Allotropes of Phosphorus

Dhristi JEE 2022-24

Allotropes of Phosphorus - Properties, Structure, Comparison, and FAQs

Allotropes are also known as Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Due to this some elements of the periodic table show different physical and chemical properties. Some examples of the elements of the periodic table showing allotropism are carbon and phosphorus. 

Allotropes of Phosphorus 

Phosphorus shows different forms of allotropes. Some of the most important phosphorus allotropes are: 

  • Black Phosphorus

  • White Phosphorus 

  • Red Phosphorus

Black Phosphorus

Black phosphorus is formed by heating red phosphorus at 416 degrees celsius. Red Phosphorus is converted into black phosphorus only while heated in a sealed tube and at the right temperature.  When the white form is heated under high pressure at 473 k then also black phosphorus is formed. 

Physical properties of Black Phosphorus 

Following are some of the physical properties of Black Phosphorus : 

  • Black Phosphorus is black in color. 

  • Melting point of black phosphorus is 416 degree celsius. 

  • The specific gravity of black phosphorus is 2.69.

  • The exact mass of black phosphorus is 30.973762 g/mol.

  • The density of black phosphorus is 2.34 g/cm3

  • The solubility of black phosphorus in water (H2O) is 0.3 g/l. 

  • The molecular weight of Black phosphorus is 30.97. 

  • Black phosphorus exists in both crystalline and amorphous forms. 

Chemical properties of Black Phosphorus

Following are some of the chemical properties of Black Phosphorus : 

  • Black phosphorus is the most stable allotrope of all the allotrope of phosphorus. 

  • Black phosphorus is the least reactive allotrope of all the allotrope of phosphorus

Structure of Black Phosphorus 

The following bullet points summarize the brief structure of black phosphorus : 

  • The structure of black phosphorus is zig-zag lines of P - P bonds. 

  • The structure of Black phosphorus looks like a honeycomb, hence also called a honeycomb structure. 

  • The bond angle of the black phosphorus is 99 degrees. 

  • The Bond length of black phosphorus is 218 Pm.

White Phosphorus

White phosphorus is also known as Tetra Phosphorus and Yellow Phosphorus.  White Phosphorus is not obtained naturally, but it is manufactured from phosphate rocks. 

Physical properties of White Phosphorus 

Following are some of the physical properties of White  Phosphorus: 

  • White phosphorus looks like a waxy solid because it is translucent. 

  • White phosphorus has a garlic-like odor, which means it smells like garlic. 

  • White phosphorus is known as polar compound because it is not soluble in water while it is soluble in carbon dioxide. 

  • White phosphorus molecular weight is 30.97 g/mol.

  • White Phosphorus is highly toxic and corrosive in nature. 

Chemical properties of White Phosphorus

Following are some of the chemical properties of White  Phosphorus: 

  • White phosphorus results in metal phosphide when reacted with a metal. 

  • White phosphorus reacts with the oxygen present in the air and catches fire as a result. Due to this reason, white phosphorus is stored underwater. 

  • When White phosphorus is heated at 573K in an inert atmosphere for several days it results in red phosphorus. 

Structure of Black Phosphorus 

The following bullet points summarize the brief structure of White phosphorus:

  • White Phosphorus contains four atoms of phosphorus all connected in a covalent bond. 

  • The structure of white phosphorus looks like a ring. 

  • The angle of the white phosphorus bond is 60 degrees. 

Red Phosphorous 

Red phosphorus is another allotrope of phosphorus that is obtained by heating the white phosphorus at a high temperature of around 573 K in an inert atmosphere for several days. 

Physical properties of Red Phosphorus 

Following are some of the physical properties of Red Phosphorus: 

  • As the name suggests Red Phosphorus is deep Red in color. 

  • Red phosphorus is odorless.

  • Red Phosphorus is non-toxic.

  • The melting point of red phosphorus is 860 K.

  • The molecular weight of the red phosphorus is 30.97 g/ mol.

  • Red  Phosphorus does not glow in the dark. 

Chemical properties of Red Phosphorus

Following are some of the chemical properties of Red Phosphorus : 

  • Red Phosphorus reacts with the oxygen and results in phosphorus pentoxide but red phosphorus reacts with oxygen only at 565 K. 

  • Red Phosphorus reacts with Sulfur as results in sulphides.

Structure of Red Phosphorus 

The following bullet points summarize the brief structure of Red phosphorus: 

  • The structure of Red Phosphorus is very similar to Black Phosphorus. 

  • Red Phosphorus is polymeric in nature. 

  • Each atom of the P4 molecule is connected by a covalent bond to three other phosphorus atoms in a tetrahedral structure. 

Difference between Black Phosphorus, White Phosphorus and Red Phosphorus 


Sr.No

Basis

Black Phosphorus

White Phosphorus

Red Phosphorus

1.

Colour

Black

yellowish

Red

2.

Stability 

Most stable among all 

Least stable among all



Moderate stable among all

3.

Reactivity

Least Reactive among all

Most Reactive among all

Moderate Reactive among all 

FAQs on Allotropes of Phosphorus

1. What are the Main Allotropes of Phosphorus and Which One is the Most Reactive Form Among Them?

Phosphorus exists in three allotropic forms; these are white phosphorus, red phosphorus and black phosphorus. White phosphorus is the most reactive form of phosphorus. Due to angular strain in its structure, it is less stable. Therefore, more reactive.

2. Why Do We Store White Phosphorus in the Water?

White phosphorus is the most reactive form of phosphorus. It reacts with the oxygen present in the air and catches the fire. Therefore, it is stored in the water to protect it from the air.

3. Is White Phosphorus Stable or Not? Explain with Reason.

No, white phosphorus is not stable. It exists as a P4 molecule in a tetrahedral ring form. In this ring structure, the bond angle is 60°  so the distance between the bonds is very less, due to which the bond-bond repulsion takes place. This repulsion leads to the angular strain in the molecule. Therefore, the molecule becomes highly unstable. Due to this high instability, the molecule becomes very reactive.

4. Why white Phosphorus is stored underwater? 

White phosphorus reacts with the oxygen present in the air and catches fire as a result.  Due to this reason, white phosphorus is stored under water. 

5.  Which is the most reactive allotrope of Phosphorus ?

White Phosphorus is the most reactive phos[hpus among all the allotropes of phosphorus. 

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