There are six nearest neighbors in the same close packed layer, three in the layer above and three in the layer below.

The coordination number of the atoms in this structure is 12. The image on the right above attempts to show several hcp unit cells in a larger lattice. Each of the 12 atoms in the corners of the top and bottom layers contribute 1/6 atom to the unit cell, the two atoms in the center of the hexagon of both the top and bottom layers each contribute ½ atom and each of the three atom in the middle layer contribute 1 atom. As shown in the middle image above, there are six atoms in the hcp unit cell. Note that there are six of these 'grooves' surrounding each atom in the hexagonal plane, but only three of them can be filled by atoms.

The middle layer has three atoms nestle in the triangular 'grooves' of the top and bottom plane. In each the top and bottom layer, there are six atoms that arrange themselves in the shape of a hexagon and a seventh atom that sits in the middle of the hexagon. The hcp structure has three layers of atoms. Some of the metals that have the fcc structure include aluminum, copper, gold, iridium, lead, nickel, platinum and silver. The packing factor (the volume of atoms in a cell per the total volume of a cell) is 0.74 for fcc crystals. As more balls are added they will pack together to fill up all the pockets. When a few additional balls are tossed in the box, they will not balance directly on top of the balls in the first layer but instead will come to rest in the pocket created between four balls of the bottom layer. To picture packing arrangement, imagine a box filled with a layer of balls that are aligned in columns and rows. The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer. In the fcc structure (and the hcp structure) the atoms can pack closer together than they can in the bcc structure. The image below highlights a unit cell in a larger section of the lattice. The fcc unit cell consists of a net total of four atoms eight eighths from corners atoms and six halves of the face atoms as shown in the middle image above.

Since 12 of its atoms are shared, it is said to have a coordination number of 12. Additionally, each of its six face centered atoms is shared with an adjacent atom. Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells. The face centered cubic structure has atoms located at each of the corners and the centers of all the cubic faces (left image below).

Atomic packing factor for bcc and fcc