Polymorphism and Isomorphism: Definition and 5 Reliable Differences

The term polymorphism and isomorphism are related to the external appearance of the compound. The solid substances which exist in more than one form are called polymorphic substances and this phenomenon of occurring solid substances in different physical forms is called polymorphism. On the other hand, when two or more different solids crystallize in the same crystalline form, they are said to be isomorphous with each other, and the phenomenon is called an isomorphism.

Compounds may appear in various forms within the natural world. Various forms may exhibit distinct morphologies or structures. The physical properties of a chemical compound are determined by its structure. The structural configuration can also influence the chemical characteristics in certain cases. The term “morphism” is used as a designation for the concept of “morphology“. It’s a way to explain how a substance appears to the naked eye. These resemblances and dissimilarities, at first sight, are referred to as isomorphism and polymorphism, respectively. When a substance has more than one crystalline form, this is called polymorphism. When two or more compounds have the same shape, we say that they are isomorphisms.

The basic difference between isomorphism and polymorphism lies in the fact that isomorphism relates to the existence of two or more compounds that possess identical morphologies, while polymorphism relates to the existence of distinct morphologies of a single substance.


Polymorphism definition

Polymorphism refers to the occurrence of a solid substance in more than one physical form, and the substances that exhibit this property are known as polymorphic substances.

Examples of polymorphism

Carbon, sulfur, silica, calcium carbonate, etc. show this phenomenon.

  • There exist two polymorphic forms of carbon, namely graphite, and diamond.
  • There exist two distinct polymorphic forms of calcium carbonate, namely orthorhombic aragonite and hexagonal calcite.
  • Six different polymorphs of silicon oxide exist.
  • There are two different sulfur polymorphic forms- the rhombohedral and the monoclinic.

Properties of polymorphism

Polymorphic substances exhibit distinct variations in their physical and chemical properties. Polymorphic substances exhibit variations in their physical properties such as melting point, density, hardness, solubility, and electrical conductivity. The chemical reactivity of a substance is subject to variation across distinct polymorphic forms.

Types of Polymorphism

Polymorphism can be categorized into two primary classifications based on the stability of solid crystals when subjected to various factors such as temperature, pressure, and so on.

  • Monotropic polymorphism: The term “monotropic polymorphism” refers to a phenomenon in which a compound appears in multiple polymorphic forms, yet only one of these forms demonstrates thermodynamic stability across all temperature ranges. Some examples include metolazone, chloramphenicol palmitate, and glyceryl stearate.
  • Enantiotropic polymorphism: At specific combinations of temperature and pressure, a particular polymorph exhibits thermodynamic stability, whereas alternative polymorphs exhibit stability over distinct ranges of pressure and temperature. Sulfur is one example of it.

Application of polymorphism

The application of polymorphism in the pharmaceutical sector is extensive, as it facilitates the production of diverse drugs and medications. The polymorphic forms’ reliance is dependent upon their respective chemical properties. The varying chemical properties exhibited by distinct polymorphic forms enable the determination of drug efficacy in the human body.

Some of the major applications are:

  • Common salt (sodium chloride) is a polymorphic dietary ingredient. NaCl is cubes or needles. The needles are too big to disintegrate, but the cube can be recovered from seawater. Entropy causes chaotic sodium and chlorine atoms to organize when more energy is added.
  • Agrochemicals like malathion exhibit polymorphism as well. They are also useful as a form of regulation. Commercial dyes and pigments employ it, as do medications like salicylic acid and cholesterol-lowering treatments like phthalocyanine green (blue).
  • Many scientists have tried to use polymorphism to create drugs for specific patients. Some persons react negatively to drugs due to polymorphic metabolization variations. Polymorphism could target difficulties by taking diverse shapes and treating a wider spectrum of symptoms and diseases than conventional treatments.


Isomorphism definition

When multiple solids crystallize in the same crystalline form despite being different, they are considered isomorphous with each other. This phenomenon is referred to as isomorphism.

Examples of isomorphism

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

Calcium carbonate and sodium nitrate exhibit a trigonal molecular geometry. Each compound exhibits an atomic ratio of 1:1:3 among its constituent atoms. However, these compounds exhibit distinct physical and chemical characteristics. The molar masses of calcium carbonate and sodium nitrate exhibit dissimilarity, with calcium carbonate having a molar mass of 100 g/mol and sodium nitrate having a molar mass of 85 g/mol.

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

These compounds have a tetrahedral crystal structure and an atomic ratio of 3:1:4 in their empirical formula. However, their chemical and physical properties are distinct from one another.

  • Epsom salt and white vitriol (ZnSO4.7H2O)

Conditions for isomorphism

For polar organic compound

  • Related chemical formulas are required.
  • Ions in unit cells should always be arranged in the same chemical and geometric pattern.
  • The ions’ sizes shouldn’t fluctuate too much.
  • The polarization of ions ought to be uniform.

For non-polar organic compound

  • There should not be significant variations in molecular volume.
  • There should be a resemblance in the crystal structure.
  • The chemical constituents are identical.

Properties of isomorphism

Isomorphic compounds have the same empirical formula because the ratio of atoms in each compound is the same; this implies that the compounds have the same empirical formula. However, the compounds differ in accordance with their atomic structure, and as a result, they have different physical properties. Density, mass, and chemical reactivity are a few examples of the many distinct types of physical properties.

Application of isomorphism

  • Used for the calculation of atomic weight
  • The correction of atomic mass.
  • Identification of the valency

Limitations of isomorphism

  • Some of the compounds exhibit isomorphism because of crystal structure-based differences. Sulfates of iron and zinc are two such compounds.
  • Non-isomorphous compounds are those that resist the isomorphism condition by having the same chemical formula and the same number of atoms.
  • Isomorphous substances must adhere to the requirement that distinct compound structures possess an identical quantity of ions within their respective unit cells. Nevertheless, certain compounds continue to oppose this principle, despite their classification as isomorphous compounds.

Difference between polymorphism and Isomorphism

DefinitionA substance is said to be polymorphic if it can be found in more than one crystalline form. This phenomenon is called polymorphism.Isomorphism is a phenomenon that occurs when two or more crystals with the same chemical composition exist in the same crystalline shape.
ShapePolymorphic substances exhibit different crystal shapes from one another.Isomorphic substances exhibit the same crystal shapes as one another.
CompoundsPolymorphism is restricted to a single compound.Two or more compounds are involved in isomorphism.
ElementsElements exhibit polymorphism or several forms.Elemental isomorphism is not observed.
Atomic ratioPolymorphic substances’ atomic ratios may or may not be identical.Isomorphic substances’ have the same atomic radius

Summary of Polymorphism and Isomorphism

The physical appearance of a substance is referred to as its morphology, which can be clarified by two distinct concepts, namely isomorphism, and polymorphism. Polymorphism is the term used to describe the existence of multiple forms of a given compound. Isomorphism refers to the phenomenon where multiple compounds share an identical shape. The key difference between isomorphism and polymorphism is based on their respective definitions. Specifically, isomorphism relates to the presence of identical morphologies across distinct substances, while polymorphism relates to the presence of multiple morphologies of a single substance.

Crystal lattice parameters like atomic weight, valency, etc. can be calculated with the help of isomorphic studies. The study of polymorphism, on the other hand, is significant because of the range of its potential applications in the industry sector.

Polymorphism and Isomorphism Video


  • CaCO3-calcite-structure’By Tem5psu – Own work
  • “Isomorphism.” The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. Web.
  • “Polymorphism.” Encyclopædia Britannica. Encyclopædia Britannica, inc., n.d.
  • https://pediaa.com/difference-between-isomorphism-and-polymorphism/
  • https://www.toppr.com/ask/en-np/content/concept/isomorphism-and-polymorphism-238073/
  • https://www.differencebetween.com/difference-between-isomorphism-and-vs-polymorphism/
  • https://testbook.com/chemistry/polymorphism
  • https://unacademy.com/content/upsc/study-material/geology/isomorphism-and-polymorphism/

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