Editing Phosphorus (section)

===Allotropes===
{{Main|Allotropes of phosphorus}}
{{multiple image|perrow=2|total_width=320|caption_align=center
| header = {{font|size=100%|font=Sans-serif|text=Crystalline structures of the main phosphorus allotropes}}
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|image1=White phosphorus molecule.jpg
 |alt1=
 |caption1=White
|image2=redPhosphorus.jpg
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 |caption2=Red
|image3=Hittorf's violet phosphorus.png
 |alt3=
 |caption3=Violet
|image4=BlackPhosphorus.jpg
 |alt4=
 |caption4=Black
}}
Phosphorus has several [[allotropy|allotropes]] that exhibit very diverse properties.{{r|Holleman1985}} The most useful and therefore common is [[white phosphorus]], followed by [[red phosphorus]]. The two other main allotropes, violet and black phosphorus, have either a more fundamental interest or specialised applications. Many other allotropes have been theorised and synthesised, with the search for new materials an active area of research.{{r|Tian2023}} Commonly mentioned "yellow phosphorus" is not an allotrope, but a result of the gradual degradation of white phosphorus into red phosphorus, accelerated by light and heat. This causes white phosphorus that is aged or otherwise impure (e.g. weapons-grade) to appear yellow.

White phosphorus is a soft, waxy [[molecular solid]] that is insoluble in water.{{r|Greenwood1997}} It is also very toxic, highly [[flammable]] and [[pyrophoricity|pyrophoric]], igniting in air at about {{convert|30|C|K}}.{{r|Mellor1939|pp=721-722}} Structurally, it is composed of {{chem2|P4}} [[tetrahedra]]. The nature of bonding in a given {{chem2|P4}} tetrahedron can be described by [[spherical aromaticity]] or cluster bonding, that is the electrons are highly [[Delocalized electron|delocalized]]. This has been illustrated by calculations of the magnetically induced currents, which sum up to 29&nbsp;nA/T, much more than in the archetypical [[Aromaticity|aromatic]] molecule [[benzene]] (11&nbsp;nA/T).{{r|Cossairt2010}} The {{chem2|P4}} molecule in the gas phase has a P-P bond length of 2.1994(3)&nbsp;Å as determined by [[gas electron diffraction]].{{r|Cossairt2010}} White phosphorus exists in two crystalline forms named α (alpha) and β (beta), differing in terms of the relative orientation of the constituent {{chem2|P4}} tetrahedra.{{r|Roberts1992|Averbuch-Pouchot1996}} The α-form is most stable at room temperature and has a [[cubic crystal structure]]. When cooled down to {{convert|195.2|K|C}} it transforms into the β-form, turning into an [[hexagonal crystal structure]]. When heated up, the tetrahedral structure is conserved after melting at {{convert|317.3|K|C}} and boiling at {{convert|553.7|K|C}}, before facing [[thermal decomposition]] at {{convert|1100|K|C}} where it turns into gaseous [[diphosphorus]] ({{chem2|P2}}).{{r|Arndt1997}} This molecule contains a triple bond and is analogous to {{chem2|N2}}; it can also be generated as a transient intermediate in solution by thermolysis of organophosphorus precursor reagents.{{r|Piro2006}} At still higher temperatures, {{chem2|P2}} dissociates into atomic P.{{r|Greenwood1997}}

[[File:White phosphorus glowing e17.png|thumb|right|White phosphorus exposed to air glows in the dark.]]
When exposed to air, white phosphorus faintly glows green and blue due to [[oxidation]], a phenomenon best visible in the dark. This reaction with oxygen takes place at the surface of the solid (or liquid) phosphorus, forming the short-lived molecules {{chem2|HPO}} and {{chem2|P2O2}} that both emit visible light.{{r|Vanzee1976}} However, in a pure-oxygen environment phosphorus does not glow at all, with the oxidation happening only in a range of [[partial pressure]]s.{{r|Ölander1956}} Derived from this phenomenon, the terms ''[[phosphor]]s'' and ''[[phosphorescence]]'' have been loosely used to describe substances that shine in the dark. However, phosphorus itself is not phosphorescent but [[chemiluminescent]], since it glows due to a chemical reaction and not the progressive reemission of previously absorbed light.{{r|Sommers2007}}

Red phosphorus is [[polymer]]ic in structure. It can be viewed as a derivative of {{chem2|P4}} wherein one P-P bond is broken and one additional bond is formed with the neighbouring tetrahedron, resulting in chains of {{chem2|P21}} molecules linked by [[van der Waals force]]s.{{r|Shen2016}} Red phosphorus may be formed by heating white phosphorus to {{convert|250|C|K}} in the absence of air or by exposing it to sunlight.{{r|Mellor1939|p=717}} In this form phosphorus is [[amorphous]], but can be crystallised upon further heating into violet phosphorus or fibrous red phosphorus depending on the reaction conditions. Red phosphorus is therefore not an allotrope in the strictest sense of the term, but rather an intermediate between other crystalline allotropes of phosphorus, and consequently most of its properties have a range of values. Freshly prepared, bright red phosphorus is highly reactive and ignites at about {{convert|300|C|K}}.{{r|Wiberg2001}} After prolonged heating or storage, the color darkens; the resulting product is more stable and does not spontaneously ignite in air.{{r|Hammond2000}}

Violet phosphorus or α-metallic phosphorus can be produced by day-long annealing of red phosphorus above {{convert|550|C|K}}. In 1865, [[Johann Wilhelm Hittorf]] discovered that when phosphorus was recrystallised from molten [[lead]], a red/purple form is obtained. Therefore, this form is sometimes known as "Hittorf's phosphorus" .{{r|Berger1996}}

Black phosphorus or β-metallic phosphorus is the least reactive allotrope and the thermodynamically stable form below {{convert|550|C|K}}. In appearance, properties, and structure, it resembles [[graphite]], being black and flaky, a conductor of electricity, and having puckered sheets of linked atoms.{{r|Engel2003|Brown1965|Cartz1979}} It is obtained by heating white phosphorus under high pressures (about {{convert|12000|atm|GPa|disp=or}}). It can also be produced at ambient conditions using metal salts, e.g. mercury, as catalysts.{{r|Lange2007}} Single-layer black phosphorus is called [[phosphorene]], and is therefore predictably analogous to [[graphene]].