There are different allotropic forms of phosphorus in nature. The important allotropic forms of phosphorus are white phosphorus, black phosphorus and red phosphorous. In this chapter we will learn about the Phosphorus allotropic forms along with their properties.
White Phosphorus properties:
- Translucent in appearance like white waxy solid
- Poisonous in nature
- Less stable and more reactive
- Does not dissolve in water
- When mixed with carbon disulphide it dissolves
- Glows in the dark(chemiluminescence)
- Further, the white form of it dissolves in NaOH in an inert atmosphere giving PH3. The reaction that takes place is as follows:
- \( P_4 + 3NaOH + 3H_20 \rightarrow PH_3 + 3NaH_2PO_2\)
- Due to the angular strain in P4 molecule (wherein the angle is 60∘), In the presence of air, it readily catches fire and gives out dense white fumes of P4O10. The reaction is as follows: P4+5O2→P4O10
- The below figure shows that it has a discrete tetrahedral P4 molecule:
- On heating white P4 at 573K in an inert atmosphere for several days, red phosphorus is obtained. Further, on heating it under high pressure, a series of phases of black form is formed.
Red phosphorus properties:
- Has iron grey lustre
- Solubility properties: does not dissolve in water and carbon disulphide
- Does not glow in the dark
- Chemically, white phosphorus is more reactive than red phosphorus
- It is polymeric and consists of chains of P4 tetrahedra linked together as shown in the figure given below
Black Phosphorus properties:
α-black and β-black are two forms of black phosphorus. When red phosphorus is heated in a sealed tube at 803K, a α-black form is formed. It can be sublimed in the air and has either rhombohedral crystals or opaque monoclinic. It does not oxidise in the air. When white form is heated under high pressure at 473 K β-Black is formed. It does not burn in the air up to 673K. Structure of black form of this substance is shown below:
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