Chemistry
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Molecular solids
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Molecular solids
Molecular solids are composed of discrete molecules. The cohesive forces that bind the molecules together are van der Waals forces, dipole-dipole interactions, quadrupole interactions, π-π interactions, hydrogen bonding, halogen bonding, London dispersion forces, and in some molecular solids, coulombic interactions.
The strengths of the attractive forces between the units present in different crystals vary widely, as indicated by the melting points of the crystals. Small symmetrical molecules (nonpolar molecules), such as H2, N2, O2, and F2, have weak attractive forces and form molecular solids with very low melting points (below -200 °C). Substances consisting of larger, nonpolar molecules have larger attractive forces and melt at higher temperatures. Molecular solids composed of molecules with permanent dipole moments (polar molecules) melt at still higher temperatures.
Most molecular solids are nonpolar. These nonpolar molecular solids will not dissolve in water but will dissolve in a nonpolar solvent, such as benzene and octane. Polar molecular solids, such as sugar and salt, dissolve easily in water. Molecular solids are nonconductive.
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