In our surroundings, we can find numerous types of compounds, which are different in their appearances, or we can say that their morphologies differ. Some chemical properties are determined just by appearances. The morphology of a compound describes its external structure. The expressions isomorphism and polymorphism are utilised to describe the external features of compounds. The major difference between the two is that in isomorphism, two or more compounds show identical morphologies, whereas in the case of polymorphism, the same compound shows different morphologies.

Introduction to Isomorphism

The law of isomorphism was first given by Mitscherlich in 1819. When identical morphology is found in two or more compounds, they are called isomorphous compounds. This implies that crystal structure is present in not only a single compound but may be present in other compounds as well. This property is called isomorphism. 

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Properties of Isomorphic Substances

The ratio of atoms found in isomorphic compounds is the same, which indicates the same empirical formula but the compounds differ in accordance with their atomic structure; therefore, they possess different physical properties. The different physical properties include density, mass, chemical reactivity, etc.

Conditions of Isomorphism

Polar organic compound:

• Chemical formulas must be comparable.
• There should be the same chemical and geometrical arrangement of ions in unit cells.
• The size of the ions must not vary as much.
• The polarisation of ions should be the same.

 Non-polar organic compound:

• Molecular volume does not contain major differences.
• The similarity in the crystal structure.
• Similar chemical constituents are used.

Examples

The following examples show isomorphic nature:

• Calcium carbonate and sodium nitrate

The shape of both the compounds is trigonal. The atomic ratio of all the elements present in them is 1:1:2. But it should be noted that the physical properties, chemical properties, and molar mass are different in both compounds.

• Another example is potassium chromate and potassium sulphate.

Applications of Isomorphism

It is used for the following purposes:

• Determination of atomic weight
• Identifying the valency
• Correction of atomic weight

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Limitations of isomorphism

• Isomorphism is shown by some of the compounds having differences in accordance with their crystalline structure. Examples of such compounds are ferrous sulphate and zinc sulphate.
• It is mandatory for isomorphous substances that different structures of compounds have an equal number of ions in their unit cell. However, some of the compounds still do not obey this rule, yet they are isomorphous compounds.
• Compounds with similar chemical formulas and the same number of atoms disobey the isomorphism condition and are non-isomorphous compounds.

Introduction to Polymorphism

When the same compound shows different morphologies, it is termed as polymorphism, and that particular substance is called a polymorphic substance. The shape and structure may differ for the same compound.

Properties of Polymorphic Substances

There is a difference in the physical and chemical properties of polymorphic substances. The physical properties like melting point, density, hardness, solubility, electrical conductivity differ in various polymorphic substances. The chemical reactivity of a substance also changes in different polymorphic substances. 

Factors Affecting Polymorphism

• The polymorphism is affected by temperature and humidity. For example, solid ice melts down to liquid with an increase in temperature.
• The polymorphism is affected by photostability. On exposure to visible light, the chemical and physical properties of a substance change.
• The polymorphism is also affected by grinding. On grinding, the bond between the atom gets weaker. That is the reason why the anhydrous form is less stable than the dihydrate form.

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Types of Polymorphism

• Monotropic polymorph: Monotropic polymorph means that the compound exhibits several polymorphic forms, but out of them, only one form shows stability at all temperatures. Examples: Glyceryl stearate, Chloramphenicol palmitate, Metolazone.

• Enantiotropic polymorph: At a certain temperature and pressure, one polymorph is stable, while others are stable at different ranges of pressure and temperature. Example: Sulphur 

Examples

• Carbon has two polymorphs, which are graphite and diamond.
• Calcium carbonate has two polymorphs, one is orthorhombic aragonite, and the other is hexagonal calcite.
• Silicon oxides have six polymorphs.

Application of Polymorphism

Polymorphism has a wide range of applications in the pharmaceutical industries for the production of various kinds of drugs and medicines. It depends upon the chemical properties of the polymorphic forms. Since different polymorphic forms show different chemical properties, the effectiveness of drugs on the body can be determined accordngly.

Differences in Isomorphism and Polymorphism

Conclusion

The external appearance of a substance is known as morphology which is further explained by two different terms, isomorphism and polymorphism. The isomorphic substances and polymorphic substances show different chemical properties and physical properties.

The study of isomorphic form is important in the determination of crystal lattice properties such as atomic weight, valency etc. On the other hand, the study of the polymorphic form is important as it has a wide application across different industries.