The structure of most of the metals (from s and d Blocks of the periodical chart) belong either to one or more of the three simple type of structures:
1. Cubic close packed (face centered cubic)
2. Hexagonal close packed
3. Body centered cubic
Cubic close packed (face centered cubic):In this structure, atoms are arranged at the corners and at the centres of all six faces of a cube. This type of arrangement has been discussed in detail before. In this structure each atom has 12 nearest neighbours as shown in figure. For example, the atom at the center of the middle face has four nearest neighbours at the corners of that face and eight more at the same distance at the center or four faces of adjoining cubes.
2 Hexagonal close packed
In this arrangement, atoms are located at the corners and the center of two hexagons which are placed parallel to each other and three more atoms in a parallel plane midway between these two planes. This arrangement is obtained when we have ABABA... type of close packing of atoms. Each atom in this arrangement has also 12 nearest neighbours as shown in the figure.