SEMJCOXDl CrORS 



.^M 



covalent bonds. These neighbors are arranged on the corners of a regular 

 tetrahedron in conformity witli the known chemical behavior of the tetra- 

 hedral carbon atom.^ For purposes of discussion of conductivity in these 

 crystals, we shall represent the three-dimensional array in two dimensions 

 as is shown in Fig. 3, indicating that each carbon atom forms an electron- 

 l)air bond with four neighbors. 



On the basis of this valence bond structure we can intuitively see why 

 diamond should be an insulator. Although it contains a large number of 



1 A = 



I ANGSTROM 

 = 10"® CM 



Fig. 1. 



electrons, as does a metal, the covalent bond is a quite different structure 

 from the metallic bond. In an ideally perfect diamond crystal, each valence 

 bond would contain its two electrons; therefore, every electron would be 

 tightly bound and thus unable to enter into the conduction process. 



Conductivity can be produced in diamond, however, in a number of ways, 

 all of which involve destroying the perfection of the valence bond structure. 



3 Long before the arrangement of atoms in the diamond crystal was estabUshed b>- 

 X-rays, the organic chemists had coiickided that carlwn formed four bonds at the Ictra- 

 hedral angles— a truly remarkat)le result of inductive reasoning based on observations of 

 the ojitical properties of solutions of organic compounds. 



