directional bonding and simple covalent structures

The number of adjacent atoms covalently bonded to specific atoms, in the limit by covalent valence of the atom. Covalent valence is usually small, most of the covalent structures of molecular crystals, in which covalent bonds are found only on the molecule in question, but the molecules themselves each is bound by the bond dipole, which is different from covalent bonds. Many organic compounds have this structure, but a lot of bonding structure entirely based on covalent bonding. In general, the structure has covalent bonds stronger than ionic bonding structure, and compound of this structure is more stable and stronger at high temperatures. The number of atoms in a volume less than in the structure of the meeting because of the directional bonding thus its density is lower.

Bond distance between atoms for covalent bond different from ionic bonding. This is called covalent bonding radius of the atoms, a little to the coordination and orthohedral tetrahedral, and form hybrid orbital are also changed.
Typical structure can be seen below:

Diamond structure 
Lighter elements in group IV such as diamond (C), Si, Ge, gray tin, etc., has a diamond structure. In this structure every atom is bound by its overlap with each other and the sp3 hybrid orbital connective tissue in the direction of the peak octahedron.

The structure of graphite 
Graphite is a crystalline polymorph of carbon. Carbon atoms form a covalent bond coating composition of the meeting, which was tied with weak pi bond. The upper layer of dense array of covalent bonds formed by sp2 hybrid orbital.