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Group 15: Phosphorus - Allotropes

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Group 15: Phosphorus - Allotropes - Lesson Summary

The phenomenon of the existence of an element in more than one physical form is called allotropy. The different physical forms of the same element are called allotropes.
Allotropes have different physical properties, but similar chemical properties.
Phosphorus exists in several allotropic forms.  Of these, the three main forms are white phosphorus, red phosphorus and black phosphorus.

White phosphorus:
It is a soft, waxy and translucent solid.
It is insoluble in water, but soluble in carbon tetrachloride or carbon disulphide.
It dissolves in boiling caustic soda in an inert atmosphere to give phosphine and sodium hypophosphite.

P4            +     3NaOH  + 3H2O   →  PH3     +   3NaH2PO2
White              Caustic                    Phosphine       Sodium
phosphorus        soda                                             hydrophosphite
It is highly toxic and extremely reactive.
It exists as tetrahedral P 4 molecules.

Each phosphorus atom is covalently bonded to three other atoms of phosphorus.
These molecules   are held together by weak Van der Waal’s forces of attraction. It has a very low melting point of 440c
The bond angle in a P 4 molecule is   600, which is much less than the expected or theoretical bond angle.
This angular strain in the molecule makes white phosphorus   unstable and, therefore, highly reactive.
White phosphorus ignites spontaneously in air at about 350c, that is, at a temperature slightly higher than room temperature, to form phosphorus pentoxide. Hence it is stored in water.
P4 + 5O2 → 2P2O5 or P4O10
                       Phosphorus pentoxide
In contact with humid air, white phosphorus gets oxidised. This reaction is associated with the emission of light. As a consequence, it glows in the dark.
White phosphorus exhibits chemiluminescence.
Red phosphorus:
When white phosphorus is heated to about 250 degrees Celsius in the presence of sunlight red phosphorus is obtained.
Red phosphorus is   an iron-grey lustrous crystalline solid.
It is odourless, non-toxic and insoluble in water as well as in carbon tetrachloride.  It doesn’t dissolve in boiling caustic soda, but   dissolves in alcoholic potash.
It exists as a polymeric solid.

It is stable under normal conditions and doesn’t ignite in air.
It undergoes combustion only when heated to about 4000c.
Red phosphorus doesn’t exhibit chemiluminescence.
Black phosphorus:
It is the most stable allotrope of phosphorus.
It is a highly polymerised form of phosphorus. It has a layered   structure. 

It exists in   two forms, namely, alpha black phosphorus and beta black phosphorus.



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