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Difference Between Isomers and Allotropes for JEE Main 2024

Last updated date: 17th May 2024
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What are Isomers?

In this article, students will be able to learn all the major points about the difference between allotropes and isomers. First, we need to learn about the answer to the question of what is allotropes and isomers.

In Chemistry, isomers can be defined as the molecules or polyatomic ions that have identical molecular formulas. This means that the same number of atoms exists in each element. However, the arrangement of atoms in space is different. Hence, in other words, it can be said that isomerism is the possibility of the existence of different isomers.


One might also be interested in finding out that isomers do not usually share similar physical or chemical properties. Further, the two main forms of isomerism are spatial isomerism and structural or constitutional isomerism.


Structural or constitutional isomerism can be defined as a form of isomerism in which the bonds between all the atoms are different. On the other hand, spatial or stereoisomers are the type of isomerism in which the bonds are the same, but the atoms differ in terms of their relative positions to one another.


When it comes to the topic of the difference between isomers isotopes and allotropes, it is important for students to remember that isomeric relationships constitute a hierarchy. This means that two chemical compounds might have the same constitutional isomer. However, if a more thorough analysis is carried out, then they would appear to be stereoisomers of each other.


Further, if there are two molecules that are similar stereoisomers of each other, then they might also be indifferent conformational forms. In other words, those molecules are different isotopologues. In this case, the depth of analysis is directly dependent on the field of study or the physical and chemical properties of the molecules.


Another interesting point to note is that the English word ‘isomer’ is a back-formation of the word ‘isomeric.’ This word is borrowed from the German word ‘isomerisch’ and from the Swedish word ‘isomerisk.’ Further, these words were coined from the Greek word ‘isomeros,’ and this word has its roots in the words ‘isos,’ which means equal and ‘meros’ that means part.


Examples of Isomers

To get a clearer picture of what isomers are, it is important to have some visual aid. This is why, in this section, we will look at some examples of isomers.

1. Butane and Isobutane

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2. Propanal and Propanone

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What are Allotropes?

According to experts, allotropes can be defined as the structural modifications of an element. This means that these compounds are different forms of the same element. This phenomenon can happen because of various different bonding of the atoms that are arranged in a particular sequence. This results in a new structure.

It is important for readers to remember that all these structures possess various chemical and physical properties. Some good examples of allotropes include diamond and graphite. Further, in general, allotropes occur in different elements belonging to Group 13, 14, 15, and 16 of the Periodic Table.

It should also be noted that allotropism of elements only tends to occur within the same phase, including solid, liquid, or gas forms. This means that these allotropic elements cannot interchange or transfer to another phase or state.

There are also some elements for which the allotropes have other dissimilar molecular formulas. Some examples of this topic include allotropes of oxygen molecules like dioxide (O2) and ozone (O3). Both of these molecules exist in all three states, including solid, liquid, and gas.

These are all the major points that students should know about the topic of allotropes. In the next section, the focus will be on learning the difference between isomers isotopes and allotropes.

Examples of Allotropes

For better understanding, let’s look at some examples of allotropes. These examples are mentioned below.

1. Diamond and Graphite

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2. Diatomic Oxygen and Ozone

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What is the Difference Between Isomers and Allotropes?

Now, before discussing the difference between isomers and allotropes, let’s do a recap. Till now, we have learned the answer to the question of what is allotropes and isomers. We have also discussed the examples of allotropes and isomers.

Now, it’s time to take this discussion about allotropes and isomers further. In this section, we will focus on the difference between allotropes and isomers. For ease of understanding, the difference between isomers isotopes and allotropes are illustrated in the table that is mentioned below.





Isomers can be explained as the type of chemical compounds that have a similar molecular formula. But these compounds have the different structural formula

Allotropes can be defined as the different types of compounds that are made out of the same element. However, these compounds have different chemical formulas and different arrangements


2-bromopropane and 1-bromopropane

Diamond and graphite

Fun Facts about Allotropes and Isomers

Did you know that isomerism as a phenomenon was first observed in 1827? According to sources, this was done when Friedrich Wohler prepared silver cyanate and found out that even though this elemental composition has a molecular formula of AgCNO, it still was identical to silver fulminate that was prepared by Justus von Liebig in 1826. However, the properties of both compounds were different.

This finding was monumental as it challenged the chemical understanding of that time, which stated that chemical compounds could only be distinct when their elemental compositions are different. In the following years, other examples were also found. For example, when Wohler discovered in 1828 that urea has the same atomic composition of CH₄N₂O as ammonium cyanate.

Eventually, Jons Jacob Berzelius introduced the term ‘isomerism’ to describe this phenomenon in 1830. Another important in history is when Louis Pasteur discovered that tartaric acid crystals are available in two kinds of shapes. Both of these shapes were mirror images of one another. This discovery was made in 1848.

FAQs on Difference Between Isomers and Allotropes for JEE Main 2024

1. What are the Different Types of Structural Isomerism?

Answer: There are different types of structural isomerism. And those different types of structural isomerism are mentioned below.

  • Chain Isomerism: This type of isomers have the same molecular formula, but the difference exists in terms of the bonding arrangements of carbon atoms. For example, 2-methylpropane and butane

  • Positional Isomerism: These isomers have the same carbon skeleton or molecular formula. However, the point of difference is in terms of the position of the functional group. For example, isomers of butane, including But-2-ene and But-1-ene

  • Functional Isomerism: These isomers have similar molecular formulas. However, they belong to different functional groups. For example, dimethyl ether and ethanol

  • Metamerism Isomerism: This type of isomers have similar molecular formulas, but the difference is in terms of the nature of alkyl groups that are attached to the same functional group. For example, methyl propyl ether and diethyl ether

  • Tautomerism Isomerism: These types of isomers have similar molecular formulas and also exist in dynamic equilibrium with one another. For example, nitro-ethane and isonitroethane

2. Mention Five Common Examples of Allotropes.

Answer: Five main examples of allotropes are mentioned below.

  • Graphites, fullerenes, and diamonds are allotropes of carbon

  • Crystalline boron and amorphous boron are allotropes of boron

  • Crystalline silicon and amorphous silicon are allotropes of silicon

  • Rhombic sulphur, monoclinic sulphur, and plastic or amorphous sulphur are allotropes of sulphur

  • Red phosphorus, white phosphorus, black phosphorus, and violet phosphorus are allotropes of phosphorus

3. What is the Meaning of Stereoisomerism, and What are the Types of Stereoisomerism?

Answer: Stereoisomer is a type of isomerism in which the compounds have the same molecular and structural formula, but the difference lies in the fact that the spatial arrangement of the atoms or groups in space is different.

There are two types of stereoisomers. Those two types of stereoisomers are mentioned below.

  1. Cis - Trans or Geometrical Isomerism: This type of isomers have the same molecular formula, but they differ in terms of their spatial arrangement of groups of atoms around the carbon atoms (C=C). This type of stereoisomer can further be divided into cis isomers and trans-isomers. Cis isomers have the same group on the same side, and trans isomers have the same groups on opposite sides

  2. Optical Isomerism: This type of isomers have a similar molecular formula but are also non-superimposable mirror images of one another. This type of isomers is also known as enantiomers. For example, L-alaline and D-alanine