PRINCIPLES OF STRUCTURE 



55 



paradoxical that the soft, plastic paraffin crystals should have a lattice 

 structure so similar to the diamond model represented by Fig. 31a. 

 Notwithstanding the apparent analogy, however, there exist funda- 

 mental differences which explain the differences in the physical be- 

 haviour of the two substances. In particular, the lattice of the paraffin 

 crystals is built much more loosely. This is caused by the fact that 



cleavage< 

 plane 



homogeneous 

 lattice region 



<> ^l <> I S 2 

 a b 



Fig. 32. Aliphatic chains, a) Molecule lattice; b) chain lattice. 



these crystals possess not a main valency lattice, but a molecule 

 lattice. The chains are joined by Van der Waals forces only, since 

 the CH2 groups are able to bind only two neighbouring groups by 

 primary valencies. Thus in the paraffin lattice we have two types 

 of distances : molecular distances of the order of magnitude 5 A and 

 atomic ones of the order of magnitude 1.5 A (Fig. 31b). The fact that 

 in the diamond lattice all C-atoms touch each other explains its great 

 density and hardness. The paraffin lattice, on the other hand, has a 

 much lower density and layers of molecules can be shifted with respect 

 to each other with relative ease (Fig. 32). This accounts for the soft- 

 ness and plasticity of paraffin crystals. 



As long as the paraffin chains are short, they easily crystallize into 

 a molecular lattice. This leads to crystals in the form of flakes, which 



