crystal lattice and unit cell:

(i) H-bonding is the intermolecular force present in ice.
(ii) Covalent bonds are present in network solids.

In ionic crystals, the units occupying lattice points are positive and negative ions.
The ionic crystals have the following characteristics :
i) The heat of vapourisation is high.
ii) The vapour pressure of ionic crystals at ordinary temperature are very low.
iii) Their melting and boiling points are very high.
iv) They are hard and brittle.
v) They are insulators in the solid state.
vi) Ionic crystals are soluble in water and also in other polar solvents.
vii) Ionic solids are good conductors when dissolved in water.

The fundamental equation that gives a simple relation between the wavelength of the X-rays, the interplanar distance in the crystal and the angle of reflection is known as Bragg's equation.
nλ = 2dsinθ
Where n is the order of reflection
λ is the wavelength of X-rays
d is the interplanar distance in the crystal
θ is the angle of reflection 

(i) A face centred cubic lattice has 4 atoms per I unit cell.
(ii) The lattice has 8 atoms in the corners (N_c) and 6 atoms on the faces (N_f) (one on each face)
Contribution by atoms on the corners to unit cell =\(\frac{N_c}{8}=\frac{8}{8}=1\)
Contribution by atoms on the faces to unit cell =\(\frac{N_f}{2}=\frac{6}{2}=3\)
∴ The number of atoms per unit cell in fcc
\(=\frac { { N }_{ c } }{ 8 } +\frac { { N }_{ f } }{ 2 } =\frac { 8 }{ 8 } +\frac { 6 }{ 2 } =1+3=4\)