Broken natural diamond

Thus we have shown that when s and p orbitals are available and s—p quantization is broken an atom can form four (or fewer) equivalent bonds which are directed towards tetrahedron corners. To the approximation involved in these calculations the strength of a bond is independent of the nature of other bonds. This result gives us at once the justification for the tetrahedral carbon atom and other tetrahedral atoms, such as silicon, germanium, and tin in the diamond-type crystals of the elements and, in general, all atoms in tetrahedral structures. 

Diamond is the purest and crystallo-graphically the most perfect substance that occurs in nature though composed of one of the commonest elements on earth, i.e., carbon. It is the hardest natural material and yet can be broken with the blow of a hammer. It can withstand attack by the strongest chemicals and harshest radiation and yet thermodynamically it is metastable. It is the best thermal conductor as well as a good electrical insulator. 

From this tabulation, it is obvious, that the (111) surface has the lowest surface free energy because it exhibits the smallest number of dangling bonds per unit area. This simple bond counting picture indicates correctly that the minimum energy surface of a covalent diamond structure crystal is the (111) surface and also the natural shape of a diamond crystal. The (111) plane is also the cleavage plane for C, Si, and Ge because the number of broken bonds is minimal as shown in Table 9.4. The dependence of the surface free energy with crystal orientation has... 

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