Phosphorus

Phosphorus is a chemical element, with atomic number Z = 15, that is present in all living organisms in the form of organophosphates and as calcium phosphates such as hydroxyapetite (Ca₁₀(PO₄)₆(OH)₂) and fluoroapatite (Ca₁₀(PO₄)₆F₂) found in teeth and bones. Many cell signaling cascades in living organisms operate by a series of phosphorylation events in which a phosphate group (PO₄)^2- is either added to a protein by a kinase or removed from a protein by a phosphorylase. Unlike other elements in group VA, phosphorus is never found as a pure element in nature, but only in combination with other elements. Both red phosphorus and tetraphosphorus trisulfide are used in common matches because they are easily ignited by heat. However, the agricultural industry is the largest user of phosphorus in the form of fertilizers. Phosphorus and arsenic share many chemical properties.

production of elemental phosphorus

Calcium phosphate (phosphate rock), mostly mined in Florida and North Africa, can be heated to 1200-1500 Celcius with sand, which is mostly SiO₂, and coke (impure carbon) to produce vaporized tetraphosphorus, P₄, (mp. 44.2 C) which is subsequently condenced into a white power under water to prevent oxidation. Even under water, white phosphorus is slowly converted to the more stable red phosphorus allotrope (mp. 597C). Both the white and red allotropes of phosphoruus are insoluble in water.

fertilizers

Due to the essential nature of phosphorus to living organisms, the low solubility of natural phosphorus-containing compounds, and the slow natural cycle of phosphorous, the agricultural industry is heavily reliant on fertilizes which contain phosphate, mostly in the form of superphosphate of lime. Superphosphate of lime is a mixture of two phosphate salts, calcium dihyrogen phosphate (Ca(H₂PO₄)₂) and calcium sulfate dihydrate CaSO₄•2H₂O produced by the reaction of sulfuric acid and water with calcium phosphate.

allotropes of phosphorus

Both phosphorus and arsenic have many allotropes, but only two forms predominate. White phosphorus and yellow arsenic both have four atoms arranged in a tetrahedral structure in which each atom is bound to the other three atoms by a single bond. This form of the elements are the least stable, most reactive, more volatile, less dense, and more toxic than the other allotropes. The toxicity of white phosphorus lead to its discontinued use in matches. In red phosphorus, one of the bonds in P4 described above have been broken, and one additional bond is formed with a neighboring tetrahedron.

phosphine, diphosphine and phosphonium salts

Phosphine (PH₃) and arsine (AsH₃) are structural analogs with ammomia (NH₃) and form pyramidal structures with the phosphorus or arsenic atom in the center bound to three hydrogen atoms and one lone electron pair. Both are colorless, ill-smelling, toxic compounds. Phosphine is produced in a manner similar to the production of ammonia. Hydrolysis of calcium phosphide, Ca₃P₂, or calcium nitride, Ca₃N₂ produce phosphine or ammonia, respectively. Unlike ammonia, phosphine in unstable and it reacts instantly with air giving off phosphoric acid clouds. Arsenic is even less stable. Although phosphine is less basic than ammonia, it can form a few some phosphonium salts (PH₄I), analogs of ammonium salts, but these salts immediately decompose in water and do not yield PH₄⁺ ions. Diphosphine (P₂H₄ or H₂P-PH₂) is an analog of hydrazine (N2H4) that is a colorless liquid which spontaneously ignites in air and can disproportionate into phosphine and complex hydrides.

phosphorus halides

The trihalides PF₃, PCl₃, PBr₃ and PI₃ and the pentahalides PF₅, PCl₅ and PBr₅ are all known and mixed halides can also be formed. The trihalides can be formed simply by mixing the appropriate stoichiometric amounts of phosphorus and a halide. For safety reasons, however, PF3 is typically made by reacting PCl₃ with AsF₅ and fractional distillation because the direct reaction of phosphorus with fluorine can be explosive.

The pentahalides, PX₅, are synthesized by reacting excess halide with either elemental phosphorus or with the corresponding trihalide. Mixed phosphorus halides are unstable and decompose to form simple halides. Thus 5PF₃Br₂ decomposes into 3PF₅ and 2PBr₅.