Molecular Solids (College Board AP® Chemistry): Revision Note
Molecular Solids
Molecular solids such as ice, sucrose (table sugar) and iodine are composed of neutral molecules
Molecular solids are held together by relatively weak intermolecular forces, so they generally have low melting points; the exact melting point depends on the types and strengths of the forces present
Small nonpolar molecules such as carbon dioxide, hydrogen, fluorine and nitrogen
These have weak attractive forces and form molecular solids with very low melting points
Solid substances like iodine, fullerenes (C60), sulfur crystals (S8)
These larger nonpolar molecules have more electrons and larger, more polarizable electron clouds, so their London dispersion forces are stronger and they melt at higher temperatures than small nonpolar molecules
Molecular solids such as ice and sucrose
These are polar molecules that also form hydrogen bonds, which act in addition to London dispersion forces. Ice melts at 0 °C; sucrose decomposes before it melts, at about 186 °C
A polar molecule does not always melt at a higher temperature than a nonpolar one: iodine (nonpolar, 114 °C) melts far higher than ice (hydrogen bonded, 0 °C) because its large electron cloud gives strong London dispersion forces
Structure of molecular solids
Solid carbon dioxide (CO2) is made of small nonpolar molecules held by weak London dispersion forces, so it sublimes at −78 °C. Iodine (I2) molecules have many more electrons, so their London dispersion forces are stronger and iodine melts at 114 °C
Properties of molecular solids
Molecular solids are generally non-conductors of electricity
This is because their valence electrons are held tightly in the covalent bonds and lone pairs of each molecule
Unlike covalent network solids, molecular solids have variable hardness and brittleness but can be considered to be generally soft and brittle
Some molecular solids are made of very large molecules or polymers, such as polyethylene
Examiner Tips and Tricks
List every intermolecular force present. All molecules have London dispersion forces; polar molecules have dipole-dipole forces in addition, and molecules with H–N, H–O or H–F bonds also form hydrogen bonds
Do not assume dipole-dipole forces or hydrogen bonds always beat London dispersion forces. Compare the specific forces for the molecules given
Name the specific force: 'stronger intermolecular forces' on its own does not earn the point
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