Atoms are centered at the corners of the unit cell and at the center of each face.There are 6 faces to a cube. Each atom in the center of a face is part of 2 unit cells. Each face atom contributes 1/2 of an atom to the unit cell. Thus the 6 faces contribute 1/2 of an atom per face, or 3 atoms to the unit cell.The 8 corners of the cube again contribute 1/8 atom each, making 1 atom per unit cell.Face-centered cubic unit cells have collectively 1 atom from the eight corner atoms and 3 atoms from the six face-centered atoms (those in the centers of the four side cube faces) for a total of 4 atoms per unit cell.The second layer has the centers of atoms at an elevation along the z axis of 1/2 the unit cell length.
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