SOLIDS: PROPERTIES AND STRUCTURES:
Try this: What is the difference between and amorphous and a crystalline solid? answer: Amorphous solids, like glass and rubber have no long term order to their structures. Crystalline solids, like salts and metals, have a well-ordered 3-D structure.
Because of their repeat structure, crystalline solids are the easiest to describe. Consider the following annalogy to the wall paper in Dr. Science's livingroom:
Rather than draw out the entire wallpaper (its a big livingroom!), we can convey the same information with the following square if we know that it keeps repeating. This repeat unit is called the unit cell. When extended in 3 dimensions, the same process works for the packing of atoms or molecules in the solid state. You can imagine that some unit cells can be quite complicated like . Don't worry though because we will focus on the simplest family of unit cells: the cubic unit cell system where all sides are the same length and all angles are 90o. There are three types of cubic unit cell: simple cubic (SC), body-centered cubic (BCC) and face-centered cubic (FCC)
For atoms that pack together in the siple cubic arrangement, unit cell will have one atom on each corner of the cube. For the sake of clarity, sometimes we represent the SC unit cell as but it is important to think of the atoms coming in contact along the edges of the cube. Even though we know atoms don't have hard surfaces, sometimes it is helpful to picture them as solid spheres:. Each unit cell only has one atom since each corner atom contributes only 1/8 to the cell (x 8 corners = 1). See figure 11.33 in your text.
Simple cubic unit cells are rare in nature because they have a lot of empty space (in the center of the cube). In nature most metals and ions prefer to pack together more tightly in the BCC or FCC unit cells like these fruit:
As with the SC, the BCC and FCC have atoms in the corners of the cell (again 1/8 contribution each). In addition, the BCC has an atom in the center and the FCC has atoms on each of the 6 faces of the cube. Center atoms count for 1 whole giving a total of 2 atoms/cell for BCC. Face atoms count for 1/2 each giving a total of 4 atoms/cell for FCC.
For section 11.8 make sure that you know the information in TABLE 11.6.