You must first determine the formation of the aluminum. This will depend on the crystallization, so a different number of atoms may be present in the cell. Because aluminum crystallizes in a face-centered cubic arrangement, the coordination number should be 12, with four atoms within each face-centered cubic unit cell. Thus, four complete spheres are formed, three from the face center and one from the corners.
If there are four atoms (one is from eight corners and the three are from six faces), take the molar mass of aluminum, which is 26.98 grams per mole--or molecular weight--and divide by Avogadro's number, which is 6.022x10^23 (or 6.022 multiplied by 10 to the power of 23). This figure equals one aluminum atom in grams. Keep in mind that the faced-centered cell has four atoms, so multiply this figure by four.
The answer to this equation contains the units of grams per unit cell of aluminum, and the answer indicates how many grams are in one unit cell of aluminum. Your final answer should be approximately 1.79 x 10^-22 (or 1.79 multiplied by 10 to the power of negative 22) grams per unit cell.