aAl = 0.4050 nm
FCC
Aw = 26.98 g/mole
aAg = 0.4086 nm
FCC
Aw = 107.87 g/mole
Density corresponds to mass per unit volume. In an FCC unit cell this corresponds to the mass of 4 atoms
in the volume of a single unit cell.
Al =
Ag =
1mole
26.98 g

23
6.022 10 atoms mole  2.7 g / cm 3 (agrees with tabulated value)
3
0.4050nm 3  121cm 3
10 nm
4atoms 
4atoms 
1mole
107.87
6.022 10 23 atoms
 10.50 g / mole (agrees with tabulated value)
0.40863 1021
If we model Al and Ag atoms as spheres, the fact that both elements are FCC and have comparable lattice
parameters indicates that the spheres are about the same size. Indeed, the tabulated atomic radii of Al
and Ag are very similar. The difference in density is thus due to the fact that there is a lot more stuff –
electrons, protons, and neutrons – inside the Ag atomic sphere than in the Al atomic sphere.