The percentage of unoccupied volume in simple cubic cell is

Key Idea Percentage of unoccupied volume = 100 – Packing efficiency Packing efficiency
$=\frac{\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}}{\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{o}\mathrm{f}\mathrm{c}\mathrm{u}\mathrm{b}\mathrm{i}\mathrm{c}\mathrm{u}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{l}}\times 100\%$
For simple cubic cell,
Packing efficiency $=\frac{\frac{4}{3}\pi r^3}{8r^3}\times 100=524\%$
$\therefore$ Percentage of unoccupied volume in Scc = 100 – 52.4 = 47.6%