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

Molecular Solids: Molecular solids are a type of crystalline solid in which the constituent particles are molecules held together by intermolecular forces, such as van der Waals forces, dipole-dipole interactions, and hydrogen bonds. Molecular solids are typically soft, have low melting and boiling points, and are poor conductors of heat and electricity.

Types of Molecular Solids

Molecular solids can be classified into two main types:

  • Nonpolar molecular solids: Nonpolar molecular solids are composed of molecules with no permanent dipole moment, such as methane (CH4) and carbon dioxide (CO2). They are held together by weak van der Waals forces and are typically gases or liquids at room temperature. However, at low temperatures and high pressures, nonpolar molecular solids can form, such as solid methane (CH4) and solid carbon dioxide (dry ice).
  • Polar molecular solids: Polar molecular solids are composed of molecules with a permanent dipole moment, such as water (H2O) and ammonia (NH3). They are held together by stronger intermolecular forces, such as hydrogen bonding and dipole-dipole interactions, and are typically solids at room temperature. Polar molecular solids often have higher melting and boiling points than nonpolar molecular solids.

Properties of Molecular Solids

The properties of molecular solids depend on the specific type of molecules that make up the solid, as well as the strength of the intermolecular forces between them. Here are some general properties of molecular solids:

  • Low melting and boiling points: Molecular solids generally have low melting and boiling points, due to the relatively weak intermolecular forces that hold the molecules together. This means that they can often be easily melted or vaporized.
  • Brittle: Molecular solids are usually brittle, meaning they tend to shatter or break apart when subjected to stress or impact. This is because the molecules in the solid are held together by relatively weak intermolecular forces.
  • Poor conductors of heat and electricity: Molecular solids are generally poor conductors of heat and electricity, as the electrons in the solid are typically localized around the individual molecules and do not move freely.
  • Solubility: The solubility of a molecular solid in a particular solvent depends on the polarity of the molecules and the solvent. Polar molecular solids are typically soluble in polar solvents, while nonpolar molecular solids are typically soluble in nonpolar solvents.
  • Crystal structure: Molecular solids often have a well-defined crystal structure, due to the regular arrangement of the constituent molecules. This structure can be used to predict the properties of the solid, such as its melting point and solubility.
  • Optical properties: Some molecular solids exhibit interesting optical properties, such as fluorescence or phosphorescence, due to the interactions between the constituent molecules.

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By Team Learning Mantras