ANALYZE The ratio of the atomic rad

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.
THINK ABOUT IT The guideline for two metals forming a substitutional alloy is that their atomic radii are within 15% of each other. The radii of Ag and Cu differ by only 11%, so we would expect copper and silver to form a substitutional alloy. Moreover, copper and silver are in the same group in the periodic table have the same crystal structures (Figure 11.7).