d = ZM / ($N_A$ x $a^3$). Z = atoms per unit cell, M = molar mass $\frac{g}{mol}$, $N_A$ = 6.022 x 10^23, a = edge length in cm. Convert pm to cm: 1 pm = 10^-10 cm. Example: Ag (FCC, a = 408 pm, M = 108): d = 4(108) / (6.022 x 10^23 x (4.08 x 10^-8)^3) = 10.57 g/$cm^3$. Can rearrange to find M (identify element), a (find edge length), or Z (determine structure type).