At the atomic level, pure metals exist as crystalline arrays (Figure 7-2) that are continuous in three dimensions. In these arrays, the nuclei and core electrons occupy the atomic center with the ionizable electrons floating freely among the atomic positions. The mobility of the valence electrons is responsible for many properties of metals, such as electrical conductivity. It is important to note that the positively charged atomic centers are held together by the electrons and their positive charge is simultaneously neutralized by the negative electrons. Thus pure metals have no net charge.

The relationships between the atomic centers in a metallic crystalline array are not always uniform in all directions. The distances in the x, y (horizontal), and z (vertical) axes may be the same or different, and the angles between these axes may or may not be 90 degrees. In all, six crystal systems occur (Figure 7-3), and they can be further divided into 14 crystalline arrays. Metallic nuclei may occur at the center of faces or vertices of the crystal. Within each array, the smallest repeating unit that captures all the relationships among atomic centers is called a unit cell (see Figure 7-2). The unit cells for the most common arrays in dental metals are shown in Figure 7-4. In the body-centered cubic (BCC) array, all angles are 90 degrees and all atoms are equidistant from one another in the horizontal and vertical directions. Metallic atoms are located at the corners of the unit cell, and one atom is at the center of the unit cell (hence the name body-centered cubic). This is the crystal structure of iron and is common for many iron alloys. The face-centered cubic (FCC) array has 90-degree angles and atomic centers that are equidistant horizontally and vertically (as does the BCC), but atoms are located in the centers of the faces with no atom in the center of the unit cell (hence the name face-centered cubic). Most pure metals and alloys of gold, palladium, cobalt, and nickel exhibit the FCC array. Titanium exhibits the more complex hexagonal close-pack array. In this array, the atoms are equidistant from each other in the horizontal plane, but not in the vertical direction.

In a metallic crystal, the atomic centers are positively charged because the free valence electrons float in the crystal. Although we might expect the atomic centers to repel each other, the freely floating electrons bind the centers together and create a strong force between the atomic centers. This is known as the metallic bond and is a fundamentally important type of primary bond. The metallic bond is fundamentally different from other primary bonds, such as covalent bonds that occur in organic compounds, and ionic bonds that occur in ceramics.

Because the distances between metal atoms in a crystal lattice may be different in the horizontal and vertical directions (see Figure 7-4), properties such as conductivity of electricity and heat, magnetism, and strength may also vary by direction if a single crystal is observed. These directional properties of metals and metalloids have been exploited in the semiconductor industry to fabricate microchips for computers. However, in dentistry, a single crystal is rarely observed. Rather, a collection of randomly oriented crystals, each called a grain, generally make up a dental alloy. In this case, the directional properties are averaged across the material. In general, a fine-grained structure is desirable to encourage alloys with uniform properties in any direction. Nonuniformity of directional properties is termed anisotropy.

Like the physical properties, the mechanical properties of metals are also a result of the metallic crystal structure and metallic bonds. Metals generally have good ductility (ability to be drawn into a wire) and malleability (ability to be hammered into a thin sheet) relative to polymers and ceramics. To a large extent, these properties result from the ability of the atomic centers to slide against each other into new positions within the same crystal lattice. Because the metallic bonds are essentially nondirectional, such sliding is possible.

If the metallic crystals were perfect, calculations have shown that the force required to slide the atoms in the lattice would be hundreds of times greater than experiments indicate. Less force is necessary because the crystals are not perfect; they have flaws called dislocations. Dislocations allow the atomic centers to slide past each other one plane at a time (Figure 7-5). Because movement can occur one plane at a time, the force required is much less than if the forces of all the planes have to be overcome simultaneously. An analogy is moving a large heavy rug by forming a small fold or kink in the rug and pushing the fold from one end of the rug to the other. Dislocations are of several types, but all serve to allow the relatively easy deformation of metals. All methods for increasing the strength of metals act by impeding the movement of dislocations.