(i) The defects in ionic solids is by adding impurity ions.
(ii) If the impurity ions are in different valance state from that of host, vacancies are created in the crystal lattice of the host.
(iii) For example, addition of CdCl₂ to silver chloride yields solid solutions where the divalent cation Cd²⁺ occupies the position of Ag⁺.
(iv) This will disturb the electrical neutrality of the crystal.
(v) In order to maintain the same, proportional number of Ag⁺ ions leaves the lattice.
(vi) This produces a cation vacancy in the lattice, such kind of crystal defects are called impurity defects.

Crystal defects are classified as follows
(i) Point defects
(ii) Line defects
(iii) Interstitial defects
(iv) Volume defects
Point defects are further classified as follows

(i) By knowing the density of an unknown metal and the dimension of its unit cell, the atomic mass of the metal can be: determined.
(ii) Let 'a' be the edge length of a unit cell of a crystal, 'd' be the density of the metal, 'm' be the atomic mass of the metal and 'z' be the number of atoms in the unit cell.
(iii) Now,
Density of the unit cell
\(=\frac{Mass\ of\ the\ unit\ cell}{Volume\ of\ the\ unit\ cell}\)
\(d=\frac{Z\times m}{a^3}\) ...(1)
[Since, mass of the unit cell = Number of atoms in the unit cell x Atomic mass]
[Volume of the unit cell = (edge length of the cubic unit cell)³]
(iv) From equation (1), We have
\(m=\frac{d\times a^3}{Z}\) ....(2)
(v) Now, 
Mass of the metal (M) \(=\frac{Atomic\ mass(M)}{Avogadro's\ number(N_A)}\)
M=\(\frac{d\times a^3 \times N_A}{Z}\)
(vi) From equation (3), we can determine the atomic mass of the unknown metal.

Molecular solids: 
In molecular solids, the constituents are neutral molecules. They are held together by weak Vander Waals forces. Generally molecular solids are soft and they do not conduct electricity. These molecular solids are further classified into three types.
(i) Non-polar molecular solids:
(a) In non-polar molecular solids constituent molecules are held together by weak dispersion forces or London forces.
(b) They have low melting points and are usually in liquids or gaseous state at room temperature.
Ex: Naphthalene, anthracene etc.,
(ii) Polar molecular solids:
(a) The constituents are molecules formed by polar covalent bonds. 
(b) They are held together by relatively strong dipole-dipole interactions.
(c) They have higher melting points than the nonpolar molecular solids. 
Ex: Solid CO₂, solid NH₃ etc.

(iii) Hydrogen bonded molecular solids:
(a) The constituents are held together by hydrogen bonds.
(b) They are generally soft solids under room temperature.
(c) Examples: solid ice (H₂O), glucose, urea etc.

(i) It is given that the atoms of A are present at the corners of the cube.
(ii) Therefore, number of atoms of Q in one unit cell = 8 x \(\frac{1}{8}\) = 1
(iii) It is also given that the atoms of P are present at the body - centre.
(iv) Therefore, number of atoms of P in one unit cell.
(v) This means that the ratio of the number of P atoms to the number of Q atoms, P: Q = 1:1.
(vi) Hence, the formula of the compound is PQ.
(vii) The coordination number of both P and Q is 8.