ANALYZE The ratio of the atomic radii of Cu to Ag in 128 pm/ 144 pm = 0.89, or 89%
SOLVE Atoms of Cu can substitute for atoms of Ag in a ccp structure of the latter with some room to spare. Both elements form fcc unit cells, so little disruption to the Ag lattice should result from incorporating Cu atoms, which are nearly the same size. Copper atoms are too big, however, to fit into the octahedral holes in the lattice of silver atoms. Thus, the Cu-Ag alloy, sterling silver, is a substitutional alloy.