The vacant space in bcc lattice unit cell is:

Step 1: Packing efficiency of BCC unit cell - Packing efficiency of BCC unit cell is given by, $$\begin{aligned} & \text{ Packing efficiency }=\frac{\text{ Volume occupied by two spheres } \times 100}{\text{ Volume of unit cell }} \\ & =\frac{2 \times \frac{4}{3} \pi r^3 \times 100}{a^3} \\ & =68 \% \end{aligned}$$) - The (\%) volume occupied by atoms in a unit cell is (68 \%) Step 2: % Vacant space in BCC unit cell - % vacant space is given by,$$\begin{aligned} & \% \text{ Vacant space }=100-\% \text{ of the volume occupied by atoms } \\ & =100-68 \\ & =32 \% \end{aligned}$$) - % Vacant space is (32 \%)