Crystalline Solids The Fundamental Types

Composite units are formula units (cations and anions) 

Molecular solids are solids whose composite units are molecules. The lattice sites in a crystalline molecular solid are therefore occupied by molecules. Ice (solid H2O) and dry ice (solid CO2) are examples of molecular solids. Molecular solids are held together by the kinds of intermolecular forces—dispersion forces, dipole-dipole forces, and hydrogen bonding—discussed earlier in this chapter. Molecular solids as a whole tend to have low to moderately low melting points. However, strong intermolecular forces (such as the hydrogen bonds in water) can increase the melting points of some molecular solids. 

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