closed   packed   structure     The  metallic  bonds  are  non directional.  A  metal atom in a crystal surrounds itself with maximum  number of atoms. This results in maximum packing of the atoms.  Consider closed packed atoms in a plane, as shown in the fig. c8.  One central atom is surrounded by six other atoms. Let us  call this layer `A'. On the layer A, the next layer of atoms can  occupy  any three alternate valleys, X X X or Y Y Y. Let us have  the  next  layer of atoms of X X X, which we call the layer `B'.  On the layer B, the next layer can have a configuration of A,  that is  the  third layer  of  atoms directly over those in the first.  This  will  result  in Hexagonal closed packed structure with layers ABAB.

The  other possibility is, the third layer filled with atoms above  the  position Y Y Y, which we name C. Next to C, layer A, then  B and C can come in sequence. This results in Face Centerd Cubic structure, with closed packed layers ABC ABC......