FCC crystal structure

One of the classic metal structures is FCC, face-centered cubic. Where do the atoms touch? Along the face diagonal. What is the lattice parameter if we know the atomic radius? (2)^(1/2)*a=4*R. What is the coordination number? 12. Each atom has 12 nearest neighbors. What is the atomic packing fraction? There are 4 atoms per cell and the volume of each divided by the cell volume gives us 0.74, or 74% as the atomic packing fraction. This video goes over these core concepts.