We can again use the Pythagorean theorem to define the edge length (l) with respect to the body diagonal. A right triangle is formed by an edge, a face diagonal, and a body diagonal serving as the hypotenuse of the triangle. In such a triangle the body diagonal relates to the edge length as follows:
In the following Sample Exercises we use Equations 11.1 and 11.2 to calculate the radii of the atoms in a unit cell and to calculate the densities of element that crystallize in the unit cells depicted in Figure 11.10. For both calculations we need the edge lengths of the unit cell, which are obtained using an analytical technique called X-ray diffraction ( described in detail later in Section 11.9). As you do these calculations, you may wish to consult the summary in Table 11.1 of how atoms in different locations in fcc.bcc.and simple cubic unit cells contribute to the contents of those cells.