Difference Between Isomorphism and Polymorphism

Key Difference – Isomorphism vs Polymorphism
 

Compounds can exist in different forms in nature. These different forms can be different morphologies or different structures. The structure of a chemical compound determines the physical properties of that compound. Sometimes the chemical properties are also determined by the structure. “Morphism” is a term used to name the term “morphology”. It describes the external appearance of a compound. The terms isomorphism and polymorphism are used to describe these external appearances. Polymorphism means the existence of a compound in more than one crystalline form. Isomorphism is the existence of two or more compounds having the same morphology. The key difference between isomorphism and polymorphism is that isomorphism refers to the presence of two or more compounds with identical morphologies whereas polymorphism refers to the presence of different morphologies of the same substance.

CONTENTS

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

What is Isomorphism?

Isomorphism refers to the presence of two or more compounds with identical morphologies. This means the existence of the same crystal structure in different compounds. These compounds are known as isomorphous substances. Isomorphous substances have nearly the same shape and structure.

These compounds have the same ratio between atoms present in those compounds. This indicates that the empirical formulas of isomorphous substances are identical. But the physical properties of the isomorphous substances are different from each other because they have different atomic combinations.  Ex: mass, density, chemical reactivity are some of the physical properties that differ in isomorphous substances. Let us consider some examples in order to understand what isomorphism in chemistry is.

• Calcium carbonate (CaCO₃) and sodium nitrate (NaNO₃).

Figure 01: The Comparison of Calcium Carbonate and Sodium Nitrate as Isomorphous Substances

Both calcium carbonate and sodium nitrate are trigonal in shape. The atomic ratio of the atoms present in each compound is 1:1:3. But these compounds have different physical and chemical properties. Their molar masses are also different from each other (Calcium carbonate = 100g/mol and sodium nitrate = 85 g/mol).

• Sodium phosphate (Na₃PO₄) and sodium arsenate (Na₃AsO₄).

These compounds exist in tetrahedral shape, and the empirical formula of these compounds have the atomic ratio 3:1:4. But they have different chemical and physical properties.

What is Polymorphism?

Polymorphism refers to the presence of different morphologies of the same substance. The substance that shows polymorphism is known as polymorphic substances. Here, a particular compound exists in different shapes and crystalline forms.

Polymorphic substance show similarities and differences based on the structure. But most of the time, the chemical properties are the same since it is the same compound that exists in different forms. But the physical properties are different. , CaCO3 (calcium carbonate) exist in either orthorhombic form or in hexagonal form.

Figure 02: Allotropes of Carbon

Allotropy is a related term to the polymorphism. Allotropy refers to the polymorphism of a particular element. The compounds showing allotropy are known as allotropes. Allotropes occur when an element forms a crystalline structure with different arrangements. For example carbon form allotropes such as diamond or graphite. These allotropes have different chemical and physical properties.

What is Difference Between Isomorphism and Polymorphism?

Summary – Isomorphism vs Polymorphism

Isomorphism and polymorphism express two opposite ideas. The difference between isomorphism and polymorphism is that isomorphism refers to the presence of two or more compounds with identical morphologies whereas polymorphism refers to the presence of different morphologies of the same substance.

Reference:

1.“Crystallization and Polymorphism.” American Chemical Society. Available here 
2.“Isomorphism (Crystallography).” Wikipedia, Wikimedia Foundation, 17 Feb. 2018. Available here
3.“Allotropy.” Wikipedia, Wikimedia Foundation, 3 Mar. 2018. Available here  

Image Courtesy:

1.’CaCO3-calcite-structure’By Tem5psu – Own work, (CC BY-SA 4.0) via Commons Wikimedia   (Modified)
2.’Sodium-nitrate-unit-cell-3D-balls’By Ben Mills – Own work, (Public Domain) via Commons Wikimedia   (Modified)
3.’Allotropes of Carbon’By Mstroeck (talk) (Uploads) – Own work, (CC BY-SA 3.0) via Commons Wikimedia  

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