It is possible for pure silicon to be arranged in a mono crystaline structure. That is, all of the silicon atoms align in a very specific, well-ordered manner, without any voids or breaks in the pattern. As silicon has only four electrons in its valence shell, four more electrons would be needed to fill the shell. In the crystal, any given atom of silicon effectively “shares” an electron from its four closest neighbors through a covalent bond (meaning “with or among the valence”). Each atom does this, therefore each atom is tightly bound to its neighbors.