Allotropy 1839 226120293 2008-07-16T22:17:26Z Arkuat 29003 /* Lantanides and actinides */ sp [[Image:Diamond and graphite.jpg|thumb|300px|Diamond and graphite are two allotropes of carbon: pure forms of the same element that differ in structure.]] '''Allotropy''' is the property of some [[chemical element]]s to be able to take two or more different forms, where the [[atoms]] are arranged differently by [[chemical bond]]s. The forms are known as ''allotropes'' of that element.<ref>Allotrope in ''IUPAC Compendium of Chemical Terminology'', Electronic version, http://goldbook.iupac.org/A00243.html. Accessed March 2007.</ref> 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 language|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]], O<sub>2</sub>, and [[ozone]], O<sub>3</sub>), 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 P<sub>4</sub> 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.<ref>Jensen W.B., "The Origin of the Term Allotrope", Journal of Chemical Education, 2006, '''83''', 838-9 </ref> 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 O<sub>2</sub> and O<sub>3</sub>. In the early 20th century it was recognized that other cases such as carbon were due to differences in crystal structure. By 1912, [[Wilhelm Ostwald|Ostwald]] noted that the allotropy of elements is just a special case of the phenomenon of [[Polymorphism (materials science)|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]], [[photochemistry|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 (iron)|(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 [[catenation|catenate]]. Allotropes are typically more noticeable in [[non-metal]]s and [[metalloid]]s. Nevertheless, [[metal]]s tend to have many allotropes. Examples of allotropes include: ===Non-metals=== ''[[Allotropes of carbon|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]] * [[fullerene]]s, including "[[buckyball]]s", such as C<sub>60</sub>, and [[carbon nanotube]]s ''[[Allotropes of phosphorus|Phosphorus]]'': * White phosphorus - crystalline solid {{chem|P|4}} * Red phosphorus - polymeric solid * Scarlet phosphorus * Violet phosphorus * Black phosphorus - semiconductor, analogous to graphite * [[Diphosphorus]] ''[[Allotropes of oxygen|Oxygen]]'': * [[dioxygen]], O<sub>2</sub> - colorless * [[ozone]], O<sub>3</sub> - blue * [[tetraoxygen]], O<sub>4</sub> - [[Metastability|metastable]] * [[solid oxygen|octaoxygen]], {{chem|O|8}} - red ''[[Nitrogen#Properties|Nitrogen]]: * [[dinitrogen]] * [[tetranitrogen]] * trinitrogen * two solid forms: one hexagonal close-packed and the other alpha cubic ''[[Allotropes of sulfur|Sulfur]]'': * Plastic (amorphous) sulfur - polymeric solid * Rhombic sulfur - large crystals composed of S<sub>8</sub> molecules * Monoclinic sulfur - fine needle-like crystals * Other ring molecules such as S<sub>7</sub> and S<sub>12</sub> ''[[Selenium]]'': * "Red selenium," cyclo-Se<sub>8</sub> * 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 As<sub>4</sub> * 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=== {{Expand-section|date=January 2008}} ''[[Tin#Allotropes|Tin]]'' * grey tin (alpha-tin) * white tin (beta tin) * rhombic tin (gamma) ''[[Allotropes of iron|Iron]]'' * [[Ferrite (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#Physical|Titanium]]'' has two allotropes ''[[Strontium#Occurrence|Strontium]]'' has three allotropes ====Lanthanides and actinides==== *''[[Allotropes of plutonium|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.{{Fact|date=January 2008}} *''[[Ytterbium#Notable characteristics|Ytterbium]]'' has three allotropes *''[[Terbium#Notable characteristics|Terbium]]'' has two crystalline allotropes *''[[Promethium#Notable characteristics|Promethium]]'' has two allotropic forms *''[[Curium]] has 3 allotropes (also [[Americium]], [[Berkelium]], [[Californium]] do) <ref>http://www.iop.org/EJ/article/0305-4608/15/2/002/jfv15i2pL29.pdf?request-id=AFlRqDDL3BGhbarg2wi7Kg</ref> [[Image:Actinide phases.png|right|thumb|250px|Phase diagram of the actinide elements.]] ==References== {{reflist}} ==External links== * http://www.physics.uoguelph.ca/summer/scor/articles/scor40.htm [[Category:Allotropy]] [[Category:Inorganic chemistry]] [[ar:تآصل]] [[be:Алатропія]] [[bs:Alotropske modifikacije]] [[bg:Алотропия]] [[ca:Al·lotropia]] [[cs:Alotropie]] [[cy:Alotrop]] [[de:Allotropie]] [[et:Allotroopia]] [[el:Αλλότροπα]] [[es:Alotropía]] [[eo:Alotropo]] [[eu:Alotropia]] [[fa:دگرشکلی]] [[fr:Allotropie]] [[gl:Alotropía]] [[ko:동소체]] [[hr:Alotropija]] [[is:Fjölgervi]] [[it:Allotropia (chimica)]] [[he:אלוטרופיה]] [[ku:Allotrop]] [[lv:Alotropija]] [[hu:Allotrópia]] [[ms:Alotrop]] [[mn:Аллотропи]] [[nl:Allotropie]] [[ja:同素体]] [[no:Allotropi]] [[nn:Allotrope former]] [[pl:Alotropia]] [[pt:Alotropia]] [[ru:Аллотропия]] [[simple:Allotrope]] [[sk:Alotropia]] [[sl:Alotropija]] [[fi:Allotropia]] [[sv:Allotropi]] [[vi:Thù hình]] [[tr:Allotrop]] [[uk:Алотропія]] [[zh:同素异形体]]