The cubic unit ceIl of A1(molar mass 27 g mol^-1) has an edge length of 405 pm. Its density is 2.7 g cm^-3. The cubic unit cell is

$\begin{array}{l}\text{ Density }\left(\mathrm{d}\right)=\frac{\mathrm{Z}\times \mathrm{M}}{{\mathrm{a}}^3\times {\mathrm{N}}_{\mathrm{A}}}\\ 2.7=\frac{\mathrm{Z}\times 27}{{\left(405\times {10}^{-10}\right)}^3\times 6.022\times {10}^{23}}\\ \mathrm{Z}=\frac{2.7\times (405{)}^3\times {10}^{-30}\times 6.022\times {10}^{23}}{27}=4\end{array}$
For face centred cubic unit cell, number of atoms are 4.