Molecular Solids (College Board AP Chemistry)

• Molecular solids such as ice, sucrose (table sugar) and iodine are composed of neutral molecules
• Molecular solids are held together by attractive intermolecular forces with varying strengths, which is indicated by the different melting points of the solids
  • Small nonpolar molecules such as carbon dioxide, water hydrogen gas, fluorine and nitrogen gas
    • These have weak attractive forces and form molecular solids with very low melting points
  • Solid substances like iodine, fullerenes (C₆₀), sulfur crystals (S₈)
    • These are made of larger, nonpolar molecules that have larger attractive forces and melt at higher temperatures
  • Molecular solids such as ice and sucrose
    • These contain permanent dipole moments which means that they melt/decompose(as in the case of sucrose) at higher temperatures

 

Molecular Solid structure

Solid carbon dioxide (CO₂) consists of small, nonpolar molecules forming a molecular solid with a melting point of −78 °C, while iodine (I₂) consists of larger, nonpolar molecules forms a molecular solid with a melting point of 114 °C

Properties of Molecular Solids

• Molecular solids are generally non-conductors of electricity
  • This is because their valence electrons are not free to move
• Unlike covalent network solids, molecular solids have variable hardness and brittleness but can be considered to be generally soft and brittle