If the number of tetrahedral voids in a crystal lattice is $N$,then the number of close-packed spheres will be ..........

The percentage of vacant space for $bcc$ unit cell is ............. $\%$

The fraction of the total volume occupied by the atoms in a simple cubic unit cell is

$A$ metal forms an $fcc$ structure. Calculate the volume of the $fcc$ unit cell in $\text{cm}^3$ if the void volume is $1.66 \times 10^{-23} \text{ cm}^3$.

What is the percentage of unoccupied volume in $BCC$ structure (in $\%$)?

What is the number of tetrahedral voids present in $0.5 \text{ mole}$ of a compound forming an $hcp$ structure?