what types of close packed unit cells do most metals assume?
Most metals assume close-packed unit cells, which include the face-centered cubic (FCC) and body-centered cubic (BCC) structures.
To understand why metals prefer close-packed structures, one must consider the nature of metallic bonding. Metals consist of positive metal ions surrounded by a sea of delocalized electrons. The metal ions arrange themselves in a regular pattern to maximize the attractive forces between the positive ions and the delocalized electrons, thereby minimizing the overall energy of the system.
The face-centered cubic (FCC) structure is one of the most common close-packed arrangements. This structure consists of a cube with an atom centered at each corner and an additional atom located at the center of each face. Each atom is in contact with twelve neighboring atoms. Examples of metals that adopt the FCC structure include aluminum (Al), copper (Cu), and gold (Au).
The body-centered cubic (BCC) structure is another close-packed arrangement. It consists of a cube with an atom centered at each corner and an additional atom located at the center of the cube. Each atom is in contact with eight neighboring atoms. Examples of metals that adopt the BCC structure include iron (Fe), chromium (Cr), and tungsten (W).
To determine the type of close-packed structure that a specific metal assumes, you can refer to a crystal structure database or consult reference materials that provide information on the crystal structures of various metals. These resources will typically specify the crystal structure, lattice parameters, and space group of each metal.