its atoms arrange themselves in ordered three-dimensional arrays, forming a crystalline solid. Think of these arrays as stacked layers (two-dimensional planes) of Cu atoms packed together as tightly as possible. Each atom in a layer touches 6 others in that layer. The atoms in a layer directly above nestle into the spaces created by the atoms below, as shown in Figure 11.5 Similarly, the atoms in a third layer nestle among those in the second. However, two different alignments are possible for the atoms in the third layer.