Molecular Solids (College Board AP® Chemistry)

Molecular Solids

• 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