Phosphorus-containing molecules, force

Since force constants are approximately proportional to the bond order, the following comparison of molecules containing multiply bonded phosphorus is informative ... 

So far we have considered solids in which atoms occupy the lattice positions. In some of these substances (network solids), the solid can be considered to be one giant molecule. In addition, there are many types of solids that contain discrete molecular units at each lattice position. A conunon example is ice, where the lattice positions are occupied by water molecules [see Fig. 10.12(c)], Other examples are dry ice (solid carbon dioxide), some forms of sulfur that contain Sg molecules [Fig. 10.32(a)], and certain forms of phosphorus that contain P4 molecules [Fig. 10.32(b)]. These substances are characterized by strong covalent bonding within the molecules but relatively weak forces between the molecules. For example, it takes only 6 kJ of energy to melt 1 mole of solid water (ice) because only intermolecular (H2O—H2O) interactions must be overcome. However, 470 kJ of energy is required to break 1 mole of covalent O—H bonds. The differences between the covalent bonds within the molecules and the forces between the molecules are apparent from the comparison of the interatomic and intermolecular distances in solids shown in Table 10.6. 

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