Crystalline Solids – Class 12 | Chapter – 1 | Chemistry Short Notes Series PDF for NEET & JEE

Crystalline Solids: Crystalline solids are solids with a well-defined, ordered, and repetitive arrangement of constituent particles such as atoms, molecules, or ions. They have a specific crystal structure and characteristic physical properties such as sharp melting points and anisotropy. The constituent particles in crystalline solids are arranged in a regular, repeating pattern called a crystal lattice, which extends throughout the entire solid.

Crystalline Solids

There are several types of crystalline solids, classified based on the types of intermolecular or interatomic bonding present in the solid:

  • Ionic crystals: These are solids composed of positively and negatively charged ions held together by electrostatic forces. They have high melting points and are brittle. Examples of ionic crystals include NaCl (table salt), MgO, and CaF2.
  • Covalent crystals: These are solids composed of atoms held together by covalent bonds, forming a three-dimensional network structure. They have high melting points, are hard and brittle, and have excellent thermal and electrical conductivity. Examples of covalent crystals include diamond, silicon, and silicon carbide.
  • Molecular crystals: These are solids composed of discrete molecules held together by weak intermolecular forces such as van der Waals forces, hydrogen bonding, or dipole-dipole interactions. They have low melting points and are often soft and brittle. Examples of molecular crystals include solid carbon dioxide (dry ice), iodine, and naphthalene.
  • Metallic crystals: These are solids composed of metal atoms held together by metallic bonds, which are a type of delocalized bonding. They have high thermal and electrical conductivity, are ductile and malleable, and have lustrous surfaces. Examples of metallic crystals include copper, silver, and iron.

Properties of Crystalline Solids

The properties of crystalline solids depend on the types of bonding and the arrangement of constituent particles in the crystal lattice. Some general properties of crystalline solids include:

  • Anisotropy: Crystalline solids have different properties in different directions due to the ordered arrangement of their constituent particles. This anisotropy is evident in properties such as thermal and electrical conductivity, refractive index, and mechanical strength.
  • Melting point: Crystalline solids have well-defined, sharp melting points due to the regular and ordered arrangement of their constituent particles.
  • Hardness: Crystalline solids are often hard and brittle due to the strong covalent or ionic bonds between their constituent particles.
  • Density: Crystalline solids have a specific density that depends on the size and arrangement of their constituent particles.
  • Electrical conductivity: Some types of crystalline solids such as metals and semiconductors have high electrical conductivity due to the delocalized electrons in their crystal lattice.
  • Optical properties: Some types of crystalline solids exhibit optical properties such as birefringence, which is the splitting of light into two polarized beams, and pleochroism, which is the selective absorption of different polarizations of light.
  • Magnetic properties: Some types of crystalline solids exhibit magnetic properties such as ferromagnetism, antiferromagnetism, and paramagnetism, depending on the arrangement of their constituent particles and their spin.

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By Team Learning Mantras