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Металлоид

Металлоид — это химический элемент , который имеет преобладание промежуточных свойств или представляет собой смесь свойств металлов и неметаллов . Не существует стандартного определения металлоида и полного согласия относительно того, какие элементы являются металлоидами. Несмотря на отсутствие конкретики, этот термин продолжает использоваться в химической литературе . Металлоиды обычно имеют металлический вид, но часто хрупкие и не такие податливые, как металлы.

Шесть общепризнанных металлоидов — это бор , кремний , германий , мышьяк , сурьма и теллур . Реже так классифицируют пять элементов: углерод , алюминий , селен , полоний и астат . В стандартной таблице Менделеева все одиннадцать элементов находятся в диагональной области p-блока , простирающейся от бора вверху слева до астата внизу справа. Некоторые таблицы Менделеева включают разделительную линию между металлами и неметаллами , и металлоиды могут быть найдены рядом с этой линией.

Типичные металлоиды имеют металлический вид, могут быть хрупкими и являются хорошими проводниками электричества . Они могут образовывать сплавы с металлами, а многие другие их физические и химические свойства занимают промежуточное положение между металлическими и неметаллическими элементами. Они и их соединения используются в сплавах, биологических агентах, катализаторах , антипиренах , стеклах , оптических накопителях и оптоэлектронике , пиротехнике , полупроводниках и электронике.

Электрические свойства легированного кремния позволили создать полупроводниковую промышленность в 1950-х годах и разработать твердотельную электронику с начала 1960-х годов. [1]

Термин металлоид первоначально относился к неметаллам. Его более позднее значение как категории элементов с промежуточными или гибридными свойствами получило широкое распространение в 1940–1960 гг. Металлоиды иногда называют полуметаллами, но эта практика не поощряется, [2] поскольку термин «полуметалл» имеет другое значение в физике , чем в химии. В физике это относится к определенному типу электронной зонной структуры вещества. В этом контексте только мышьяк и сурьма являются полуметаллами и обычно считаются металлоидами.

Определения

Основанный на суждении

Металлоид — это элемент, который обладает преобладанием промежуточных свойств или представляет собой смесь свойств металлов и неметаллов, и поэтому его трудно классифицировать как металл или неметалл. Это общее определение, основанное на свойствах металлоидов, постоянно цитируемых в литературе. [n 2] Сложность категоризации является ключевым атрибутом. Большинство элементов обладают смесью металлических и неметаллических свойств [9] и могут быть классифицированы в зависимости от того, какой набор свойств более выражен. [10] [n 3] Только элементы на краях или вблизи них, не имеющие достаточно явного преобладания металлических или неметаллических свойств, классифицируются как металлоиды. [14]

Бор, кремний, германий, мышьяк, сурьма и теллур обычно считаются металлоидами. [15] [n 4] В зависимости от автора в список иногда добавляются один или несколько селена , полония или астата . [17] Бор иногда исключается сам по себе или вместе с кремнием. [18] Иногда теллур не считают металлоидом. [19] Включение сурьмы , полония и астата в качестве металлоидов было подвергнуто сомнению. [20]

Другие элементы иногда классифицируются как металлоиды. К этим элементам относятся [21] водород, [22] бериллий , [23] азот , [24] фосфор , [25] сера , [26] цинк , [27] галлий , [28] олово , йод , [29] свинец , [30] висмут , [19] и радон. [31] Термин «металлоид» также использовался для элементов, которые обладают металлическим блеском и электропроводностью и являются амфотерными , например, мышьяк, сурьма, ванадий , хром , молибден , вольфрам , олово, свинец и алюминий. [32] Металлы p-блока , [33] и неметаллы (такие как углерод или азот), которые могут образовывать сплавы с металлами [34] или изменять их свойства [35], также иногда рассматривались как металлоиды.

На основе критериев

Не существует общепринятого определения металлоида или какого-либо разделения таблицы Менделеева на металлы, металлоиды и неметаллы; [38] Хоукс [39] поставил под сомнение возможность установления конкретного определения, отметив, что аномалии могут быть обнаружены в нескольких попытках построения. Классификация элемента как металлоида была описана Шарпом [40] как «произвольная».

Количество и идентичность металлоидов зависят от того, какие критерии классификации используются. Эмсли [41] выделил четыре металлоида (германий, мышьяк, сурьму и теллур); Джеймс и др. [42] перечислили двенадцать (эмсли плюс бор, углерод, кремний, селен, висмут, полоний, московий и ливерморий ). В среднем в такие списки входит семь элементов ; отдельные механизмы классификации, как правило, имеют общие основания и различаются в нечетких [43] пределах. [n 5] [n 6]

Обычно используется единый количественный критерий, такой как электроотрицательность : [46] металлоиды имеют значения электроотрицательности от 1,8 или 1,9 до 2,2. [47] Дополнительные примеры включают эффективность упаковки (долю объема кристаллической структуры, занимаемую атомами) и критерий Гольдхаммера-Герцфельда. [48] ​​Общеизвестные металлоиды имеют эффективность упаковки от 34% до 41%. [n 7] Отношение Голдхаммера-Герцфельда, примерно равное кубу атомного радиуса, разделенному на молярный объем , [56] [n 8] является простой мерой металличности элемента, признанные металлоиды имеют соотношения примерно от 0,85 до 1,1 и в среднем 1,0. [58] [n 9] Другие авторы опирались, например, на атомную проводимость [n 10] [62] или объемное координационное число . [63]

Джонс, писая о роли классификации в науке, заметил, что «[классы] обычно определяются более чем двумя атрибутами». [64] Мастертон и Словински [65] использовали три критерия для описания шести элементов, обычно называемых металлоидами: металлоиды имеют энергию ионизации около 200 ккал/моль (837 кДж/моль) и значения электроотрицательности, близкие к 2,0. Они также сказали, что металлоиды, как правило, являются полупроводниками, хотя сурьма и мышьяк (полуметаллы с точки зрения физики) имеют электропроводность, приближающуюся к металлической. Предполагается, что селен и полоний не включены в эту схему, а статус астата неясен. [№ 11]

В этом контексте Вернон предположил, что металлоид — это химический элемент, который в стандартном состоянии имеет (а) зонную электронную структуру полупроводника или полуметалла; и (b) промежуточный первый потенциал ионизации «(скажем, 750-1000 кДж/моль)»; и (в) промежуточная электроотрицательность (1,9–2,2). [68]

Территория периодической таблицы

Расположение

Металлоиды лежат по обе стороны от разделительной линии между металлами и неметаллами . В различных конфигурациях его можно найти в некоторых таблицах Менделеева . Элементы в левом нижнем углу линии обычно демонстрируют усиление металлического поведения; элементы в правом верхнем углу отображают усиление неметаллического поведения. [69] Если представить ее в виде обычной ступеньки, элементы с самой высокой критической температурой для своих групп (Li, Be, Al, Ge, Sb, Po) лежат чуть ниже линии. [70]

Диагональное расположение металлоидов представляет собой исключение из наблюдения, согласно которому элементы со схожими свойствами имеют тенденцию встречаться в вертикальных группах . [71] Подобный эффект можно увидеть и в других диагональных сходствах между некоторыми элементами и их соседями в правом нижнем углу, в частности, литием-магнием, бериллием-алюминием и бор-кремнием. Рейнер-Кэнэм [72] утверждал, что это сходство распространяется на углерод-фосфор, азот-серу и на три серии d-блоков .

Это исключение возникает из-за конкурирующих горизонтальных и вертикальных тенденций в ядерном заряде . В течение периода заряд ядра увеличивается с ростом атомного номера , как и число электронов. Дополнительное притяжение внешних электронов по мере увеличения заряда ядра обычно перевешивает экранирующий эффект наличия большего количества электронов. Поэтому при некоторых нарушениях атомы становятся меньше, энергия ионизации увеличивается, и с течением времени происходит постепенное изменение характера элементов от сильно металлических к слабометаллическим, к слабонеметаллическим и к сильно неметаллическим. [73] В основной группе эффект увеличения заряда ядра обычно перевешивается эффектом нахождения дополнительных электронов дальше от ядра. Атомы обычно становятся крупнее, энергия ионизации падает, а металлический характер увеличивается. [74] Конечным эффектом является то, что расположение переходной зоны металл-неметалл смещается вправо при движении вниз по группе, [71] и, как уже отмечалось, аналогичные диагональные сходства наблюдаются в других частях периодической таблицы. [75]

Альтернативные методы лечения

Элементы, граничащие с разделительной линией металл-неметалл, не всегда классифицируются как металлоиды, отмечая, что бинарная классификация может облегчить установление правил для определения типов связи между металлами и неметаллами. [76] В таких случаях авторы при принятии классификационных решений сосредотачиваются на одном или нескольких интересующих признаках, а не беспокоятся о маргинальном характере рассматриваемых элементов. Их соображения могут быть или не быть явными и временами могут казаться произвольными. [40] [n 12] Металлоиды могут быть сгруппированы с металлами; [77] или считаются неметаллами; [78] или рассматриваться как подкатегория неметаллов. [79] [n 13] Другие авторы предложили классифицировать некоторые элементы как металлоиды, «подчеркивая, что свойства изменяются постепенно, а не резко, когда человек перемещается по периодической таблице или вниз по ней». [81] В некоторых таблицах Менделеева различаются элементы, которые являются металлоидами, и не имеют формальной разделительной линии между металлами и неметаллами. Вместо этого металлоиды показаны в виде диагональной полосы [82] или диффузной области. [83] Ключевым моментом является объяснение контекста используемой таксономии.

Характеристики

Металлоиды обычно выглядят как металлы, но ведут себя во многом как неметаллы. Физически это блестящие хрупкие твердые тела с электропроводностью от средней до относительно хорошей и электронной зонной структурой полуметалла или полупроводника. Химически они в основном ведут себя как (слабые) неметаллы, имеют промежуточные энергии ионизации и значения электроотрицательности, а также амфотерные или слабокислотные оксиды . Большинство других их физических и химических свойств имеют промежуточный характер .

По сравнению с металлами и неметаллами

Характерные свойства металлов, металлоидов и неметаллов сведены в таблицу. [84] Физические свойства перечислены в порядке облегчения определения; химические свойства идут от общего к частному, а затем к описательным.

Приведенная выше таблица отражает гибридную природу металлоидов. Свойства формы, внешнего вида и поведения при смешивании с металлами больше напоминают металлы. Эластичность и общее химическое поведение больше похожи на неметаллы. Электропроводность, зонная структура, энергия ионизации, электроотрицательность и оксиды занимают промежуточное положение между ними.

Общие приложения

Основное внимание в этом разделе уделяется признанным металлоидам. Элементы, которые реже называют металлоидами, обычно классифицируют либо как металлы, либо как неметаллы; некоторые из них включены сюда для сравнения.

Металлоиды слишком хрупкие, чтобы иметь какое-либо структурное применение в чистом виде. [105] Они и их соединения используются в сплавах, биологических агентах (токсикологических, пищевых и медицинских), катализаторах, антипиренах, стеклах (оксидных и металлических), оптических носителях информации и оптоэлектронике, пиротехнике, полупроводниках и электронике. [№ 19]

Сплавы

Several dozen metallic pellets, reddish-brown. They have a highly polished appearance, as if they had a cellophane coating.
Таблетки медно-германиевого сплава, вероятно, ~84% Cu; 16% Ге. [107] В сочетании с серебром получается стерлинговое серебро, устойчивое к потускнению . Также показаны две серебряные гранулы.

В начале истории интерметаллических соединений британский металлург Сесил Деш заметил, что «некоторые неметаллические элементы способны образовывать соединения отчетливо металлического характера с металлами, и поэтому эти элементы могут входить в состав сплавов». К сплавообразующим элементам он относил, в частности, кремний, мышьяк и теллур. [108] Филлипс и Уильямс [109] предположили, что соединения кремния, германия, мышьяка и сурьмы с металлами группы B «вероятно, лучше всего классифицировать как сплавы».

Среди более легких металлоидов широко представлены сплавы с переходными металлами . Бор может образовывать с такими металлами состава MnB интерметаллиды и сплавы , если n > 2. [110] Ферробор (15% бора) применяют для введения бора в сталь ; Никель-борные сплавы входят в состав сварочных сплавов и цементирующих композиций для машиностроения. Сплавы кремния с железом и алюминием широко используются в сталелитейной и автомобильной промышленности соответственно. Германий образует множество сплавов, особенно с металлами, используемыми для чеканки монет . [111]

The heavier metalloids continue the theme. Arsenic can form alloys with metals, including platinum and copper;[112] it is also added to copper and its alloys to improve corrosion resistance[113] and appears to confer the same benefit when added to magnesium.[114] Antimony is well known as an alloy-former, including with the coinage metals. Its alloys include pewter (a tin alloy with up to 20% antimony) and type metal (a lead alloy with up to 25% antimony).[115] Tellurium readily alloys with iron, as ferrotellurium (50–58% tellurium), and with copper, in the form of copper tellurium (40–50% tellurium).[116] Ferrotellurium is used as a stabilizer for carbon in steel casting.[117] Of the non-metallic elements less often recognised as metalloids, selenium – in the form of ferroselenium (50–58% selenium) – is used to improve the machinability of stainless steels.[118]

Biological agents

A clear glass dish on which is a small mound of a white crystalline powder.
Arsenic trioxide or white arsenic, one of the most toxic and prevalent forms of arsenic. The antileukaemic properties of white arsenic were first reported in 1878.[119]

All six of the elements commonly recognised as metalloids have toxic, dietary or medicinal properties.[120] Arsenic and antimony compounds are especially toxic; boron, silicon, and possibly arsenic, are essential trace elements. Boron, silicon, arsenic, and antimony have medical applications, and germanium and tellurium are thought to have potential.

Boron is used in insecticides[121] and herbicides.[122] It is an essential trace element.[123] As boric acid, it has antiseptic, antifungal, and antiviral properties.[124]

Silicon is present in silatrane, a highly toxic rodenticide.[125] Long-term inhalation of silica dust causes silicosis, a fatal disease of the lungs. Silicon is an essential trace element.[123] Silicone gel can be applied to badly burned patients to reduce scarring.[126]

Salts of germanium are potentially harmful to humans and animals if ingested on a prolonged basis.[127] There is interest in the pharmacological actions of germanium compounds but no licensed medicine as yet.[128]

Arsenic is notoriously poisonous and may also be an essential element in ultratrace amounts.[129] During World War I, both sides used "arsenic-based sneezing and vomiting agents…to force enemy soldiers to remove their gas masks before firing mustard or phosgene at them in a second salvo."[130] It has been used as a pharmaceutical agent since antiquity, including for the treatment of syphilis before the development of antibiotics.[131] Arsenic is also a component of melarsoprol, a medicinal drug used in the treatment of human African trypanosomiasis or sleeping sickness. In 2003, arsenic trioxide (under the trade name Trisenox) was re-introduced for the treatment of acute promyelocytic leukaemia, a cancer of the blood and bone marrow.[131] Arsenic in drinking water, which causes lung and bladder cancer, has been associated with a reduction in breast cancer mortality rates.[132]

Metallic antimony is relatively non-toxic, but most antimony compounds are poisonous.[133]Two antimony compounds, sodium stibogluconate and stibophen, are used as antiparasitical drugs.[134]

Elemental tellurium is not considered particularly toxic; two grams of sodium tellurate, if administered, can be lethal.[135] People exposed to small amounts of airborne tellurium exude a foul and persistent garlic-like odour.[136] Tellurium dioxide has been used to treat seborrhoeic dermatitis; other tellurium compounds were used as antimicrobial agents before the development of antibiotics.[137] In the future, such compounds may need to be substituted for antibiotics that have become ineffective due to bacterial resistance.[138]

Of the elements less often recognised as metalloids, beryllium and lead are noted for their toxicity; lead arsenate has been extensively used as an insecticide.[139] Sulfur is one of the oldest of the fungicides and pesticides. Phosphorus, sulfur, zinc, selenium, and iodine are essential nutrients, and aluminium, tin, and lead may be.[129] Sulfur, gallium, selenium, iodine, and bismuth have medicinal applications. Sulfur is a constituent of sulfonamide drugs, still widely used for conditions such as acne and urinary tract infections.[140] Gallium nitrate is used to treat the side effects of cancer;[141] gallium citrate, a radiopharmaceutical, facilitates imaging of inflamed body areas.[142] Selenium sulfide is used in medicinal shampoos and to treat skin infections such as tinea versicolor.[143] Iodine is used as a disinfectant in various forms. Bismuth is an ingredient in some antibacterials.[144]

Catalysts

Boron trifluoride and trichloride are used as homogeneous catalysts in organic synthesis and electronics; the tribromide is used in the manufacture of diborane.[145] Non-toxic boron ligands could replace toxic phosphorus ligands in some transition metal catalysts.[146] Silica sulfuric acid (SiO2OSO3H) is used in organic reactions.[147] Germanium dioxide is sometimes used as a catalyst in the production of PET plastic for containers;[148] cheaper antimony compounds, such as the trioxide or triacetate, are more commonly employed for the same purpose[149] despite concerns about antimony contamination of food and drinks.[150] Arsenic trioxide has been used in the production of natural gas, to boost the removal of carbon dioxide, as have selenous acid and tellurous acid.[151] Selenium acts as a catalyst in some microorganisms.[152] Tellurium, its dioxide, and its tetrachloride are strong catalysts for air oxidation of carbon above 500 °C.[153] Graphite oxide can be used as a catalyst in the synthesis of imines and their derivatives.[154] Activated carbon and alumina have been used as catalysts for the removal of sulfur contaminants from natural gas.[155] Titanium doped aluminium has been suggested as a substitute for noble metal catalysts used in the production of industrial chemicals.[156]

Flame retardants

Compounds of boron, silicon, arsenic, and antimony have been used as flame retardants. Boron, in the form of borax, has been used as a textile flame retardant since at least the 18th century.[157] Silicon compounds such as silicones, silanes, silsesquioxane, silica, and silicates, some of which were developed as alternatives to more toxic halogenated products, can considerably improve the flame retardancy of plastic materials.[158]Arsenic compounds such as sodium arsenite or sodium arsenate are effective flame retardants for wood but have been less frequently used due to their toxicity.[159] Antimony trioxide is a flame retardant.[160] Aluminium hydroxide has been used as a wood-fibre, rubber, plastic, and textile flame retardant since the 1890s.[161] Apart from aluminium hydroxide, use of phosphorus based flame-retardants – in the form of, for example, organophosphates – now exceeds that of any of the other main retardant types. These employ boron, antimony, or halogenated hydrocarbon compounds.[162]

Glass formation

A bunch of pale yellow semi-transparent thin strands, with bright points of white light at their tips.
Optical fibers, usually made of pure silicon dioxide glass, with additives such as boron trioxide or germanium dioxide for increased sensitivity

The oxides B2O3, SiO2, GeO2, As2O3, and Sb2O3 readily form glasses. TeO2 forms a glass but this requires a "heroic quench rate"[163] or the addition of an impurity; otherwise the crystalline form results.[163] These compounds are used in chemical, domestic, and industrial glassware[164] and optics.[165] Boron trioxide is used as a glass fibre additive,[166] and is also a component of borosilicate glass, widely used for laboratory glassware and domestic ovenware for its low thermal expansion.[167] Most ordinary glassware is made from silicon dioxide.[168] Germanium dioxide is used as a glass fibre additive, as well as in infrared optical systems.[169] Arsenic trioxide is used in the glass industry as a decolourizing and fining agent (for the removal of bubbles),[170] as is antimony trioxide.[171] Tellurium dioxide finds application in laser and nonlinear optics.[172]

Amorphous metallic glasses are generally most easily prepared if one of the components is a metalloid or "near metalloid" such as boron, carbon, silicon, phosphorus or germanium.[173][n 20] Aside from thin films deposited at very low temperatures, the first known metallic glass was an alloy of composition Au75Si25 reported in 1960.[175] A metallic glass having a strength and toughness not previously seen, of composition Pd82.5P6Si9.5Ge2, was reported in 2011.[176]

Phosphorus, selenium, and lead, which are less often recognised as metalloids, are also used in glasses. Phosphate glass has a substrate of phosphorus pentoxide (P2O5), rather than the silica (SiO2) of conventional silicate glasses. It is used, for example, to make sodium lamps.[177] Selenium compounds can be used both as decolourising agents and to add a red colour to glass.[178] Decorative glassware made of traditional lead glass contains at least 30% lead(II) oxide (PbO); lead glass used for radiation shielding may have up to 65% PbO.[179] Lead-based glasses have also been extensively used in electronic components, enamelling, sealing and glazing materials, and solar cells. Bismuth based oxide glasses have emerged as a less toxic replacement for lead in many of these applications.[180]

Optical storage and optoelectronics

Varying compositions of GeSbTe ("GST alloys") and Ag- and In- doped Sb2Te ("AIST alloys"), being examples of phase-change materials, are widely used in rewritable optical discs and phase-change memory devices. By applying heat, they can be switched between amorphous (glassy) and crystalline states. The change in optical and electrical properties can be used for information storage purposes.[181] Future applications for GeSbTe may include, "ultrafast, entirely solid-state displays with nanometre-scale pixels, semi-transparent 'smart' glasses, 'smart' contact lenses, and artificial retina devices."[182]

Pyrotechnics

A man is standing in the dark. He is holding out a short stick at mid-chest level. The end of the stick is alight, burning very brightly, and emitting smoke.
Archaic blue light signal, fuelled by a mixture of sodium nitrate, sulfur, and (red) arsenic trisulfide[183]

The recognised metalloids have either pyrotechnic applications or associated properties. Boron and silicon are commonly encountered;[184] they act somewhat like metal fuels.[185] Boron is used in pyrotechnic initiator compositions (for igniting other hard-to-start compositions), and in delay compositions that burn at a constant rate.[186] Boron carbide has been identified as a possible replacement for more toxic barium or hexachloroethane mixtures in smoke munitions, signal flares, and fireworks.[187] Silicon, like boron, is a component of initiator and delay mixtures.[186] Doped germanium can act as a variable speed thermite fuel.[n 21] Arsenic trisulfide As2S3 was used in old naval signal lights; in fireworks to make white stars;[189] in yellow smoke screen mixtures; and in initiator compositions.[190] Antimony trisulfide Sb2S3 is found in white-light fireworks and in flash and sound mixtures.[191] Tellurium has been used in delay mixtures and in blasting cap initiator compositions.[192]

Carbon, aluminium, phosphorus, and selenium continue the theme. Carbon, in black powder, is a constituent of fireworks rocket propellants, bursting charges, and effects mixtures, and military delay fuses and igniters.[193][n 22] Aluminium is a common pyrotechnic ingredient,[184] and is widely employed for its capacity to generate light and heat,[195] including in thermite mixtures.[196] Phosphorus can be found in smoke and incendiary munitions, paper caps used in toy guns, and party poppers.[197] Selenium has been used in the same way as tellurium.[192]

Semiconductors and electronics

A small square plastic piece with three parallel wire protrusions on one side; a larger rectangular plastic chip with multiple plastic and metal pin-like legs; and a small red light globe with two long wires coming out of its base.
Semiconductor-based electronic components. From left to right: a transistor, an integrated circuit, and an LED. The elements commonly recognised as metalloids find widespread use in such devices, as elemental or compound semiconductor constituents (Si, Ge or GaAs, for example) or as doping agents (B, Sb, Te, for example).

All the elements commonly recognised as metalloids (or their compounds) have been used in the semiconductor or solid-state electronic industries.[198]

Some properties of boron have limited its use as a semiconductor. It has a high melting point, single crystals are relatively hard to obtain, and introducing and retaining controlled impurities is difficult.[199]

Silicon is the leading commercial semiconductor; it forms the basis of modern electronics (including standard solar cells)[200] and information and communication technologies.[201] This was despite the study of semiconductors, early in the 20th century, having been regarded as the "physics of dirt" and not deserving of close attention.[202]

Germanium has largely been replaced by silicon in semiconducting devices, being cheaper, more resilient at higher operating temperatures, and easier to work during the microelectronic fabrication process.[107] Germanium is still a constituent of semiconducting silicon-germanium "alloys" and these have been growing in use, particularly for wireless communication devices; such alloys exploit the higher carrier mobility of germanium.[107] The synthesis of gram-scale quantities of semiconducting germanane was reported in 2013. This consists of one-atom thick sheets of hydrogen-terminated germanium atoms, analogous to graphane. It conducts electrons more than ten times faster than silicon and five times faster than germanium, and is thought to have potential for optoelectronic and sensing applications.[203] The development of a germanium-wire based anode that more than doubles the capacity of lithium-ion batteries was reported in 2014.[204] In the same year, Lee et al. reported that defect-free crystals of graphene large enough to have electronic uses could be grown on, and removed from, a germanium substrate.[205]

Arsenic and antimony are not semiconductors in their standard states. Both form type III-V semiconductors (such as GaAs, AlSb or GaInAsSb) in which the average number of valence electrons per atom is the same as that of Group 14 elements, but they have direct band gaps. These compounds are preferred for optical applications.[206] Antimony nanocrystals may enable lithium-ion batteries to be replaced by more powerful sodium ion batteries.[207]

Tellurium, which is a semiconductor in its standard state, is used mainly as a component in type II/VI semiconducting-chalcogenides; these have applications in electro-optics and electronics.[208] Cadmium telluride (CdTe) is used in solar modules for its high conversion efficiency, low manufacturing costs, and large band gap of 1.44 eV, letting it absorb a wide range of wavelengths.[200] Bismuth telluride (Bi2Te3), alloyed with selenium and antimony, is a component of thermoelectric devices used for refrigeration or portable power generation.[209]

Five metalloids – boron, silicon, germanium, arsenic, and antimony – can be found in cell phones (along with at least 39 other metals and nonmetals).[210] Tellurium is expected to find such use.[211] Of the less often recognised metalloids, phosphorus, gallium (in particular) and selenium have semiconductor applications. Phosphorus is used in trace amounts as a dopant for n-type semiconductors.[212] The commercial use of gallium compounds is dominated by semiconductor applications – in integrated circuits, cell phones, laser diodes, light-emitting diodes, photodetectors, and solar cells.[213] Selenium is used in the production of solar cells[214] and in high-energy surge protectors.[215]

Boron, silicon, germanium, antimony, and tellurium,[216] as well as heavier metals and metalloids such as Sm, Hg, Tl, Pb, Bi, and Se,[217] can be found in topological insulators. These are alloys[218] or compounds which, at ultracold temperatures or room temperature (depending on their composition), are metallic conductors on their surfaces but insulators through their interiors.[219] Cadmium arsenide Cd3As2, at about 1 K, is a Dirac-semimetal – a bulk electronic analogue of graphene – in which electrons travel effectively as massless particles.[220] These two classes of material are thought to have potential quantum computing applications.[221]

Nomenclature and history

Derivation and other names

The word metalloid comes from the Latin metallum ("metal") and the Greek oeides ("resembling in form or appearance").[222] Several names are sometimes used synonymously although some of these have other meanings that are not necessarily interchangeable: amphoteric element,[223] boundary element,[224] half-metal,[225] half-way element,[226] near metal,[227] meta-metal,[228] semiconductor,[229] semimetal[230] and submetal.[231] "Amphoteric element" is sometimes used more broadly to include transition metals capable of forming oxyanions, such as chromium and manganese.[232] "Half-metal" is used in physics to refer to a compound (such as chromium dioxide) or alloy that can act as a conductor and an insulator. "Meta-metal" is sometimes used instead to refer to certain metals (Be, Zn, Cd, Hg, In, Tl, β-Sn, Pb) located just to the left of the metalloids on standard periodic tables.[225] These metals are mostly diamagnetic[233] and tend to have distorted crystalline structures, electrical conductivity values at the lower end of those of metals, and amphoteric (weakly basic) oxides.[234] "Semimetal" sometimes refers, loosely or explicitly, to metals with incomplete metallic character in crystalline structure, electrical conductivity or electronic structure. Examples include gallium,[235] ytterbium,[236] bismuth[237] and neptunium.[238] The names amphoteric element and semiconductor are problematic as some elements referred to as metalloids do not show marked amphoteric behaviour (bismuth, for example)[239] or semiconductivity (polonium)[240] in their most stable forms.

Origin and usage

The origin and usage of the term metalloid is convoluted. Its origin lies in attempts, dating from antiquity, to describe metals and to distinguish between typical and less typical forms. It was first applied in the early 19th century to metals that floated on water (sodium and potassium), and then more popularly to nonmetals. Earlier usage in mineralogy, to describe a mineral having a metallic appearance, can be sourced to as early as 1800.[241] Since the mid-20th century it has been used to refer to intermediate or borderline chemical elements.[242][n 23] The International Union of Pure and Applied Chemistry (IUPAC) previously recommended abandoning the term metalloid, and suggested using the term semimetal instead.[244] Use of this latter term has more recently been discouraged by Atkins et al.[2] as it has a different meaning in physics – one that more specifically refers to the electronic band structure of a substance rather than the overall classification of an element. The most recent IUPAC publications on nomenclature and terminology do not include any recommendations on the usage of the terms metalloid or semimetal.[245]

Elements commonly recognised as metalloids

Properties noted in this section refer to the elements in their most thermodynamically stable forms under ambient conditions.

Boron

Several dozen small angular stone like shapes, grey with scattered silver flecks and highlights.
Boron, shown here in the form of its β-rhombohedral phase (its most thermodynamically stable allotrope)[246]

Pure boron is a shiny, silver-grey crystalline solid.[247] It is less dense than aluminium (2.34 vs. 2.70 g/cm3), and is hard and brittle. It is barely reactive under normal conditions, except for attack by fluorine,[248] and has a melting point of 2076 °C (cf. steel ~1370 °C).[249] Boron is a semiconductor;[250] its room temperature electrical conductivity is 1.5 × 10−6 S•cm−1[251] (about 200 times less than that of tap water)[252] and it has a band gap of about 1.56 eV.[253][n 24] Mendeleev commented that, "Boron appears in a free state in several forms which are intermediate between the metals and the nonmmetals."[255]

The structural chemistry of boron is dominated by its small atomic size, and relatively high ionization energy. With only three valence electrons per boron atom, simple covalent bonding cannot fulfil the octet rule.[256] Metallic bonding is the usual result among the heavier congenors of boron but this generally requires low ionization energies.[257] Instead, because of its small size and high ionization energies, the basic structural unit of boron (and nearly all of its allotropes)[n 25] is the icosahedral B12 cluster. Of the 36 electrons associated with 12 boron atoms, 26 reside in 13 delocalized molecular orbitals; the other 10 electrons are used to form two- and three-centre covalent bonds between icosahedra.[259] The same motif can be seen, as are deltahedral variants or fragments, in metal borides and hydride derivatives, and in some halides.[260]

The bonding in boron has been described as being characteristic of behaviour intermediate between metals and nonmetallic covalent network solids (such as diamond).[261] The energy required to transform B, C, N, Si, and P from nonmetallic to metallic states has been estimated as 30, 100, 240, 33, and 50 kJ/mol, respectively. This indicates the proximity of boron to the metal-nonmetal borderline.[262]

Most of the chemistry of boron is nonmetallic in nature.[262] Unlike its heavier congeners, it is not known to form a simple B3+ or hydrated [B(H2O)4]3+ cation.[263] The small size of the boron atom enables the preparation of many interstitial alloy-type borides.[264] Analogies between boron and transition metals have been noted in the formation of complexes,[265] and adducts (for example, BH3 + CO →BH3CO and, similarly, Fe(CO)4 + CO →Fe(CO)5),[n 26] as well as in the geometric and electronic structures of cluster species such as [B6H6]2− and [Ru6(CO)18]2−.[267][n 27] The aqueous chemistry of boron is characterised by the formation of many different polyborate anions.[269] Given its high charge-to-size ratio, boron bonds covalently in nearly all of its compounds;[270] the exceptions are the borides as these include, depending on their composition, covalent, ionic, and metallic bonding components.[271][n 28] Simple binary compounds, such as boron trichloride are Lewis acids as the formation of three covalent bonds leaves a hole in the octet which can be filled by an electron-pair donated by a Lewis base.[256] Boron has a strong affinity for oxygen and a duly extensive borate chemistry.[264] The oxide B2O3 is polymeric in structure,[274] weakly acidic,[275][n 29] and a glass former.[281] Organometallic compounds of boron[n 30] have been known since the 19th century (see organoboron chemistry).[283]

Silicon

A lustrous blue grey potato-shaped lump with an irregular corrugated surface.
Silicon has a blue-grey metallic lustre.

Silicon is a crystalline solid with a blue-grey metallic lustre.[284] Like boron, it is less dense (at 2.33 g/cm3) than aluminium, and is hard and brittle.[285] It is a relatively unreactive element.[284] According to Rochow,[286] the massive crystalline form (especially if pure) is "remarkably inert to all acids, including hydrofluoric".[n 31] Less pure silicon, and the powdered form, are variously susceptible to attack by strong or heated acids, as well as by steam and fluorine.[290] Silicon dissolves in hot aqueous alkalis with the evolution of hydrogen, as do metals[291] such as beryllium, aluminium, zinc, gallium or indium.[292] It melts at 1414 °C. Silicon is a semiconductor with an electrical conductivity of 10−4 S•cm−1[293] and a band gap of about 1.11 eV.[287] When it melts, silicon becomes a reasonable metal[294] with an electrical conductivity of 1.0–1.3 × 104 S•cm−1, similar to that of liquid mercury.[295]

The chemistry of silicon is generally nonmetallic (covalent) in nature.[296] It is not known to form a cation.[297][n 32] Silicon can form alloys with metals such as iron and copper.[298] It shows fewer tendencies to anionic behaviour than ordinary nonmetals.[299] Its solution chemistry is characterised by the formation of oxyanions.[300] The high strength of the silicon–oxygen bond dominates the chemical behaviour of silicon.[301] Polymeric silicates, built up by tetrahedral SiO4 units sharing their oxygen atoms, are the most abundant and important compounds of silicon.[302] The polymeric borates, comprising linked trigonal and tetrahedral BO3 or BO4 units, are built on similar structural principles.[303] The oxide SiO2 is polymeric in structure,[274] weakly acidic,[304][n 33] and a glass former.[281] Traditional organometallic chemistry includes the carbon compounds of silicon (see organosilicon).[308]

Germanium

Greyish lustrous block with uneven cleaved surface.
Germanium is sometimes described as a metal

Germanium is a shiny grey-white solid.[309] It has a density of 5.323 g/cm3 and is hard and brittle.[310] It is mostly unreactive at room temperature[n 34] but is slowly attacked by hot concentrated sulfuric or nitric acid.[312] Germanium also reacts with molten caustic soda to yield sodium germanate Na2GeO3 and hydrogen gas.[313] It melts at 938 °C. Germanium is a semiconductor with an electrical conductivity of around 2 × 10−2 S•cm−1[312] and a band gap of 0.67 eV.[314] Liquid germanium is a metallic conductor, with an electrical conductivity similar to that of liquid mercury.[315]

Most of the chemistry of germanium is characteristic of a nonmetal.[316] Whether or not germanium forms a cation is unclear, aside from the reported existence of the Ge2+ ion in a few esoteric compounds.[n 35] It can form alloys with metals such as aluminium and gold.[329] It shows fewer tendencies to anionic behaviour than ordinary nonmetals.[299] Its solution chemistry is characterised by the formation of oxyanions.[300] Germanium generally forms tetravalent (IV) compounds, and it can also form less stable divalent (II) compounds, in which it behaves more like a metal.[330] Germanium analogues of all of the major types of silicates have been prepared.[331] The metallic character of germanium is also suggested by the formation of various oxoacid salts. A phosphate [(HPO4)2Ge·H2O] and highly stable trifluoroacetate Ge(OCOCF3)4 have been described, as have Ge2(SO4)2, Ge(ClO4)4 and GeH2(C2O4)3.[332] The oxide GeO2 is polymeric,[274] amphoteric,[333] and a glass former.[281] The dioxide is soluble in acidic solutions (the monoxide GeO, is even more so), and this is sometimes used to classify germanium as a metal.[334] Up to the 1930s germanium was considered to be a poorly conducting metal;[335] it has occasionally been classified as a metal by later writers.[336] As with all the elements commonly recognised as metalloids, germanium has an established organometallic chemistry (see Organogermanium chemistry).[337]

Arsenic

Two dull silver clusters of crystalline shards.
Arsenic, sealed in a container to prevent tarnishing

Arsenic is a grey, metallic looking solid. It has a density of 5.727 g/cm3 and is brittle, and moderately hard (more than aluminium; less than iron).[338] It is stable in dry air but develops a golden bronze patina in moist air, which blackens on further exposure. Arsenic is attacked by nitric acid and concentrated sulfuric acid. It reacts with fused caustic soda to give the arsenate Na3AsO3 and hydrogen gas.[339] Arsenic sublimes at 615 °C. The vapour is lemon-yellow and smells like garlic.[340] Arsenic only melts under a pressure of 38.6 atm, at 817 °C.[341] It is a semimetal with an electrical conductivity of around 3.9 × 104 S•cm−1[342] and a band overlap of 0.5 eV.[343][n 36] Liquid arsenic is a semiconductor with a band gap of 0.15 eV.[345]

The chemistry of arsenic is predominately nonmetallic.[346] Whether or not arsenic forms a cation is unclear.[n 37] Its many metal alloys are mostly brittle.[354] It shows fewer tendencies to anionic behaviour than ordinary nonmetals.[299] Its solution chemistry is characterised by the formation of oxyanions.[300] Arsenic generally forms compounds in which it has an oxidation state of +3 or +5.[355] The halides, and the oxides and their derivatives are illustrative examples.[302] In the trivalent state, arsenic shows some incipient metallic properties.[356] The halides are hydrolysed by water but these reactions, particularly those of the chloride, are reversible with the addition of a hydrohalic acid.[357] The oxide is acidic but, as noted below, (weakly) amphoteric. The higher, less stable, pentavalent state has strongly acidic (nonmetallic) properties.[358] Compared to phosphorus, the stronger metallic character of arsenic is indicated by the formation of oxoacid salts such as AsPO4, As2(SO4)3[n 38] and arsenic acetate As(CH3COO)3.[361] The oxide As2O3 is polymeric,[274] amphoteric,[362][n 39] and a glass former.[281] Arsenic has an extensive organometallic chemistry (see Organoarsenic chemistry).[365]

Antimony

A glistening silver rock-like chunk, with a blue tint, and roughly parallel furrows.
Antimony, showing its brilliant lustre

Antimony is a silver-white solid with a blue tint and a brilliant lustre.[339] It has a density of 6.697 g/cm3 and is brittle, and moderately hard (more so than arsenic; less so than iron; about the same as copper).[338] It is stable in air and moisture at room temperature. It is attacked by concentrated nitric acid, yielding the hydrated pentoxide Sb2O5. Aqua regia gives the pentachloride SbCl5 and hot concentrated sulfuric acid results in the sulfate Sb2(SO4)3.[366] It is not affected by molten alkali.[367] Antimony is capable of displacing hydrogen from water, when heated: 2 Sb + 3 H2O → Sb2O3 + 3 H2.[368] It melts at 631 °C. Antimony is a semimetal with an electrical conductivity of around 3.1 × 104 S•cm−1[369] and a band overlap of 0.16 eV.[343][n 40] Liquid antimony is a metallic conductor with an electrical conductivity of around 5.3 × 104 S•cm−1.[371]

Most of the chemistry of antimony is characteristic of a nonmetal.[372] Antimony has some definite cationic chemistry,[373] SbO+ and Sb(OH)2+ being present in acidic aqueous solution;[374][n 41] the compound Sb8(GaCl4)2, which contains the homopolycation, Sb82+, was prepared in 2004.[376] It can form alloys with one or more metals such as aluminium,[377] iron, nickel, copper, zinc, tin, lead, and bismuth.[378] Antimony has fewer tendencies to anionic behaviour than ordinary nonmetals.[299] Its solution chemistry is characterised by the formation of oxyanions.[300] Like arsenic, antimony generally forms compounds in which it has an oxidation state of +3 or +5.[355] The halides, and the oxides and their derivatives are illustrative examples.[302] The +5 state is less stable than the +3, but relatively easier to attain than with arsenic. This is explained by the poor shielding afforded the arsenic nucleus by its 3d10 electrons. In comparison, the tendency of antimony (being a heavier atom) to oxidize more easily partially offsets the effect of its 4d10 shell.[379] Tripositive antimony is amphoteric; pentapositive antimony is (predominately) acidic.[380] Consistent with an increase in metallic character down group 15, antimony forms salts including an acetate Sb(CH3CO2)3, phosphate SbPO4, sulfate Sb2(SO4)3 and perchlorate Sb(ClO4)3.[381] The otherwise acidic pentoxide Sb2O5 shows some basic (metallic) behaviour in that it can be dissolved in very acidic solutions, with the formation of the oxycation SbO+
2.[382] The oxide Sb2O3 is polymeric,[274] amphoteric,[383] and a glass former.[281] Antimony has an extensive organometallic chemistry (see Organoantimony chemistry).[384]

Tellurium

A shiny silver-white medallion with a striated surface, irregular around the outside, with a square spiral-like pattern in the middle.
Tellurium, described by Dmitri Mendeleev as forming a transition between metals and nonmetals[385]

Tellurium is a silvery-white shiny solid.[386] It has a density of 6.24 g/cm3, is brittle, and is the softest of the commonly recognised metalloids, being marginally harder than sulfur.[338] Large pieces of tellurium are stable in air. The finely powdered form is oxidized by air in the presence of moisture. Tellurium reacts with boiling water, or when freshly precipitated even at 50 °C, to give the dioxide and hydrogen: Te + 2 H2O → TeO2 + 2 H2.[387] It reacts (to varying degrees) with nitric, sulfuric, and hydrochloric acids to give compounds such as the sulfoxide TeSO3 or tellurous acid H2TeO3,[388] the basic nitrate (Te2O4H)+(NO3)−,[389] or the oxide sulfate Te2O3(SO4).[390] It dissolves in boiling alkalis, to give the tellurite and telluride: 3 Te + 6 KOH = K2TeO3 + 2 K2Te + 3 H2O, a reaction that proceeds or is reversible with increasing or decreasing temperature.[391]

At higher temperatures tellurium is sufficiently plastic to extrude.[392] It melts at 449.51 °C. Crystalline tellurium has a structure consisting of parallel infinite spiral chains. The bonding between adjacent atoms in a chain is covalent, but there is evidence of a weak metallic interaction between the neighbouring atoms of different chains.[393] Tellurium is a semiconductor with an electrical conductivity of around 1.0 S•cm−1[394] and a band gap of 0.32 to 0.38 eV.[395] Liquid tellurium is a semiconductor, with an electrical conductivity, on melting, of around 1.9 × 103 S•cm−1.[395] Superheated liquid tellurium is a metallic conductor.[396]

Most of the chemistry of tellurium is characteristic of a nonmetal.[397]It shows some cationic behaviour. The dioxide dissolves in acid to yield the trihydroxotellurium(IV) Te(OH)3+ ion;[398][n 42] the red Te42+ and yellow-orange Te62+ ions form when tellurium is oxidized in fluorosulfuric acid (HSO3F), or liquid sulfur dioxide (SO2), respectively.[401] It can form alloys with aluminium, silver, and tin.[402] Tellurium shows fewer tendencies to anionic behaviour than ordinary nonmetals.[299] Its solution chemistry is characterised by the formation of oxyanions.[300] Tellurium generally forms compounds in which it has an oxidation state of −2, +4 or +6. The +4 state is the most stable.[387] Tellurides of composition XxTey are easily formed with most other elements and represent the most common tellurium minerals. Nonstoichiometry is pervasive, especially with transition metals. Many tellurides can be regarded as metallic alloys.[403] The increase in metallic character evident in tellurium, as compared to the lighter chalcogens, is further reflected in the reported formation of various other oxyacid salts, such as a basic selenate 2TeO2·SeO3 and an analogous perchlorate and periodate 2TeO2·HXO4.[404] Tellurium forms a polymeric,[274] amphoteric,[383] glass-forming oxide[281] TeO2. It is a "conditional" glass-forming oxide – it forms a glass with a very small amount of additive.[281] Tellurium has an extensive organometallic chemistry (see Organotellurium chemistry).[405]

Elements less commonly recognised as metalloids

Carbon

A shiny grey-black cuboid nugget with a rough surface.
Carbon (as graphite). Delocalized valence electrons within the layers of graphite give it a metallic appearance.[406]

Carbon is ordinarily classified as a nonmetal[407] but has some metallic properties and is occasionally classified as a metalloid.[408] Hexagonal graphitic carbon (graphite) is the most thermodynamically stable allotrope of carbon under ambient conditions.[409] It has a lustrous appearance[410] and is a fairly good electrical conductor.[411] Graphite has a layered structure. Each layer consists of carbon atoms bonded to three other carbon atoms in a hexagonal lattice arrangement. The layers are stacked together and held loosely by van der Waals forces and delocalized valence electrons.[412]

Like a metal, the conductivity of graphite in the direction of its planes decreases as the temperature is raised;[413][n 43] it has the electronic band structure of a semimetal.[413] The allotropes of carbon, including graphite, can accept foreign atoms or compounds into their structures via substitution, intercalation, or doping. The resulting materials are sometimes referred to as "carbon alloys".[417] Carbon can form ionic salts, including a hydrogen sulfate, perchlorate, and nitrate (C+
24X−.2HX, where X = HSO4, ClO4; and C+
24NO–
3.3HNO3).[418][n 44] In organic chemistry, carbon can form complex cations – termed carbocations – in which the positive charge is on the carbon atom; examples are CH+3 and CH+5, and their derivatives.[419]

Graphite is an established solid lubricant and behaves as a semiconductor in a direction perpendicular to its planes.[413] Most of its chemistry is nonmetallic;[420] it has a relatively high ionization energy[421] and, compared to most metals, a relatively high electronegativity.[422] Carbon can form anions such as C4− (methanide), C2–
2 (acetylide), and C3–
4 (sesquicarbide or allylenide), in compounds with metals of main groups 1–3, and with the lanthanides and actinides.[423] Its oxide CO2 forms carbonic acid H2CO3.[424][n 45]

Aluminium

A silvery white steam-iron shaped lump with semi-circular striations along the width of its top surface and rough furrows in the middle portion of its left edge.
High purity aluminium is much softer than its familiar alloys. People who handle it for the first time often ask if it is the real thing.[426]

Aluminium is ordinarily classified as a metal.[427] It is lustrous, malleable and ductile, and has high electrical and thermal conductivity. Like most metals it has a close-packed crystalline structure,[428] and forms a cation in aqueous solution.[429]

It has some properties that are unusual for a metal; taken together,[430] these are sometimes used as a basis to classify aluminium as a metalloid.[431] Its crystalline structure shows some evidence of directional bonding.[432] Aluminium bonds covalently in most compounds.[433] The oxide Al2O3 is amphoteric[434] and a conditional glass-former.[281] Aluminium can form anionic aluminates,[430] such behaviour being considered nonmetallic in character.[69]

Classifying aluminium as a metalloid has been disputed[435] given its many metallic properties. It is therefore, arguably, an exception to the mnemonic that elements adjacent to the metal–nonmetal dividing line are metalloids.[436][n 46]

Stott[438] labels aluminium as a weak metal. It has the physical properties of a metal but some of the chemical properties of a nonmetal. Steele[439] notes the paradoxical chemical behaviour of aluminium: "It resembles a weak metal in its amphoteric oxide and in the covalent character of many of its compounds ... Yet it is a highly electropositive metal ... [with] a high negative electrode potential". Moody[440] says that, "aluminium is on the 'diagonal borderland' between metals and non-metals in the chemical sense."

Selenium

A small glass jar filled with small dull grey concave buttons. The pieces of selenium look like tiny mushrooms without their stems.
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[441]

Selenium shows borderline metalloid or nonmetal behaviour.[442][n 47]

Its most stable form, the grey trigonal allotrope, is sometimes called "metallic" selenium because its electrical conductivity is several orders of magnitude greater than that of the red monoclinic form.[445] The metallic character of selenium is further shown by its lustre,[446] and its crystalline structure, which is thought to include weakly "metallic" interchain bonding.[447] Selenium can be drawn into thin threads when molten and viscous.[448] It shows reluctance to acquire "the high positive oxidation numbers characteristic of nonmetals".[449] It can form cyclic polycations (such as Se2+
8) when dissolved in oleums[450] (an attribute it shares with sulfur and tellurium), and a hydrolysed cationic salt in the form of trihydroxoselenium(IV) perchlorate [Se(OH)3]+·ClO–
4.[451]

The nonmetallic character of selenium is shown by its brittleness[446] and the low electrical conductivity (~10−9 to 10−12 S•cm−1) of its highly purified form.[93] This is comparable to or less than that of bromine (7.95×10–12 S•cm−1),[452] a nonmetal. Selenium has the electronic band structure of a semiconductor[453] and retains its semiconducting properties in liquid form.[453] It has a relatively high[454] electronegativity (2.55 revised Pauling scale). Its reaction chemistry is mainly that of its nonmetallic anionic forms Se2−, SeO2−
3 and SeO2−
4.[455]

Selenium is commonly described as a metalloid in the environmental chemistry literature.[456] It moves through the aquatic environment similarly to arsenic and antimony;[457] its water-soluble salts, in higher concentrations, have a similar toxicological profile to that of arsenic.[458]

Polonium

Polonium is "distinctly metallic" in some ways.[240] Both of its allotropic forms are metallic conductors.[240] It is soluble in acids, forming the rose-coloured Po2+ cation and displacing hydrogen: Po + 2 H+ → Po2+ + H2.[459] Many polonium salts are known.[460] The oxide PoO2 is predominantly basic in nature.[461] Polonium is a reluctant oxidizing agent, unlike its lightest congener oxygen: highly reducing conditions are required for the formation of the Po2− anion in aqueous solution.[462]

Whether polonium is ductile or brittle is unclear. It is predicted to be ductile based on its calculated elastic constants.[463] It has a simple cubic crystalline structure. Such a structure has few slip systems and "leads to very low ductility and hence low fracture resistance".[464]

Polonium shows nonmetallic character in its halides, and by the existence of polonides. The halides have properties generally characteristic of nonmetal halides (being volatile, easily hydrolyzed, and soluble in organic solvents).[465] Many metal polonides, obtained by heating the elements together at 500–1,000 °C, and containing the Po2− anion, are also known.[466]

Astatine

As a halogen, astatine tends to be classified as a nonmetal.[467] It has some marked metallic properties[468] and is sometimes instead classified as either a metalloid[469] or (less often) as a metal.[n 48] Immediately following its production in 1940, early investigators considered it a metal.[471] In 1949 it was called the most noble (difficult to reduce) nonmetal as well as being a relatively noble (difficult to oxidize) metal.[472] In 1950 astatine was described as a halogen and (therefore) a reactive nonmetal.[473] In 2013, on the basis of relativistic modelling, astatine was predicted to be a monatomic metal, with a face-centred cubic crystalline structure.[474]

Several authors have commented on the metallic nature of some of the properties of astatine. Since iodine is a semiconductor in the direction of its planes, and since the halogens become more metallic with increasing atomic number, it has been presumed that astatine would be a metal if it could form a condensed phase.[475][n 49] Astatine may be metallic in the liquid state on the basis that elements with an enthalpy of vaporization (∆Hvap) greater than ~42 kJ/mol are metallic when liquid.[477] Such elements include boron,[n 50] silicon, germanium, antimony, selenium, and tellurium. Estimated values for ∆Hvap of diatomic astatine are 50 kJ/mol or higher;[481] diatomic iodine, with a ∆Hvap of 41.71,[482] falls just short of the threshold figure.

"Like typical metals, it [astatine] is precipitated by hydrogen sulfide even from strongly acid solutions and is displaced in a free form from sulfate solutions; it is deposited on the cathode on electrolysis."[483][n 51] Further indications of a tendency for astatine to behave like a (heavy) metal are: "... the formation of pseudohalide compounds ... complexes of astatine cations ... complex anions of trivalent astatine ... as well as complexes with a variety of organic solvents".[485] It has also been argued that astatine demonstrates cationic behaviour, by way of stable At+ and AtO+ forms, in strongly acidic aqueous solutions.[486]

Some of astatine's reported properties are nonmetallic. It has been extrapolated to have the narrow liquid range ordinarily associated with nonmetals (mp 302 °C; bp 337 °C),[487] although experimental indications suggest a lower boiling point of about 230±3 °C. Batsanov gives a calculated band gap energy for astatine of 0.7 eV;[488] this is consistent with nonmetals (in physics) having separated valence and conduction bands and thereby being either semiconductors or insulators.[489] The chemistry of astatine in aqueous solution is mainly characterised by the formation of various anionic species.[490] Most of its known compounds resemble those of iodine,[491] which is a halogen and a nonmetal.[492] Such compounds include astatides (XAt), astatates (XAtO3), and monovalent interhalogen compounds.[493]

Restrepo et al.[494] reported that astatine appeared to be more polonium-like than halogen-like. They did so on the basis of detailed comparative studies of the known and interpolated properties of 72 elements.

Related concepts

Near metalloids

Shiny violet-black coloured crystalline shards.
Iodine crystals, showing a metallic lustre. Iodine is a semiconductor in the direction of its planes, with a band gap of ~1.3 eV. It has an electrical conductivity of 1.7 × 10−8 S•cm−1 at room temperature.[495] This is higher than selenium but lower than boron, the least electrically conducting of the recognised metalloids.[n 52]

In the periodic table, some of the elements adjacent to the commonly recognised metalloids, although usually classified as either metals or nonmetals, are occasionally referred to as near-metalloids[498] or noted for their metalloidal character. To the left of the metal–nonmetal dividing line, such elements include gallium,[499] tin[500] and bismuth.[501] They show unusual packing structures,[502] marked covalent chemistry (molecular or polymeric),[503] and amphoterism.[504] To the right of the dividing line are carbon,[505] phosphorus,[506] selenium[507] and iodine.[508] They exhibit metallic lustre, semiconducting properties[n 53] and bonding or valence bands with delocalized character. This applies to their most thermodynamically stable forms under ambient conditions: carbon as graphite; phosphorus as black phosphorus;[n 54] and selenium as grey selenium.

Allotropes

Many small, shiny, silver-coloured spheres on the left; many of the same sized spheres on the right are duller and darker than the ones of the left and have a subdued metallic shininess.
White tin (left) and grey tin (right). Both forms have a metallic appearance.

Different crystalline forms of an element are called allotropes. Some allotropes, particularly those of elements located (in periodic table terms) alongside or near the notional dividing line between metals and nonmetals, exhibit more pronounced metallic, metalloidal or nonmetallic behaviour than others.[514] The existence of such allotropes can complicate the classification of the elements involved.[515]

Tin, for example, has two allotropes: tetragonal "white" β-tin and cubic "grey" α-tin. White tin is a very shiny, ductile and malleable metal. It is the stable form at or above room temperature and has an electrical conductivity of 9.17 × 104 S·cm−1 (~1/6th that of copper).[516] Grey tin usually has the appearance of a grey micro-crystalline powder, and can also be prepared in brittle semi-lustrous crystalline or polycrystalline forms. It is the stable form below 13.2 °C and has an electrical conductivity of between (2–5) × 102 S·cm−1 (~1/250th that of white tin).[517] Grey tin has the same crystalline structure as that of diamond. It behaves as a semiconductor (as if it had a band gap of 0.08 eV), but has the electronic band structure of a semimetal.[518] It has been referred to as either a very poor metal,[519] a metalloid,[520] a nonmetal[521] or a near metalloid.[501]

The diamond allotrope of carbon is clearly nonmetallic, being translucent and having a low electrical conductivity of 10−14 to 10−16 S·cm−1.[522] Graphite has an electrical conductivity of 3 × 104 S·cm−1,[523] a figure more characteristic of a metal. Phosphorus, sulfur, arsenic, selenium, antimony, and bismuth also have less stable allotropes that display different behaviours.[524]

Abundance, extraction, and cost

Abundance

The table gives crustal abundances of the elements commonly to rarely recognised as metalloids.[525] Some other elements are included for comparison: oxygen and xenon (the most and least abundant elements with stable isotopes); iron and the coinage metals copper, silver, and gold; and rhenium, the least abundant stable metal (aluminium is normally the most abundant metal). Various abundance estimates have been published; these often disagree to some extent.[526]

Extraction

The recognised metalloids can be obtained by chemical reduction of either their oxides or their sulfides. Simpler or more complex extraction methods may be employed depending on the starting form and economic factors.[527] Boron is routinely obtained by reducing the trioxide with magnesium: B2O3 + 3 Mg → 2 B + 3MgO; after secondary processing the resulting brown powder has a purity of up to 97%.[528] Boron of higher purity (> 99%) is prepared by heating volatile boron compounds, such as BCl3 or BBr3, either in a hydrogen atmosphere (2 BX3 + 3 H2 → 2 B + 6 HX) or to the point of thermal decomposition. Silicon and germanium are obtained from their oxides by heating the oxide with carbon or hydrogen: SiO2 + C → Si + CO2; GeO2 + 2 H2 → Ge + 2 H2O. Arsenic is isolated from its pyrite (FeAsS) or arsenical pyrite (FeAs2) by heating; alternatively, it can be obtained from its oxide by reduction with carbon: 2 As2O3 + 3 C → 2 As + 3 CO2.[529] Antimony is derived from its sulfide by reduction with iron: Sb2S3 → 2 Sb + 3 FeS. Tellurium is prepared from its oxide by dissolving it in aqueous NaOH, yielding tellurite, then by electrolytic reduction: TeO2 + 2 NaOH → Na2TeO3 + H2O;[530] Na2TeO3 + H2O → Te + 2 NaOH + O2.[531] Another option is reduction of the oxide by roasting with carbon: TeO2 + C → Te + CO2.[532]

Production methods for the elements less frequently recognised as metalloids involve natural processing, electrolytic or chemical reduction, or irradiation. Carbon (as graphite) occurs naturally and is extracted by crushing the parent rock and floating the lighter graphite to the surface. Aluminium is extracted by dissolving its oxide Al2O3 in molten cryolite Na3AlF6 and then by high temperature electrolytic reduction. Selenium is produced by roasting the coinage metal selenides X2Se (X = Cu, Ag, Au) with soda ash to give the selenite: X2Se + O2 + Na2CO3 → Na2SeO3 + 2 X + CO2; the selenide is neutralized by sulfuric acid H2SO4 to give selenous acid H2SeO3; this is reduced by bubbling with SO2 to yield elemental selenium. Polonium and astatine are produced in minute quantities by irradiating bismuth.[533]

Cost

The recognised metalloids and their closer neighbours mostly cost less than silver; only polonium and astatine are more expensive than gold, on account of their significant radioactivity. As of 5 April 2014, prices for small samples (up to 100 g) of silicon, antimony and tellurium, and graphite, aluminium and selenium, average around one third the cost of silver (US$1.5 per gram or about $45 an ounce). Boron, germanium, and arsenic samples average about three-and-a-half times the cost of silver.[n 55] Polonium is available for about $100 per microgram.[534] Zalutsky and Pruszynski[535] estimate a similar cost for producing astatine. Prices for the applicable elements traded as commodities tend to range from two to three times cheaper than the sample price (Ge), to nearly three thousand times cheaper (As).[n 56]

Notes

  1. ^ For a related commentary see also: Vernon RE 2013, 'Which Elements Are Metalloids?', Journal of Chemical Education, vol. 90, no. 12, pp. 1703–1707, doi:10.1021/ed3008457
  2. ^ Definitions and extracts by different authors, illustrating aspects of the generic definition, follow:
    • "In chemistry a metalloid is an element with properties intermediate between those of metals and nonmetals."[3]
    • "Between the metals and nonmetals in the periodic table we find elements ... [that] share some of the characteristic properties of both the metals and nonmetals, making it difficult to place them in either of these two main categories"[4]
    • "Chemists sometimes use the name metalloid ... for these elements which are difficult to classify one way or the other."[5]
    • "Because the traits distinguishing metals and nonmetals are qualitative in nature, some elements do not fall unambiguously in either category. These elements ... are called metalloids ..."[6]
    More broadly, metalloids have been referred to as:
    • "elements that ... are somewhat of a cross between metals and nonmetals";[7] or
    • "weird in-between elements".[8]
  3. ^ Gold, for example, has mixed properties but is still recognised as "king of metals". Besides metallic behaviour (such as high electrical conductivity, and cation formation), gold shows nonmetallic behaviour:On halogen character, see also Belpassi et al.,[12] who conclude that in the aurides MAu (M = Li–Cs) gold "behaves as a halogen, intermediate between Br and I"; on aurophilicity, see also Schmidbaur and Schier.[13]
  4. ^ Mann et al.[16] refer to these elements as "the recognized metalloids".
  5. ^ Jones[44] writes: "Though classification is an essential feature in all branches of science, there are always hard cases at the boundaries. Indeed, the boundary of a class is rarely sharp."
  6. ^ The lack of a standard division of the elements into metals, metalloids, and nonmetals is not necessarily an issue. There is more or less, a continuous progression from the metallic to the nonmetallic. A specified subset of this continuum could serve its particular purpose as well as any other.[45]
  7. ^ The packing efficiency of boron is 38%; silicon and germanium 34; arsenic 38.5; antimony 41; and tellurium 36.4.[49] These values are lower than in most metals (80% of which have a packing efficiency of at least 68%),[50] but higher than those of elements usually classified as nonmetals. (Gallium is unusual, for a metal, in having a packing efficiency of just 39%.)[51] Other notable values for metals are 42.9 for bismuth[52] and 58.5 for liquid mercury.[53]) Packing efficiencies for nonmetals are: graphite 17%,[54] sulfur 19.2,[55] iodine 23.9,[55] selenium 24.2,[55] and black phosphorus 28.5.[52]
  8. ^ More specifically, the Goldhammer–Herzfeld criterion is the ratio of the force holding an individual atom's valence electrons in place with the forces on the same electrons from interactions between the atoms in the solid or liquid element. When the interatomic forces are greater than, or equal to, the atomic force, valence electron itinerancy is indicated and metallic behaviour is predicted.[57] Otherwise nonmetallic behaviour is anticipated.
  9. ^ As the ratio is based on classical arguments[59] 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.[60] Even so it offers a first order rationalization for the occurrence of metallic character amongst the elements.[61]
  10. ^ Atomic conductance is the electrical conductivity of one mole of a substance. It is equal to electrical conductivity divided by molar volume.[5]
  11. ^ Selenium has an ionization energy (IE) of 225 kcal/mol (941 kJ/mol) and is sometimes described as a semiconductor. It has a relatively high 2.55 electronegativity (EN). Polonium has an IE of 194 kcal/mol (812 kJ/mol) and a 2.0 EN, but has a metallic band structure.[66] Astatine has an IE of 215 kJ/mol (899 kJ/mol) and an EN of 2.2.[67] Its electronic band structure is not known with any certainty.
  12. ^ Jones (2010, pp. 169–71): "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."
  13. ^ Oderberg[80] argues on ontological grounds that anything not a metal is therefore a nonmetal, and that this includes semi-metals (i.e. metalloids).
  14. ^ Copernicium is reportedly the only metal thought to be a gas at room temperature.[86]
  15. ^ Metals have electrical conductivity values of from 6.9 × 103 S•cm−1 for manganese to 6.3 × 105 for silver.[90]
  16. ^ Metalloids have electrical conductivity values of from 1.5 × 10−6 S•cm−1 for boron to 3.9 × 104 for arsenic.[92] If selenium is included as a metalloid the applicable conductivity range would start from ~10−9 to 10−12 S•cm−1.[93]
  17. ^ Nonmetals have electrical conductivity values of from ~10−18 S•cm−1 for the elemental gases to 3 × 104 in graphite.[94]
  18. ^ Chedd[101] defines metalloids as having electronegativity values of 1.8 to 2.2 (Allred-Rochow scale). He included boron, silicon, germanium, arsenic, antimony, tellurium, polonium, and astatine in this category. In reviewing Chedd's work, Adler[102] described this choice as arbitrary, as other elements whose electronegativities lie in this range include copper, silver, phosphorus, mercury, and bismuth. He went on to suggest defining a metalloid as "a semiconductor or semimetal" and to include bismuth and selenium in this category.
  19. ^ Olmsted and Williams[106] commented that, "Until quite recently, chemical interest in the metalloids consisted mainly of isolated curiosities, such as the poisonous nature of arsenic and the mildly therapeutic value of borax. With the development of metalloid semiconductors, however, these elements have become among the most intensely studied".
  20. ^ Research published in 2012 suggests that metal-metalloid glasses can be characterised by an interconnected atomic packing scheme in which metallic and covalent bonding structures coexist.[174]
  21. ^ The reaction involved is Ge + 2 MoO3 → GeO2 + 2 MoO2. Adding arsenic or antimony (n-type electron donors) increases the rate of reaction; adding gallium or indium (p-type electron acceptors) decreases it.[188]
  22. ^ Ellern, writing in Military and Civilian Pyrotechnics (1968), comments that carbon black "has been specified for and used in a nuclear air-burst simulator."[194]
  23. ^ For a post-1960 example of the former use of the term metalloid to refer to nonmetals see Zhdanov,[243] who divides the elements into metals; intermediate elements (H, B, C, Si, Ge, Se, Te); and metalloids (of which the most typical are given as O, F, and Cl).
  24. ^ Boron, at 1.56 eV, has the largest band gap amongst the commonly recognised (semiconducting) metalloids. Of nearby elements in periodic table terms, selenium has the next highest band gap (close to 1.8 eV) followed by white phosphorus (around 2.1 eV).[254]
  25. ^ The synthesis of B40 borospherene, a "distorted fullerene with a hexagonal hole on the top and bottom and four heptagonal holes around the waist" was announced in 2014.[258]
  26. ^ The BH3 and Fe(CO4) species in these reactions are short-lived reaction intermediates.[266]
  27. ^ On the analogy between boron and metals, Greenwood[268] 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 ..."
  28. ^ The bonding in boron trifluoride, a gas, has been referred to as predominately ionic[272] a description which was subsequently described as misleading.[273]
  29. ^ Boron trioxide B2O3 is sometimes described as being (weakly) amphoteric.[276] It reacts with alkalies to give various borates.[277] In its hydrated form (as H3BO3, boric acid) it reacts with sulfur trioxide, the anhydride of sulfuric acid, to form a bisulfate B(HSO3) 4.[278] In its pure (anhydrous) form it reacts with phosphoric acid to form a "phosphate" BPO4.[279] The latter compound may be regarded as a mixed oxide of B2O3 and P2O5.[280]
  30. ^ Organic derivatives of metalloids are traditionally counted as organometallic compounds.[282]
  31. ^ In air, silicon forms a thin coating of amorphous silicon dioxide, 2 to 3 nm thick.[287] This coating is dissolved by hydrogen fluoride at a very low pace – on the order of two to three hours per nanometre.[288] Silicon dioxide, and silicate glasses (of which silicon dioxide is a major component), are otherwise readily attacked by hydrofluoric acid.[289]
  32. ^ The bonding in silicon tetrafluoride, a gas, has been referred to as predominately ionic[272] a description which was subsequently described as misleading.[273]
  33. ^ Although SiO2 is classified as an acidic oxide, and hence reacts with alkalis to give silicates, it reacts with phosphoric acid to yield a silicon oxide orthophosphate Si5O(PO4)6,[305] and with hydrofluoric acid to give hexafluorosilicic acid H2SiF6.[306] The latter reaction "is sometimes quoted as evidence of basic [that is, metallic] properties".[307]
  34. ^ Temperatures above 400 °C are required to form a noticeable surface oxide layer.[311]
  35. ^ Sources mentioning germanium cations include: Powell & Brewer[317] who state that the cadmium iodide CdI2 structure of germanous iodide GeI2 establishes the existence of the Ge++ ion (the CdI2 structure being found, according to Ladd,[318] in "many metallic halides, hydroxides, and chalcides"); Everest[319] who comments that, "it seems probable that the Ge++ ion can also occur in other crystalline germanous salts such as the phosphite, which is similar to the salt-like stannous phosphite and germanous phosphate, which resembles not only the stannous phosphates, but the manganous phosphates also"; Pan, Fu & Huang[320] who presume the formation of the simple Ge++ ion when Ge(OH)2 is dissolved in a perchloric acid solution, on the basis that, "ClO4− has little tendency to enter complex formation with a cation"; Monconduit et al.[321] who prepared the layer compound or phase Nb3GexTe6 (x ≃ 0.9), and reported that this contained a GeII cation; Richens[322] who records that, "Ge2+ (aq) or possibly Ge(OH)+(aq) is said to exist in dilute air-free aqueous suspensions of the yellow hydrous monoxide…however both are unstable with respect to the ready formation of GeO2.nH2O"; Rupar et al.[323] who synthesized a cryptand compound containing a Ge2+ cation; and Schwietzer and Pesterfield[324] who write that, "the monoxide GeO dissolves in dilute acids to give Ge+2 and in dilute bases to produce GeO2−2, all three entities being unstable in water". Sources dismissing germanium cations or further qualifying their presumed existence include: Jolly and Latimer[325] who assert that, "the germanous ion cannot be studied directly because no germanium (II) species exists in any appreciable concentration in noncomplexing aqueous solutions"; Lidin[326] who says that, "[germanium] forms no aquacations"; Ladd[327] who notes that the CdI2 structure is "intermediate in type between ionic and molecular compounds"; and Wiberg[328] who states that, "no germanium cations are known".
  36. ^ Arsenic also exists as a naturally occurring (but rare) allotrope (arsenolamprite), a crystalline semiconductor with a band gap of around 0.3 eV or 0.4 eV. It can also be prepared in a semiconducting amorphous form, with a band gap of around 1.2–1.4 eV.[344]
  37. ^ Sources mentioning cationic arsenic include: Gillespie & Robinson[347] who find that, "in very dilute solutions in 100% sulphuric acid, arsenic (III) oxide forms arsonyl (III) hydrogen sulphate, AsO.HO4, which is partly ionized to give the AsO+ cation. Both these species probably exist mainly in solvated forms, e.g., As(OH)(SO4H)2, and As(OH)(SO4H)+ respectively"; Paul et al.[348] who reported spectroscopic evidence for the presence of As42+ and As22+ cations when arsenic was oxidized with peroxydisulfuryl difluoride S2O6F2 in highly acidic media (Gillespie and Passmore[349] noted the spectra of these species were very similar to S42+ and S82+ and concluded that, "at present" there was no reliable evidence for any homopolycations of arsenic); Van Muylder and Pourbaix,[350] who write that, "As2O3 is an amphoteric oxide which dissolves in water and in solutions of pH between 1 and 8 with the formation of undissociated arsenious acid HAsO2; the solubility…increases at pH's below 1 with the formation of 'arsenyl' ions AsO+…"; Kolthoff and Elving[351] who write that, "the As3+ cation exists to some extent only in strongly acid solutions; under less acid conditions the tendency is toward hydrolysis, so that the anionic form predominates"; Moody[352] who observes that, "arsenic trioxide, As4O6, and arsenious acid, H3AsO3, are apparently amphoteric but no cations, As3+, As(OH)2+ or As(OH)2+ are known"; and Cotton et al.[353] who write that (in aqueous solution) the simple arsenic cation As3+ "may occur to some slight extent [along with the AsO+ cation]" and that, "Raman spectra show that in acid solutions of As4O6 the only detectable species is the pyramidal As(OH)3".
  38. ^ The formulae of AsPO4 and As2(SO4)3 suggest straightforward ionic formulations, with As3+, but this is not the case. AsPO4, "which is virtually a covalent oxide", has been referred to as a double oxide, of the form As2O3·P2O5. It consists of AsO3 pyramids and PO4 tetrahedra, joined together by all their corner atoms to form a continuous polymeric network.[359] As2(SO4)3 has a structure in which each SO4 tetrahedron is bridged by two AsO3 trigonal pyramida.[360]
  39. ^ As2O3 is usually regarded as being amphoteric but a few sources say it is (weakly)[363] acidic. They describe its "basic" properties (its reaction with concentrated hydrochloric acid to form arsenic trichloride) as being alcoholic, in analogy with the formation of covalent alkyl chlorides by covalent alcohols (e.g., R-OH + HCl → RCl + H2O)[364]
  40. ^ Antimony can also be prepared in an amorphous semiconducting black form, with an estimated (temperature-dependent) band gap of 0.06–0.18 eV.[370]
  41. ^ Lidin[375] asserts that SbO+ does not exist and that the stable form of Sb(III) in aqueous solution is an incomplete hydrocomplex [Sb(H2O)4(OH)2]+.
  42. ^ Cotton et al.[399] note that TeO2 appears to have an ionic lattice; Wells[400] suggests that the Te–O bonds have "considerable covalent character".
  43. ^ Liquid carbon may[414] or may not[415] be a metallic conductor, depending on pressure and temperature; see also.[416]
  44. ^ For the sulfate, the method of preparation is (careful) direct oxidation of graphite in concentrated sulfuric acid by an oxidising agent, such as nitric acid, chromium trioxide or ammonium persulfate; in this instance the concentrated sulfuric acid is acting as an inorganic nonaqueous solvent.
  45. ^ Only a small fraction of dissolved CO2 is present in water as carbonic acid so, even though H2CO3 is a medium-strong acid, solutions of carbonic acid are only weakly acidic.[425]
  46. ^ A mnemonic that captures the elements commonly recognised as metalloids goes: Up, up-down, up-down, up ... are the metalloids![437]
  47. ^ Rochow,[443] who later wrote his 1966 monograph The metalloids,[444] commented that, "In some respects selenium acts like a metalloid and tellurium certainly does".
  48. ^ A further option is to include astatine both as a nonmetal and as a metalloid.[470]
  49. ^ A visible piece of astatine would be immediately and completely vaporized because of the heat generated by its intense radioactivity.[476]
  50. ^ The literature is contradictory as to whether boron exhibits metallic conductivity in liquid form. Krishnan et al.[478] found that liquid boron behaved like a metal. Glorieux et al.[479] characterised liquid boron as a semiconductor, on the basis of its low electrical conductivity. Millot et al.[480] reported that the emissivity of liquid boron was not consistent with that of a liquid metal.
  51. ^ Korenman[484] similarly noted that "the ability to precipitate with hydrogen sulfide distinguishes astatine from other halogens and brings it closer to bismuth and other heavy metals".
  52. ^ The separation between molecules in the layers of iodine (350 pm) is much less than the separation between iodine layers (427 pm; cf. twice the van der Waals radius of 430 pm).[496] This is thought to be caused by electronic interactions between the molecules in each layer of iodine, which in turn give rise to its semiconducting properties and shiny appearance.[497]
  53. ^ For example: intermediate electrical conductivity;[509] a relatively narrow band gap;[510] light sensitivity.[509]
  54. ^ White phosphorus is the least stable and most reactive form.[511] It is also the most common, industrially important,[512] and easily reproducible allotrope, and for these three reasons is regarded as the standard state of the element.[513]
  55. ^ Sample prices of gold, in comparison, start at roughly thirty-five times that of silver. Based on sample prices for B, C, Al, Si, Ge, As, Se, Ag, Sb, Te, and Au available on-line from Alfa Aesa; Goodfellow; Metallium; and United Nuclear Scientific.
  56. ^ Based on spot prices for Al, Si, Ge, As, Sb, Se, and Te available on-line from FastMarkets: Minor Metals; Fast Markets: Base Metals; EnergyTrend: PV Market Status, Polysilicon; and Metal-Pages: Arsenic metal prices, news, and information.

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Sources

Further reading