1.6 Binding forces in molecular and non-molecular substances - a first look
As we shall see in Section 4, elementary bonding theories imply that materials as different as salt, iodine and aluminium are held together by different types of chemical bond. However, all binding forces between atoms are essentially electrical, and arise from a balance of forces acting between positively charged nuclei and negatively charged electrons. As electrical forces are stronger at short distances, in solid iodine (Figure 6b) the short distances between the pairs of atoms (I2 molecules) suggest that the forces holding these atoms together are strong. By contrast, the longer distance between different pairs (molecules) tells us that the forces acting between one I2 molecule and another are much weaker.
Now, iodine melts at only 114 °C and boils at 185 °C.
Why does iodine have low melting and boiling temperatures?
In solid iodine, different I2 molecules are held together by weak forces, so only a little thermal energy is needed to separate them and create first a liquid, and then a gas. Both liquid and gaseous iodine also contain I2 molecules. To melt and then boil iodine it is not necessary to break up the I2 molecules themselves.
This also explains another property of iodine: it dissolves fairly easily in an organic solvent like petrol. The solid crystal falls apart and individual I2 molecules drift off into solution. As organic compounds are molecular, they too, often dissolve in petrol. The organic polymers you meet in everyday life have unusually large molecules, but being molecular they may also be vulnerable (Figure 14).
By contrast, in salt, silica or aluminium, the bonding is more evenly distributed through the crystal, and there are no points of weakness where discrete molecules can be prised apart. So the melting and boiling temperatures of non-molecular substances tend to be greater than those of molecular ones. Salt, silica and aluminium, for example, melt at 801 °C, 1 713 °C and 660 °C, respectively.