Difference Between Allotropes and Isomers

In organic chemistry, these terms have a lot of importance. Although they seem pretty much similar, they have a vast difference in their nature and functionality.


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Some elements are stable at room temperature and can occur in different formulas or different arrangements. It can be made up of a single element or several elements. 

Allotropes and isomers are the best examples of such elements. Isomers have the same molecular formula but a different structural formula.

On the other hand, allotropes are chemical compounds with structural modification of an element. They work on different parameters.

Allotropes and isomers are two different types of elements or compounds. Most of the time, these are stable and naturally found compounds. 

Key Takeaways

  1. Allotropes are different forms of an element with distinct physical and chemical properties.
  2. Isomers are compounds with the same molecular formula but different molecular structures.
  3. Both concepts demonstrate how atoms can arrange themselves to create unique substances.

Allotropes vs Isomers 

Allotropes are composed of different number of atoms, while isomers are composed of the same number of atoms. Allotropes are composed of the same single element, while Isomers are composed of different elements. Allotropes have different structures, while Isomers have similar structures.

Allotropes vs Isomers

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Allotropes are composed of a single element. Many chemical compounds are formed by a single element that can be existed in more than one form.

Due to their composition, they have similar properties. The structural modification of the same element results in different compounds. 

On the other hand, isomers are usually composed of different elements. Moreover, they have a similar molecular formula and the same number of atoms.

But, these compounds have a different structural formula that results in the formation of isomers. And properties may not be the same due to composition.

Comparison Table

Parameters of ComparisonAllotropes Isomers 
Definition Chemical compounds having the same elements but distinct chemical formulae in different arrangements are known as allotropes.Isomers are compounds that include various elements and have the same chemical formula but are arranged differently
Number of atoms It is composed of different numbers of atoms It has the same number of atoms. 
Types of elements It has the same single element It has different elements 
Structure It always has different structures.It may have similar or different structures. 
Presence Usually, present in metals, non-metals, metalloids.It can be seen in organic molecules and inorganic molecules. 
Major types It includes metal allotropes, non-metal allotropes, and metalloids allotropes.Mainly, two forms are present; structural isomers and stereoisomerism. 

What are Allotropes?

Allotropes are chemical elements that can exist in more than one form in the same physical state. These are known as allotropes.

The same element with different structural modifications results in different compounds. Usually, elements are bonded together differently.

Allotropy is only used for elements, not compounds. However, the use is restricted to only solid materials such as crystals. It’s usually referred to as different forms of an element within the same physical state.

The only difference in the physical state will constitute allotropes. Moreover, there’s also a difference in molecular formulae or a difference in the crystalline structure. 

For example, two allotropes of oxygen can exist in solid, liquid, and gaseous states (O2 and O3).

Other elements, such as phosphorus, do not preserve discrete allotropes in various physical states; for example, when phosphorus is melted into a liquid state, all of its solid allotropes revert to the identical P4 form.


What are Isomers?

Chemical compounds that have similar molecular formulae but different structural formulas are known as isomers. Although they have different atomic arrangements, they have the same number of atoms. Usually, they are constituted of different elements.

Isomers have the same number and types of atoms, but these atoms are arranged in different ways. As a result, isomers’ chemical structures differ from one another.

They don’t necessarily share the same physical or chemical properties unless their functional group is similar. 

Two types of isomerism are found; structural or constitutional isomerism, and stereoisomerism or spatial isomerism. Structural isomerism results due to atoms and functional groups being attached in different ways.

On the other hand, stereoisomerism results due to differences in geometrical positioning. 

constitutional isomers

Main Differences Between Allotropes and Isomers

Allotropes and isomers are distinguished by the fact that they are compounds made up of the same single element but with various chemical formulae in different arrangements.

Whereas, isomers are chemical compounds composed of different elements that have a similar molecular formula but a different structural formula. 

  1. Both are chemical compounds, but allotropes are composed of the same single elements, and on the other hand, isomers are composed of different elements. 
  2. Allotropes have different chemical formulas in different arrangements, while isomers have a different structural formulas. 
  3. Isomers have the same number of atoms, whereas allotropes have a different number of atoms. 
  4. The same properties can be seen in allotropes due to similar compositions. Although, isomers don’t show similar properties as it depends on the functionality group. 
  5. Allotropes are generally seen in chemical elements, while isomers are generally found in molecules or polyatomic ions.

Allotropy results are due to many factors, such as pressure, light, and temperature. Isomerism is caused by differences in the mechanisms of atoms connecting as well as differences in the configurations of atoms or groups in space.

Difference Between Allotropes and Isomers
  1. https://pubs.acs.org/doi/pdf/10.1021/ed064p404
  2. https://www.nature.com/articles/nmat2885
  3. https://pubs.acs.org/doi/full/10.1021/cr940246k
  4. https://link.springer.com/article/10.1007/s10698-020-09383-3
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