Phosphorus, atomic number 15, is a p-block element of the periodic table. It is a member of group 15. It is generally not found in a pure state in nature. It can be obtained from phosphate rocks. Several allotropic forms of phosphorus are known – white phosphorous, red phosphorous, violet, and black phosphorus. Red and white phosphorus are the most common examples of allotropes of phosphorus. All the allotropic forms of phosphorous show various properties. Let us study them in detail.
White phosphorus, the least stable allotrope of phosphorus, is also known as yellow phosphorous or tetraphosphorus (P4).
Industrial method – A phosphorite (or phosphate) rock is heated in a furnace. Carbon and silicon are used as catalysts. Vapours of phosphorus are liberated. They can be collected under phosphoric acid. The following reaction shows how white phosphorus is obtained from calcium phosphate.
2 Ca3 (PO4)2 + 6 SiO2 + 10C → 6 CaSiO3 + 10 CO + P4
It is made up of four atoms and is tetrahedral in structure.
The bond angle is 600.
The bond length is 225pm.
The molecule has six P-P bonds.
It has a body-centred cubic (BCC) structure.
It exists as a translucent, waxy solid.
It turns yellow when exposed to light.
In the dark, it glows greenish. This property is called chemiluminescence.
It is very poisonous and toxic.
It sinks in water as its density is more.
On burning, it smells like garlic.
It is highly flammable and reactive.
It does not dissolve in water which is a polar solvent.
It acts as a reducing agent.
It is soluble in carbon disulfide and benzene.
Its boiling point is 440C
In an inert atmosphere, white phosphorus dissolves in a sodium hydroxide (NaOH) to give phosphine (PH3) and sodium hypophosphite (NaH2PO2).
P4 + 3 NaOH + 3 H2O → PH3 + 3 NaH2PO2
Because of the angular strain in the tetraphosphorus molecule (bond angle 600), it readily catches fire in the presence of air. It emits white, dense fumes of phosphorous pentoxide (P4O10).
P4 + 5O2 → P4O10
It is used to manufacture fertilisers and cleaning compounds.
It is used in ammunition.
It is also used as a smoke agent in the military.
Red phosphorus is manufactured by heating white phosphorus at 573K (300°C) in an inert atmosphere. It is also exposed to sunlight for several days. Iodine (I2) is used as a catalyst.
Red phosphorus exists as a polymeric chain of P4 tetrahedral molecules. Further heating crystallises it.
It possesses an iron-grey lustre.
It is odourless.
It is less reactive and more stable than white phosphorus.
It has a boiling point of 860K
It ignites only above 240 °C (or 464 °F).
It is insoluble in water and carbon disulfide.
It forms sulphides in reaction with sulphur.
It does not glow in the dark, i.e. and it doesn’t show chemiluminescence.
It is a very effective flame retardant. It prevents or slows further ignition.
It is used as a catalyst in the formation of hydrogen from water.
It is helpful for manufacturing semiconductors.
It is used to make matchsticks.
It is a vital component of steel production.
Violet phosphorus is also known as monoclinic phosphorus or Hirtoff’s phosphorus.
It can be manufactured by dissolving white or yellow phosphorus in the molten lead for about 18 hours. The temperature taken here is 500-550 °C. When cooled down slowly, violet phosphorus crystallises.
It is a polymer. It breaks down into P2 molecules upon heating which dimerises to give P4.
It does not dissolve in any solvent.
It ignites at temperatures above 300 °C (573K)
It does not react with alkali.
Nitric acid oxidises violet phosphorus to phosphoric acid.
It is the most stable allotrope of phosphorus. It has two forms – α-black phosphorus and β-black phosphorus.
Both of them are thermodynamically stable and resemble graphite.
It is produced by heating red phosphorus in a sealed tube at 803K (530 °C).
It is formed by heating white phosphorus at 473K (200 °C). High pressure is applied.
Structure
It is opaque.
Its crystals have a rhombohedral or monoclinic structure.
It does not oxidise in air.
It does not conduct electricity.
It does not ignite in the air below 673K.
The crystals have an orthorhombic structure.
It conducts electricity.
Phosphorus forms numerous allotropes, like white phosphorus (the most reactive) and black phosphorus (the most stable). Black phosphorus exists in two forms – alpha and beta black phosphorus. Red and violet phosphorus are also the types of allotropic forms of phosphorus. Phosphorus is also essential for biochemical processes. It is found in the bones of human beings. The sugar-phosphate chain in the DNA and RNA are made of phosphates.
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