In its crystalline
structure, every Si or Ge atom tends to share one of its
four valence electrons with each of its four nearest
neighbour atoms, and also to take share of one electron
from each such neighbour These shared electron pairs
are referred to as forming a  covalent bond or simply a
valence bond The two shared electrons can be assumed
to shuttle back-and-forth between the associated atoms
holding them together strongly Figure 14