Gold-copper alloy, molecular model. Gold atoms are gold, copper atoms are red. The crystal lattice of this alloy has a face centred cubic (FCC) structure, similar to that adopted by both copper and gold in their pure state. This can be seen in the unit cell of the crystal, outlined in blue at lower left. A gold atom is found at each vertex of a cube, with a copper atom in the centre of each of the cube's faces.