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Allotropes and Isomers both represent different forms of a compound, yet they are much different from each other. Allotropes are those forms of an element which differs in structure, for example arrangement of atoms, bonding etc. On the other hand, isomers are those compounds that share the same molecular formula but different structural formulas.
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Key Terms: Allotropes, Isomers, Structural Isomerism, Spatial Isomerism, Tautomerism, Metamerism
What are Allotropes?
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When the physical state of the elements are the same in every form of element whether it be solid, liquid, or gas, they are called allotropes. Herein, the bonding of the atoms is different from each other and the chemical properties of the elements also differ. There are only a few elements belonging to a specific periodic table group that contains allotropes. Diamond is the best example of allotropes of carbon. Oxygen dioxide and three ozone are also good forms of allotropes that are formed out of oxygen. Plastic sulphur, monoclinic sulfur are also examples of allotropes. Similarly, you will find different kinds of allotropes of silicon, boron, carbon, oxygen and even phosphorus.
Allotropes of Carbon
Properties of Allotropes
The important properties of allotropes are given below:
- Allotropes are the different structural forms of the same element.
- Allotropes are formed under the influence of different factors including light, pressure and temperature.
- Stability of allotropes depends upon a number of factors like pressure, temperature etc.
- They are found in different states (solid, liquid or gas) and shapes due to differences in the arrangement of the atoms.
- They share the same chemical but different physical properties.
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What are Isomers?
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Isomers are the different structural forms of a compound. A compound is made up of different elements. Isomerism is a phenomenon in which different structural forms of compounds arise but their molecular formula remains the same i.e. same number of atoms of each element with difference in their structural arrangement. All isomers do not have the same composition. They are different physically and also chemically.
Isomers of Butane
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Types of Isomerism
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The two main types of isomerism are: Structural isomerism and Spatial isomerism.
Classification of Isomerism
Structural Isomerism
In structural isomerism the bonds of the atoms are not the same and are unique from every element. It is also called constitutional isomerism. Structural isomerism is again divided into 6 forms depending on its molecular formula:
- Chain isomerism - Here the molecules are the same but the bondings of the atom in the elements are different.
- Functional isomerism - In this, molecules are the same but the functional groups are different.
- Positional isomerism - The molecule formula is the same but their positions in all the groups are different.
- Metamerism - The molecular formulas are the same but the alkyl groups are different based on their functional groups.
- Tautomerism - The molecular formulas are the same but there is a dynamic equilibrium between the atoms.
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Spatial Isomerism
Spatial isomerism is also called stereoisomerism where the bonds of every atom in every element are the same. The positions of the atom may be different but the bond and the nature of the same do not differ from each other. They are of mainly two types:
- Geometric isomerism - It is also known as cis-trans isomerism. It has different arrangements of carbon atoms. Cis isomers have the same atoms on the same side of the double bond, whereas the trans isomers have the same atoms on opposite sides of the double bond.
- Optical isomerism - Optical isomers differ only in the spatial arrangements of the atoms. Such isomers are non-superimposable mirror images of each other. They are also known as enantiomers.
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Differences Between Isomers and Allotropes
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The main differences between allotropes and isomers are as follows:
| Allotropes | Isomers |
|---|---|
| The atom numbers are not the same. | Number of atoms are the same. |
| They have the same element. | They are made up of different elements. |
| The elements are the same but the chemical formulas are different. | Here, the elements are different but the chemical formulas are the same. |
| Allotropes differ in the bonding and number of atoms. | Isomers have different spatial arrangements of the atoms. |
| Allotropes are found mainly in metallic and non-metallic items. | Isomerism is found in organic and inorganic matters. |
| Examples of Allotropes can be diamonds. | Examples of isomers can be Bromopropane. |
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Things to Remember
- Isomerism was first developed in 1827 while preparing silver cyanate.
- Isomers are compounds having the same molecular but different structural formulas.
- Allotropes are the different forms of the same element.
- Two types of isomerism can be seen in compounds: structural isomerism and spatial isomerism.
- Allotropes have different physical properties due to differences in their structures.
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Sample Questions
Ques. Mention three differences between isomers and allotropes. (3 Marks)
Ans. The main differences are as follows:
| Allotropes | Isomers |
|---|---|
| The atom numbers are not the same. | The atom numbers are the same. |
| They have the same element. | They are made up of different elements. |
| The elements are the same but the chemical formulas are not the same. | Here, the elements are different but the chemical formulas are the same. |
Ques. What are the different kinds of isomerism? Explain. (3 Marks)
Ans. There are mainly two kinds of isomerism:
- Structural - In structural isomerism, the bonds of the atoms are not the same and are unique from every element. It is also called constitutional Isomerism.
- Spatial - Spatial isomerism is also called stereoisomers where the bonds of every atom in every element are the same. The positions of the atom may be different but the bond and the nature of the same do not differ from each other.
Ques. Define the structural modifications of allotropes. (3 Marks)
Ans. Allotropes are the different forms of the same element. There are only a few elements belonging to a specific periodic table group that contains allotropes. Diamond is the best form of allotropes made of carbon. Oxygen dioxide and three ozone are also good forms of allotropes of oxygen. Plastic sulphur, monoclinic sulphur are also allotropes.
Ques. Explain the different kinds of structural isomerism. (5 Marks)
Ans. Structural isomerism is divided into 6 forms depending on its molecular formula:
- Chain isomerism- The molecules are the same but the bondings of the atom in the elements are different.
- Functional isomerism- The molecules are the same but the functional groups are different.
- Positional isomerism- The molecule formula is the same but their positions in all the groups are different.
- Metamerism - The molecular formulas are the same but the alkyl groups are different based on their functional groups.
- Tautomerism - The molecular formulas are the same but there is a dynamic equilibrium between the atoms.
Ques. Give proper examples of carbon Allotropes, boron allotropes, silicon allotropes, and sulphur allotropes. (2 Marks)
Ans. The examples are as follows:
- Carbon - Graphites, Diamonds.
- Boron - Amorphous and crystalline boron.
- Silicon - Crystalline silicon
- Sulphur - Monoclinic sulphur, rhombic sulphur
Ques. Explain geometrical isomerism. (2 Marks)
Ans. It is also known as cis-trans isomerism that has different arrangements of carbon atoms. The cis-isomers contains same atoms on the same side while the trans-isomers contains same atoms on the different sides.
Ques. What are isomers? (2 Marks)
Ans. In Isomers, the molecules or atomic ions are almost similar in nature. It consists of the same elements but different spatial arrangements. All isomers do not have the same composition. Isomers differ in their physical and chemical properties.
Ques. Explain allotropes with periodic tables. (3 Marks)
Ans. Allotropes are found in Group 13, 14, 15, and 16 of the periodic table. They belong to the same element group but of different phases such as either solid, liquid, or gas forms. They cannot change into any other form from their existing ones. Although the elements are the same, the molecules and the molecular formulas are different. For example, oxygen O is the same but the molecular combination is different, for oxygen dioxide O2 and for ozone O3.
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