised metalloids occupymiddle groundin terms of their abundance, extraction methods, andcosts. Silicon is the second most abundant element in theEarths crust afteroxygen; tellurium is rarer thangold,but more abundant thanrhenium, the rarest of the stablemetals. Extraction may be achieved by ordinary chemical

reductionof theoxidesorsulfides.

1 Definitions

See also:List of metalloid lists

1.1 Judgement-based

A metalloid is an element with properties that are inbetween, or a mixture of, the properties of metals andnonmetals and thus, is hard to classify as either a metalor a nonmetal. This is a generic definition that drawson metalloid attributes that consistently are cited in theliterature.[n 2] Difficulty of categorisation is a key at-tribute. Most elements have a mixture of metallic andnonmetallic properties,[10] and can be classified accord-ing to which set of properties is more pronounced.[11][n 3]

Only the elements at or near the margins, lacking a suf-ficiently clear preponderance of either metallic or non-metallic properties, are classified as metalloids.[15]

Boron, silicon, germanium, arsenic, antimony, and tel-lurium are recognised commonly as metalloids.[16][n 4]

Depending on the author, one or more from selenium,polonium, or astatine sometimes are added to the list.[18]

Boron sometimes is excluded, by itself, or with silicon.[19]

Sometimes tellurium is not regarded as a metalloid.[20]

The inclusion of antimony, polonium, and astatine asmetalloids also has been questioned.[21]

Other elements occasionally are classified as met-alloids. These elements include,[22] hydrogen,[23]

beryllium,[24] nitrogen,[25] phosphorus,[26] sulfur,[27]

zinc,[28] gallium,[29] tin, iodine,[30] lead,[31] bismuth,[20]

and radon.[32] The term metalloid also has been usedfor elements that exhibit metallic lustre and electricalconductivity, and that areamphoteric, such as arsenic,antimony,vanadium,chromium,molybdenum,tungsten,tin, lead, and aluminium.[33] Thep-block metals,[34] andnonmetals (such as carbon or nitrogen) that can formalloyswith metals[35] or modify their properties[36] alsohave occasionally been considered as metalloids.

1.2 Criteria-based

No widely accepted definition of a metalloid exists, norany division of the periodic table into metals, metalloids

and nonmetals;[39] Hawkes[40] questioned the feasibilityof establishing a specific definition, noting that anomaliescan be found in several attempted constructs. Classifying

1
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2 3 PROPERTIES

an element as a metalloid has been described by Sharp[41]

as arbitrary.

The number and identities of metalloids depend on whatclassification criteria are used. Emsley[42] recognisedfour metalloids (germanium, arsenic, antimony and tel-

lurium); James et al.[43]

listed twelve (Emsleys plusboron, carbon, silicon, selenium, bismuth, polonium,ununpentiumandlivermorium). On average, seven el-ements are included insuch lists; individual classificationarrangements tend to share common ground and vary inthe ill-defined[44] margins.[n 5][n 6]

A single quantitative criterion such as electronegativityis commonly used,[47] metalloids having electronega-tivity values from 1.8 or 1.9 to 2.2.[48] Further ex-amples includepacking efficiency(the fraction of vol-ume in acrystal structureoccupied by atoms) and theGoldhammer-Herzfeld criterion ratio.[49] The commonly

recognised metalloids have packing efficiencies of be-tween 34% and 41%.[n 7] The Goldhammer-Herzfeld ra-tio, roughly equal to the cube of the atomic radius di-vided by themolar volume,[57][n 8] is a simple measureof how metallic an element is, the recognised metal-loids having ratios from around 0.85 to 1.1 and averag-ing 1.0.[59][n 9] Other authors have relied on, for example,atomic conductance[n 10][63] orbulk coordination num-ber.[64]

Jones, writing on the role of classification in science, ob-served that extquotedbl[classes] are usually defined bymorethantwoattributes.[65] Masterton and Slowinski[66]

used three criteria to describe the six elements commonlyrecognised as metalloids: metalloids haveionization en-ergiesaround 200 kcal/mol (837 kJ/mol) and electroneg-ativity values close to 2.0. They also said that metalloidsare typically semiconductors, though antimony and ar-senic (semimetals from a physics perspective) have elec-trical conductivities approaching those of metals. Sele-nium and polonium are suspected as not in this scheme,while astatines status is uncertain.[n 11]

2 Periodic table territory

2.1 Location

Metalloids lie on either side of thedividing line betweenmetals and nonmetals. This can be found, in varyingconfigurations, on someperiodic tables. Elements to thelower left of the line generally display increasing metallicbehaviour; elements to the upper right display increasingnonmetallic behaviour.[69] When presented as a regularstairstep, elements with the highestcritical temperaturefor their groups (Li, Be, Al, Ge, Sb, Po) lie just belowthe line.[70]

The diagonal positioning of the metalloids represents anexception to the observation that elements with simi-lar properties tend to occur in vertical groups.[71] A re-

lated effect can be seen in otherdiagonal similaritiesbe-tween some elements and their lower right neighbours,specifically lithium-magnesium, beryllium-aluminium,and boron-silicon. Rayner-Canham[72] has argued thatthese similarities extend to carbon-phosphorus, nitrogen-sulfur, and into threed-blockseries.

This exception arises due to competing horizontal andvertical trends in the nuclear charge. Going along aperiod, thenuclear chargeincreases withatomic num-beras do the number of electrons. The additional pullon outer electrons as nuclear charge increases gener-ally outweighs the screening effect of having more elec-trons. With some irregularities, atoms therefore becomesmaller, ionization energy increases, and there is a grad-ual change in character, across a period, from stronglymetallic, to weakly metallic, to weakly nonmetallic, tostrongly nonmetallic elements.[73] Going down a maingroup, the effect of increasing nuclear charge is gener-

ally outweighed by the effect of additional electrons be-ing further away from the nucleus. Atoms generally be-come larger, ionization energy falls, and metallic charac-ter increases.[74] The net effect is that the location of themetalnonmetal transition zone shifts to the right in goingdown a group,[71] and analogous diagonal similarities areseen elsewhere in the periodic table, as noted.[75]

2.2 Alternative treatments

Depictions of metalloids vary according to the author.

Some do not classify elements bordering the metalnonmetal dividing line as metalloids, noting that a bi-nary classification can facilitate the establishment ofrules for determining bond types between metals andnonmetals.[76] Metalloids are variously grouped withmetals,[77] regarded as nonmetals[78] or treated as a sub-category of nonmetals.[79][n 12] Other authors have sug-gested that classifying some elements as metalloids em-phasizes that properties change gradually rather thanabruptly as one moves across or down the periodictable.[81] Some periodic tables distinguish elements thatare metalloids and display no formal dividing line be-

tween metals and nonmetals. Metalloids are shown as oc-curring in a diagonal band[82] or diffuse region.[83]

3 Properties

Metalloids usually look like metals but behave largely likenonmetals. Physically, they are shiny, brittle solids withintermediate to relatively good electrical conductivity andthe electronic band structure of a semimetal or semicon-ductor. Chemically, they mostly behave as (weak) non-metals, have intermediate ionization energies and elec-

tronegativity values, and amphoteric or weakly acidicoxides. They can form alloys with metals. Most of theirother physical and chemical properties are intermediate
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4.2 Biological agents 3

in nature.

3.1 Compared to metals and nonmetals

Main article: Metalloid (comparison of properties with

those of metals and nonmetals)

Characteristic properties of metals, metalloids and non-metals are summarized in the table.[84] Physical proper-ties are listed in order of ease of determination; chemicalproperties run from general to specific, and then to de-scriptive.

The above table reflects the hybrid nature of metalloids.The properties ofform, appearance,andbehaviour whenmixed with metalsaremorelikemetals. Elasticity andgen-eral chemical behaviourare more like nonmetals. Electri-cal conductivity, band structure, ionization energy, elec-

tronegativity, and oxides are intermediate between thetwo.

4 Common applications

The focus of this section is on the recognised

metalloids. Elements less often recognised as

metalloids are ordinarily classified as either

metals or nonmetals; some of these are included

here for comparative purposes.

Metalloids are too brittle to have any structural uses intheir pure forms.[105] They and their compounds are usedas (or in) alloying components, biological agents (toxico-logical, nutritional and medicinal), catalysts, flame retar-dants, glasses (oxide and metallic), optical storage mediaand optoelectronics, pyrotechnics, semiconductors andelectronics.[n 18]

4.1 Alloys

Writing early in the history ofintermetallic compounds,

the British metallurgist Cecil Desch observed that cer-tain non-metallic elements are capable of forming com-pounds of distinctly metallic character with metals, andthese elements may therefore enter into the compositionof alloys. He associated silicon, arsenic and tellurium, inparticular, with the alloy-forming elements.[108] Phillipsand Williams[109] suggested that compounds of silicon,germanium, arsenic and antimony with B metals, areprobably best classed as alloys.

Among the lighter metalloids, alloys withtransition met-alsare well-represented. Boron can form intermetalliccompounds and alloys with such metals of the composi-

tionMnB,if n > 2.[110] Ferroboron (15% boron) is used tointroduce boron intosteel; nickel-boron alloys are ingre-dients in welding alloys andcase hardeningcompositions

Copper-germanium alloy pellets, likely ~84% Cu; 16% Ge.[107]

When combined withsilverthe result is atarnish resistant sterling

silver. Also shown are two silver pellets.

for the engineering industry. Alloys of silicon withironand with aluminium are widely used by thesteel and auto-motive industries, respectively. Germanium forms manyalloys, most importantly with thecoinage metals.[111]

The heavier metalloids continue the theme. Arseniccan form alloys with metals, including platinum andcopper;[112] it is also added to copper and its alloys toimprove corrosion resistance[113] and appears to conferthe same benefit when added to magnesium.[114] Anti-mony is well known as an alloy-former, including withthe coinage metals. Its alloys includepewter(a tin al-loy with up to 20% antimony) and type metal(a lead

alloy with up to 25% antimony).

[115]

Tellurium readilyalloys with iron, as ferrotellurium (5058% tellurium),and with copper, in the form of copper tellurium (4050% tellurium).[116] Ferrotellurium is used as a stabilizerfor carbon in steel casting.[117] Of the non-metallic ele-ments less often recognised as metalloids, seleniuminthe form of ferroselenium (5058% selenium)is usedto improve themachinabilityof stainless steels.[118]

4.2 Biological agents

All six of the elements commonly recognised as met-alloids have toxic, dietary or medicinal properties.[120]

Arsenic and antimony compounds are especially toxic;boron, silicon, and possibly arsenic, are essential traceelements. Boron, silicon, arsenic and antimony havemedical applications, and germanium and tellurium arethought to have potential.

Boron is used in insecticides[121] and herbicides.[122] It isan essential trace element.[123] Asboric acid, it has anti-septic, antifungal, and antiviral properties.[124]

Silicon is present in silatrane, a highly toxicrodenticide.[125] Long-term inhalation of silica dust

causessilicosis, a fatal disease of the lungs. Silicon is anessential trace element.[123] Siliconegel can be appliedto badly burned patients to reduce scarring.[126]
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4 4 COMMON APPLICATIONS

Arsenic trioxide orwhitearsenic,oneof themost toxic andpreva-lent forms of arsenic. Theantileukaemicproperties of white ar-

senic were first reported in 1878.[119]

Saltsof germanium are potentially harmful to humansand animals if ingested on a prolonged basis.[127] Thereis interest in the pharmacological actions of germaniumcompounds but no licensed medicine as yet.[128]

Arsenic is notoriously poisonous and may also be anessential elementin ultratrace amounts.[129] It has beenused as a pharmaceutical agent since antiquity, includ-ing for the treatment of syphilis before the develop-ment of antibiotics.[130] Arsenic is also a componentof melarsoprol, a medicinal drug used in the treat-

ment ofhuman African trypanosomiasisor sleeping sick-ness. In 2003, arsenic trioxide (under the trade nameTrisenox) was re-introduced for the treatment of acutepromyelocytic leukaemia, a cancer of the blood and bonemarrow.[130]

Metallic antimony is relatively non-toxic, but most anti-mony compounds are poisonous.[131] Twoantimony com-pounds, sodium stibogluconate and stibophen,areusedasantiparasiticaldrugs.[132]

Elemental tellurium is not considered particularly toxic;two grams of sodium tellurate, if administered, canbe lethal.[133] People exposed to small amounts of air-

borne tellurium exude a foul and persistent garlic-likeodour.[134] Tellurium dioxide has been used to treatseborrhoeic dermatitis; other tellurium compounds wereused asantimicrobialagents before the development ofantibiotics.[135] In future, such compounds may need tobe substituted for antibiotics that have become ineffec-tive due to bacterial resistance.[136]

Of the elements less often recognised as metalloids,beryllium and lead are noted for their toxicity;lead ar-senatehas been extensively used as an insecticide.[137]

Sulfur is one of the oldest of the fungicides and pes-ticides. Phosphorus, sulfur, zinc, selenium and iodine

are essential nutrients, and aluminium, tin and lead maybe.[129] Sulfur, gallium, selenium, iodine and bismuthhave medicinal applications. Sulfur is a constituent of

sulfonamide drugs, still widely used for conditions suchas acne and urinary tract infections.[138] Gallium ni-trateis used to treat the side effects of cancer;[139] gal-lium citrate, a radiopharmaceutical, facilitates imagingof inflamed body areas.[140] Selenium sulfideis used inmedicinal shampoos and to treat skin infections such

as tinea versicolor.[141] Iodine is used as a disinfectantin various forms. Bismuth is an ingredient in someantibacterials.[142]

4.3 Catalysts

Boron trifluorideandtrichlorideare used ascatalystsinorganic synthesis and electronics; thetribromideis usedin the manufacture of diborane.[143] Non-toxic boronligandscan replace toxic phosphorus ligands in transitionmetal catalysts.[144] Silica sulfuric acid (SiO2OSO3H)

is used in organic reactions.[145] Germanium dioxide issometimes used as a catalyst in the production ofPETplastic for containers;[146] cheaper antimony compounds,such as the trioxide or triacetate, are more commonly em-ployed for the same purpose[147] despite concerns aboutantimony contamination of food and drinks.[148] Arsenictrioxide has been used in the production of natural gas,to boost the removal ofcarbon dioxide, as haveselenousacidand tellurous acid.[149] Selenium acts as a catalystin some microorganisms.[150] Tellurium, and its diox-ide andtetrachloride, are strong catalysts for air oxida-tion of carbon above 500 C.[151] Graphite oxidecan

be used as a catalyst in the synthesis of imines andtheir derivatives.[152] Activated carbonandaluminahavebeen used as catalysts for the removal of sulfur con-taminants from natural gas.[153] Titanium doped alu-minium has been identified as a substitute for expensivenoble metalcatalysts used in the production of industrialchemicals.[154]

4.4 Flame retardants

Compounds of boron, silicon, arsenic and antimony have

been used asflame retardants. Boron, in the form ofborax, has been used as a textile flame retardant sinceat least the 18th century.[155] Silicon compounds suchas silicones, silanes, silsesquioxane,silicaand silicates,some of which were developed as alternatives to moretoxic halogenated products, can considerably improve theflame retardancy of plastic materials.[156] Arsenic com-pounds such assodium arseniteorsodium arsenateareeffective flame retardants for wood but have been less fre-quently used due to their toxicity.[157] Antimony trioxideis a flame retardant.[158] Aluminium hydroxidehas beenused as a wood-fibre, rubber, plastic and textile flame re-tardant since the 1890s.[159] Apart from aluminium hy-

droxide, use of phosphorus based flame-retardantsinthe form of, for example, organophosphatesnow ex-ceeds that of any of the other main retardant types.
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4.6 Optical storage and optoelectronics 5

These employ boron, antimony orhalogenated hydrocar-boncompounds.[160]

4.5 Glass formation

Optical fibres, usually made of pure silicon dioxide glass, with

additives such as boron trioxide or germanium dioxide for in-

creased sensitivity

The oxides B2O3, SiO2, GeO2, As2O3 and Sb2O3readily form glasses. TeO2 forms a glass but this re-quires a heroic quench rate[161] or the addition ofan impurity, otherwise the crystalline form results.[161]

These compounds are used in chemical, domestic andindustrial glassware[162] and optics.[163] Boron trioxideis used as aglass fibreadditive,[164] and is also a com-ponent of borosilicate glass, widely used for labora-

tory glassware and domestic ovenware for its low ther-mal expansion.[165] Most ordinary glassware is madefrom silicon dioxide.[166] Germanium dioxide is usedas a glass fibre additive, as well as in infrared opticalsystems.[167] Arsenic trioxide is used in the glass indus-try as adecolourizingand fining agent (for the removal ofbubbles),[168] as is antimony trioxide.[169] Tellurium diox-ide finds application in laser andnonlinear optics.[170]

Amorphousmetallic glasses are generally mosteasilypre-pared if one of the components is a metalloid or nearmetalloid such as boron, carbon, silicon, phosphorus orgermanium.[171][n 19] Aside from thin films deposited at

very low temperatures, the first known metallic glass wasan alloy of composition Au75Si25reported in 1960.[173]

A metallic glass having a strength and toughness not pre-viously seen, of composition Pd.P6Si.Ge2, was re-ported in 2011.[174]

Phosphorus, selenium and lead, which are less of-ten recognised as metalloids, are also used in glasses.Phosphate glasshas a substrate of phosphorus pentoxide(P2O5), rather than the silica (SiO2) of conventional sil-icate glasses. It is used, for example, to makesodiumlamps.[175] Selenium compounds can be used both as de-colourising agents and to add a red colour to glass.[176]

Decorative glassware made of traditionallead glasscon-tainsatleast30% lead(II)oxide (PbO); lead glassused forradiation shielding may have up to 65% PbO.[177] Lead-

based glasses have also been extensively used in electron-ics components; enamelling; sealing and glazing materi-als; and solar cells. Bismuth based oxide glasses haveemerged as a less toxic replacement for lead in many ofthese applications.[178]

4.6 Optical storage and optoelectronics

Varyingcompositions of GeSbTe (GST alloys) and Ag-and In- doped Sb2Te(AIST alloys), being examplesofphase-change materials, are widely used in rewritableoptical discsandphase-change memorydevices. By ap-plying heat, they can be switched between amorphous(glassy) and crystalline states. The change in opticaland electrical properties can be used for informationstorage purposes.[179] Future applications for GeSbTemay include, ultrafast, entirely solid-state displays with

nanometre-scale pixels, semi-transparent smart glasses,smart contact lenses and artificial retina devices.[180]

4.7 Pyrotechnics

Archaic blue light signal, fuelled by a mixture of sodium nitrate,

sulfur and (red) arsenic trisulfide[181]

The recognised metalloids have either pyrotechnic appli-cations or associated properties. Boron and silicon arecommonly encountered;[182] they act somewhat like metal

fuels.[183] Boron is used inpyrotechnic initiatorcomposi-tions (for igniting other hard-to-start compositions), andin delay compositionsthat burn at a constant rate.[184]
http://-/?-https://en.wikipedia.org/wiki/Delay_compositionhttps://en.wikipedia.org/wiki/Pyrotechnic_initiatorhttp://-/?-https://en.wikipedia.org/wiki/Crystallinehttps://en.wikipedia.org/wiki/Phase-change_memoryhttps://en.wikipedia.org/wiki/Optical_dischttps://en.wikipedia.org/wiki/Phase-change_materialhttps://en.wikipedia.org/wiki/AgInSbTehttps://en.wikipedia.org/wiki/AgInSbTehttps://en.wikipedia.org/wiki/GeSbTehttps://en.wikipedia.org/wiki/Lead(II)_oxidehttps://en.wikipedia.org/wiki/Lead_glasshttps://en.wikipedia.org/wiki/Sodium_lamphttps://en.wikipedia.org/wiki/Sodium_lamphttps://en.wikipedia.org/wiki/Phosphate_glasshttps://en.wikipedia.org/wiki/Metallic_glasshttps://en.wikipedia.org/wiki/Amorphoushttps://en.wikipedia.org/wiki/Nonlinear_opticshttps://en.wikipedia.org/wiki/Glass_coloring_and_color_markinghttps://en.wikipedia.org/wiki/Borosilicate_glasshttps://en.wikipedia.org/wiki/Glass_fibrehttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Tellurium_dioxidehttps://en.wikipedia.org/wiki/Glasshttps://en.wikipedia.org/wiki/Antimony_trioxidehttps://en.wikipedia.org/wiki/Arsenic_trioxidehttps://en.wikipedia.org/wiki/Germanium_dioxidehttps://en.wikipedia.org/wiki/Silicon_dioxidehttps://en.wikipedia.org/wiki/Boron_trioxidehttps://en.wikipedia.org/wiki/Optical_fiberhttps://en.wikipedia.org/wiki/Halogenated_hydrocarbonhttps://en.wikipedia.org/wiki/Halogenated_hydrocarbon


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6 4 COMMON APPLICATIONS

Boron carbidehas been identified as a possible replace-ment for more toxicbariumor hexachloroethanemix-tures in smoke munitions, signal flares and fireworks.[185]

Silicon, like boron, is a component of initiator and de-lay mixtures.[184] Doped germanium can act as a vari-able speed thermitefuel.[n 20] Arsenic trisulfideAs2S3was used in oldnaval signal lights; in fireworks to makewhite stars;[187] in yellowsmoke screen mixtures; andin initiator compositions.[188] Antimony trisulfideSb2S3is found in white-light fireworks and inflash and soundmixtures.[189] Tellurium has been used in delay mixturesand inblasting capinitiator compositions.[190]

Carbon, aluminium, phosphorus and selenium continuethe theme. Carbon, inblack powder, is a constituent offireworks rocket propellants, bursting charges, and effectsmixtures, and military delay fuses and igniters.[191][n 21]

Aluminium is a common pyrotechnic ingredient,[182] andis widely employed for its capacity to generate light and

heat,[193] including in thermite mixtures.[194] Phosphoruscan be found in smoke and incendiary munitions, papercapsused intoy guns, andparty poppers.[195] Seleniumhas been used in the same way as tellurium.[190]

4.8 Semiconductors and electronics

Semiconductor-based electronic components. From left to right:

a transistor, an integrated circuitand an LED. Theelementscom-

monly recognised as metalloids find widespread use in such de-

vices, as elemental orcompoundsemiconductor constituents (Si,

Ge orGaAs, for example) or asdoping agents (B, Sb, Te, for

example).

All the elements commonly recognised as metalloids (ortheir compounds) have been used in the semiconductoror solid-state electronic industries.[196]

Some properties of boron have limited its use as a semi-conductor. It has a high melting point, singlecrystalsarerelatively hard to obtain, and introducing and retainingcontrolled impurities is difficult.[197]

Silicon is the leading commercial semiconductor; itforms the basis of modern electronics (including stan-dard solar cells)[198] and information and communica-

tion technologies.[199] This was despite the study of semi-conductors, early in the 20th century, having been re-garded as the physics of dirt and not deserving of close

attention.[200]

Germanium has largely been replaced by silicon in semi-conducting devices, being cheaper, more resilient athigher operating temperatures, and easier to work dur-ing the microelectronic fabrication process.[107] Germa-

nium is still a constituent of semiconducting silicon-germaniumalloys and these have been growing in use,particularly for wireless communication devices; such al-loys exploit the highercarrier mobility of germanium.[107]

The synthesis of gram-scale quantities of semiconduct-inggermananewas reported in 2013. This comprisesone-atom thick sheets of hydrogen-terminated germa-nium atoms, analogous to graphane. It conducts elec-trons more than ten times faster than silicon and five timesfaster than germanium, and is thought to have potentialfor optoelectronic and sensing applications.[201] The de-velopment of a germanium-wire based anode that morethan doubles the capacity oflithium-ion batterieswas re-

ported in 2014.[202] In the same year, Lee at al. reportedthat defect-free crystals of graphene large enough to haveelectronic uses could be grown on, and removed from, agermanium substrate.[203]

Arsenic and antimony are not semiconductors in theirstandard states. Both formtype III-V semiconductors(such as GaAs,AlSbor GaInAsSb) in which the averagenumber of valence electrons per atom is the same as thatofGroup 14elements. These compounds are preferredfor some special applications.[204] Antimony nanocrystalsmay enablelithium-ion batteriesto be replaced by morepowerfulsodium ion batteries.[205]

Tellurium, which is a semiconductor in its standardstate, is used mainly as a component in type II/VIsemiconducting-chalcogenides; these have applicationsin electro-optics and electronics.[206] Cadmium telluride(CdTe) is used in solar modules for its high conversion ef-ficiency, low manufacturing costs, and largeband gapof1.44eV,lettingitabsorbawiderangeofwavelengths.[198]

Bismuth telluride(Bi2Te3), alloyed with selenium andantimony, is a component ofthermoelectric devicesusedfor refrigeration or portable power generation.[207]

Five metalloidsboron, silicon, germanium, arsenic and

antimonycan be found in cell phones (along with atleast 39 other metals and nonmetals).[208] Tellurium is ex-pected to find such use.[209] Of the less often recognisedmetalloids, phosphorus, gallium (in particular) and se-lenium have semiconductor applications. Phosphorus isused in trace amounts as a dopant for n-type semiconduc-tors.[210] The commercial use of gallium compounds isdominated by semiconductor applicationsin integratedcircuits; cell phones;laser diodes;light-emitting diodes;photodetectors; andsolar cells.[211] Selenium is used inthe production of solar cells[212] and in high-energysurgeprotectors.[213]
https://en.wikipedia.org/wiki/Surge_protectorhttps://en.wikipedia.org/wiki/Surge_protectorhttps://en.wikipedia.org/wiki/Solar_cellhttps://en.wikipedia.org/wiki/Photodetectorhttps://en.wikipedia.org/wiki/Light-emitting_diodehttps://en.wikipedia.org/wiki/Laser_diodehttps://en.wikipedia.org/wiki/N-type_semiconductorhttps://en.wikipedia.org/wiki/N-type_semiconductorhttps://en.wikipedia.org/wiki/Dopanthttps://en.wikipedia.org/wiki/Thermoelectric_materialshttps://en.wikipedia.org/wiki/Bismuth_telluridehttp://-/?-https://en.wikipedia.org/wiki/Band_gaphttps://en.wikipedia.org/wiki/Cadmium_telluridehttps://en.wikipedia.org/wiki/Chalcogenidehttps://en.wikipedia.org/wiki/List_of_semiconductor_materialshttps://en.wikipedia.org/wiki/Sodium-ion_batteryhttps://en.wikipedia.org/wiki/Lithium-ion_batterieshttps://en.wikipedia.org/wiki/Group_14https://en.wikipedia.org/wiki/AlSbhttps://en.wikipedia.org/wiki/Compound_semiconductorhttps://en.wikipedia.org/wiki/Standard_state#Liquids_and_solidshttps://en.wikipedia.org/wiki/Lithium-ion_batteryhttps://en.wikipedia.org/wiki/Graphanehttps://en.wikipedia.org/wiki/Germananehttp://-/?-https://en.wikipedia.org/wiki/Silicon-germaniumhttps://en.wikipedia.org/wiki/Silicon-germaniumhttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Crystalhttps://en.wikipedia.org/wiki/Doping_(semiconductor)https://en.wikipedia.org/wiki/GaAshttps://en.wikipedia.org/wiki/Compound_semiconductorhttps://en.wikipedia.org/wiki/LEDhttps://en.wikipedia.org/wiki/Integrated_circuithttps://en.wikipedia.org/wiki/Transistorhttp://-/?-https://en.wikipedia.org/wiki/Party_poppershttps://en.wikipedia.org/wiki/Cap_gunhttps://en.wikipedia.org/wiki/Armstrong%27s_mixturehttps://en.wikipedia.org/wiki/Armstrong%27s_mixturehttp://-/?-https://en.wikipedia.org/wiki/Black_powderhttp://-/?-https://en.wikipedia.org/wiki/Blasting_caphttps://en.wikipedia.org/wiki/Flash_powderhttps://en.wikipedia.org/wiki/Stibnitehttps://en.wikipedia.org/wiki/Smoke_screenhttps://en.wikipedia.org/wiki/Blue_light_(pyrotechnic_signal)https://en.wikipedia.org/wiki/Arsenic_trisulfidehttps://en.wikipedia.org/wiki/Thermitehttp://-/?-https://en.wikipedia.org/wiki/Hexachloroethanehttps://en.wikipedia.org/wiki/Bariumhttps://en.wikipedia.org/wiki/Boron_carbide


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7

5 Nomenclature and history

5.1 Derivation and other names

The word metalloid comes from the Latin metallum

(metal) and the Greek oeides (resembling in formor appearance).[214] Several names are sometimes usedsynonymously although some of these have other mean-ings that are not necessarily interchangeable: amphotericelement,[215] boundary element,[216] half-metal,[217] half-

way element,[218] near metal,[219] meta-metal,[220] semi-

conductor,[221] semimetal[222] and submetal.[223] Ampho-teric element is sometimes used more broadly to in-clude transition metals capable of forming oxyanions,such as chromium andmanganese.[224] extquotedblHalf-metalextquotedbl is used in physics to refer to a com-pound (such aschromium dioxide) or alloy that can actas a conductor and aninsulator. Meta-metal is some-times used instead to refer to certain metals (Be, Zn,Cd,Hg,In,Tl,-Sn,Pb) located just to the left of themetalloids on standard periodic tables.[217] These met-als are mostlydiamagnetic[225] and tend to have distortedcrystalline structures, electrical conductivity values at thelower end of those of metals, and amphoteric (weakly ba-sic) oxides.[226] Semimetal sometimes refers, loosely orexplicitly, to metals with incomplete metallic character incrystalline structure, electrical conductivity or electronicstructure. Examples include gallium,[227] ytterbium,[228]

bismuth[229] andneptunium.[230] The names amphotericelementand semiconductorare problematic as some ele-

ments referred to as metalloids do not show marked am-photeric behaviour (bismuth, for example)[231] or semi-conductivity (polonium)[232] in their most stable forms.

5.2 Origin and usage

Main article:Metalloid (nomenclature origin and usage)

The origin and usage of the term metalloidis convoluted.Its origin lies in attempts, dating from antiquity, to de-

scribe metals and to distinguish between typical and lesstypical forms. It was first applied in the early 19th cen-tury to metals that floated on water (sodium and potas-sium), and then more popularly to nonmetals. Earlier us-age inmineralogy, to describe a mineral having a metal-lic appearance, can be sourced to as early as 1800.[233]

Since the mid-20th century it has been used to refer tointermediate or borderline chemical elements.[234][n 22]

TheInternational Union of Pure and Applied Chem-istry(IUPAC) previously recommended abandoning theterm metalloid, and suggested using the term semimetalinstead.[236] Useof this latter term hasmore recently beendiscouraged by Atkins et al.[3] as it has a different mean-

ing in physicsone that more specifically refers to theelectronic band structureof a substance rather than theoverall classification of an element. The most recent IU-

PAC publications on nomenclature and terminology donot include any recommendations on the usage of theterms metalloid or semimetal.[237]

6 Elements commonly recognisedas metalloids

Properties noted in this section refer to the el-

ements in their most thermodynamically stable

forms under ambient conditions.

6.1 Boron

Main article:Boron

Pure boron is a shiny, silver-grey crystalline solid.[239]

Boron, shown here in the form of its -rhombohedralphase (its

most thermodynamically stableallotrope)[238]

It is less dense than aluminium (2.34 vs. 2.70 g/cm3),and is hard and brittle. It is barely reactive under normalconditions, except for attack byfluorine,[240] and has amelting point of 2076 C (cf. steel ~1370 C).[241] Boronis a semiconductor;[242] its room temperature electricalconductivity is 1.5 106 Scm1[243] (about 200 times

less than that of tap water)[244] and a band gap of about1.56 eV.[245][n 23]

The chemistry of boron is dominated by its small atomicsize, and relatively high ionization energy. With onlythree valence electrons, simple covalent bonding cannotfulfil the octet rule.[247] Metallic bonding is the usualresult among the heavier congenors of boron but thisgenerally requires a low ionization energy.[248] Beingsmall and having a high ionization energy boron insteadforms delocalized covalent bonds,[249] in which threeatoms share two electrons. The associated structuralunit, theicosahedralB12cluster, pervades theallotropic

forms of boron. The same motif can be seen, as aredeltahedral variants or fragments, in metal boridesandhydride derivatives, and in somehalides.[250]
https://en.wikipedia.org/wiki/Halidehttps://en.wikipedia.org/wiki/Derivative_(chemistry)https://en.wikipedia.org/wiki/Hydridehttps://en.wikipedia.org/wiki/Boridehttps://en.wikipedia.org/wiki/Deltahedronhttps://en.wikipedia.org/wiki/Allotropes_of_boronhttps://en.wikipedia.org/wiki/Allotropehttps://en.wikipedia.org/wiki/Icosahedronhttps://en.wikipedia.org/wiki/Three-center_two-electron_bondhttps://en.wikipedia.org/wiki/Metallic_bondinghttps://en.wikipedia.org/wiki/Octet_rulehttps://en.wikipedia.org/wiki/Siemens_(unit)https://en.wikipedia.org/wiki/Fluorinehttps://en.wikipedia.org/wiki/Allotropehttps://en.wikipedia.org/wiki/Rhombohedralhttps://en.wikipedia.org/wiki/Boronhttps://en.wikipedia.org/wiki/Electronic_band_structurehttp://-/?-https://en.wikipedia.org/wiki/International_Union_of_Pure_and_Applied_Chemistryhttps://en.wikipedia.org/wiki/International_Union_of_Pure_and_Applied_Chemistryhttp://-/?-https://en.wikipedia.org/wiki/Mineralogyhttps://en.wikipedia.org/wiki/Metalloid_(nomenclature_origin_and_usage)http://-/?-https://en.wikipedia.org/wiki/Neptuniumhttps://en.wikipedia.org/wiki/Ytterbiumhttps://en.wikipedia.org/wiki/Diamagnetismhttp://-/?-https://en.wikipedia.org/wiki/Leadhttps://en.wikipedia.org/wiki/Tin#Physical_propertieshttps://en.wikipedia.org/wiki/Thalliumhttps://en.wikipedia.org/wiki/Indiumhttps://en.wikipedia.org/wiki/Mercury_(element)https://en.wikipedia.org/wiki/Cadmiumhttps://en.wikipedia.org/wiki/Zinchttps://en.wikipedia.org/wiki/Berylliumhttps://en.wikipedia.org/wiki/Insulator_(electricity)https://en.wikipedia.org/wiki/Chromium_dioxidehttps://en.wikipedia.org/wiki/Half-metalhttps://en.wikipedia.org/wiki/Half-metalhttps://en.wikipedia.org/wiki/Manganesehttps://en.wikipedia.org/wiki/Oxyanionhttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Greek_languagehttps://en.wikipedia.org/wiki/Latin_language


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8 6 ELEMENTS COMMONLY RECOGNISED AS METALLOIDS

The bonding in boron has been described as beingcharacteristic of behaviour intermediate between met-als and nonmetallic covalent network solids (such asdiamond).[251] The energy required to transform B, C, N,Si and P from nonmetallic to metallic states has been es-timated as 30, 100, 240, 33 and 50 kJ/mol, respectively.

This indicates how close boron is to the metal-nonmetalborderline.[252]

Most of the chemistry of boron is nonmetallic innature.[252] The small size of the boron atom enables thepreparation of many interstitial alloy-type borides.[253]

Analogies between boron and transition metals have beennoted in the formation of complexes,[254] andadducts(for example, BH3 + CO BH3CO and, similarly,Fe(CO)4 + CO Fe(CO)5), as well as in the geo-metric and electronic structures ofcluster speciessuchas [B6H6]2 and [Ru6(CO)18]2.[255][n 24] The aqueouschemistry of boron is characterised by the formation

of many differentpolyborate anions.[257] Given its highcharge-to-size ratio, nearly all compounds of boron arecovalent, with a few complexed anionic and cationicspecies.[258] Boron has a strong affinity foroxygenanda duly extensiveboratechemistry.[253] The oxide B2O3ispolymericin structure,[259] weakly acidic,[260] and a glassformer.[261] Organometallic compounds of boron[n 25]

havebeenknownsincethe19thcentury(see organoboronchemistry).[263]

6.2 Silicon

Main article:SiliconSilicon is a crystalline solid with a blue-grey metallic

Silicon has a blue-grey metallic lustre.

lustre.[264] Like boron, it is less dense (at 2.33 g/cm3)thanaluminium, andis hard andbrittle.[265] Itisarelativelyun-reactive element.[264] According to Rochow,[266] the mas-sive crystalline form (especially if pure) is remarkablyinert to all acids, includinghydrofluoricextquotedbl.[n 26]

Less pure silicon, and the powdered form, are variously

susceptible to attack by strong or heated acids, as wellas by steam and fluorine.[270] Silicon dissolves in hotaqueous alkalis with the evolution of hydrogen, as do

metals[271] such as beryllium, aluminium, zinc, galliumor indium.[272] It melts at 1414 C. Silicon is a semicon-ductor with an electrical conductivity of 104 Scm1[273]

and a band gap of about 1.11 eV.[267] When it melts, sil-icon becomes a reasonable metal[274] with an electricalconductivity of 1.01.3 104 Scm1, similar to that of

liquid mercury.[275]

The chemistry of silicon is generally nonmetallic (cova-lent) in nature.[276] It can form alloys with metals suchas iron and copper.[277] Silicon shows fewer tendenciesto anionic behaviour than ordinary nonmetals.[278] Itssolution chemistry is characterised by the formation ofoxyanions.[279] The high strength of the silicon-oxygenbond dominates the chemical behaviour of silicon.[280]

Polymeric silicates, built up by tetrahedral SiO4 unitssharing their oxygen atoms, are the most abundant andimportant compounds of silicon.[281] The polymeric bo-rates, comprising linked trigonal and tetrahedral BO3or

BO4units, are built on similar structural principles. [282]The oxide SiO2 is polymeric in structure,[259] weaklyacidic,[283][n 27] and a glass former.[261] Traditionalorganometallicchemistry includes the carbon compoundsof silicon (seeorganosilicon).[287]

6.3 Germanium

Main article:GermaniumGermanium is a shiny grey-white solid.[288] It has a den-

Germanium is sometimes described as a metal

sity of 5.323 g/cm3 andis hard and brittle.[289] It is mostlyunreactive at roomtemperature[n 28] but is slowlyattackedby hot concentratedsulfuricor nitric acid.[291] Germa-nium also reacts with moltencaustic sodato yield sodiumgermanate Na2GeO3 and hydrogen gas.[292] It melts at

938 C. Germanium is a semiconductor with an electricalconductivity of around 2 102 Scm1[291] and a bandgap of 0.67 eV.[293] Liquid germanium is a metallic con-
http://-/?-https://en.wikipedia.org/wiki/Sodium_hydroxidehttp://-/?-https://en.wikipedia.org/wiki/Nitric_acidhttps://en.wikipedia.org/wiki/Sulfuric_acidhttps://en.wikipedia.org/wiki/Germaniumhttps://en.wikipedia.org/wiki/Organosiliconhttp://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-https://en.wikipedia.org/wiki/Hydrogenhttps://en.wikipedia.org/wiki/Alkalihttps://en.wikipedia.org/wiki/Hydrofluoric_acidhttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Siliconhttps://en.wikipedia.org/wiki/Organoboron_chemistryhttps://en.wikipedia.org/wiki/Organoboron_chemistryhttps://en.wikipedia.org/wiki/Organometallic_chemistryhttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Polymerichttp://-/?-https://en.wikipedia.org/wiki/Boratehttps://en.wikipedia.org/wiki/Oxygenhttps://en.wikipedia.org/wiki/Anion#Anions_and_cationshttps://en.wikipedia.org/wiki/Borate#Polymeric_ionshttps://en.wikipedia.org/wiki/Cluster_compoundhttps://en.wikipedia.org/wiki/Carbon_monoxidehttps://en.wikipedia.org/wiki/Adducthttps://en.wikipedia.org/wiki/Complex_(chemistry)http://-/?-https://en.wikipedia.org/wiki/Interstitial_compoundhttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Diamondhttps://en.wikipedia.org/wiki/Covalent_network


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6.5 Antimony 9

ductor, with an electrical conductivity similar to that ofliquid mercury.[294]

Most of the chemistry of germanium is characteristic of anonmetal.[295] It can form alloys with metals such as alu-minium andgold.[296] Germanium shows fewer tenden-

cies to anionic behaviour than ordinary nonmetals.[278]

Its solution chemistry is characterised by the formationof oxyanions.[279] Germanium generally forms tetrava-lent (IV) compounds, and it can also form less sta-ble divalent (II) compounds, in which it behaves morelike a metal.[297] Germanium analogues of all of themajor types of silicates have been prepared.[298] Themetallic character of germanium is also suggested bythe formation of various oxoacid salts. A phosphate[(HPO4)2GeH2O] and highly stable trifluoroacetateGe(OCOCF3)4have been described, as haveGe2(SO4)2,Ge(ClO4)4 and GeH2(C2O4)3.[299] The oxide GeO2 ispolymeric,[259] amphoteric,[300] and a glass former.[261]

The dioxide is soluble in acidic solutions (the monox-ide GeO, is even more so), and this is sometimes usedto classify germanium as a metal.[301] Up to the 1930sgermanium was considered to be a poorly conductingmetal;[302] it has occasionally been classified as a metalby later writers.[303] As with all the elements commonlyrecognised as metalloids, germanium has an establishedorganometallic chemistry (seeorganogermanium chem-istry).[304]

6.4 Arsenic

Main article:ArsenicArsenic is a grey, metallic looking solid. It has a density

Arsenic, sealed in a container to prevent tarnishing

of 5.727 g/cm3 and is brittle, and moderately hard (morethan aluminium; less thaniron).[305] It is stable in dry airbut develops a golden bronze patina in moist air, whichblackens on further exposure. Arsenic is attacked by ni-tric acid and concentrated sulfuric acid. It reacts withfused caustic soda to give the arsenate Na3AsO3and hy-drogen gas.[306] Arsenicsublimesat 615 C. The vapouris lemon-yellow and smells like garlic.[307] Arsenic only

melts under a pressure of 38.6atm, at 817 C.[308] It isa semimetal with an electrical conductivity of around 3.9 104 Scm1[309] and a band overlap of 0.5 eV.[310][n 29]

Liquid arsenic is a semiconductor with a band gap of 0.15eV.[312]

The chemistry of arsenic is predominatelynonmetallic.[313] Its many metal alloys are mostlybrittle.[314] Arsenic shows fewer tendencies to anionic

behaviour than ordinary nonmetals.[278]

Its solu-tion chemistry is characterised by the formation ofoxyanions.[279] Arsenic generally forms compounds inwhich it has an oxidation state of +3 or +5.[315] Thehalides, and the oxides and their derivatives are illustra-tive examples.[281] In the trivalent state, arsenic showssome incipient metallic properties.[316] The halides arehydrolysed by water but these reactions, particularlythose of the chloride, are reversible with the additionof a hydrohalic acid.[317] The oxide is acidic but, asnoted below, (weakly) amphoteric. The higher, lessstable, pentavalent state has strongly acidic (nonmetallic)properties.[318] Compared to phosphorus, the stronger

metallic character of arsenic is indicated by the forma-tion of oxoacid salts such as AsPO4, As2(SO4)3[n 30]

and arsenic acetate As(CH3COO)3.[322] The oxideAs2O3is polymeric,[259] amphoteric,[323][n 31] and a glassformer.[261] Arsenic has an extensive organometallicchemistry (seeorganoarsenic chemistry).[326]

6.5 Antimony

Main article:AntimonyAntimony is a silver-white solid with a blue tint and a

Antimony, showing its brilliant lustre

brilliant lustre.[306] It has a density of 6.697 g/cm3 and isbrittle, and moderately hard (more so than arsenic; lessso than iron; about the same as copper).[305] It is stablein air and moisture at room temperature. It is attacked

by concentrated nitric acid, yielding the hydrated pen-toxide Sb2O5.Aqua regiagives the pentachloride SbCl5and hot concentrated sulfuric acid results in thesulfate
https://en.wikipedia.org/wiki/Sulfatehttps://en.wikipedia.org/wiki/Aqua_regiahttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Antimonyhttps://en.wikipedia.org/wiki/Organoarsenic_chemistryhttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Hydrohalic_acidhttps://en.wikipedia.org/wiki/Hydrolysedhttp://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-https://en.wikipedia.org/wiki/Atmosphere_(unit)https://en.wikipedia.org/wiki/Sublimation_(phase_transition)http://-/?-http://-/?-https://en.wikipedia.org/wiki/Ironhttps://en.wikipedia.org/wiki/Arsenichttps://en.wikipedia.org/wiki/Organogermanium_chemistryhttps://en.wikipedia.org/wiki/Organogermanium_chemistryhttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Oxoacidhttp://-/?-http://-/?-http://-/?-https://en.wikipedia.org/wiki/Gold


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10 6 ELEMENTS COMMONLY RECOGNISED AS METALLOIDS

Sb2(SO4)3.[327] It is not affected by molten alkali.[328]

Antimony is capable of displacing hydrogen from water,when heated: 2 Sb+ 3 H2OSb2O3+ 3 H2.[329] It meltsat 631 C. Antimony is a semimetal with an electricalconductivity of around 3.1 104 Scm1[330] and a bandoverlap of 0.16 eV.[310][n 32] Liquid antimony is a metallic

conductor with an electrical conductivity of around 5.3 104 Scm1.[332]

Most of the chemistry of antimony is characteristic of anonmetal.[333] It can form alloys with one or more metalssuch as aluminium,[334] iron, nickel, copper, zinc, tin,lead and bismuth.[335] Antimony has fewer tendenciesto anionic behaviour than ordinary nonmetals.[278] Itssolution chemistry is characterised by the formationof oxyanions.[279] Like arsenic, antimony generallyforms compounds in which it has an oxidation stateof +3 or +5.[315] The halides, and the oxides and theirderivatives are illustrative examples.[281] The +5 state is

less stable than the +3, but relatively easier to attain thanwith arsenic. This is explained by the poor shieldingafforded the arsenic nucleus by its 3d10 electrons. Incomparison, the tendency of antimony (being a heavieratom) tooxidizemore easily partially offsets the effectof its 4d10 shell.[336] Tripositive antimony is amphoteric;pentapositive antimony is (predominately) acidic.[337]

Consistent with an increase in metallic character downgroup 15, antimony forms salts or salt-like compoundsincluding a nitrate Sb(NO3)3, phosphate SbPO4, sul-fate Sb2(SO4)3 and perchlorate Sb(ClO4)3.[338] Theotherwise acidic pentoxide Sb2O5 shows some basic

(metallic) behaviour in that it can be dissolved in veryacidic solutions, with the formation of the oxycationSbO+2.[339] The oxide Sb2O3 is polymeric,[259]

amphoteric,[340] and a glass former.[261] Antimony has anextensive organometallic chemistry (seeorganoantimonychemistry).[341]

6.6 Tellurium

Main article:Tellurium

Tellurium is a silvery-white shiny solid.[343]

It has adensity of 6.24 g/cm3, is brittle, and is the softest ofthe commonly recognised metalloids, being marginallyharder than sulfur.[305] Large pieces of tellurium are sta-ble in air. The finely powdered form is oxidized by airin the presence of moisture. Tellurium reacts with boil-ing water, or when freshly precipitated even at 50 C, togive the dioxide and hydrogen: Te + 2 H2O TeO2+2 H2.[344] It reacts (to varying degrees) with nitric, sul-furic and hydrochloric acids to give compounds such asthesulfoxideTeSO3 ortellurous acidH2TeO3,[345] thebasic nitrate (Te2O4H)+(NO3),[346] or the oxide sulfateTe2O3(SO4).[347] It dissolves in boiling alkalis, to give

thetelluriteandtelluride: 3 Te + 6 KOH = K2TeO3+ 2K2Te + 3 H2O, a reaction that proceeds or is reversiblewith increasing or decreasing temperature.[348]

Tellurium, described by Dmitri Mendeleev as forming a transition

between metals and nonmetals[342]

At higher temperatures tellurium is sufficiently plastic toextrude.[349] It melts at 449.51 C. Crystalline telluriumhas a structure consisting of parallel infinite spiral chains.The bonding between adjacent atoms in a chain is cova-lent, but there is evidence of a weak metallic interactionbetween the neighbouring atoms of different chains.[350]

Tellurium is a semiconductor with an electrical conduc-

tivity of around 1.0 Scm

1[351]

and a band gap of 0.32 to0.38 eV.[352] Liquid tellurium is a semiconductor, withan electrical conductivity, on melting, of around 1.9 103 Scm1[352] Superheatedliquid tellurium is a metal-lic conductor.[353]

Most of the chemistry of tellurium is characteristicof a nonmetal.[354] It can form alloys with aluminium,silverand tin.[355] Tellurium shows fewer tendencies toanionic behaviour than ordinary nonmetals.[278] Its so-lution chemistry is characterised by the formation ofoxyanions.[279] Tellurium generally forms compounds inwhich it has an oxidation state of 2, +4 or +6. The

+4 state is the most stable.[344]

Tellurides of compo-sition XxTey are easily formed with most other ele-ments and represent the most common tellurium miner-als.Nonstoichiometry is pervasive, especially with transi-tion metals. Many tellurides can be regarded as metallicalloys.[356] The increase in metallic character evident intellurium, as compared to the lighterchalcogens, is fur-ther reflected in the reported formation of various otheroxyacid salts, such as a basic selenate 2TeO2SeO3and ananalogous perchlorate andperiodate2TeO2HXO4.[357]

Tellurium forms a polymeric,[259] amphoteric,[340] glass-forming oxide[261] TeO2. The latter is a conditionalglass-forming oxideit forms a glass with a very small

amount of additive.[261] Tellurium has an extensiveorganometallic chemistry (see organotellurium chem-istry).[358]
https://en.wikipedia.org/wiki/Organotellurium_chemistryhttps://en.wikipedia.org/wiki/Organotellurium_chemistryhttp://-/?-http://-/?-http://-/?-http://-/?-https://en.wikipedia.org/wiki/Periodatehttps://en.wikipedia.org/wiki/Basic_salthttps://en.wikipedia.org/wiki/Chalcogenhttps://en.wikipedia.org/wiki/Nonstoichiometryhttp://-/?-http://-/?-http://-/?-https://en.wikipedia.org/wiki/Silverhttps://en.wikipedia.org/wiki/Superheatedhttp://-/?-http://-/?-http://-/?-https://en.wikipedia.org/wiki/Dmitri_Mendeleevhttps://en.wikipedia.org/wiki/Telluride_(chemistry)https://en.wikipedia.org/wiki/Telluritehttps://en.wikipedia.org/wiki/Tellurous_acidhttps://en.wikipedia.org/wiki/Sulfoxidehttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Telluriumhttps://en.wikipedia.org/wiki/Organoantimony_chemistryhttps://en.wikipedia.org/wiki/Organoantimony_chemistryhttp://-/?-http://-/?-http://-/?-https://en.wikipedia.org/wiki/Oxycationhttps://en.wikipedia.org/wiki/Perchloratehttps://en.wikipedia.org/wiki/Phosphatehttps://en.wikipedia.org/wiki/Nitratehttps://en.wikipedia.org/wiki/Pnictogenhttps://en.wikipedia.org/wiki/Penta-https://en.wikipedia.org/wiki/Redoxhttps://en.wikipedia.org/wiki/D_electron_counthttp://-/?-http://-/?-http://-/?-http://-/?-https://en.wikipedia.org/wiki/Nickelhttp://-/?-http://-/?-


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7.2 Aluminium 11

7 Elements less commonly recog-

nised as metalloids

7.1 Carbon

Main article:CarbonCarbon is ordinarily classified as a nonmetal[360] but

Carbon (as graphite). Delocalized valence electronswithin the

layers of graphite give it a metallic appearance. [359]

has some metallic properties and is occasionally clas-

sified as a metalloid.[361] Hexagonal graphitic carbon(graphite) is the most thermodynamically stableallotropeof carbon under ambient conditions.[362] It has a lus-trous appearance[363] and is a fairly good electricalconductor.[364] Graphite has a layered structure. Eachlayer comprises carbon atoms bonded to three other car-bon atoms in ahoneycomb latticearrangement. The lay-ers are stacked together and held loosely by van der Waalsforces anddelocalized valence electrons.[365]

Like a metal, the conductivity of graphite in the di-rection of its planes decreases as the temperature israised;[366][n 33] it has the electronic band structure of

a semimetal.[366] The allotropes of carbon, includ-ing graphite, can accept foreign atoms or compoundsinto their structures via substitution, intercalation ordoping. The resulting materials are referred to as car-bon alloys.[370] Carbon can form ionic salts, including ahydrogen sulfate, perchlorate, and nitrate (C+24X.2HX, where X = HSO4, ClO4; and C+24NO3.3HNO3).[371][n 34] In organic chemistry, carbon canform complex cationstermed carbocationsin whichthe positive charge is on the carbon atom; examples areCH+

3andCH+5, and their derivatives.[372]

Carbon is brittle,[373] and behaves as a semiconductor in

a direction perpendicular to its planes.[366] Most of itschemistry is nonmetallic;[374] it has a relatively high ion-ization energy[375] and, compared to most metals, a rela-tively high electronegativity.[376] Carbon can form anionssuch as C4 (methanide), C22 (acetylide) and C3

4 (sesquicarbide or allylenide), in compounds with met-als of main groups 13, and with the lanthanidesand actinides.[377] Its oxide CO2 forms carbonic acidH2CO3.[378][n 35]

7.2 Aluminium

Main article:AluminiumAluminium is ordinarily classified as a metal.[381] Itislus-

High purity aluminium is very much softer than its familiar al-

loys. People who handle it for the first time often ask if it is the

real thing.[380]

trous, malleable and ductile, and has high electrical andthermal conductivity. Like most metals it has aclose-packedcrystalline structure,[382] and forms a cation in

aqueous solution.[383]

It has some properties that are unusual for a metal; takentogether,[384] these are sometimes used as a basis to clas-sify aluminium as a metalloid.[385] Its crystalline struc-ture shows some evidence of directional bonding.[386]

Aluminium bonds covalently in most compounds.[387]

The oxide Al2O3 is amphoteric,[388] and a condi-tional glass-former.[261] Aluminium can form anionicaluminates,[384] such behaviour being considered non-metallic in character.[69]

Classifying aluminium as a metalloid has beendisputed[389] given its many metallic properties. It

is therefore, arguably, an exception to the mnemonic thatelements adjacent to the metalnonmetal dividing lineare metalloids.[390][n 36]
http://-/?-http://-/?-https://en.wikipedia.org/wiki/Aluminatehttp://-/?-https://en.wikipedia.org/wiki/Aluminium_oxidehttps://en.wikipedia.org/wiki/Bonding_in_solids#Propertieshttp://-/?-https://en.wikipedia.org/wiki/Close-packedhttps://en.wikipedia.org/wiki/Close-packedhttps://en.wikipedia.org/wiki/Aluminiumhttps://en.wikipedia.org/wiki/Carbonic_acidhttps://en.wikipedia.org/wiki/Carbon_dioxidehttps://en.wikipedia.org/wiki/Actinidehttps://en.wikipedia.org/wiki/Lanthanidehttps://en.wikipedia.org/wiki/Sesquicarbidehttps://en.wikipedia.org/wiki/Acetylidehttps://en.wikipedia.org/wiki/Methanidehttp://-/?-https://en.wikipedia.org/wiki/Carbonium_ionhttps://en.wikipedia.org/wiki/Carbonium_ionhttps://en.wikipedia.org/wiki/Carbenium_ionhttps://en.wikipedia.org/wiki/Carbenium_ionhttps://en.wikipedia.org/wiki/Carbocationhttps://en.wikipedia.org/wiki/Organic_chemistryhttps://en.wikipedia.org/wiki/Dopanthttps://en.wikipedia.org/wiki/Intercalation_(chemistry)http://-/?-http://-/?-https://en.wikipedia.org/wiki/Delocalized_electronhttps://en.wikipedia.org/wiki/Van_der_Waalhttps://en.wikipedia.org/wiki/Honeycomb_latticehttps://en.wikipedia.org/wiki/Allotropehttps://en.wikipedia.org/wiki/Graphitehttps://en.wikipedia.org/wiki/Delocalized_electronhttps://en.wikipedia.org/wiki/Carbon


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12 7 ELEMENTS LESS COMMONLY RECOGNISED AS METALLOIDS

Stott[392] labels aluminium as a weak metal. It has thephysical properties of a metal but some of the chemicalproperties of a nonmetal. Steele[393] notes the paradox-ical chemical behaviour of aluminium: It resembles aweak metal in its amphoteric oxide and in the covalentcharacter of many of its compounds ... Yet it is a highly

electropositivemetal ... [with] ahigh negativeelectrodepotential.

7.3 Selenium

Main article:SeleniumSelenium shows borderline metalloid or nonmetal

Grey selenium, being a photoconductor, conducts electricity

around 1,000 times better when light falls on it, a property used

since the mid-1870s in various light-sensing applications[394]

behaviour.[395][n 37]

Its most stable form, the greytrigonalallotrope, is some-times called metallic selenium because its electricalconductivity is several orders of magnitude greater thanthat of the redmonoclinicform.[398] The metallic char-acter of selenium is further shown by its lustre, [399]

and its crystalline structure, which is thought to includeweakly metallic interchain bonding.[400] Selenium canbe drawn into thin threads when molten and viscous. [401]

It shows reluctance to acquire the high positive oxidationnumbers characteristic of nonmetals.[402] It can formcyclic polycations (such as Se2+

8) when dissolved inoleums[403] (an attribute it shareswith sulfur and tellurium), and a hydrolysed cationicsalt in the form of trihydroxoselenium (IV) perchlorate

[Se(OH)3]+ClO4.[404]

The nonmetallic character of selenium is shown by itsbrittleness[399] and the low electrical conductivity (~109

to 1012 Scm1) of its highly purified form.[93] This is

comparable to or less than that of bromine(7.951012

Scm1),[405] a nonmetal. Selenium has the electronicband structure of a semiconductor[406] and retains itssemiconducting properties in liquid form.[406] It has a rel-atively high[407] electronegativity (2.55 revised Paulingscale). Its reaction chemistry is mainly that of its non-metallic anionic forms Se2, SeO23 and SeO24.[408]

Selenium is commonly described as a metalloid inthe environmental chemistry literature.[409] It movesthrough the aquatic environment similarly to arsenic and

antimony;[410]

its water-soluble salts, in higher concen-trations, have a similar toxicological profileto that ofarsenic.[411]

7.4 Polonium

Main article:Polonium

Polonium is distinctly metallic in some ways.[232] Bothof its allotropic forms are metallic conductors.[232] It issoluble in acids, forming the rose-coloured Po2+ cation

and displacing hydrogen: Po + 2 H+

Po2+

+ H2.[412]

Many polonium salts are known.[413] The oxidePoO2ispredominantly basic in nature.[414] Polonium is a reluctantoxidizing agent, unlike its lightercongener oxygen: highlyreducing conditionsare required for the formation of thePo2 anion in aqueous solution.[415]

Whether polonium is ductile or brittle is unclear. It ispredicted to be ductile based on its calculated elasticconstants.[416] It has a simplecubic crystalline structure.Such a structure has fewslip systemsand leads to verylow ductility and hence low fracture resistance.[417]

Polonium shows nonmetallic character in its halides, and

by the existence ofpolonides. The halides have prop-erties generally characteristic of nonmetal halides (beingvolatile, easily hydrolyzed, and soluble in organic sol-vents).[418] Many metal polonides, obtained by heatingthe elements together at 5001,000 C, and containingthe Po2 anion, are also known.[419]

7.5 Astatine

Main article:Astatine

As a halogen, astatine tends to be classified as anonmetal.[420] It has some marked metallic properties[421]

and is sometimes instead classified as either a
https://en.wikipedia.org/wiki/Halogenhttps://en.wikipedia.org/wiki/Astatinehttps://en.wikipedia.org/wiki/Organic_solventhttps://en.wikipedia.org/wiki/Organic_solventhttps://en.wikipedia.org/wiki/Polonidehttps://en.wikipedia.org/wiki/Slip_(materials_science)#slip_systemshttps://en.wikipedia.org/wiki/Cubic_crystal_systemhttps://en.wikipedia.org/wiki/Young%27s_modulus#Relation_among_elastic_constantshttps://en.wikipedia.org/wiki/Young%27s_modulus#Relation_among_elastic_constantshttps://en.wikipedia.org/wiki/Reducing_agenthttp://-/?-https://en.wikipedia.org/wiki/Polonium_dioxidehttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Poloniumhttps://en.wikipedia.org/wiki/Toxicologyhttps://en.wikipedia.org/wiki/Environmental_chemistryhttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Brominehttp://-/?-http://-/?-https://en.wikipedia.org/wiki/Oleumhttp://-/?-https://en.wikipedia.org/wiki/Monoclinic_crystal_systemhttps://en.wikipedia.org/wiki/Trigonal_crystal_systemhttps://en.wikipedia.org/wiki/Photoconductivityhttps://en.wikipedia.org/wiki/Seleniumhttps://en.wikipedia.org/wiki/Table_of_standard_electrode_potentialshttps://en.wikipedia.org/wiki/Electronegativity#Electropositivity


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8.1 Near metalloids 13

metalloid[422] or (less often) as a metal.[n 38] Immediatelyfollowing its production in 1940, early investigatorsconsidered it a metal.[424] In 1949 it was called the mostnoble (difficult toreduce) nonmetal as well as being arelatively noble (difficult to oxidize) metal.[425] In 1950astatine was described as a halogen and (therefore) a

reactive nonmetal.[426] In 2013, on the basis of relativisticmodelling, astatine was predicted to be a monatomicmetal, with aface-centred cubic crystalline structure.[427]

Several authors have commented on the metallic natureof some of the properties of astatine. Since iodine is asemiconductor in the direction of its planes, and since thehalogens become more metallic with increasing atomicnumber, it has been presumed that astatine would be ametal if it could form a condensed phase.[428][n 39] Asta-tine may be metallic in the liquid state on the basis thatelements with an enthalpy of vaporization (H a) greaterthan ~42 kJ/mol are metallic when liquid.[430] Such ele-

ments include boron,[n 40] silicon, germanium, antimony,selenium and tellurium. Estimated values for H ofdiatomicastatine are 50 kJ/mol or higher;[434] diatomiciodine, with a H of 41.71,[435] falls just short of thethreshold figure.

Like typical metals, it [astatine] is precipitated byhydrogen sulfideeven from strongly acid solutions andis displaced in a free form from sulfate solutions; it isdeposited on thecathodeon electrolysis.[436][n 41] Fur-ther indications of a tendency for astatine to behavelike a(heavy) metalare: extquotedbl... the formationof pseudohalidecompounds ... complexes of astatinecations ... complex anions of trivalent astatine ... as wellas complexes with a variety of organic solvents. [438] Ithas also been argued that astatine demonstrates cationicbehaviour, by way of stable At+ and AtO+ forms, instrongly acidic aqueous solutions.[439]

Some of astatines reported properties are nonmetallic.It has the narrow liquid range ordinarily associated withnonmetals (mp 302 C; bp 337 C).[440] Batsanov givesa calculated band gap energy for astatine of 0.7 eV;[441]

this is consistent with nonmetals (in physics) having sep-aratedvalenceandconduction bandsand thereby beingeither semiconductors or insulators.[442] The chemistryof astatine in aqueous solution is mainly characterised bythe formation of various anionic species.[443] Most of itsknown compounds resemble those of iodine,[444] whichis a halogen and a nonmetal.[445] Such compounds in-clude astatides (XAt), astatates (XAtO3), and monovalentinterhalogen compounds.[446]

Restrepo et al.[447] reported that astatine appeared to bemore polonium-like than halogen-like. They did so onthe basis of detailed comparative studies of the knownand interpolated properties of 72 elements.

8 Related concepts

8.1 Near metalloids

Iodine crystals, showing a metallic lustre. Iodine is a semicon-

ductor in the direction of its planes, with a band gap of ~1.3

eV. It has an electrical conductivity of 1.7 108 Scm1 at

room temperature.[448] This is higher than selenium but lower

than boron, the least electrically conducting of the recognised

metalloids.[n 42]

In the periodic table, some of the elements adjacent to thecommonly recognised metalloids, although usually clas-sified as either metals or nonmetals, are occasionally re-ferred to as near-metalloids[451] or noted for their metal-loidal character. To the left of the metalnonmetal divid-ing line, such elements include gallium,[452] tin[453] andbismuth.[1] They show unusual packing structures,[454]

marked covalent chemistry (molecular or polymeric),[455]

and amphoterism.[456] To the right of the dividing

line are carbon,[457] phosphorus,[458] selenium[459] andiodine.[460] They exhibit metallic lustre, semiconductingproperties[n 43] and bonding or valence bands with delo-calized character. This applies to their most thermody-namically stable forms under ambient conditions: carbonas graphite; phosphorus as black phosphorus;[n 44] and se-lenium as grey selenium.

8.2 Allotropes

White tin (left) and grey tin (right). Both forms have a metallic

appearance.

Different crystalline forms of an element are calledallotropes. Some allotropes, particularly those of ele-
https://en.wikipedia.org/wiki/Allotropyhttp://-/?-https://en.wikipedia.org/wiki/Interhalogen_compoundhttps://en.wikipedia.org/wiki/Monovalent_ionhttps://en.wikipedia.org/wiki/Conduction_bandhttps://en.wikipedia.org/wiki/Valence_bandhttps://en.wikipedia.org/wiki/Pseudohalidehttps://en.wikipedia.org/wiki/Heavy_metal_(chemistry)https://en.wikipedia.org/wiki/Electrolysishttps://en.wikipedia.org/wiki/Cathodehttps://en.wikipedia.org/wiki/Hydrogen_sulfidehttps://en.wikipedia.org/wiki/Diatomic_moleculehttp://-/?-https://en.wikipedia.org/wiki/Enthalpy_of_vaporizationhttps://en.wikipedia.org/wiki/Face-centred_cubichttps://en.wikipedia.org/wiki/Relativistic_quantum_chemistryhttps://en.wikipedia.org/wiki/Reactivity_(chemistry)https://en.wikipedia.org/wiki/Redox


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14 10 NOTES

ments located (in periodic table terms) alongside or nearthe notional dividing line between metals and nonmetals,exhibit more pronounced metallic, metalloidal or non-metallic behaviour than others.[466] The existence of suchallotropes can complicate the classification of the ele-ments involved.[467]

Tin, for example, has two allotropes: tetragonalwhite-tin and cubic grey -tin. White tin is a very shiny,ductile and malleable metal. It is the stable form at orabove room temperature and has an electrical conduc-tivity of 9.17 104 Scm1 (~1/6th that of copper).[468]

Grey tin usually has the appearance of a grey micro-crystalline powder, and can also be prepared in brittlesemi-lustrous crystalline or polycrystalline forms. It is thestable form below 13.2 C and has an electrical conduc-tivity of between (25) 102 Scm1 (~1/250th that ofwhite tin).[469] Grey tin has the same crystalline structureas that of diamond. It behaves as a semiconductor (with

a band gap of 0.08 eV), but has the electronic band struc-ture of a semimetal.[470] It has been referred to as eithera very poor metal,[471] a metalloid,[472] a nonmetal[473] ora near metalloid.[1]

The diamond allotrope of carbon is clearly nonmetallic,being translucent and having a low electrical conductivityof 1014 to 1016 Scm1.[474] Graphite has an electricalconductivity of 3 104 Scm1,[475] a figure more charac-teristic of a metal. Phosphorus, sulfur, arsenic, selenium,antimony and bismuth also have less stable allotropes thatdisplay different behaviours.[476]

9 Abundance, extraction and cost

9.1 Abundance

The table givescrustal abundancesof the elements com-monly to rarely recognised as metalloids.[477] Some otherelements are included for comparison: oxygen and xenon(the most and least abundant elements with stable iso-topes); iron and the coinage metals copper, silver andgold; and rhenium, the least abundant stable metal (alu-

minium is normally the most abundant metal). Variousabundance estimates have been published; these oftendisagree to some extent.[478]

9.2 Extraction

The recognised metalloids can be obtained bychemicalreductionof either their oxides or theirsulfides. Simpleror more complex extraction methods may be employeddepending on the starting form and economic factors.[479]

Boron is routinely obtained by reducing the trioxide withmagnesium: B2O3 + 3 Mg 2 B + 3MgO; after sec-

ondary processing the resulting brown powder has a pu-rity of up to 97%.[480] Boron of higher purity (> 99%)is prepared by heating volatile boron compounds, such as

BCl3or BBr3, either in a hydrogen atmosphere (2 BX3+ 3 H2 2 B + 6 HX) or to the point ofthermal de-composition. Silicon and germanium are obtained fromtheir oxides byheating the oxide with carbonor hydrogen:SiO2+ C Si + CO2; GeO2+ 2 H2 Ge + 2 H2O. Ar-senic is isolated fromits pyrite (FeAsS) or arsenicalpyrite

(FeAs2) by heating; alternatively, it can be obtained fromits oxide by reduction with carbon: 2 As2O3+ 3 C 2As + 3 CO2.[481] Antimony is derived from its sulfide byreduction with iron: Sb2S3 2 Sb + 3 FeS. Tellurium isprepared from its oxide bydissolving it in aqueous NaOH,yielding tellurite, then byelectrolytic reduction: TeO2+2 NaOH Na2TeO3+ H2O;[482] Na2TeO3 + H2O Te + 2 NaOH + O2.[483] Another option is reduction ofthe oxide by roasting with carbon: TeO2 + C Te +CO2.[484]

Production methods for the elements less frequentlyrecognised as metalloids involve natural processing, elec-

trolytic or chemical reduction, or irradiation. Carbon (asgraphite) occurs naturally and is extracted by crushing theparent rock andfloating the lighter graphite to the surface.Aluminium is extracted by dissolving its oxide Al2O3inmoltencryoliteNa3AlF6 and then by high temperatureelectrolytic reduction. Selenium is produced by roastingits coinage metal selenides X2Se (X = Cu, Ag, Au) withsoda ashto give the selenite: X2Se + O2+ Na2CO3Na2SeO3 + 2 X + CO2; the selenide is neutralized bysulfuric acid H2SO4to giveselenous acidH2SeO3; thisis reduced by bubbling withSO2 to yield elemental se-lenium. Polonium and astatine are produced in minute

quantities by irradiating bismuth.[485]

9.3 Cost

The recognised metalloids and their closer neighboursmostly cost less than silver; only polonium and astatineare more expensive than gold. As of 5 April 2014, pricesfor small samples (up to 100 g) of silicon, antimony andtellurium, and graphite, aluminium and selenium, aver-age around one third the cost of silver (US$1.5 per gramor about $45 an ounce). Boron, germanium and arsenicsamples average about three-and-a-half times the costof silver.[n 45] Polonium is available for about $100 permicrogram, which is $100,000,000 a gram.[486] Zalutskyand Pruszynski[487] estimate a similar cost for produc-ing astatine. Prices for the applicable elements tradedas commodities tend to range from two to three timescheaper than the sample price (Ge), to nearly three thou-sand times cheaper (As).[n 46]

10 Notes

[1] See also Vernon

[1]

for a related commentary.[2] Definitions and extracts by different authors, illustrating

aspects of the generic definition, follow:
http://-/?-https://en.wikipedia.org/wiki/Microgramhttps://en.wikipedia.org/wiki/Sulfur_dioxidehttps://en.wikipedia.org/wiki/Selenous_acidhttps://en.wikipedia.org/wiki/Soda_ashhttps://en.wikipedia.org/wiki/Cryolitehttps://en.wikipedia.org/wiki/Electrolytic_reductionhttps://en.wikipedia.org/wiki/Thermal_decompositionhttps://en.wikipedia.org/wiki/Thermal_decompositionhttps://en.wikipedia.org/wiki/Sulfidehttps://en.wikipedia.org/wiki/Redoxhttps://en.wikipedia.org/wiki/Redoxhttps://en.wikipedia.org/wiki/Crustal_abundancehttp://-/?-https://en.wikipedia.org/wiki/Polycrystallinehttps://en.wikipedia.org/wiki/Tetragonal_crystal_system


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15

In chemistry a metalloid is an element with prop-erties intermediate between those of metals andnonmetals.[4]

Between the metals and nonmetals in the periodictable we find elements ... [that] share some of thecharacteristic properties of both the metals and non-

metals, making it difficult to place them in either ofthese two main categories[5]

Chemists sometimes use the name metalloid ... forthese elements which are difficult to classify oneway or the other.[6]

Because the traits distinguishing metals and non-metals are qualitative in nature, some elements donot fall unambiguously in either category. These el-ements ... are called metalloids ...[7]

More broadly, metalloids have been referred to as:

elements that ... are somewhat of a cross between

metals and nonmetals extquotedbl;[8]

or weird in-between elements.[9]

[3] Gold, for example, has mixed properties but is still recog-nised as king of metals. Besides metallic behaviour(such as high electrical conductivity, and cationforma-tion), gold shows nonmetallic behaviour:

It has thehighest electrode potential

It has the third-highestionization energyamong themetals (afterzincandmercury)

It has the lowestelectron affinity

Its electronegativity of 2.54 is highest among

the metals and exceeds that of some nonmetals(hydrogen2.2;phosphorus2.19; andradon2.2)

ItformstheAu auride anion, acting in this way likeahalogen

It sometimes has a tendency, known asextquotedblaurophilicity extquotedbl, to bondto itself;[12]

on halogen character, see also Belpassi et al,[13] who con-clude that in the aurides MAu (M =LiCs) gold behavesas a halogen, intermediate betweenBrandIextquotedbl;on aurophilicity, see also Schmidbaur and Schier. [14]

[4] Mann et al.[17]

refer to these elements as the recognizedmetalloids.

[5] Jones[45] writes: Though classification is an essential fea-ture in all branches of science, there are always hard casesat the boundaries. Indeed, the boundary of a class is rarelysharp.

[6] The lack of a standard division of the elements into met-als, metalloids and nonmetals is not necessarily an issue.There is more or less, a continuous progression from themetallic to the nonmetallic. Potentially, a specified subsetof this continuum can serve its particular purpose as wellas any other.[46]

[7] The packing efficiency of boron is 38%; silicon and ger-manium 34; arsenic 38.5; antimony 41; and tellurium36.4.[50] These values are lower than in most metals (80%

of which have a packing efficiency of at least 68%), [51]

but higher than those of elements usually classified asnonmetals. (Gallium is unusual, for a metal, in havinga packing efficiency of just 39%.[52] Other notable val-ues for metals are 42.9 for bismuth[53] and 58.5 for liq-uid mercury.[54]) Packing efficiencies for nonmetals are:

graphite 17%,[55]

sulfur 19.2,[56]

iodine 23.9,[56]

selenium24.2,[56] and black phosphorus 28.5.[53]

[8] More specifically, the Goldhammer-Herzfeldcriterionisthe ratio of the force holding an individual atomsvalenceelectronsin place with the forces on the same electronsfrom interactionsbetweenthe atoms in the solid or liquidelement. When the interatomic forces are greater than, orequal to, the atomic force, valence electron itinerancy isindicated and metallic behaviour is predicted.[58] Other-wise nonmetallic behaviour is anticipated.

[9] As the ratio is based on classical arguments [60] it doesnot accommodate the finding that polonium, which has

a value of ~0.95, adopts a metallic (rather thancovalent)crystalline structure, onrelativisticgrounds.[61] Even so itoffers afirst orderrationalization for the occurrence ofmetallic character amongst the elements.[62]

[10] Atomic conductance is the electrical conductivity of onemole of a substance. It is equal to electrical conductivitydivided by molar volume.[6]

[11] Selenium has an ionization energy (IE) of 225 kcal/mol(941 kJ/mol) and is sometimes described as a semicon-ductor. It has a relatively high 2.55 electronegativity (EN).Polonium has an IE of 194 kcal/mol (812 kJ/mol) and a2.0 EN, but has a metallic band structure.[67] Astatine has

an IE of 215 kJ/mol (899 kJ/mol) and an EN of 2.2.[68]

Itselectronic band structure is not known with any certainty.

[12] Oderberg[80] argues onontologicalgrounds that anythingnot a metal is therefore a nonmetal, and that this includessemi-metals (i.e. metalloids).

[13] Coperniciumis reportedly the only metal known to be agas at room temperature.[86]

[14] Metals have electrical conductivity values of from 6.9 103 Scm1 formanganeseto 6.3 105 forsilver.[90]

[15] Metalloids have electrical conductivity values of from 1.5 106 Scm1 for boron to 3.9 104 for arsenic.[92] Ifselenium is included as a metalloid the applicable conduc-tivity range would start from ~109 to 1012 Scm1.[93]

[16] Nonmetals have electrical conductivity values of from~1018 Scm1 for the elemental gases to 3 104 ingraphite.[94]

[17] Chedd[101] defines metalloids as having electronegativ-ity values of 1.8 to 2.2(Allred-Rochow scale). He in-cluded boron, silicon, germanium, arsenic, antimony, tel-lurium, polonium andastatine in this category. In re-viewing Chedds work, Adler[102] described this choice asarbitrary, as other elements whose electronegativities liein this range includecopper, silver, phosphorus, mercuryand bismuth. He went on to

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