Classification of crystals

Classification of Crystals

Crystals are solids characterized by a specific and repetitive three-dimensional arrangement of their constituent particles, which may be atoms, ions, or molecules. These solids exhibit long-range order and typically possess flat surfaces, sharp edges, and distinct angles. Based on the nature of bonding and constituent particles, crystals are classified into several types.

Types of Crystalline Solids

• Ionic Crystals: Formed by strong electrostatic forces between positively and negatively charged ions. They are hard, brittle, and have high melting points. Example: Sodium chloride (NaCl), Magnesium oxide (MgO).
• Covalent or Network Crystals: Consist of atoms bonded covalently in a continuous three-dimensional network. These crystals are extremely hard and have high melting points. Example: Diamond, Silicon carbide (SiC).
• Molecular Crystals: These consist of molecules bonded by weak intermolecular forces like van der Waals attractions, hydrogen bonding, or dipole-dipole interactions. They are typically soft and melt at relatively low temperatures. Example: Ice, Iodine, Dry ice (solid CO₂).
• Metallic Crystals: Made up of metal atoms surrounded by a sea of delocalized electrons. The presence of free-moving electrons enables metals to efficiently transfer heat and conduct electricity. Example: Copper (Cu), Silver (Ag), Iron (Fe).

Properties of Crystalline Crystals

• Exhibit well-defined geometric shapes and symmetry.
• Exhibit well-defined and consistent melting temperatures.
• Show cleavage—break along specific planes.
• Show directional dependence in physical properties, known as anisotropy.

Comparison of Different Crystal Types