AskDefine | Define allotropy

Dictionary Definition

allotropy n : the phenomenon of an element existing in two or more physical forms [syn: allotropism]

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English

Noun

  1. A property, exhibited by some elements of existing in multiple forms with different atomic structures.

Related terms

Translations

having multiple atomic structures

Extensive Definition

Allotropy is the property of some chemical elements to be able to take two or more different forms, where the atoms are arranged differently by chemical bonds. The forms are known as allotropes of that element. The phenomenon of allotropy is sometimes also called allotropism. For example, carbon has two common allotropes: diamond, where the carbon atoms are bonded together in a tetrahedral lattice arrangement, and graphite, where the carbon atoms are bonded together in sheets of a hexagonal lattice.
The word allotropy comes from the Greek allos, meaning "other", and tropos, "manner".
Allotropy refers only to different forms of an element within the same phase or state of matter (i.e. different solid, liquid or gas forms). Changes of state (between solid, liquid and gas) are not considered allotropy. Some elements have allotropes that persist in different phases - for example, the two allotropes of oxygen (dioxygen, O2, and ozone, O3), can both exist in the solid, liquid and gaseous states. Other elements maintain distinct allotropes only in some phases - for example phosphorus has many solid allotropes, which all revert to the same P4 form when melted to the liquid state.

History

The concept of allotropy was originally proposed in 1841 by the Swedish scientist Baron Jons Jakob Berzelius (1779-1848) who offered no explanation. After the acceptance of Avogadro's hypothesis in 1860 it was understood that elements could exist as polyatomic molecules, and the two allotropes of oxygen were recognized as O2 and O3. In the early 20th century it was recognized that other cases such as carbon were due to differences in crystal structure.
By 1912, Ostwald noted that the allotropy of elements is just a special case of the phenomenon of polymorphism known for compounds, and proposed that the terms allotrope and allotropy be abandoned and replaced by polymorph and polymorphism. Although many other chemists have repeated this advice, IUPAC and most chemistry texts still favour the usage of allotrope and allotropy for elements only.

Differences in properties of an element's allotropes

Allotropes are different structural forms of the same element and can exhibit quite different physical properties and chemical behaviours. The change between allotropic forms is triggered by the same forces that affect other structures, i.e. pressure, light, and temperature. Therefore the stability of the particular allotropes depends on particular conditions. For instance, iron changes from a body-centered cubic structure (ferrite) to a face-centered cubic structure (austenite) above 906 °C, and tin undergoes a transformation known as tin pest from a metallic phase to a semiconductor phase below 13.2 °C.

List of allotropes

Typically, elements capable of variable coordination number and/or oxidation states tend to exhibit greater numbers of allotropic forms. Another contributing factor is the ability of an element to catenate. Allotropes are typically more noticeable in non-metals and metalloids. Nevertheless, metals tend to have many allotropes.
Examples of allotropes include:

Non-metals

Carbon:
  • diamond - an extremely hard, transparent crystal, with the carbon atoms arranged in a tetrahedral lattice. A poor electrical conductor. An excellent thermal conductor.
  • graphite - a soft, black, flaky solid, a moderate electrical conductor. The C atoms are bonded in flat hexagonal lattices, which are then layered in sheets.
  • amorphous carbon
  • fullerenes, including "buckyballs", such as C60, and carbon nanotubes
Phosphorus:
  • White phosphorus - crystalline solid
  • Red phosphorus - polymeric solid
  • Scarlet phosphorus
  • Violet phosphorus
  • Black phosphorus - semiconductor, analogous to graphite
  • Diphosphorus
Oxygen:
Nitrogen:
Sulfur:
  • Plastic (amorphous) sulfur - polymeric solid
  • Rhombic sulfur - large crystals composed of S8 molecules
  • Monoclinic sulfur - fine needle-like crystals
  • Other ring molecules such as S7 and S12
Selenium:
  • "Red selenium," cyclo-Se8
  • Gray selenium, polymeric Se
  • Black selenium

Metalloids

Boron
  • amorphous boron - brown powder
  • crystalline boron - black, hard (9.3 on Mohs' scale), and a weak conductor at room temperature.
Silicon''
  • amorphous silicon - brown powder
  • nanocrystalline silicon - similar to the amorphous silicon
  • crystalline silicon - has a metallic luster and a grayish color. Single crystals of crystalline silicon can be grown with a process known as the Czochralski process
Arsenic:
  • Yellow arsenic - molecular non-metallic As4
  • Gray arsenic, polymeric As (metalloid)
  • Black arsenic (metalloid) and several similar other ones.
Antimony:
  • blue-white antimony - the stable form (metalloid)
  • yellow antimony (non-metallic)
  • black antimony (non-metallic)
  • (a fourth one too)
Polonium has two metallic allotropes.

Metals

Tin
  • grey tin (alpha-tin)
  • white tin (beta tin)
  • rhombic tin (gamma)
Iron
  • ferrite (alpha iron) - forms below 770°C (the Curie point, Tc ); the iron becomes magnetic in its alpha form; BCC
  • beta - forms below 912°C (BCC)
  • gamma - forms below 1401°C; face centred cubic (FCC) crystal structure
  • delta - forms from cooling down molten iron below 1535°C; has a body-centred cubic (BCC) crystal structure
Titanium has two allotropes
Strontium has three allotropes

Lantanides and actinides

  • Plutonium has six distinct solid allotropes under "normal" pressures. Their densities vary within a ratio of some 4:3, which vastly complicates all kinds of work with the metal (particularly casting, machining, and storage). A seventh plutonium allotrope exists at very high pressures, which adds further difficulties in exotic applications.
allotropy in Arabic: تآصل
allotropy in Belarusian: Алатропія
allotropy in Bosnian: Alotropske modifikacije
allotropy in Bulgarian: Алотропия
allotropy in Catalan: Al·lotropia
allotropy in Czech: Alotropie
allotropy in Welsh: Alotrop
allotropy in German: Allotropie
allotropy in Estonian: Allotroopia
allotropy in Modern Greek (1453-): Αλλότροπα
allotropy in Spanish: Alotropía
allotropy in Esperanto: Alotropo
allotropy in Basque: Alotropia
allotropy in Persian: دگرشکلی
allotropy in French: Allotropie
allotropy in Galician: Alotropía
allotropy in Korean: 동소체
allotropy in Croatian: Alotropija
allotropy in Icelandic: Fjölgervi
allotropy in Italian: Allotropia (chimica)
allotropy in Hebrew: אלוטרופיה
allotropy in Kurdish: Allotrop
allotropy in Latvian: Alotropija
allotropy in Hungarian: Allotrópia
allotropy in Malay (macrolanguage): Alotrop
allotropy in Mongolian: Аллотропи
allotropy in Dutch: Allotropie
allotropy in Japanese: 同素体
allotropy in Norwegian: Allotropi
allotropy in Norwegian Nynorsk: Allotrope former
allotropy in Polish: Alotropia
allotropy in Portuguese: Alotropia
allotropy in Russian: Аллотропия
allotropy in Simple English: Allotrope
allotropy in Slovak: Alotropia
allotropy in Slovenian: Alotropija
allotropy in Finnish: Allotropia
allotropy in Swedish: Allotropi
allotropy in Vietnamese: Thù hình
allotropy in Turkish: Allotrop
allotropy in Ukrainian: Алотропія
allotropy in Chinese: 同素异形体

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