Allotropes of Phosphorus

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Some elements in the periodic table show different structural forms and this process is known as allotropism. Due to different structures, they differ in their physical and chemical properties. These structural forms are known as allotropes. Many elements like phosphorus, carbon, sulphur show an allotropism process.


Allotropes of Phosphorus

Phosphorus found in many allotropic forms. The most common forms from all the allotropes of phosphorus are black phosphorus, white phosphorus and red phosphorus. In chapter allotropes of phosphorus, we will learn about the allotropic forms of phosphorus, physical and chemical properties of allotropes of phosphorus.

The three main allotropes of phosphorus are:

  1. White phosphorous

  2. Red phosphorus

  3. Black phosphorus

Let’s discuss the above-given allotropes of phosphorus one by one.

1. White Phosphorus

White phosphorus exists in P4 state. All four phosphorus atoms are connected with a covalent bond in a ring form. In the ring form bond angle is 60 degrees, which causes angular strain in the molecule. Another name of White phosphorus is yellow phosphorus and tetra phosphorus.  


White Phosphorus Structure 

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In white phosphorus structure, there are four phosphorus atoms present. All four phosphorus atoms are covalently bonded with each other and form a closed ring structure in a tetrahedral arrangement. The angle between each bond of phosphorus in this form is 60 degrees. This low bond angle causes the repulsion between the bonds and causes angular strain. Therefore, this is the most reactive allotrope of phosphorus under normal conditions.


White Phosphorus Properties


(i). Physical Properties of White Phosphorus

  • It is a translucent waxy white solid.

  • It has a sharp pungent, garlic-like odour.

  • Its density is 15.2 lb/gal. It means it is denser than water. Hence, sinks in water.

  • It is corrosive in nature.

  • Its molecular weight is 30.97 g/mol.

  • It is highly toxic in nature.

  • It is not soluble in water (polar compound).

  • It is soluble in carbon disulphide (non-polar compound)

  • It glows in the dark, which means it possesses chemiluminescent properties.

(ii). Chemical Properties of White Phosphorus

The chemical properties of phosphorus depend upon its structure. Due to angular strain in its structure, it is less stable. Therefore,  more reactive.

  • It reacts with boiling NaOH solution in an inert atmosphere and forms phosphene as a product and sodium hypophosphite as a byproduct.

P4 + 3NaOH + 3H20 → PH3 + 3NaH2PO2

  • It readily reacts with the oxygen present in the air. Therefore, catches fire. In this reaction, it forms dense white fumes of P4O10.

P4 + 5O2 → P4O10

  • It reacts with metals to form metal phosphide.

12Na + P4 → 4Na3P

P4 + 10H2SO4 → 10 SO2 + 4H3PO4 + 4H2O

2. Red Phosphorus

Red phosphorus is another allotrope of phosphorus. Red phosphorus is obtained by heating the white phosphorus at a high temperature around 573 K. The conversion of white phosphorus to red phosphorus takes place in an inert atmosphere for several days.


Red Phosphorus Structure

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Red phosphorus is polymeric in nature. In red phosphorus structure, P4 tetrahedral molecules are interlinked in a straight chain manner. The structure of red phosphorus is very similar to the P4 molecule. Each phosphorus atom in P4 molecule is connected by a covalent bond to three other phosphorus atoms in a tetrahedral structure. When one of these covalent bonds is broken, these tetrahedral structures can proceed to bond with neighbouring phosphorus atoms, leading to a polymer-like structure.


Red Phosphorus Properties


(i) Physical Properties of Red Phosphorus

  • Red phosphorus is deep red in colour and has iron-grey lustre.

  • It has no odour. Therefore, odourless.

  • It exists in an amorphous state.

  • It is non-toxic.

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

  • The density of red phosphorus is 2.34 g/cm3.

  • The melting point of red phosphorus is 860 K.

(ii) Chemical Properties of Red Phosphorus

Reactivity of white phosphorus is lesser than the white phosphorus.

  • Red phosphorus reacts with the oxygen at 565 K and forms phosphorus pentoxide.

P4 + 5O2 → P4O10

  • Red phosphorus being less reactive than white phosphorus reacts with group 17 elements and forms phosphorus halide.

P4 + 6Cl2 → 4PCl3

  • Red phosphorus reacts with sulphur to produce sulphides.

4P4 + 3S8 → 8P2S3


3. Black Phosphorus

Black phosphorus exists in two forms; alpha-black phosphorus and beta-black phosphorus. These two forms have different formation paths. 

  • Red phosphorus when heated in a sealed tube at 803 K temperature it forms alpha-black phosphorus.

              Red phosphorus → alpha black phosphorus. (heated at 803 K temperature)

  • White phosphorus, when hated at 473 K temperature under 4000- 1200 atmospheric pressure beta-black phosphorus is formed.

  • White phosphorus → beta-black phosphorus (at 473K temperature and 4000-1200 atm. pressure)   

Black Phosphorus Structure

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Structure of black phosphorus consists of the double-layered crystal lattice. The structure of black phosphorus has zig-zag chains between P-P-P bonds. The bond angle of the black phosphorus is 99 degrees. Bond length is 218 Pm.


Black Phosphorus Properties


(i) Physical Properties of Black Phosphorus

  • Black phosphorus is black in colour and poses black metallic lustre.

  • It has a melting point of about 860 K.

  • It exists in both crystalline and amorphous forms.

  • The specific gravity of black phosphorus is 2.69.

(ii) Chemical Properties of Black Phosphorus

Black phosphorus is the least reactive form among all the allotropes of phosphorus and most stable allotrope of phosphorus. Therefore, it does not show any reactivity towards oxygen, halogens, sulphur, alkalies etc. 


Comparison of the Allotropes of Phosphorus

Properties

White Phosphorus

Red Phosphorus

Black Phosphorus

colour

White turns yellow on exposure

Dark red

black

State

Waxy solid

amorphous

Crystalline and amorphous

Density (g/cm³)

1.84

2.1

2.69

Can Store In

water

Open in air

Open in air

Stability

Least stable

Moderately stable

Most stable

Chemical Reactivity

Most reactive

Moderately reactive

Least reactive


Did you know?

  • Phosphorus is highly reactive. Therefore, It does not found free in nature.

  • White phosphorus is very poisonous. Therefore, it is considered lethal.

  • Red phosphorus is found on the sides of the matchboxes.

  • Phosphorus is essential to life. Phosphates form a  major part of DNA, RNA and membranes. 

  • Phosphorus is present in ADP and ATP, these molecules responsible for energy generation in the cell.

  • Phosphorus is the seventh most common element found in the Earth’s crust.

  • White phosphorus glows in the dark, which means it possesses chemiluminescent properties.

FAQ (Frequently Asked Questions)

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

Answer: 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. Question: Why Do We Store White Phosphorus in the Water?

Answer: 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. Question: Is White Phosphorus Stable or Not? Explain with Reason.

Answer: 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.