Phosphorus: Allotropic Forms

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.

About White Phosphorous 

White phosphorus, the least stable allotrope of phosphorus,  is also known as yellow phosphorous or tetraphosphorus (P4). 

Methods of Preparation

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

Structure  

1. It is made up of four atoms and is tetrahedral in structure.

2. The bond angle is 600.

3. The bond length is 225pm.

4. The molecule has six P-P bonds.

5. It has a body-centred cubic (BCC) structure.

Properties 

1. It exists as a translucent, waxy solid.

2. It turns yellow when exposed to light.

3. In the dark, it glows greenish. This property is called chemiluminescence.

4. It is very poisonous and toxic.

5. It sinks in water as its density is more.

6. On burning, it smells like garlic.

7. It is highly flammable and reactive.

8. It does not dissolve in water which is a polar solvent.

9. It acts as a reducing agent. 

10. It is soluble in carbon disulfide and benzene.

11. Its boiling point is 440C

Some Important Reactions of White Phosphorous

1. 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

2. 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

Uses of White Phosphorous

1. It is used to manufacture fertilisers and cleaning compounds.

2. It is used in ammunition. 

3. It is also used as a smoke agent in the military. 

All about Red Phosphorous

Methods of preparation

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.

Structure

Red phosphorus exists as a polymeric chain of P4 tetrahedral molecules. Further heating crystallises it. 

Properties 

1. It possesses an iron-grey lustre.

2. It is odourless.

3. It is less reactive and more stable than white phosphorus.

4. It has a boiling point of 860K

5. It ignites only above 240 °C (or 464 °F).

6. It is insoluble in water and carbon disulfide.

7. It forms sulphides in reaction with sulphur.

8. It does not glow in the dark, i.e. and it doesn’t show chemiluminescence.

Uses of Red Phosphorus

1. It is a very effective flame retardant. It prevents or slows further ignition.

2. It is used as a catalyst in the formation of hydrogen from water. 

3. It is helpful for manufacturing semiconductors.

4. It is used to make matchsticks.

5. It is a vital component of steel production. 

All about Violet phosphorus

Violet phosphorus is also known as monoclinic phosphorus or Hirtoff’s phosphorus.

Methods of Preparation

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. 

Properties

1. It is a polymer. It breaks down into P2 molecules upon heating which dimerises to give P4.

2. It does not dissolve in any solvent.

3. It ignites at temperatures above 300 °C (573K)

4. It does not react with alkali.

5. Nitric acid oxidises violet phosphorus to phosphoric acid.

About Black Phosphorus

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.

Methods of Preparation

α-black phosphorus

It is produced by heating red phosphorus in a sealed tube at 803K (530 °C).

β-black phosphorus

It is formed by heating white phosphorus at 473K (200 °C). High pressure is applied. 

Structure

Properties

α-black phosphorus

1. It is opaque.

2. Its crystals have a rhombohedral or monoclinic structure.

3. It does not oxidise in air.

4. It does not conduct electricity.

β-black Phosphorus

1. It does not ignite in the air below 673K.

2. The crystals have an orthorhombic structure.

3. It conducts electricity. 

Conclusion

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.