Symbol: P
Atomic number: 15
Molecular formula: P4
Atomic mass: 31 amu
Electronic configuration: 1s2 2s2 2p6 3s2 3p3
Electronegativity: 2.05

Allotropes of phosphorus:

There are many allotropic forms of phosphorous. Among them, white or yellow and red phosphorus are most important.

1. White or yellow phosphorus: This is generally obtained by either electro-thermal process or retort process.

2. Red phosphorous: It is obtained by heating white phosphorous at 240-250°C in presence of an inert atmosphere and a little iodine.

preparation of red phosphorous

Since the reaction is exothermic, red phosphorus is more stable and less reactive than white phosphorus. This is because the exothermic compounds are formed by releasing heat energy from the reactants. So, the change in enthalpy is -ve for exothermic compounds which means it has less energy. Lesser the energy, more the stability of the compounds. So, red phosphorus is more stable than white phosphorus.

3. Black phosphorus

4. Violet phosphorous

5. Scarlet phosphorus

Laboratory preparation of phosphine gas:


When white phosphorous is heated with caustic soda, phosphine gas is formed.

P4(white) + 3NaOH + 3H2O → PH3(phosphine) + 3NaH2PO2(Sodium hypophosphite)

Physical properties:
  1. It is a colorless gas with a rotten fish smell.
  2. It is heavier than air.
  3. It is insoluble in water.
  4. It is highly poisonous.
Chemical properties:

1. Action of air: Phosphine is a non-supporter of combustion, but when phosphine is heated with oxygen, phosphorous pentoxide or phosphoric acid is formed.

2PH3 + 4O2 → P2O5 + 3H2O
PH3 + 2O2 → H3PO3

2. Basic nature: Phosphine is less basic than ammonia because its lone pair of electrons is less available for donation.

PH3 + HCl → PH4Cl(Phosphonium chloride)
PH3 + H2O ⇌ PH4+(Phosphonium ion) + OH

3. Action with Cl2:

PH3 + 3Cl2 → PCl3 + 3HCl
PH3 + 4Cl2 → PCl5 + 3HCl

4. Phosphine as a reducing agent:

  • With CuSO4
    3CuSO4 + 2PH3 → Cu3P2(black ppt.) + 3H2SO4
  • With HgCl2:
    3HgCl2 + 2PH3 → Hg3P2(red ppt.) + 6HCl
  • As reducing agents
  • To prepare PCl3, PCl5,P2O5,etc.
Some Important Questions
  1. Write any two allotropes of phosphorous.
  2. How phosphine gas is prepared in the lab? Write its action with CuSO4 and HgCl2.

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