Copper 3: the copper unit cell

Other elements that adopt this structure: Al, Ni, Rh, Pd, Ag, Ir, Pt, Au, Ac, Ce and Yb.

The black wireframe just marks out the repeating unit of space in the structure - the lines are not supposed to depict any kind of chemical bond. Copper atoms in the bulk of the structure have a coordination number of 12. This is not readily apparent from the unit cell as there is no atom in the centre of cube: copper atoms occupy the centres of the (six) faces and the (eight) corner positions. This arrangement is therefore also known as "face-centred cubic" (FCC). Note that only a fraction of each atom is actually inside the unit cell as the vertices of the cell are in the centres of the corner atoms, and the cube faces pass through the centre of the face-centred atoms. Only one eighth of each corner atom and one half of each face-centred atom is inside the unit cell. That is to say that each corner atom is shared between eight neighbouring unit cells and each face-centred atom is shared between two neighbouring unit cells. Therefore the atom occupancy in the unit cell is (8 x 1/8) + (6 x 1/2) = 4.

Page 4 shows the close-packed layers with the unit cell.

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