Discrete molecules held by intermolecular forces, giving soft low-melting solids such as ice and iodine.
Practise Bonding & Molecular Structure All Bonding & Molecular Structure topics
Molecular solids are crystals composed of discrete molecules held together by intermolecular forces such as London dispersion, dipole-dipole or hydrogen bonding. The forces between molecules are much weaker than covalent bonds, so these solids are soft, have low melting points and are poor conductors of heat and electricity. Packing in the crystal depends on molecular shape and symmetry, which influence density and melting point. Many molecular solids sublime, passing directly to the gas phase, when their intermolecular forces are weak. Examples include ice, iodine and solid carbon dioxide.
Key ideas: Classify by dominant force: nonpolar (Ar, I2, C60, soft, low melting), polar (sugar, ice), and hydrogen bonded (ice, boric acid). Melting points range from near 0 K for helium to a few hundred degrees C. Compare enthalpy of fusion: ice 6.0 kJ/mol versus Ar 1.2 kJ/mol. Volatile solids like I2 sublime easily.
Common mistakes and connections: A common mistake is to say covalent bonds in molecular solids break when they melt. Another is expecting all molecular solids to be soft. Also molecular solids usually do not conduct, but some organic solids such as TTF-TCNQ do. Links to intermolecular forces, phase diagrams, sublimation and crystal packing.
Molecular solids melt at low temperatures because only weak intermolecular forces hold the discrete molecules together. The interactive lesson shows this as a live animation or chart.
Dry ice, solid CO2, sublimes at about -78 degrees Celsius at atmospheric pressure.
Discrete molecules held by intermolecular forces, giving soft low-melting solids such as ice and iodine.
Molecular Solids is a beginner topic in Bonding & Molecular Structure. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Bonding & Molecular Structure first: Intramolecular Hydrogen Bonding, Water's Anomalous Properties, Covalent Network Solids.
The next topics in Bonding & Molecular Structure are Ionic Solids and Metallic Solids.
Yes. The interactive lesson includes a self-check question on Molecular Solids, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Molecular Solids is topic 80 of 104 in Bonding & Molecular Structure.