The Solid State

 It is basically a vacancy defect in ionic solids. In order to maintain electrical neutrality, the number of missing cations and anions are equal.

Thus there is equal number of positive and negtive ion leave correct lattice point and go outside the lattice creating  a pair of vacancy. So there are less number of ions thannbefore whichresult in the decrease in its density.

Silicon forms four covalent bonds thus two types of impurities can be added to silicon.
for example : boron can be added or phosphorus can be added.
In case of boron : boron form three bonds with silicon, it will result in an electron deficient bond andwill create a hole. These holes can move through crystal like positive charge giving rise electrical conductivity.
In case of phosphorus: when silicon forms four bonds with phosphorus one electron of phosphorous atom will remain unbonded due to which it become delocalizedn and contributes to electrical conductivity.

We have,
Edge of length of cell a = 4.07x10^–8cm
Density p = 10.5 g /cm³
Number of atoms in unit cell of fcc lattice = 4
Avogadro number NA = 6.022x10²³
By using formula,

We have give that,
Density (d)=8.55 g Cm^-3
Atomic mass (M) =93u =93 gMol^-1
Atomic radius (r) = ?

We know that, Avogadro number N_a =6.022 x 10 ²³ mol^-1

since given lattice is bcc

therefore
Number of atoms per unit cell (z) =2

we know that