Phosphorus is also called “the devil’s element” because of its use in making explosives and poison.
It is found in many allotropic forms, the important ones being white, red and black phosphorus.
Let’s study the properties of white phosphorus.
White phosphorus consists of a single P4 molecule with a tetrahedral geometry.
It is a translucent solid, that is white and waxy.
Also, it is poisonous and chemiluminescent. Chemiluminescence is the ability of a substance to glow in the dark.
Elemental white phosphorus gets oxidised when exposed to moist air, giving a glow.
It is insoluble in water as shown, but dissolves in carbon disulphide.
Moreover, it dissolves in boiling NaOH solution to give phosphine.
This reaction is possible in an inert atmosphere of CO2 and is used in the lab preparation of phosphine.
White phosphorus catches fire easily when exposed to air, and gives dense white fumes. These fumes comprise of P4O10, which is a dimer of phosphorus pentoxide.
Next is another allotrope of phosphorus - red phosphorus.
Red phosphorus possesses an iron-grey lustre.
It is odourless, non-poisonous, and insoluble in both water and carbon disulphide.
Also, it is not chemiluminescent like white phosphorus.
It has a polymeric structure consisting of chains of P4 tetrahedra linked together.
Moving on, let’s study the properties of black phosphorus.
Black phosphorus, as the name suggests, is black in colour.
It has two forms: 𝜶-black phosphorus and 𝛃-black phosphorus.
It can be sublimed in air but cannot oxidise in air. Also, it does not burn in air up to 673K.
It has opaque monoclinic or rhombohedral crystals.
Black phosphorus has a layered structure with phosphorus arranged in hexagonal rings.
We’ll now compare the stability and reactivity of white, red, and black phosphorus.
White phosphorus is less stable and more reactive than the red and black allotropes.
It is less stable due to the angular strain in the P4 molecule where the angles are only 60o.
It is possible to prepare one allotrope of phosphorus from another.
When white phosphorus is heated at 573K for several days in an inert atmosphere, red phosphorus is obtained.
With further heating under high pressure, a series of phases of black phosphorus is formed.
𝜶-Black phosphorus is formed when red phosphorus is heated in a sealed tube at 803K.
On the other hand, 𝛃-black phosphorus is obtained when white phosphorus is heated at 473K under high pressure.
Revision
Phosphorus is found in many allotropic forms, important ones being white, red and black phosphorus.
White phosphorus is a translucent, white, waxy solid, consisting of a single P4 molecule with a tetrahedral geometry.
It is poisonous and chemiluminescent. Also, it is insoluble in water but dissolves in carbon disulphide.
It dissolves in boiling NaOH solution to give phosphine.
It catches fire easily when exposed to air and gives dense white fumes.
Red phosphorus possesses an iron-grey lustre.
It is odourless, non-poisonous, and insoluble in both water and carbon disulphide.
It has a polymeric structure consisting of chains of P4 tetrahedra linked together.
Black phosphorus is black in colour and has two forms: 𝜶-black phosphorus and 𝛃-black phosphorus.
It can be sublimed in air but cannot oxidise in air. Also, it does not burn in the air up to 673K.
It has opaque monoclinic or rhombohedral crystals. Its structure of black phosphorus is a polymeric structure with phosphorus arranged in hexagonal rings.
White phosphorus is less stable and more reactive than red and black allotropes.
When white phosphorus is heated at 573K for several days in an inert atmosphere, red phosphorus is obtained.
𝜶-Black phosphorus is formed when red phosphorus is heated in a sealed tube at 803K.
𝛃-black phosphorus is obtained when white phosphorus is heated at 473K under high pressure.