
135
Fig. 9.5 Diagrammatic representation of the four-fold linkages of tetravalent atoms
such as silicon
of squashing a bit of such a model flat so as to get it on a sheet of
paper. Because every atom is identical with every other, the structure
so formed is made up of atoms in perfectly regular array, like a case
of tennis balls packed for export. But we must remember that the
atoms are inconceivably small and numerous. Broken-off pieces of
such an atomic structure naturally tend to have flat faces at certain
definite angles to one another, and this description fits what we call
crystals.
9.6 Holes
A crystal of this kind, if perfect, would