Phosphorous: allotropic forms, compounds of phosphorus

p-Block Elements: Group 15

Phosphorous: allotropic forms, compounds of phosphorus

Phosphorous is used extensively in pyrotechnics, steel making and alloy making while phosphoric acid makes an essential part of fertilizers. Phosphorous is also used in electrode-less plating.

Phosphorous is naturally found in three allotropes:

White Phosphorus = P₄ (tetrahedral) – very reactive
Black Phosphorus = crystalline structure – much less reactive
Red Phosphorus = amorphous with P₄ chains

White P is stored underwater to protect from the air because it reacts vigorously with oxygen so it must be stored under water. Red and black P for a stable chain like structure in air, burn on heating. P is soluble in organic solvents.

White phosphorus reacts with sodium hydroxide and water to produce its salt, sodium hypophosphite.

P₄ +3NaOH + 3H₂O PH₃ + 3NaH₂PO₂

Red phosphorus is produced by heating white phosphorus at 573K in an inert atmosphere. This process may take several days to let the transformation to occur and create a crystalline structure with more metallic bonding. It has a chain-like structure. Black phosphorous has two forms alpha-black phosphorous and beta-black phosphorous. Alpha black is formed when red phosphorus is heated in a sealed tube at 803K. It can be sublimed in air and has an opaque monoclinic or rhombohedral crystal structure. It does not oxidise in air. Beta black us prepared by heating white phosphorus at 473 K under high pressure. It does not burn in the air upto 673K.

White phosphorus	Red phosphorus	Black phosphorus
Discrete tetrahedral P4 molecules	Polymeric structure consisting of chains of P4 units linked together	Exists in two forms – α black P and β black P
Very reactive	Less reactive than white P	Stable compound
Has chemiluminescence, Glows in the dark	No chemiluminescence	No chemiluminescence
Translucent or white waxy solid	Has an iron-grey lustre	Has an opaque monoclinic or rhombohedral crystals
Soluble in CS₂ but insoluble in water	Insoluble in water as well as CS₂	Insoluble in water as well as CS₂
It has low ignition temperature, therefore, kept under water	High ignition temperature	Very high ignition temperature
Fig1: (a) P4 molecule found in white phosphorous (b) crystalline network structure of black Phosphorous (c) chain structure of red phosphorous

Phosphorous hexoxide (P₄O₆)

It has P in its +5 oxidation state. This compound is a powerful drying agent.

Phosphorus pentoxide (P₄O₁₀)

It is produced when white phosphorus reacts with oxygen.

P₄ + 5O₂ P₄O₁₀

It is one of the most useful drying agents for industrial purposes. Reacts with water to form phosphoric acid. It has P in its +3 oxidation state.

Fig 2: Structures of oxides of phosphorous

The phosphate ion, (PO₄)^3-, is a tetrahedron ion. It is the conjugate base of phosphoric acid and it is a weak oxidizing agent. This ion forms an essential component of nucleotides. It also forms a bright yellow precipitate with ammonium molybdate:

(PO₄)^3-+3NH₄⁺+12MoO₄²⁻+24H⁺(NH₄)₃PO₄⋅12MoO₃+12H₂O

Fig 3: The phosphate ion