5.1.2          Involatile nonmetallic compounds

Some nonmetallic compounds are involatile because their molecules are very large (e.g. higher paraffins).  Although the van der Waals interactions between atoms in different molecules are small (they are the same as the interactions between atoms of small molecules), there are so many atoms that the total interaction between molecules is large.  Compounds of this type nevertheless dissolve in volatile solvents (e.g. higher paraffins in lower ones), because each molecule can interact with as many solvent molecules as it can get round it.  These interactions add up to overcome the large interaction between molecules of the compound.  Very large molecules are called “macromolecules”.

Other involatile nonmetallic compounds are different in being insoluble in volatile solvents (e.g. diamond, silicon carbide, and silica).  The nature of these compounds can be inferred as follows:

(i)  From the above discussion of volatile compounds, the molecules of these compounds in the vapour phase must interact very strongly in the liquid and solid phases.

(ii)  In many volatile compounds, carbon atoms are known to bond to four other atoms in directions defined by a tetrahedron (see above). Now if carbon atoms were to bond to each other in this kind of way, instead of a small molecule being produced, a continuous three-dimensional network would be formed, each atom being held in the network by very strong bonds - the same kind of bonds that hold the atoms of each molecule together in volatile compounds. In two dimensions this may be drawn:

Such a structure would satisfy (i) and suggests itself for diamond.

(iii)  A determination of the structure of diamond by X-ray crystallography shows that it indeed has this structure, with carbon-carbon bond lengths of 1.54 Å, as compared with 1.53 Å in C₂H₆. (Click here for a picture.)

(iv)  In a similar way we can “guess” the structures of silicon carbide and silica. Silicon also forms volatile compounds in which the silicon atom is surrounded by a tetrahedron of other atoms  (e.g. simple molecules like SiH₄ and SiCl₄, and resolvable ones of the type SiWXYZ). Silicon carbide could thus have a similar structure to that of diamond:

Further, since oxygen forms compounds in which its atom has two neighbours, silica could have a structure of the diamond type, only with an oxygen atom between every pair of silicon atoms:

That silicon carbide and silica indeed have structures of this kind has been shown by X-ray crystallography. (Both compounds are polymorphic, but in each case the different forms have the same arrangement of nearest neighbours, and differ only in the relative orientation of more distant neighbours.)

Compounds of this type thus consist of giant assemblies of atoms tightly held together. Such assemblies of atoms are called “frameworks” and such compounds “nonmolecular”.

Liquefaction of compounds of this type involves the breaking of some of the bonds, for only in this way can atoms move positions. Clearly, however, the system will resist whole-scale breaking of bonds, and fluidity will rely on a process whereby the breaking of one bond is compensated for by the remaking of another bond somewhere else. Consequently the fluidity of such compounds in the liquid state is low: that is, their viscosity is high.

Vaporization of compounds of this type does require a break-up of the network of bonds, and the vapour effectively consists of fragments of the solid. In the case of diamond, these are C₂ molecules; for silica, they are SiO₂ molecules. Silicon carbide decomposes.

Note

It is important to be able to work out the overall composition of a compound from diagrams of the kind given in the discussion above.  This can be done by picking out the central atom in the diagram, counting the number of nearest neighbours to it (x), and then working out the share that the central atom has of these neighbours. This is done by counting the number of other atoms of the same type that the central atom has to share each neighbour with (y), from which the share the central atom has of each neighbour is 1/y.  The composition is then AB_z, where A represents the central atom, B its neighbour, and z = x/y.  The method can readily be extended to cases where there is more than on kind of neighbour.