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Allotropes of Phosphorus

Last Updated : 29 Nov, 2021
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Despite the fact that people have been using the periodic table with phosphorus (P) in it for thousands of years, it was not until 1669 that it was isolated and named by a chemist named Brand. Phosphorus is an element that cannot be found naturally in our environment. It is highly reactive. Phosphorus is a chemical element with an atomic number of 15, which means it has 15 protons and 15 electrons in its atomic structure. Phosphorus has the chemical symbol P.

Uses Of Phosphorus

  • Nowadays, four types of phosphorous are commonly used: white, black, red, and violet.
  • Phosphorus is easily located on the periodic table at position fifteen, just below nitrogen (N).
  • Fireworks, fertilisers, and baking powder all have phosphorus.
  • Phosphorus is also used in the production of steel.
  • Phosphates are also utilised to make fine china and specialized glasses.

Compounds of Phosphorus

Phosphorus Trichloride (PCl3)

Phosphorus trichloride is a colourless oily liquid that is extremely toxic. The compound has a triangular pyramidal shape and is sp3 hybridized with phosphorus.


  • When dry chlorine is passed over hot white phosphorus, phosphorus trichloride is created.

P4 + 6Cl2 → 4PCl3

  • Phosphorus trichloride is formed when thionyl chloride reacts with white phosphorus.

P + 8SOCl2 → 4PCl3 + 4SO2 + 2S2Cl2

Chemical properties

  • In the presence of moisture, phosphorus trichloride hydrolyzes.

PCl3 + 3H2O → H3PO3 + 3HCl

  • It reacts with organic compounds that contain a –OH group to form chloro- derivatives.

3C2H5OH + PCl3 → 3C2H5Cl + H3PO3

Phosphorus Pentachloride (PCl5)

Phosphorus pentachloride is a water-sensitive yellowish-white solid. Carbon tetrachloride, carbon disulfide, benzene, and diethyl ether are all soluble in it. In both gaseous and liquid phases, it has a trigonal bipyramidal structure. It exists in solid form as an ionic solid, [PCl4]+[PCl6], with the cation [PCl4]+ being tetrahedral and the anion [PCl6] being octahedral. Three equatorial P-Cl bonds and two axial P-Cl bonds exist. Because of the greater repulsion at axial positions versus equatorial positions, the two axial bonds are longer than the equatorial bonds.


  • When white phosphorus combines with an excess of dry chlorine, phosphorus pentachloride is formed.

P4 + 10Cl2→4PCl5

  • It can also be made by combining SO2Cl2 and phosphorus.

P4 + 10SO2Cl2→4PCl5+10SO2

Chemical properties

  • Phosphorus pentachloride hydrolyzes to POCl3 in moist air, which then converts to phosphoric acid.

PCl5 + H2O→POCl3 + 2HCl

POCl3 + 3H2O→H3PO4 + 3HCl

  • It sublimes when heated and decomposes further to phosphorus trichloride when heated further.

PCl5→PCl3 + Cl2

  • Under the influence of heat, it reacts with finely divided metals to produce metal chlorides.

2Ag + PCl5→2AgCl + PCl3

  • It reacts with organic compounds that contain a –OH group to form chloro- derivatives.

C2H5OH + PCl5→C2H5Cl + POCl3 + HCl

Uses of phosphorus halides

  • It is used to chlorinate water.
  • It is used as a precursor in the production of phosphorus acid, chloro-anhydrides, and phosphoric acid derivatives.
  • It is used as an intermediate for making organophosphorus pesticides, water treatment agents, lube oil and paint additives etc.

Allotropes of Phosphorus

White Phosphorus 

  • It is white in colour and exists as a waxy solid.
  • It has a translucent appearance, is poisonous, and is insoluble in water.
  • It is soluble in CS2 and exhibits chemiluminescence (glows in dark).
  • It is more reactive than other carbon allotropes. When burned, it emits white P4O10 fumes.
  • It has a tetrahedral structure.

Red Phosphorus 

  • It’s made from white phosphorus. To produce red phosphorus, white phosphorus is heated at 573K in an inert atmosphere for several days.
  • Red phosphorus is odourless, colourless, non-poisonous, and lustrous.
  • It is not soluble in water or CS2.
  • It has a lower reactivity than white phosphorus.
  • It has chains made up of P4 tetrahedra that are linked together.

Black Phosphorus 

  • There are two types of black phosphorus: alpha black phosphorus and beta black phosphorus.
  • The crystal structure of alpha black phosphorus is opaque monoclinic or rhombohedral. It is made by heating red phosphorus in a sealed tube at 803K.
  • Beta black phosphorus has a low reactivity. It is made by heating white phosphorus under high pressure at 473K.

Sample Problems

Problem 1: What is unique about phosphorus?


Phosphorus, with the exception of black phosphorus, is a poor heat and electrical conductor. At room temperature, all forms of phosphorus are stable. The white allotrope (also known as yellow phosphorus) is similar to wax, the red and purple allotropes are non-crystalline solids, and the black allotrope is similar to graphite in pencil lead.

Problem 2: What are the main sources of phosphorus?


Phosphorus is found in high concentrations in protein foods such as milk and milk products, as well as meat alternatives such as beans, lentils, and nuts. Phosphorus is found in grains, particularly whole grains. Phosphorus is found in lower concentrations in vegetables and fruits.

Problem 3: Why is phosphorus vital for us?


Phosphorus’ primary function is the formation of bones and teeth. It is essential for the body’s utilisation of carbohydrates and fats. Protein is also required by the body for cell and tissue growth, maintenance, and repair.

Problem 4: What are the three important allotropes of phosphorus?


The three important allotrope of phosphorus are red, black and white phosphorus

Problem 5: What are the two types of black phosphorus?


The two types of black phosphorus are alpha black phosphorus and beta black phosphorus.

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