Allotropy in Chemistry with Definition and Examples

Allotropes are the different forms of the same element. These different forms may differ in the arrangement. Let’s discuss the allotropy meaning. When the chemical element exists in more than one form, this statement defines allotropy process. Another allotropy definition can be written as “allotropy is the capacity of the element to exist in more than one form. Each form of the element has different physical properties. But these allotropic forms of elements are identical in their chemical properties. These different forms are called allotropes. Some of the famous elements that exhibit allotropy are Carbon, Phosphorous, and sulphur. Allotropy meaning in Urdu can be written as: 

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Elements Showing Allotropy in chemistry are:

1. Carbon

2. Phosphorus

3. Sulphur 

Let’s Define Allotropy of Carbon

Carbon occur in various allotropic forms, some of them are:

1. Crystalline Form- Examples: diamond, graphite, and fullerene.

2. Amorphous Form- This form is also known as the micro-crystalline form. Examples: coal, lampblack, and charcoal.

Diamond- diamond is a colourless, transparent carbon solid form. It shines in the dark. Diamond does not conduct electricity. When a diamond is heated strongly, it produces carbon dioxide only. This proves that the diamond is made up of only carbon atoms. It is a very expensive compound. In a diamond, each carbon atom is bonded with the other four carbon atoms by the covalent bond. These covalent bonds are very strong in nature. Each carbon atom is present on the vertices of the regular tetrahedron. The structure of the diamond is a very strong and rigid structure. It is used for making glass cutters and strong drills. 

Graphite- Graphite is greyish black in colour. Graphite is opaque in nature. It is soft and slippery to touch. Graphite is a good conductor of electricity. 

Fullerenes- it is an allotrope of carbon. It exists in different shapes like a hollow sphere and ellipsoid. 

Spherical fullerenes or buckminsterfullerene or buckyball. In this allotrope carbon atoms are arranged in a football shape. This molecule contains 60 atoms of carbon. Therefore, its chemical formula is C60. two types of carbon ring exist in it. Five carbon rings and six-carbon rings.

Let’s Define Allotropy of Phosphorus

Allotropy of Phosphorus- Phosphorus exists in many allotropic forms. Of these, three main allotropic forms are:

1. White phosphorus

2. Red phosphorus

3. Black phosphorus.

1. White Phosphorus- It is a common variety of phosphorus and is obtained from phosphorite rock with coke and sand in an electric furnace at 1775 K. It consists of a P4 unit. In this case, the four phosphorus atoms lie at the corners of a rectangular tetrahedron with an angle PPP = 60 degrees. Each phosphorus atom is bonded with each of the other three P- atoms by the non-metallic covalent bonds so that each P-atom completes its valence shell. 

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2. Red Phosphorus- It is obtained by heating white phosphorus in an inert atmosphere at 573 K for several days. Like white phosphorus, red phosphorus also exists as P4 tetrahedra but has a polymeric structure consisting of P4 tetrahedra linked together. In the above-shown structure, P4 molecules are interconnected by the non-metallic covalent bonds. 

3. Black Phosphorus- Black phosphorus occurs in two forms. These forms of phosphorus are alpha black phosphorus and beta- black phosphorus. Alpha-black phosphorus can be formed by simple heating of the red phosphorus in a sealed tube at a temperature around 803 K. It can be sublimed in air and has opaque monoclinic or rhombohedral crystals. Beta-black phosphorus can be produced by heating the white phosphorus at a temperature of around 473 K and under very high pressure (4000-12000 atm) in an inert or neutral atmosphere. It has a double-layered crystal lattice. Each layer is made up of zig-zag chains with P-P-P bond angles of 99 degrees and a P-P bond distance of 218 pm. Since it is highly polymeric, therefore, it has a high density.

Allotropy of 16th Group

All the elements of the group exhibit allotropy. For example, oxygen exists as an oxygen molecule and an ozone molecule. Sulphur exists in a number of allotropic forms of which yellow ortho-rhombic, alpha and beta-monoclinic forms are most important. All these allotropic forms of sulphur are non-metallic. Selenium occurs in eight allotropic forms. From these eight allotropic forms, three forms are red monoclinic forms. These allotropic forms of the selenium contain Se8 rings. The thermodynamically stable form is grey-hexagonal ‘metallic’ selenium which consists of polymeric helical chains. The element exists as common amorphous black selenium. The grey selenium is the only allotrope of selenium that conducts electricity.  Tellurium has only one crystalline form having a chain structure similar to that of grey selenium.

Sulphur defines the term allotropy very well by existing in three different forms, rhombic, monoclinic, and plastic sulphur. 

• Rhombic Sulphur- This form of sulphur is also known as octahedral and alpha sulphur. It exists in rhombic form. But on increasing temperature, it gets converted into monoclinic form. Rhombic sulphur exists at low temperature (around normal room temperature). 

• Monoclinic Sulphur- This form of sulphur exists in equilibrium with rhombic form. It occurs at a high temperature (around 96 degrees celsius). This temperature is known as transition temperature.

Rhombic sulphur form ⇆ monoclinic sulphur form. (reaction takes place at 96 degrees celsius).

• Plastic Sulphur- When we put the boiling sulphur in the cold water, the boiling sulphur starts to get arranged in a zig-zag form. This zig-zag form of sulphur is known as plastic sulphur. After some time this zig-zag sulphur again gets converted into rhombic sulphur.

Difference Between Polymorphism and Allotropy

Polymorphism is the phenomena of the existence of the same substance with different crystalline forms. Such forms or shapes are called polymorphs. This phenomenon is applicable only to crystalline substances. Examples of polymorphism are:

1. Silver Nitrate (AgNO₃)- It exists in two forms; rhombohedral and orthorhombic

2. Potassium Nitrate (KNO₃)- It exists in two crystalline forms; rhombohedral and orthorhombic. 

3. Calcium Carbonate (CaCO₃)- It exists in two crystalline forms; trigonal and orthorhombic.

Properties of Polymorphism:

• They have different shapes due to different arrangements of particles.

• They have different physical properties.

• They have the same chemical properties. As the chemical composition of the compound is the same.

Allotropy is a special case of polymorphism that is limited to only a few elements. If the elements show polymorphism then it is known as allotropy.

Properties of Allotropes:

• Allotropes have different crystalline shapes due to different arrangements of particles.

• Allotropes have different physical properties.

• Allotropes have the same chemical properties due to the same chemical composition.

Did You Know?

• Diamond is the hardest natural substance that occurs in nature.

• Do you know that pressure, light, and temperature are the external forces that trigger the allotropes formation process?

• The highest number of allotropic forms are exhibited by the Carbon.

• Black phosphorus is the most stable form of phosphorus.