Figure 11.12 illustrates the structure of bronze in which tin (Sn) atoms have replaced some of the copper atoms in what would have been a face-centered cubic unit cell of Cu. The radii of copper and tin atoms are similar--128 pm and 140 pm, respectively. Inserting the slightly larger Sn atoms in the cubic closest-packed structure of copper atoms disturbs the Cu lattice structure a little, making the planes of copper atoms "bumpy" instead of uniform (Figure 11.13). This atomic-scale roughness makes it more difficult for the copper atoms to slip past one another. Less slippage makes bronze less malleable than copper, but being less malleable also means that bronze is harder and stronger.