atoms added to the cube's cornersA crystal is one small block, the unit cell, repeated in every direction. Most metals use one of two cubic cells. Body-centred cubic adds an atom in the middle of the cube, and face-centred cubic adds one in the middle of each face, which packs the atoms as tightly as spheres can go. The packing matters: face-centred metals like copper, aluminium and gold have many planes along which layers can slide, so they are soft and easy to shape.