Hexagonal close-packed:

In a HCP (hexagonal close-packed) arrangement of atoms, a unit cell consists of three layers of atoms.  A top and bottom layers hold six atoms at the corners of a hexagon and one atom at the middle of each hexagon. A middle layer holds three atoms nestled among the atoms of the top and bottom layers, therefore, the name close-packed.

Most diagrams of the structural cells for the FCC and BCC forms of iron are drawn as by they are of the same size, as displays within Figure, but they are not. Within the BCC arrangement, the structural cell, that uses just nine atoms, is much smaller.

Figure: Common Lattice Types

Metals such as -iron (Fe) (ferrite), molybdenum (Mo), vanadium (V), chromium (Cr), and tungsten (W) possess BCC structures.  Those BCC metals have two characteristics in general, high strength and low ductility (that allows permanent deformation).  Face-centered cubic metals like as  -iron (Fe) (austenite), aluminum (Al), lead (Pb), copper (Cu), silver (Ag), nickel (Ni), gold (Au),  platinum (Pt), and thorium (Th) are, in common, of lower strength and higher ductility than BCC metals.  HCP structures are found in magnesium (Mg), beryllium (Be), zinc (Zn), cobalt (Co), cadmium (Cd), zirconium (Zr) and thallium (Tl).