When you mention crystal to most people, they think of fine glassware. Metal is not the first thing that comes to mind. But, in fact, most metals are crystalline, and it is rather difficult to make noncrystalline metals. Crystalline materials have their atoms arranged in a periodic, ordered 3D array. Typically, all of the metals, many ceramics, and some polymers are crystalline. Noncrystalline materials have atoms with no periodic arrangement, i.e., a random order. Noncrystalline material can result when you have complex structures or you rapidly cool from the liquid state to the solid state. Amorphous material is another name for noncrystalline material.
Why do metals form crystals? It turns out that the lowest energy for metal atoms occurs when the atoms are packed together as tightly as possible. If you’ve ever tried to put many small pieces into a large box, you know that if you put the pieces in the box in an ordered fashion you can fit much more in the box than if you just throw things into the box in a disorderly fashion. So, for metals, ordered structures tend to be nearer the minimum energy and are more stable. In addition, since metallic bonds are nondirectional it is much simpler for metal atoms to densely pack than it is for ceramics and polymers. So how do metal atoms pack together? In the next section, we will look at one of the ways that metal atoms pack together.