Difference Between Polymorphism and Allotropy

The key difference between polymorphism and allotropy is that the polymorphism occurs in chemical compounds whereas the allotropy occurs in chemical elements.

Polymorphism is the presence of several different forms of the same solid material. It means that the compounds of this type may have more than one crystal structure. Allotropy, on the other hand, is a similar chemical concept, but it describes the presence of several different forms of the same chemical element.

1. Overview and Key Difference
2. What is Polymorphism
3. What is Allotropy
4. Side by Side Comparison – Polymorphism vs Allotropy in Tabular Form
5. Summary

What is Polymorphism?

Polymorphism is the ability of a solid material to exist in more than one form or crystal structure. We can find this characteristic in any crystalline material such as polymers, mineral, metal, etc. There are several forms of polymorphism as follows:

Packing polymorphism – depending on the differences in the crystal packing
Conformational polymorphism – the presence of different conformers of the same molecule
Pseudopolymorphism – the presence of different crystal types as a result of hydration or solvation.

The variation of the conditions during the crystallization process is the main reason that is responsible for the occurrence of the polymorphism in crystalline materials. These variable conditions are as follows:

Polarity of solvent
Presence of impurities
The level of supersaturation at which the material starts to crystalize
Temperature
Changes in stirring conditions

What is Allotropy?

Allotropy is the existence of two or more different physical forms of a chemical element. These forms exist in the same physical state, mostly in the solid state. Therefore, these are different structural modifications of the same chemical element. Allotropes contain atoms of the same chemical element that binds with each other in different ways.

Difference Between Polymorphism and Allotropy

Figure 01: Diamond and Graphite are Allotropes of Carbon

Moreover, these different forms may have different physical properties because they have different structures and chemical behavior may vary as well. One allotrope may convert into another when we change some factors such as pressure, light, temperature, etc. Therefore these physical factors affect the stability of these compounds. Some common examples for allotropes are as follows:

Carbon – diamond, graphite, graphene, fullerenes, etc.
Phosphorous – white phosphorous, red phosphorous, diphosphorous, etc.
Oxygen – dioxygen, ozone, tetraoxygen, etc.
Boron – amorphous boron, alpha rhombohedral boron, etc.
Arsenic – yellow arsenic, grey arsenic, etc.

What is the Difference Between Polymorphism and Allotropy?

Polymorphism is the ability of a solid material to exist in more than one form or crystal structure. It occurs only in chemical compounds. Moreover, it describes the differences in crystal structures of compounds. Allotropy is the existence of two or more different physical forms of a chemical element. It occurs only in chemical elements. In addition to that, it describes the differences in the atomic arrangement of compounds having the atoms of the same chemical element. The infographic below gives the difference between polymorphism and allotropy in a tabular form.

Summary – Polymorphism vs Allotropy

Polymorphism and allotropy are two related terms in inorganic chemistry. The difference between polymorphism and allotropy is that the polymorphism occurs in chemical compounds whereas allotropy occurs in chemical elements.

Reference:

1. “Polymorphism (Materials Science).” Wikipedia, Wikimedia Foundation, 3 July 2018. Available here
2. Britannica, The Editors of Encyclopaedia. “Allotropy.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 21 Apr. 2017. Available here