Class 11 Chapter 6 Anisotropy vs Isotropy Properties of Crystalline Solids
Introduction
In Chapter 6 of Class 11 Chemistry, we study how crystalline substances can exhibit similar structures (isomorphism) or exist in more than one form (polymorphism). These phenomena influence physical properties, industrial applications, and identification of minerals.
Isomorphism
Definition
Isomorphism is the phenomenon where two or more crystalline compounds have the same crystal structure and similar cell dimensions, despite having different chemical compositions.
Examples
- NaCl, KCl, and LiCl all crystallize in the cubic rock-salt structure.
- Calcite (CaCO3) and aragonite (another CaCO3 polymorph) are not isomorphs, but calcite and siderite (FeCO3) are isomorphs.
- ZnS (zinc blende) and GaAs (gallium arsenide) share the cubic sphalerite structure.
Factors Affecting Isomorphism
- Similarity in ionic radii: Typically within 15% of each other.
- Same crystal charge: Ions have similar charges and coordination numbers.
- Similar electronegativities: Leads to comparable bonding character.
Polymorphism
Definition
Polymorphism is the ability of a substance to exist in more than one crystalline form, each having different physical properties, while the chemical composition remains the same.
Examples
- Carbon: diamond and graphite.
- Calcium carbonate: calcite, aragonite, and vaterite.
- Titanium dioxide: rutile, anatase, and brookite.
Significance
- Affects solubility and bioavailability of pharmaceuticals.
- Determines hardness and conductivity in materials.
- Used in tailoring properties for catalysts, pigments, and ceramics.
Properties of Crystalline Substances
- Definite Melting Point: Sharp, characteristic temperature.
- Anisotropy: Direction-dependent properties (optical or mechanical).
- Cleavage & Fracture: Break along specific planes in isomorphous series similarly.
- Distinct Habit: Characteristic external shapes, modified by polymorphic form.
- Density & Hardness: Vary with crystal form and composition.
Conclusion
Understanding isomorphism and polymorphism helps in predicting and controlling the properties of crystalline materials. These concepts are fundamental in mineralogy, materials science, and pharmaceutical development.
