CBSE Class 12 Chemistry Solid State Chapter Notes

18. Types of voids:
a. Tetrahedral voids: This type of void is formed at the centre when four spheres are joined in the form of a tetrahedron.
b. Octahedral void: This type of void is surrounded by six spheres.

19. In hcp or ccp arrangement, octahedral and tetrahedral voids are present.
The number of octahedral voids present in a lattice is equal to the number of close packed particles. The number of tetrahedral voids is twice the number of octahedral voids.
Example: If the number of close packed particles = n
Number of particles present in octahedral voids = n
Number of particles present in tetrahedral voids = 2n

20. Packing efficiency is the percentage of total space occupied by constituent particles (atoms, molecules or ions)
Packingefficiency = Volume occupiedby spheres / Total volume of unit cell x 100%
a. Packing efficiency for face centred unit cell =74%
b. Packing efficiency for body centred cubic unit cell =68% 
c. Packing efficiency for simple cubic unit cell =52.4%

21. Radius ratio in an octahedral void: For an atom to occupy an octahedral void, its radius must be 0.414 times the radius of the sphere. r/R = 0.414

22. Radius ratio for tetrahedral void: For an atom to occupy a tetrahedral void, its radius must be 0.225 times the radius of the sphere.  r/R = 0.225

23. Density of a unit cell is same as the density of the substance.

24. Relationship between radius of constituent particle (r) and edge length( a):
a. Simple cubic unit cell: a= 2r
b. Face centred unit cell: a 2√2r
c. Body centred unit cell: a= 4r√/3 

26. Number of atoms in a unit cell (z):
a. Simple cubic unit cell: z = 1
b. Face centred unit cell: z = 4
c. Body centred unit cell: z = 2

27. Density of unit cell:
Density of unit cell= zM/a₃.N_A

28. Crystal defects are basically irregularities in the arrangement of constituent particles.

29. Defects are of two types:
a. Point defects - Point defects are the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance.
b. Line defects - Line defects are the irregularities or deviations from ideal arrangement in entire rows of lattice points.

30. Point defects are of three types:
a. Stoichiometric or intrinsic or thermodynamic defects: These are the point defects that do not disturb the stoichiometry of the solid.
b. Non – stoichiometric defects: These are the point defects that disturb the stoichiometry of the solid.
c. Impurity defects: These are the defects in ionic solids due to the presence of impurities present in them.

31. Stoichiometric defects for non- ionic solids are of two types:

Vacancy defect
A crystal is said to have vacancy defect when some of the lattice sites are vacant.
This results in decrease in density of the substance.

Interstitial defect
A crystal is said to have interstitial defect when some constituent particles (atoms or molecules) occupy an interstitial site.
This results in increase in density of the substance.

32. Stoichiometric defects for ionic solids are of two types:

Schottky defect
In this defect equal number of cations and anions are missing.
It is basically a vacancy defect in ionic solids.
It decreases the density of a solid
Schottky defect is shown by ionic substances in which the cation and anion are of almost similar sizes.
For example: NaCl, KCl, CsCl and AgBr

Frenkel or dislocation defect
In this defect, the smaller ion (usually cation) is dislocated from its normal site to an interstitial site.
It creates a vacancy defect at its original site and an interstitial defect at its new location.
It does not change the density of the solid
Frenkel defect is shown by ionic substance in which there is a large difference in the size of ions.
For example: ZnS, AgCl, AgBr and AgI

34. Solids can also be classified into three types based on their electrical conductivities:
a. Conductors-The solids with conductivities ranging between 10⁴ to 10⁷ ohm^–1m^–1 are called conductors.
b. Insulators - These are the solids with very low conductivities ranging between 10^–20 to 10^–10 ohm^–1m^–1.
a. Semi- conductors - These are the solids with conductivities in the intermediate range from 10^–6 to 10⁴ ohm^–1m^–1

35. Band theory –  A metal is characterized by a band structure. The highest filled band is called valence band and the lowest unoccupied band is called conduction band. The gap between the two bands is called forbidden band.
a. In case of conductors, the valence band and conduction band overlap
b. In case of insulators, the forbidden gap is very large and the electrons are unable to excite to the conduction band.
c. In case of semiconductors, forbidden gap is small. Therefore, some electrons may jump to conduction band and show some conductivity. Electrical conductivity of semiconductors increases with rise in temperature, since more electrons can jump to the conduction band.

36. Types of semiconductors:
a. Intrinsic: These are those semiconductors in which the forbidden gap is small. Only some electrons may jump to conduction band and show some conductivity. They have very low electrical conductivity. Example: Silicon, germanium
b. Extrinsic: When an appropriate impurity is added to an intrinsic semiconductor. Their electrical conductivity is high.

38. Types of extrinsic semi conductors:

n-type semiconductors
They are formed when silicon is doped with electron rich impurity like group 15 elements.
The increase in conductivity is due to the negatively charged electrons.

p –type semiconductors
They are formed when silicon is doped with electron deficient impurity like group 13 elements.
The increase in conductivity is due to the positively charged holes.

39. Diode: It is a combination of n-type and p-type semiconductors and is used as a rectifier.

40.Transistors: They are made by sandwiching a layer of one type of semiconductor between two layers of the other type of semiconductor. npn and pnp type of transistors are used to detect or amplify radio or audio signals.

41.12- 16 compounds – These compounds are formed by the combination of group 12 and group 16 compounds. They possess an average valency of 4. Example: ZnS, CdS, CdSe and HgTe

42. 13- 15 compounds – These compounds are formed by the combination of group 13 and group 15 compounds. They possess an average valency of 4. Example: InSb, AlP and GaAs

43. Every substance has some magnetic properties associated with it.
The origin of these properties lies in the electrons. Each electron in an atom behaves like a tiny magnet. Its magnetic moment originates from two types of motions (i) its orbital motion around the nucleus and (ii) its spin around its own axis.

44.On the basis of their magnetic properties, substances can be classified into five categories:
a. Paramagnetic: These are those substances which are weakly attracted by the magnetic field. It is due to presence of one or more unpaired electrons.
b. Diamagnetic: Diamagnetic substances are weakly repelled by a magnetic field. Diamagnetism is shown by those substances in which all the electrons are paired and there are no unpaired electrons.
c. Ferromagnetic: These are those substances which are attracted very strongly by a magnetic field.
d. Antiferromagnetic: They have equal number of parallel and anti parallel magnetic dipoles resulting in a zero net dipole moment.
e. Ferrimagnetic: They have unequal number of parallel and anti parallel magnetic dipoles resulting in a net dipole moment.