Molecular Solids

Molecular solids, such as ice, sucrose (table sugar), and iodine, as shown in Figure 1 , are composed of neutral molecules. 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. Examples include ice (melting point, 0 °C) and table sugar (melting point, 185 °C).

Figure 1 : Carbon dioxide (CO₂) consists of small, nonpolar molecules and forms a molecular solid with a melting point of −78 °C. Iodine (I₂) consists of larger, nonpolar molecules and forms a molecular solid that melts at 114 °C.