View source for Allotropes of phosphorus

[[Image:Phosphor Modifikationen.png|right|160px|thumb|Diagram of transformations of modifications of phosphorus]]
Elemental [[phosphorus]] can exist in several [[allotropy|allotropes]]; the most common of which are white and red. There are also violet and black phosphorus, and gaseous [[diphosphorus]].

==White phosphorus==
[[Image:Weißer Phosphor.JPG|thumb|White phosphorus sample]]
''[[White phosphorus]]'', or ''yellow phosphorus'', or simply ''tetraphosphorus'' (P<sub>4</sub>) exists as individual [[molecules]] made up of four [[atoms]] in a [[tetrahedral]] arrangement, resulting in very high [[ring strain]] and instability. It contains 6 single bonds. Two different crystalline forms of white phosphorus are known. The α form, which is stable under standard conditions, has a body-centered cubic crystal structure. It transforms reversibly into the β form at 195.2 K. The β form is believed to have a hexagonal crystal structure.<ref>Marie-Thérèse Averbuch-Pouchot, A. Durif. ''Topics in Phosphate Chemistry''. World Scientific, '''1996'''. ISBN 9810226349. p. 3.</ref>

[[Image:White phosphrous molecule.jpg|left|175px|thumb|White phosphorus molecule]]
White phosphorus is a transparent waxy solid that quickly becomes yellow when exposed to light. For this reason it is also called yellow phosphorus. It glows greenish in the dark (when exposed to oxygen), is highly [[flammable]] and [[pyrophoricity|pyrophoric]] (self-igniting) upon contact with air as well as [[toxicity|toxic]] (causing severe liver damage on ingestion). The odour of combustion of this form has a characteristic garlic smell, and samples are commonly coated with white "(di)[[phosphorus pentoxide]]", which consists of P<sub>4</sub>O<sub>10</sub> tetrahedra with oxygen inserted between the phosphorus atoms and at their vertices. White phosphorus is insoluble in water but soluble in [[carbon disulfide]]. 
                             
The white allotrope can be produced using several different methods. In one process, [[calcium]] phosphate, which is derived from phosphate rock, is heated in an electric or fuel-fired furnace in the presence of [[carbon]] and [[silica]].<ref name="threlfall">Threlfall, R.E., (1951). ''100 years of Phosphorus Making: 1851 - 1951''. Oldbury: [[Albright and Wilson]] Ltd</ref>  Elemental phosphorus is then liberated as a vapour and can be collected under [[phosphoric acid]]. This process is similar to the first synthesis of phosphorus from calcium phosphate in urine.

[[Image:Tetraphosphorus-liquid-2D-dimensions.png|200px|thumb|Tetraphosphorus molecule]]
White phosphorus is a soft, white wax-like solid, with an appreciable [[Vapor pressure|vapour pressure]] at ordinary temperatures. The [[vapour density]] indicates that the vapour is composed of P<sub>4</sub> molecules up to about 800°C. Above that temperature, dissociation into P<sub>2</sub> molecules occurs. The P<sub>4</sub> molecule is tetrahedral, with each phosphorus atom being at the apices of the tetrahedron.

It ignites spontaneously at about 50°C, and at much lower temperatures if finely divided. White phosphorus is only slightly soluble in water and, indeed, it can be stored under water. However, it is soluble in [[benzene]], [[oils]], carbon disulfide and sulfur monochloride. The [[Molecular mass|Relative Molecular Mass]] in solution corresponds to that expected for P<sub>4</sub> molecules.

White phosphorus is chemically very reactive, and will spontaneously ignite in an abundant supply of air to give phosphorus (V) oxide:

{{chem|P|4}} + 5{{chem|O|2}} -> {{chem|P|4|O|10}}
{{clear}}

==Red phosphorus==
[[Image:Phosphor.JPG.jpg|thumb|Red phosphorus powder. 99%+]]
[[Image:Phosphor rot.jpg|thumb|Red phosphorus]]
Red phosphorus may be formed by heating white phosphorus to 250°C (482°F) or by exposing white phosphorus to sunlight. Phosphorus after this treatment exists as an [[amorphous]] network of atoms which reduces strain and gives greater stability; further heating results in the red phosphorus becoming crystalline. Red phosphorus does not catch fire in air at temperatures below 240°C, whereas white phosphorus ignites at about 30°C.

In 1865 [[Hittorf]] discovered that when phosphorus was recrystallized from molten [[lead]], a red/purple form is obtained. This purple form is sometimes known as "Hittorf's phosphorus." In addition, a fibrous form exists with similar phosphorus cages. Below is shown a chain of phosphorus atoms which exhibits both the purple and fibrous forms.

Red phosphorus is much less reactive than white phosphorus, as evidenced by:

{{chem|P|white}} -> {{chem|P|violet}} ΔH = -16.8 J

It does not ignite in air until heated to 300°C, and it is insoluble in [[water]], [[alkali]] and carbon disulfide. It does not react so readily with the [[halogens]], but can be [[oxidised]] by [[nitric acid]] to [[phosphoric acid]].

[[Image:Červený fosfor2.gif|thumb|175px|left|Red phosphorus structure]]
If it is heated in an atmosphere of inert gas, for example [[nitrogen]] or [[carbon dioxide]], it [[Sublimation (chemistry)|sublimes]] and the vapour condenses as white phosphorus. If, however, it is heated in a [[vacuum]] and the vapour condensed rapidly, violet phosphorus is obtained. It would appear that violet phosphorus is a [[polymer]] of high Relative Molecular Mass, which on heating breaks down into P2 molecules. On cooling, these would normally [[dimerize]] to give P4 molecules (i.e. white phosphorus) but, in vacuo, they link up again to form the polymeric violet allotrope.


One of the forms of red/black phosphorus is a [[cubic]] solid.<ref>R. Ahuja, ''Physica Status Solidi, Sectio B: Basic Research'', 2003, '''235''', 282-287</ref>
{{clear}}
==Violet phosphorus==
[[Image:Phosphor-rot-violett.jpg|200px|right|thumb|Violet phosphorus (right) by a sample of red phosphorus (left)]]
[[Image:Violetter Phosphor.jpg|160px|left|thumb|Violet phosphorus structure]]
[[Image:Hittoff phosphorus chain.jpg|160px|left|thumb|Hitoff phosphorus structure]]
''Monoclinic phosphorus'', or ''violet phosphorus'', is also known as "''Hittorf`s Metallic Phosphorus''".<ref>[http://www.lateralscience.co.uk/phos/index.html Lateral Science - Phosphorus Topics<!-- Bot generated title -->]</ref><ref>[http://www.freepatentsonline.com/4620968.html Monoclinic phosphorus formed from vapor in the presence of an alkali metal - Patent 4620968<!-- Bot generated title -->]</ref> In 1865 Hittorf heated red phosphorus in a sealed tube at 530&nbsp;°C. The upper part of the tube was kept at 444&nbsp;°C. Brilliant opaque [[monoclinic]], or [[rhombohedral]], crystals sublime. Can also be prepared by dissolving white phosphorus in molten [[lead]] in a sealed tube at 500&nbsp;°C for 18 hours. Upon slow cooling, Hittorf's allotrope [[crystallise]]s out. The crystals can be revealed by dissolving the lead in dilute [[nitric acid]] followed by boiling in concentrated [[hydrochloric acid]].

Violet phosphorus was obtained by [[Johann Wilhelm Hittorf]] by heating white phosphorus at 550&nbsp;°C for two weeks. He obtained a polymer that was insoluble in CS<sub>2</sub>. The crystal structure was obtained in 1960 at the [[University]] of [[Stuttgart]] by [[Herbert Thurn]]. <!-- Auch der violette Phosphor ist ungiftig. -->
{{clear}}
==Black phosphorus==
[[Image:Black phosphorus.jpg|left|170px|thumb|Black phosphorus]]
[[Image:Schwarzer Phosphor.jpg|right|170px|thumb|Black phosphorus structure]]
Black phosphorus is the thermodynamically stable form of phosphorus at room temperature and pressure. It is obtained by heating white phosphorus under very high pressures (12,000 atmospheres) In appearance, properties and structure it is very like graphite, being black and flaky, a conductor of electricity and has puckered sheets of linked atoms.

Black phosphorus has an [[Orthorhombic crystal system|orthorhombic]] structure (C<sub>mca</sub>) and is the least reactive allotrope, it consists of many six-membered rings which are interlinked. Each atom is bonded to three other atoms.<ref name="Brown">A. Brown, S. Runquist, ''Acta Crystallogr.'', '''19''' (1965) 684</ref><ref>Cartz, L.;Srinivasa, S.R.;Riedner, R.J.;Jorgensen, J.D.;Worlton, T.G., ''Journal of Chemical Physics'', 1979, '''71''', 1718-1721</ref> A recent synthesis of black phosphorus using metal salts as catalysts has been reported.<ref>Stefan Lange, Peer Schmidt, and Tom Nilges, ''Inorganic Chemistry'', 2007, '''46''', 4028</ref>
{{clear}}
==Diphosphorus==
{{main|Diphosphorus}}
[[Image:Diphosphorus-2D-dimensions.png|thumb|100px|Diphosphorus molecule]]
The diphosphorus allotrope (P<sub>2</sub>) can be obtained normally only under extreme conditions (for example, from P4 at 1100 kelvin). Nevertheless, some advancements were obtained in generating the diatomic molecule in homogenous solution, under normal condtitions with the use by some transitional metal complexes (based on for example [[tungsten]] and [[niobium]]).<ref>''Triple-Bond Reactivity of Diphosphorus Molecules'' Nicholas A. Piro, Joshua S. Figueroa, Jessica T. McKellar, Christopher C. Cummins [[Science (journal)|Science]] 1 September '''2006''':Vol. 313. no. 5791, pp. 1276 - 1279 [[DOI|10.1126/science.1129630]]</ref><ref>http://www.sciencemag.org/cgi/reprint/313/5791/1276.pdf</ref>

In fact, diphosphorus is the gaseous form of [[phosphorus]], and the thermodynamically stable form above 1200&nbsp;°C and until 2000&nbsp;°C.  The dissociation of tetraphosphorus ({{chem|P|4}}) begins at lower temperature: the percentage of {{chem|P|2}} at 800&nbsp;°C is ~&nbsp;1&nbsp;%. Around 2000&nbsp;°C the diphosphorus molecule begins to dissociate into atomic phosphorus.

==Phosphorus nanorods==
Phosphorus nanorods were synthesized as {{chem|-||P|12|-}} polymers in Germany in August 2004 in two modifications.<ref>[http://www3.interscience.wiley.com/cgi-bin/abstract/109083291/ABSTRACT?CRETRY=1&SRETRY=0]</ref><ref name="pfitzner2004">Arno Pfitzner, Michael F. Bräu, Josef Zweck, Gunther Brunklaus und Hellmut Eckert: ''Phosphorus Nanorods - Two Allotropic Modifications of a Long-Known Element'' in ''Angewandte Chemie International Edition'' 43 (2004), 4228 - 4231 {{DOI|10.1002/anie.200460244}}</ref> 

The red-brown phase is different from red phosphorus and is also stable in air for weeks. [[Electron microscope]] showed the red-brown form as having long, parallel nanorods with a diameter between 0,34&nbsp;[[Nanometre|nm]] and 0,47&nbsp;nm.
{{clear}}
==Other==
===Scarlet phosphorus===
Another form, [[Schenk's scarlet phosphorus]], is obtained by allowing a solution of white phosphorus in carbon disulfide to evaporate in sunlight.<ref>[http://www.lateralscience.co.uk/phos/index.html Lateral Science - Phosphorus Topics<!-- Bot generated title -->]</ref> It was first characterised by [[Rudolf Schenck]] in 1902 by heating white phosphorus in [[phosphorus tribromide]] ({{Chem|P||Br|3}})). It might include some [[bromine]] and has a density of about 1.88 g/cm3.

==References==
{{reflist|2}}

==See also==
*[[Allotropy]]
*[[Phosphorus]]
[[Category:Allotropy]]
[[Category:Phosphorus]]