Schottky defect:
(i) Schottky defect arises due to the missing of equal number of cations and anions from the crystal lattice.
(ii) This effect does not change the stoichiometry of the crystal.
(iii) Ionic solids in which the cation and anion are of almost of similar size show schottky defect. Ex: NaCl.
(iv) Presence of large number of schottky defects in a crystal, lowers its density.

Frenkel defect:
(i) Frenkel defect arises due to the dislocation of ions from its crystal lattice.
(ii) The ion which is missing from the lattice point occupies an interstitial position.
(iii) This defect is shown by ionic solids in which cation and anion differ in size.
(iv) Unlike Schottky defect, this defect does not affect the density of the crystal.
(v) For example AgBr, in this case, small Ag⁺ ion leaves its normal site and occupies an interstitial position.

(i) Similarities between ionic and metallic crystals:
(a) Ionic and metallic crystals have electrostatic forces of attraction.
(b) Both the crystals exhibit high melting point.
(c) The bonds in metallic and ionic crystals are non-directional.
(ii) Differences between ionic and metallic crystals:

Density of the element, d = 2.7 x 10³ kg m^-3
Molar mass, M = 2.7 x 10^-2 kg mol^-1
Edge length, a = 405 pm
= 405 x 10^-12 m
= 4.05 x 10^-10 m
Avogadro's number, N_A= 6.022 x 10²³ mol^-1
\(\therefore d=\frac { Z\times M }{ { a }^{ 3 }\times { N }_{ A } } \)
\(\Rightarrow Z=\frac { d\times { a }^{ 3 }{ N }_{ A } }{ M } \)
\(=\frac { 2.7\times { 10 }^{ 3 }kg\quad { m }^{ -3 }{ (4.05\times { 10 }^{ -10 }m) }^{ 3 }\times 6.022\times { 10 }^{ 23 }{ mol }^{ -1 } }{ 2.7\times { 10 }^{ -2 }kg\quad { mol }^{ -1 } } \)
= 4.004 = 4.
This implies that four atoms of the element are present per unit cell. Hence the unit cell is face centred cubic.

(i) Solids have definite volume and shape 
(ii) Solids are rigid and incompressible
(iii) Solids have strong cohesive forces.
(iv) Their constituents have fixed positions and can only oscillate about their mean positions.

(i) The colour develops because of the presence of electrons in the 8 anionic sites.
(ii) These electron absorb energy from the visible region of radiation and get excited. 
(iii) For example when crystals of NaCl are heated in an atmosphere of sodium vapours, the sodium atoms get deposited on the surface of the crystal and the deposited Na atoms.
(iv) During this process, the Na atoms on the surface lose electrons to form Na⁺ ions 
(v) These electrons get excited by absorbing energy from the visible light and impart yellow colour to the crystals.