Simulating a Crystal Lattice

Atoms are often simulated as spheres and so it is a simple matter to simulate groups of atoms such as molecules and crystals as groups of spheres.  The following code doesn't simulate a particular srystal or set of atoms, but it does show that it is a simple matter to create groups of such atoms.  Here, a crystal lattice is simulated by alternating small red spheres and larger yellow spheres.

gr=Graphics3D[{EdgeForm[],Yellow,Specularity[White,50],Table[Sphere[{x,y,z}],{x,0,10,5},{y,0,10,5},{z,0,10,5}],Red,Specularity[White,50],Table[Sphere[{x,y+2,z},.3],{x,0,10,5},{y,0,10,5},{z,0,10,5}]},ImageSize->600,Background->Black,Boxed->False]