Allotropes of phosphorus

Elemental phosphorus can exist in several allotropes; the most common of which are white and red. There are also violet and black phosphorus, and gaseous diphosphorus.

White phosphorus

"White phosphorus", or "yellow phosphorus", or simply "tetraphosphorus" (P₄) 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. [Marie-Thérèse Averbuch-Pouchot, A. Durif. "Topics in Phosphate Chemistry". World Scientific, 1996. ISBN 9810226349. p. 3.]

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 pyrophoric (self-igniting) upon contact with air as well as 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 P₄O₁₀ 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.Threlfall, R.E., (1951). "100 years of Phosphorus Making: 1851 - 1951". Oldbury: Albright and Wilson Ltd] 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.

White phosphorus has an appreciable vapour pressure at ordinary temperatures. The vapour density indicates that the vapour is composed of P₄ molecules up to about 800°C. Above that temperature, dissociation into P₂ molecules occurs. The P₄ 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 Relative Molecular Mass in solution corresponds to that expected for P₄ molecules.

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

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 Johann Wilhelm 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:

)). It might include some bromine and has a density of about 1.88 g/cm3.

References

ee also

*Allotropy
*Phosphorus

External links

*http://www3.interscience.wiley.com/journal/114190122/abstract?CRETRY=1&SRETRY=0