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Section 4: Molecular Solids
Molecular solids are materials whose molecules (or atoms) are held together by intermolecular forces, such as London dispersion forces, dipole-dipole forces, and hydrogen bonds. As we saw in Unit 5, these forces are much weaker than ionic or covalent bonds. As a result, many molecular substances are gases or liquids at normal temperature and pressure.
Figure 13-7. Liquid and Solid Water Molecules
Ice is less dense than water because frozen water molecules are locked together in a fairly open network by rigid hydrogen bonds.
© Science Media Group.
Molecular solids are soft and low density, and have low melting points, usually lower than 300°C. Examples include sugar, iodine, and dry ice. Dry ice has such weak bonds that it sublimates directly from a solid to a gas at -78.5°C. Iodine will sublimate at room temperature from a black solid to a purple gas. The familiar form of iodine that is widely used for disinfecting cuts is actually a small amount of iodine dissolved in a water and alcohol base.
As we learned in the discussion of phase diagrams in Unit 2, ice (a molecular solid) has an unusual property: It is less dense in its solid form than as a liquid. This occurs because weak hydrogen bonds between liquid water molecules constantly form and break, so the molecules are randomly distributed; when water freezes, however, the bonds form rigid lattices with each water molecule bonded to four others. These lattices contain relatively large gaps, which would be filled with atoms if the water were liquid. (Figure 13-7)
Figure 13-8. Morphology of Snow Crystals
Snow crystals form simple shapes at lower humidity levels and more complex shapes at higher humidity. However, all have a six-sided structure because of the way hydrogen bonding occurs between ice molecules.
© Science Media Group, adapted from image by Caltech.
Note in FIgure 13-7 that the water molecules in solid form create a six-sided structure . This happens because at temperatures and pressures that are typical on Earth's surface, water crystallizes into six-sided rings when it freezes. Snowflakes are six-sided because they form in clouds when water vapor turns into ice. As more water molecules in the cloud strike against the central snow crystal, some of them stick to its corners and freeze, growing into branches. A snowflake may be a single hexagonal ice crystal, a few crystals stuck together, or a large clump of crystals. For reasons that are not yet completely understood, snowflakes grow in different shapes (always six-sided) under different temperature and humidity conditions. (Figure 13-8)