Metallic bonding intermolecular forces compared

Note 5.6 (Van der Waals force). Even for nonpolar molecules, the polar character is generated due to instantaneous deviations in the electron orbit. Because of this electric fleld, the neighboring molecules become polarized, and the energy level of the total system becomes lower if the force is attractive rather than repulsive. Frozen carbon dioxide and crystals of iodine h are examples of crystals formed by van der Waals forces, which are known as molecular crystals. Since the van der Waals forces have no orientation, the molecular crystals occur in a closely-packed structure. The van der Waals force V(r) is inversely proportional to the sixth power of the intermolecular distance r V(r) = —C/r . The van der Waals force is extremely small compared to chemical forces such as ionic bonds, covalent bonds and metallic bonds (i.e., less than 1/100). ... 

The forces of attraction between molecules are known as intermolecular forces. Intermolecular forces vary in strength but are generally weaker than bonds that join atoms in molecules, ions in ionic compounds, or metal atoms in solid metals. Compare the boiling points of the metals and ionic compounds in Figure 5.8 (on the next page) with those of the molecular substances listed. Note that the values for ionic compounds and metals are much higher than those for molecular substances. 

Covalently bonded substances with a simple molecular structure, for example water and ammonia, are usually liquids or gases. This is because the forces between the molecules are weak. It does not take much energy to overcome these intermolecular forces, so these substances have low melting points, low boiling points and low enthalpy changes of vaporisation compared with ionic compounds. Some substances that have covalently bonded molecules maybe solids at room temperature, for example iodine and poly(ethene). These are usually molecules where the van der Waals forces are considerable. However, the melting points of these substances are still fairly low compared with ionic compounds or most metals. 

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