[A. K.

MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 1
 SOLID STATE
PART-1: GENERAL CHARACTERISTICS OF SOLID STATE, AMORPHOUS SOLID STATE
AND CRYSTALLINE SOLID, CLASSIFICATION OF CRYSTALLINE SOLIDS.
MATTER .

PHYSICAL CLASSIFICATION CHEMICAL CLASSIFICATION

Gas Liquid Solid Element Compound Mixture

 SOLID: - The main characters of solids are:
1. Why are solids rigid?
1. Have definite shape, size & volume.
 Particles in solids are closely packed, having
2. Incompressible, have high densities.
strong force of attraction among them.
3. These are rigid.
Therefore, they are incompressible and rigid.
4. Regular order of arrangement of particles
2. Why do solids have a definite volume?
5. Strong intermolecular forces.
 The particles in solids have strong
6. Smaller intermolecular distance.
intermolecular attraction and they are
7. Show vibration motion about its mean.
strongly held together at fixed position, thus
8. Translational motion absent.
solids have definte volume.
9. Low kinetic energy

SOLID
S. PROPERTIES CRYSTALLINE SOLID AMORPHOUS SOLID
1 Shape Internal arrangement of particles is Internal arrangement of particles is
regular, i.e. they have Characteristic irregular, i.e. they have irregular shape.
geometrical shape.
2 Melting point They have sharp melting point don't have sharp melting point (melt
over a wide range of temperature)
3 Cleavage When cut with a sharp edged tool, When cut with a sharp edged tool, they
Property they split into two pieces and the cut into two pieces with irregular
newly generated surfaces are plain surfaces
and smooth
4 Heat of fusion have a definite heat of fusion do not have definite heat of fusion
5 Anisotropy Anisotropic in nature because of Isotropic in nature because of these
these substances show different substances show same property in all
property in different direction directions
6 Nature True solids and symmetrical Pseudo solids or supercooled liquid &
Unsymmetrical.
7 Example Salt, sugar, NH4Cl, potassium nitrate, Glass, fibre glass, plastics, rubbers,
naphthalene, benzoic acid, copper Polyurethane, teflon, cellophane,
etc. polyvinyl chloride etc.
*Noted:- Glass is considered a as super cooled liquid or pseudo solid b/c glass has a tendency to
flow, though very, slow.

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 2
 TYPES OF CRYSTALLINE SOLIDS:-
Type of Solid Constituent Bonding/ Physical Melting Electrical Examples
Particles Attractive Nature Point Conductivity
Forces
1. Molecular Molecules
solids
 Non polar  Dispersion or  Soft  Very  Insulator  Ar, CCl4,
London forces low H2, I2, CO2
 Polar  Dipole-dipole  Soft  Low  Insulator  HCl, SO2
interactions
 Hydrogen  Hydrogen  Hard  Low  Insulator  H2O (ice)
bonded bonding
2. Ionic Ions Electrostatic Hard but High Insulators in NaCl, MgO,
solids brittle solid state ZnS, CaF2
but
conductors
in molten &
in aqueous
solutions
3. Metallic Positive ions Metallic bonding Hard but Fairly Conductors Fe, Cu, Ag, Mg
solids in a sea of malleabl High in solid state
delocalised e and as well as in
electrons ductile molten
State
4. Covalent Atoms Covalent bonding Hard Very Insulators SiO2 (quartz),
or High Conductor SiC, C(diamond),
network (exception) AlN,
solids Soft C(graphite)

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 3
 PART-2 : CRYSTAL LATTICE & UNIT CELL
A. CRYSTAL LATTICE:- The regular three dimensional arrangement of constituent particles
in space is called a crystal lattice. The positions which are occupied by atoms, ions or
molecules in the crystal lattice are called lattice points or lattice sites. There are only 14
possible three dimensional lattices. These are called Bravais Lattices. The following are
the characteristics of a crystal lattice:
i. Each point in a lattice is called lattice point or lattice site.
ii. Each point in a crystal lattice represents one constituent particle which may be an atom, a
molecule (group of atoms) or an ion.
iii. Lattice points are joined by straight lines to bring out the geometry of the lattice.
B. UNIT CELL:- The smallest portion of the crystal lattice which when repeated again and
again in different directions generates the complete space lattice.
A unit cell is characterized by:
(i) its dimensions along the three edges, a, b and c.
These edges may or may not be mutually perpendicular.
(ii) angles between the edges, α (b/w b & c)
β (b/w a & c) and γ (between a & b).
Thus, a unit cell is characterized by six parameters, a, b, c, α, β and γ.
C. TYPES OF UNIT CELLS:- Unit cells can be broadly divided into 2 categories, primitive and
centred unit cells.
1. Primitive Unit Cells:- When constituent particles are present only on the corner
positions of a unit cell, it is called as primitive unit cell or simple Unit cell.
2. Centred Unit Cells:- When a unit cell contains one or more constituent particles present at
positions other than corners in addition to those at corners, it is called a centred unit cell.
Centred unit cells are of three types:
i. Body-Centred Unit Cells: When atoms are present at 8 corners as well as in the body
centre in a cubic unit cell then this arrangement is known as BCC.
ii. Face-Centred Unit Cells: When atoms are present in all 8-corners and six face centres in
a cubic unit cell then this arrangement is known as FCC
iii.End-Centred Unit Cells: When atoms are present in all 8-corners & one constituent
particle is present at the centre of any two opposite faces

Primitive Unit Cell Body-Centred Unit Cell Face-Centred Unit Cell End-Centred Unit Cell

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 4
D. SEVEN CRYSTAL SYSTEMS AND FOURTEEN BRAVAIS LATTICE
Crystal Possible Edge Axial angles Examples Geometry
system variations lengths
1 Cubic Primitive, a = b = c α = β = γ = 900 NaCl, Zinc
Body-centred, blende,
Face-centred Cu
2 Tetragonal Primitive, a=b≠c α = β = γ = 900 White tin,
Body-centred, SnO2,
TiO2, CaSO4

3 Orthorhom Primitive, a≠b≠c α = β = γ = 900 Rhombic

bic Body-centred, sulphur,
Face-centred KNO3, BaSO4
End-centred
4 Hexagonal Primitive, a=b≠c α = β = 900, γ Graphite,
= 1200 ZnO,CdS,

5 Trigonal or Primitive, a=b=c α = β = γ ≠ 900 Calcite

Rhombohed (CaCO3), HgS
ral (cinnabar)

6 Monoclinic Primitive, a≠b≠c α = γ = 900 Monoclinic

End-centred β ≠ 1200 sulphur

7 Triclinic Primitive, a≠b≠c α ≠ β ≠ γ ≠ 900 CusSO4.5H2O,

K2Cr2O7,
H3BO3

E. NUMBER OF ATOMS PER UNIT CELL

i. There are eight (8) corners, Twelve (12) edges, and six (6) faces of a cube. Total number of
body centre in a cube = 1, Total number of edge centre in a cube = 12 & Total number of
face centre in a cube = 6.
ii. An atom at corner of unit cell is shared by eight (8) unit cells in the lattice and hence
contributes only 1/8 to a particular unit cell.
iii. An atom at edge centre of unit cell is shared by four (4) unit cells in the lattice and hence
contributes only 1/4 to a particular unit cell.
iv. An atom at face centre of unit cell is shared by two (2) unit cells in the lattice and hence
contributes only 1/2 to a particular unit cell.
v. An atom at body centre of unit cell belongs to the particular unit cell & contributes one
complete point to the cell

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 5
 Types of Lattice points Lattice points Lattice points at No. of particles
Unit Cell at corners at face-centred body-centred per unit cell

8 0 0 1
8x =1
SCC 8

8 0 1 1
8x +1x1=2
BCC 8

8 6 0 1 1
8x +6x =
FCC 8 2
4
F. RELATIONSHIP BETWEEN EDGE LENGTH (A) OF UNIT CELL AND RADIUS OF ATOM(R)
PACKING FRACTION (PACKING EFFICIENCY):-
PACKING FRACTION:- It is the percentage of total space occupied by the particles.
N x volume of one sphere
Packing Fraction= x 100
volume of one unit cell
Simple cubic unit cell Body centred cubic Face centered cubic

a = 2r and z = 1 √ 3a = 4r and = 2
❑
√ 2a = 4r and z = 4
❑

4 4 3
1× r3 r 4
2  r 3 4
= 3 = 3 = 0.52 or 52% 3
4  r 3
  0.68 i.e., 68% 3
a3 2r 3 3  0.74 i.e., 74%
 4r  3
   4r 
 3  
 2

Structure a v/s r Atom pre cell

C. N. P.F. Example
Square CP 4 52.4 %
HCP (Two D.) 6 60.4 %
Simple cubic a = 2r 1 6 52.4 %
F.C.C √2
❑
a = 4r 4 12 74 %
B.C.C √ 3a = 4r
❑
2 8 68 %
HCP (Three D.) 2 12 74 %
PART – 3 DENSITY
Mass of theUnit Cell Z xM
 DENSITY:- = = gm/cm3
Volume of theUnit Cell a 3 x Na

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 6
 Where ‘a’ = edge length, ‘Z’ is the number of particle (atoms) in unit cell, M = Molar mass
and ‘NA’ is Avogadro’s number.
PART - 4 : CLOSE PACKED STRUCTURES , COORDINATION NUMBER, VOIDS
 CLOSE PACKING IN CRYSTALLINE SOLID:- The spheres are packed in such a way that the

maximum available space is occupied leaving minimum vacant space, hence the crystal has
maximum density. The closer the packing, the greater is the stability of the packed system.
 TYPES OF CLOSE PACKING:-

A. ONE DIMENSION CLOSE PACKING:- In this arrangement, each sphere
is in contact with two of its neighbours. Coordination number is 2.
B. TWO DIMENSION CLOSE PACKING:- This can be done in two different ways.
(1) Square close packing in two dimension (2) Hexagonal cp in two dimension

 The spheres of second row are exactly above  The spheres of second row are in the
those of the first row. depressions of the first row.
 The second row is exactly same as the first  The second row is different from the
one. first row.
 AAAA...... types arrangement is obtained.  ABAB......types arrangement is obtained.
 In this arrangement, only 52.4% of the  60.4% of the available space is occupied
available space is occupied by the spheres. by the spheres.
 C.N. = 4 & Packing fraction:- 52.4%  C.N. = 6 & Packing fraction :- 60.4%
C. THREE DIMENSION CLOSE PACKING:-
1. Three DCP from two dimension square packed layers

2. Three DCP from two dimension hexagonal closed packed layers:-Three dimensional close
packed structure can be generated by placing layers one over the other.
i. Placing second layer over the first layer:- The second layer is placed in the depressions of the
first layer. This gives rise two
different types of voids i.e. tetrahedral
& octahedral void.
 A tetrahedral void is surrounded by
four spheres and when the centres of
these four spheres are joined a
tetrahedron is formed. While a
octahedral void is surrounded by six
spheres and when the centres of these
six spheres are joined a octahedron is formed.
 The number of these two types of voids depends upon the number of closed packed spheres. If
the number of closed packed spheres are N, then:
 The number of octahedral voids are = N and The number of tetrahedral voids are = 2N

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 7
 ii. Placing third layer over the second layer:- When the third layer is placed over the second
layer, there are two possibilities.
a. Covering Tetrahedral Voids:- if the Tetrahedral Voids of the second layer is covered by the
spheres of the third layer, then ABABAB……….. arrangement is obtained.
In this case, the spheres of the third layer are exactly aligned with those of the
first layer. This structure is called hexagonal close packed (hcp) structure.
 This types of arrangement of atoms is found in many metals like Mg, Zn etc.
 Coordination number = 12 and Packing fraction :- 74%
b. Covering Octahedral Voids:- If the octahedral Voids of the second layer is covered by the
spheres of the third layer, then ABCABCABC ……….. arrangement is obtained.
In this case, the spheres of the third layer are not aligned with those of the
first layer. This structure is called cubic close packed (ccp) or face-centred
cubic (fcc) structure.
Ex = Cu & Ag. C.N. = 12 and Packing fraction:- 74%
Noted:- It is not necessary that only tetrahedral or only Octahedral voids may be occupied in
given crystal lattice. Also it is not necessary that all the voids must be occupied. Only a fraction of
the total voids may be occupied. Knowing the fraction of voids occupied, the formula of the
compound can be calculated or vice versa.
 SIZE OF TETRAHEDRAL & OCTAHEDRAL VOID:-



Radius of the Tetrahedral void = 0.225 R and Radius of the Octahedral void (r) = 0.414 R.
(R is the radius of spheres forming tetrahedral void.)
In the case of ionic compounds, as usually anions are present in the packing and cations occupy
the voids. Hence we can also write:
i. For cations occupying the Tetrahedral voids, r+ = 0.225 r-
ii. For cations occupying the Octahedral voids, r+ = 0.414 r-
 COORDINATION NUMBER:- The numbers of nearest neighbours of a particle are called its

coordination number.
 RADIUS RATIO & STRUCTURE OF INORGANIC COMPOUNDS (For NEET/IIT):-

Radius Ratio Co-ordination no. Geometry of Crystal Examples.
¿ of Cation/Anion anions structure
R = r + r−¿¿ ¿
1 < .155 2/2 Linear Not formed HF2-
2 0.155 - 0.225 3/6 Trigonal Planar …….. B2O3
3 0.225 - 0.414 4/4 Zinc Blende HgS, ZnS
Tetrahedral
4/4 Wurtzite AgS, ZnS
4 0.414 - 0.732 6/6 Octahedral FCC NaCl, KCl
6/3 Rutile TiO2, MgF2
5 0.732 – 1.0 8/8 BCC CsCl, CsBr

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 8
 4/8 Cubic Fluorite CaF2, BaCl2
8/4 Antifluorite Na2S, K2S, Na2O
The hexagonal closest packed (hcp) has a coordination number of 12 and contains
6 atoms per unit cell. The face-centered cubic (fcc) has a coordination number of 12 and
contains 4 atoms per unit cell. The body-centered cubic (bcc) has a coordination number of 8
and contains 2 atoms per unit cell.
 STRUCTURE OF SOME IONIC SOLIDS (For NEET/IIT):-


 EFFECT OF TEMP & PRESSURE ON CYRSTAL STRUCTURE (Conversion of NaCl into CsCl

Structure & Vice-versa):- On applying high pressure, NaCl str. having 6 : 6 Co-ordination
number change to CsCl structure having 8 : 8 Co-ordination. Similarly, CsCl having 8 : 8 Co-
ordination on heating to 760 k changes to NaCl Structure having 6 : 6 Co-ordination. Thus,
increase of pressure increases the Co-ordination no. whereas increase of temp decreases the co-
ordination number.
Pressure
NaCl Strucute CsCl Strucute
(Co.No. 6 : 6) 760k (Co.No. 8 : 8)

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 9
 PART - 5 : CRYSTAL DEFECTS (IMPERFECTIONS IN SOLIDS)
 CRYSTAL DEFECTS:- Irregularities in the arrangement of constituent particles in a crystal. This

happens when crystallization process occurs at fast or moderate rate.
DFFECT/IMPERFECTIONS

ATOMIC DEFECT ELECTRONIC DEFECT

Point defects Line defects Plane defects

Stoichiometric defects Edge dislocation
Non-stoichiometric defects Skrew dislocation
Impurity defects
 POINT DEFECT:- The irregularities or deviations from ideal arrangement around a point or an

atom in a crystalline substance are called …… It is three types:
1. STOICHIOMETRIC DEFECTS:-The ratio of Cations & Anions remains same after defect.
Electrical neutrality remains same. They are also called Intrinsic or Thermodynamic defects.
Stoichiometric defect is also called Intrinsic defect because it is due to the deviation from
regular arrangement of atoms or ions within the crystal & no external substance is added.
Since, a perfect crystal exists at 0K. As the temperature increases the chance that a lattice may
be unoccupied by an ion increases. As the number of defects increases with temperature,
the defects are called thermodynamics defect.
Stoichiometric defects are two types-
Schottky’s defect/Lattice vacancy defects Frenkel’s defect/Lattice interstitial defects
1 These types of defect arise when equal These types of defect arise when an ion
number of cations and anions are missing from the lattice site and occupies
missing from their lattice sites. any interstitial void.

2 Density Decreases. Not affected.

3 These defects are shown by those crystals These defects are shown by those crystals in
(i) Which have almost same the sizes of which (i) there is a large difference in the
cation and anion (ii) High Co-ordination no. size of ions (ii) Low Co-ordination no.
4 Example:- NaCl, KCl, CsCl, AgBr etc. Example, ZnS, AgCl, AgBr and AgI due to
small size of Zn2+ and Ag+ ions.
It may be noted that AgBr shows both, Frenkel as well as Schottky defects.
 Noted:- In NaCl there are approximately106 Schottky pairs per cm3 at room temperature.
In1 cm3 there are about 1022 ions. Thus, there is one Schottky defect per 1016 ions.

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 10
 2. NON-STOICHIOMETRIC DEFECTS:- The ratio of Cations & Anions change after defect.
These defects are of two types: (I) Metal excess defect and (II) Metal deficiency defect.
I. METAL EXCESS DEFECT:- In this types of defect, the number of metals are more than
non-metals. These types of defects arise by two ways.
a) Metal excess defect due to anionic vacancies:- During the formation
of crystal lattice, some anions missing from its lattice site leaving a hole
which is occupied by an electron. (To maintain the electrical neutrality)
In this way, the ratio of metal ions increases.
 F-Center:- The electrons trapped in the anion vacancies are called F-center
because these centres are responsible for imparting colour.
(German word Farbenzenter for colour centre).
 For example:- Alkali halides like NaCl and KCl show this type of defect.
 When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms
are deposited on the surface of the crystal. The Cl– ions diffuse to the surface of the crystal
and combine with Na atoms to give NaCl. This happens by loss of electron by sodium atoms
to form Na+ ions. The released electrons diffuse into the crystal and occupy anionic sites. As
a result the crystal now has an excess of sodium.
These electrons absorb some energy of white light, giving the yellow colour to NaCl.
 Similarly, excess of lithium makes LiCl crystals pink and excess of potassium makes KCl
crystals violet (or lilac).
 Greater the number of F-centres, greater is the intensity of colour.
 Solids containing F-centres are paramagnetic because the electrons occupying the ‘holes’
are unpaired.
b) Metal excess defect due to presence of extra cations:- During the formation of crystal
lattice, some metal ions enter in interstitial sites. To maintain the electrical neutrality of the
crystal, some electrons also enter in interstitial sites. In this way,
the ratio of metal ions increases. Such types of crystals do not have
any holes.
For example:-Zinc oxide is white in colour at room temperature.
On heating it loses oxygen and turns yellow.
2ZnO → 2Zn+2 + O2 + 4e-
 The excess Zn2+ ions move to interstitial sites & the electrons to
neighboring interstitial sites.
Noted: - In both types of crystals, the electric current pass due to the free electrons.
So these are also called n-types semiconductors.
II. METAL DEFICIENCY DEFECT:- In this types of defect, the number of metals are less than
non-metals. These types of defects also arise by two ways.
a) Metal deficiency defect due to cationic vacancies:- During the
formation of crystal lattice, some metal ions exit out leaving their
lattice point. To maintain the electrical neutrality of the crystal,
some neighboring metals ions goes in its higher oxidation states.
For example:-FeO, FeS, NiO crystals.
This types of defect arise due to Schottky’s defect.

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 11
 For example: FeO which is mostly found with a composition of
Fe0.95O. It may actually range from Fe0.93O to Fe0.96O.

b) Metal deficiency defect due to presence of extra anion:- During the formation of
crystal lattice, some anions enter in interstitial sites. To maintain the electrical neutrality of
the crystal, some neighboring metals ions goes in its higher oxidation states. But the
possibility of this types defect is negligible, b/c due to large size of anions, it does not fit in
interstitial sites.
Noted:- Above both types of defect, arise in those crystals which show variable oxidation
state like transition metal salts.
Noted:- In both types of crystals, the electric current pass due to the movement of
hole and electrons. So these are also called p-types semiconductors.
3. IMPURITY DEFECTS:- This types of defects arise when foreign particles (ion or atom) are
introduced at the lattice site (in place of host particles) or at the vacant interstitial sites.
Addition of impurities changes the properties of the crystals.
The process of introducing impurities is called doping.
For example:- If molten NaCl containing a little amount of SrCl2
is crystallized, some of the sites of Na+ ions are occupied by Sr2+.
Each Sr2+ replaces two Na+ ions. It occupies the site of one ion &
the other site remains vacant. The cationic vacancies thus
produced are equal in number to that of Sr2+ ions.
Semiconductors are obtained when impurities are doped in
covalent solids like Si or Ge.

 LINE DEFECT (BSC):- These defects are also called dislocations.

(1) Edge dislocations & (2) Screw Dislocations.

Both defects results from the improper orientation of planes w.r.t. one another in the
crystal. The line defects reduce the strength of the metal.
 ELECTRONIC DEFECT (BSC):- At 0K temp, the electrons occupy fully the lowest energy
states in crystals. Above 0K, some of the electrons may occupy higher energy states
depending upon the temp, so excited electros & +Ve holes are created in the crystals
structure (Eg. In the Si) which are considered to be electronic defects. The electronic
defects are responsible for the electrical conductivity in metals, semiconductors etc,
because these excited electros are mobile.

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 12
 PART - 6 : PROPERTIES OF THE SOLID
A) ELECTRICAL PROPERTIES:- On the basis of electric behaviour, solid can be classified in
following types:1. Conductor 2. Insulator 3. Semiconductor 4. Super conductor
1. Conductors:- Conductivities are very high (i.e. ranging between 104 to 107 ohm–1m–1).
Metals = Flow of electricity is due to migration of electrons.
Conductor
Electrolyte = Flow of electricity is due to migration of ions.
 Metals conduct electricity in solid as well as molten state while electrolytes conduct
electricity only in aq. Solution or in molten state not in solid state.
2. Insulators:- Conductivities are very low (Ranging between 10–20 to 10–10 ohm–1m–1).
3. Semiconductors:- Conductivities are in the intermediate range from 10–6 to 104 ohm–1m–1.
 DISTINCTION AMONG, METALS, INSULATORS & SEMICONDUCTORS IN TERMS OF
BAND THEORY:-
METALS INSULATORS SEMICONDUCTORS
The energy gap The energy gap between In case of semiconductors, the gap
between filled filled valence band and between the valence band and conduction
valence band and the the conduction band is band is small. At 0k temp, they act as
conduction band is large, thus electrons insulators and at high temp, some
very small or there is cannot jump to higher electrons may jump to conduction band
overlapping between band and such a and show some conductivity. Electrical
these bands then substance has very small conductivity of semiconductors increases
electrons can flow conductivity and it with rise in temperature, since more
easily. behaves as an insulator electrons can jump to the conduction
band. Substances like silicon and
germanium show this type of behaviour
and are called intrinsic semiconductors.

1. Intrinsic semiconductors: -Examples: pure Si & Ge. At 0 k temp, | | | | | |

it acts as perfectly insulators b/c all four valence electrons are —Si—Si—Si—Si—Si—Si—
involved in bond formation i.e. there is no free electrons at 0k | | | | | |
temp. But at higher temp, some covalent bond break and —Si—Si—Si—Si—Si—Si—
electrons become free to move under applied field, hence it act | | | | | |
as semiconductor at high temp. —Si—Si—Si—Si—Si—Si—
 Electrical conductivity of semiconductors increases with increase | | | | | |
in temperature, since more electrons can jump from valence —Si—Si—Si—Si—Si—Si—
| | | | | |
band to the conduction band.
2. Extrinsic semiconductors: -The conductivity of intrinsic semiconductors is too low to be
of practical use. Their conductivity can be increased by adding impurities such as boron

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 13
 (electron deficient impurity) or phosphorus (electron rich impurity). This process is called
doping and such types of semiconductors are called extrinsic semiconductors. Such
impurities introduce electronic defects in them. On the basis of impurity, extrinsic
semiconductors are two types:
N - TYPES SEMICONDUCTORS P - TYPES SEMICONDUCTORS
(Adding of Electron – rich impurities) (Adding of Electron–deficit impurities)
When Si or Ge (Group 14 elements, containing 4 When Si or Ge (Group 14 elements,
valence electrons) are doped with P or As containing 4 valence electrons) are doped
(group 15 element containing 5 valence with B, Al or Ga (group 13 element
electrons,) n-type semiconductor are formed, containing 3 valence electrons,) p type
because Four out of five electrons are used in semiconductor is formed. Because dopant
the formation of four covalent bonds with the is having only three electrons thus an
four neighbouring silicon atoms. The fifth electron hole is created at the place of fourth
electron is extra and becomes delocalised. missing electrons. Here holes are positively
These delocalised electrons increase the charged and are moving towards negatively
conductivity of doped Si or Ge. charged plate.

The increase in conductivity is due to the The increase in conductivity is due to the
Negatively charged electron. Positively charged hole.
 Applications of n-type and p-type semiconductors:-
1. Diode is a combination of n-type and p-type semiconductors and is used as a rectifier.
2. In formation of Transistors:-Transistors 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.
3. The solar cell is an efficient photo-diode used for conversion of light energy into electrical
energy.
4. 12-16 and 13-15 group compounds:- Group 14 elements i.e. Germanium and silicon have
four valence and they can form four bonds like diamond. Similar type of compounds can be
prepared by combining group-12 and group-16 elements or group-13 and group-15
elements. These compounds have average valence of four and behave just like germanium
and silicon and thus act as semiconductors. Examples of group 12 – 16 compounds are ZnS,
CdS, CdSe and HgTe while group 13 – 15 compounds are InSb, AlP and GaAs. In these
compounds, the bonds are not perfectly covalent and the ionic character depends on the
electronegativity of the two elements.

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 14
B) MAGNETIC PROPERTIES:- Since electron is a charged particle, so that its orbital motion
generates a small magnetic field along the axis of rotation and its spinning motion produces
a small magnetic field along the spin axis. Thus, each electron in an atom behaves like a tiny
magnet.
 On the basis of their magnetic properties, substances can be classified into five categories:
1. PARAMAGNETISM:- The substances which are weakly attracted by the external magnetic
field is called ………..Paramagnetism is due to presence of one or more unpaired electrons.
Examples:- O2, Cu2+, Fe3+, Cr3+.
 They are magnetised in a magnetic field in the same direction but they lose their
magnetism in the absence of magnetic field.
2. DIAMAGNETISM:- The substances which are weakly repelled by the external magnetic
field are called ……………
 They are weakly magnetised in a magnetic field in opposite direction.
 Diamagnetism is shown by those substances in which all the electrons are paired and there
are no unpaired electrons. Pairing of electrons cancels their magnetic moments and they
lose their magnetic character. Examples :- H2O, TiO2, NaCl and C6H6
3. FERROMAGNETISM:- The substances which are strongly attracted by the external
magnetic field is called ……
 In solid state, the metal ions of ferromagnetic substances are
grouped together into small regions called domains and each domain acts as a tiny magnet.
But these domains are randomly oriented and their magnetic moments get cancelled. When
these substances are placed in a magnetic field all the domains get oriented in the direction
of the magnetic field and a strong magnetic effect is produced. This ordering of domains
persists even when the magnetic field is removed and the ferromagnetic substance
becomes a permanent magnet.
 Examples: - Iron, cobalt, nickel, gadolinium & CrO2
4. FERRIMAGNETISM:- When the substances are placed in a magnetic field the domains get
oriented in parallel and antiparallel directions in unequal
numbers resulting net magnetic moment, is called
ferrimagnetism. They are weakly attracted by magnetic field as compared to ferromagnetic
substances.
Examples: Fe3O4 (magnetite) and ferrites like MgFe2O4 & ZnFe2O4
Noted:-These substances lose ferrimagnetism on heating and become paramagnetic.
5. ANTIFERROMAGNETISM:- They have zero magnetic moment, their
domains are oppositely oriented and cancel out each other's
magnetic moment. Ex. MnO.

[A. K. MACHIWAL, PGT CHEMISTRY, KENDRIYAL VIDYALAYA, 07891632633]

Page 15