The ratio of bulk to surface atoms for a cubic centimeter of material is about . We can make this statement because most interactions are localized within a few neighbor disctances, hence surface effects die out fast. Real crystals have domains, but even these domains are large enough that we can treat the material in a single domain as a Bravais lattice. In all these three structures a is the size of the cube's side. The choice of primitive vectors is not unique, but the ones above are the standard ones.
Coordination number is the number of nearest neighbors.
We have sc 6, bcc 8, and fcc 12. Chemical bonding is short-ranged and the coordination number plays an important role in simple models. The choice of primitive cell is not unique. A useful choice for computations is with . This type of cell clearly shows the symmetry, but contains more than one lattice point.