How much is the empty space present in a primitive unit cell? Also calculate the total volume occupied.

Silicon doped with phosphorus, gives an $\mathrm{n}$ -type semiconductor, while $\mathrm{p}$ -type semiconductor is obtained when silicon is doped with gallium. Give reasons.

The electrical conductivity of a metal decreases with rise in temperature, while that of a semiconductor increases. Give reasons.

Silicon is an insulator but silicon doped with phosphorus acts as a semiconductor. Give reasons.

Explain why some of the very old glass objects appear slightly milky instead of being transparent.