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Encyclopedia > Allotropy
Diamond and graphite are two allotropes of carbon: pure forms of the same element that differ in structure.

Allotropy (Gr. allos, other, and tropos, manner) is a behavior exhibited by certain chemical elements: these elements can exist in two or more different forms, known as allotropes of that element. In each different allotrope, the element's atoms are bonded together in a different manner. Image File history File links Size of this preview: 627 × 600 pixelsFull resolution (1206 × 1154 pixel, file size: 164 KB, MIME type: image/jpeg) This file is a composite of Image:GraphiteUSGOV.jpg (public domain), en:Image:Brillanten. ... Image File history File links Size of this preview: 627 × 600 pixelsFull resolution (1206 × 1154 pixel, file size: 164 KB, MIME type: image/jpeg) This file is a composite of Image:GraphiteUSGOV.jpg (public domain), en:Image:Brillanten. ... The periodic table of the chemical elements A chemical element, or element, is a type of atom that is defined by its atomic number; that is, by the number of protons in its nucleus. ...


For example, the element 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. For other uses, see Carbon (disambiguation). ... This article is about the mineral. ... A tetrahedron (plural: tetrahedra) is a polyhedron composed of four triangular faces, three of which meet at each vertex. ... For other uses, see Graphite (disambiguation). ...


Note that 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) - the changes of state between solid, liquid and gas in themselves are not considered allotropy. For some elements, allotropes have different molecular formulae which can 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. Conversely, some elements do not maintain distinct allotropes in different phases: for example phosphorus has numerous solid allotropes, which all revert to the same P4 form when melted to the liquid state. In the physical sciences, a state of matter is one of the many ways that matter can interact with itself to form a macroscopic, homogenous phase. ... For other uses, see Solid (disambiguation). ... For other uses, see Liquid (disambiguation). ... For other uses, see Gas (disambiguation). ... General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colorless (gas) pale blue (liquid) Standard atomic weight 15. ... Dioxygen, O2, is the most common form of the element oxygen in normal conditions. ... For other uses, see Ozone (disambiguation). ... General Name, symbol, number phosphorus, P, 15 Chemical series nonmetals Group, period, block 15, 3, p Appearance waxy white/ red/ black/ colorless Standard atomic weight 30. ...

Contents

History

[1] 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. Jöns Jacob Berzelius (August 20, 1779 - August 7, 1848) was a Swedish chemist, who invented modern chemical notation and is considered one of the fathers of modern chemistry (along with John Dalton and Antoine Lavoisier). ... Count Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e Cerreto, (Turin August 9, 1776 - July 9, 1856) was a great Italian scientist. ...


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. Friedrich Wilhelm Ostwald (commonly just Wilhelm Ostwald) (September 2, 1853 - April 4, 1932) was a German chemist. ... Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. ... The International Union of Pure and Applied Chemistry (IUPAC) is an international non-governmental organization devoted to the advancement of chemistry. ...


Differences in properties of an element's allotropes

Allotropes of the same element can typically exhibit quite different physical properties and chemical behaviour, even though they contain nothing else but atoms of that element. They may have different colors, odors, hardnesses, electrical and thermal conductivities, etc.


The change between different allotropic forms of an element is often triggered by pressure and temperature, and many allotropes are only stable in the correct 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. This article is about pressure in the physical sciences. ... For other uses, see Temperature (disambiguation). ... For other uses, see Iron (disambiguation). ... In crystallography, the cubic crystal system is the most symmetric of the 7 crystal systems. ... Iron-carbon phase diagram, showing the conditions under which ferrite (α) is stable. ... In crystallography, the cubic crystal system is the most symmetric of the 7 crystal systems. ... Iron-carbon phase diagram, showing the conditions under which austenite (γ) is stable in carbon steel. ... This article is about the metallic chemical element. ... Tin pest is an allotropic transformation of the element tin, which causes deterioration of tin objects at low temperatures. ... For alternative meanings see metal (disambiguation). ... A semiconductor is a solid whose electrical conductivity is in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. ...


Examples 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. In chemistry coordination number (c. ... The oxidation state or oxidation number is defined as the sum of negative and positive charges in an atom, which indirectly indicates the number of electrons it has accepted or donated. ... Catenation is the ability of a chemical element to form covalent bonds with itself, resulting in ring, chain and cage molecules. ... Together with the metals and metalloids, a nonmetal is one of three categories of chemical elements as distinguished by ionization and bonding properties. ... Metalloid is a term used in chemistry when classifying the chemical elements. ...


Examples of allotropes include:


Carbon: For other uses, see Carbon (disambiguation). ...

Main article: Allotropes of 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.
  • fullerene - (including the "buckyball", C60)

Phosphorus: This article or section does not cite any references or sources. ... This article is about the mineral. ... For other uses, see Graphite (disambiguation). ... The Icosahedral Fullerene C540 C60 and C-60 redirect here. ... General Name, symbol, number phosphorus, P, 15 Chemical series nonmetals Group, period, block 15, 3, p Appearance waxy white/ red/ black/ colorless Standard atomic weight 30. ...

  • Red phosphorus - polymeric solid
  • White phosphorus - crystalline solid
  • Black phosphorus - semiconductor, analogous to graphite

Oxygen: General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colorless (gas) pale blue (liquid) Standard atomic weight 15. ...

Sulfur: Dioxygen, O2, is the most common form of the element oxygen in normal conditions. ... For other uses, see Ozone (disambiguation). ... Tetraoxygen, also called red oxygen (O4), is an allotrope of oxygen occuring in extremely high pressures (in order of 20 GPa). ... This article is about the chemical element. ...

  • Plastic (amorphous) sulfur - polymeric solid
  • Rhombic sulfur - large crystals composed of S8 molecules
  • Monoclinic sulfur - fine needle-like crystals
  • Molecular sulfur - sulphur tends to form ring molecules such as S7 and S12

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. General Name, Symbol, Number plutonium, Pu, 94 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery white Standard atomic weight (244) g·mol−1 Electron configuration [Rn] 5f6 7s2 Electrons per shell 2, 8, 18, 32, 24, 8, 2 Physical properties Phase solid Density (near r. ...


External links

  • Allotrope in IUPAC Compendium of Chemical Terminology, Electronic version, http://goldbook.iupac.org/A00243.html. Accessed March 2007.

phosphorous is white ,red & violet]


References

  1. ^ Jensen W.B., "The Origin of the Term Allotrope", Journal of Chemical Education, 2006, 83, 838-9

  Results from FactBites:
 
Allotropy - Wikipedia (378 words)
Some classic examples of solids showing allotropy are phosphorus (in "red" and "white" forms) and carbon (in the form of graphite, diamond, or fullerenes).
Allotropy specifically refers to the chemical bond structure between atoms and should not be confused with the existence of multiple physical states, such as with water, which can exist as a gas (steam), a liquid (water), or a solid (ice).
The word allotropy is usually restricted to the case of pure elements and is a special case of Polymorphism.
Allotropy - Wikipedia, the free encyclopedia (513 words)
Amorphous (plastic sulfur) is produced by quickly cooling the crystalline form, generating helical structure with eight atoms per spiral.
Allotropy specifically refers to the chemical bond structure between atoms of the same kind and should not be confused with the existence of multiple physical states, such as with water, which can exist as a gas (steam), a liquid (water), or a solid (ice).
As can be seen with the example of carbon allotropes, certain physical properties can vary dramatically from allotrope to allotrope.
  More results at FactBites »

 
 

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