2.5 Binding forces in molecular and non-molecular substances

Now let’s start to think a little about chemical bonding.

As you will see in Session 3 of this module, elementary bonding theories imply that materials as different as salt, iodine and aluminium are held together by different types of chemical bond, although as you’ll see these all arise from a balance of forces acting between positively charged nuclei and negatively charged electrons.

As these forces are stronger at shorter distances, in solid iodine (Figure 1b), the short distances between the pairs of atoms (I₂ molecules) suggest that the forces holding these atoms together are strong. But, 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^oC, and boils at 185^oC.

This also explains another property of iodine: it dissolves fairly easily in an organic solvent like petrol. This is telling you the solid crystal falls apart and individual I₂ molecules drift off into solution.

As organic compounds are molecular, they too, often dissolve in petrol. For example organic polymers you meet in everyday life although having unusually large molecules, being molecular this could cause problems! An example is illustrated in Figure 9. 

Figure 9. From the Daily Telegraph, 6 April 2001 (note the erroneous use of ‘melt’ for ‘dissolve’ in this extract)

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This is a short article from the Daily Telegraph with the headline 'Fuel hoarder sentenced' written by Maurice Weaver. It reads 'A taxi driver who tried to beat the fuel crisis by storing petrol in a wheelie-bin at home was given a suspended prison sentence yesterday. Saquib Bashir, 28, caused a major alert, leading to the evacuation of 60 neighbouring properties and a £100,000 clean-up bill when fuel melted the bottom of the plastic bin and leaked into the cellar of his terraced house. At Derby Crown Court, Bashir admitted storing petrol without a licence and in non-metal containers. He was sentenced to eight months' imprisonment, suspended for two years, and ordered to pay £1,000 costs. Bashir had stockpiled 90 litres of fuel in a wheelie-bin, a beer barrel and a cooking-oil container at his home in Normanton, Derby. Later he told trading standards officers that he had no idea that petrol was so flammable.

Figure 9. From the Daily Telegraph, 6 April 2001 (note the erroneous use of ‘melt’ for ‘dissolve’ in this extract)...

By contrast, in salt, silica or aluminium (all non-molecular), 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^oC, 1713^oC and 660^oC, respectively.