The edge length of unit cell of a body-centred cubic (bcc) crystal is $352 \mathrm{pm}$. Calculate the radius of the atom.

In a body-centred and face-centred arrangement of atoms of an element, what will be the numbers of atoms present in respective unit cells? Justify your answer with calculation.

Lead sulphide has face centred cubic crystal structure. If the edge length of the unit cell of lead sulphide is $495 \mathrm{pm}$, calculate the density of the crystal (at. wt. $\mathrm{Pb}=207, \mathrm{S}=32$).

Examine the defective crystal given below and answer the question that follows:

State if the above defect is stoichiometric or non- stoichiometric. How does this defect affect the density of the crystal? Also, write the term used for this type of defect.