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CBSE Class 12 Chemistry
Quick Revision Notes
Chapter 3
Electrochemistry

• Oxidation: It is defined as a loss of electrons while reduction is defined as a gain of electrons.

In a redox reaction, both oxidation and reduction reaction takes place simultaneously.

• Direct redox reaction: In a direct redox reaction, both oxidation and reduction reactions take place in the same vessel. Chemical energy is converted to heat energy in a direct redox reaction.
• Indirect redox reaction: In indirect redox reactions, oxidation and reduction take place in different vessels.

In an indirect redox reaction, chemical energy is converted into electrical energy.The device which converts chemical energy into electrical energy is known as an electrochemical cell.

• In an electrochemical cell:

1. The half-cell in which oxidation takes place is known as oxidation half-cell
2. The half-cell in which reduction takes place is known as reduction half-cell.
3. Oxidation takes place at anode which is negatively charged and reduction takes place at cathode which is positively charged.
4. Transfer of electrons takes place from anode to cathode while electric current flows in the opposite direction.
5. An electrode is made by dipping the metal plate into the electrolytic solution of its soluble salt.
6. A salt bridge is a U shaped tube containing an inert electrolyte in agar-agar and gelatine.

• Salt bridge: A salt bridge maintains electrical neutrality and allows the flow of electric current by completing the electrical circuit.
• Representation of an electrochemical cell:

1. Anode is written on the left while the cathode is written on the right.
2. Anode represents the oxidation half-cell and is written as: Metal/Metal ion (Concentration)
3. Cathode represents the reduction half-cell and is written as: Metal ion (Concentration)/Metal
4. Salt bridge is indicated by placing double vertical lines between the anode and the cathode
5. Electrode potential is the potential difference that develops between the electrode and its electrolyte. The separation of charges at the equilibrium state results in the potential difference between the metal and the solution of its ions. It is the measure of tendency of an electrode in the half cell to lose or gain electrons.
6. Standard electrode potential: When the concentration of all the species involved in a half cell is unity, then the electrode potential is known as standard electrode potential. It is denoted as E0.

• According to the present convention, standard reduction potentials are now called standard electrode potential.
• Types of electrode potential: There are 2 types of electrode potentials namely,

1. Oxidation potential
2. Reduction potential

• Oxidation potential: It is the tendency of an electrode to lose electrons or get oxidized.
• Reduction potential: It is the tendency of an electrode to gain electrons or get reduced.

Oxidation potential is the reverse of reduction potential.

• The electrode having a higher reduction potential have higher tendency to gain electrons and so it acts as a cathode whereas the electrode having a lower reduction potential acts as an anode.
• The standard electrode potential of an electrode cannot be measured in isolation.
• According to convention, the Standard Hydrogen Electrode is taken as a reference electrode and it is assigned a zero potential at all temperatures.
• Reference electrode: Standard calomel electrode can also be used as a reference electrode
• SHE: Standard hydrogen electrode consists of a platinum wire sealed in a glass tube and carrying a platinum foil at one end. The electrode is placed in a beaker containing

an aqueous solution of an acid having 1 Molar concentration of hydrogen ions. Hydrogen gas at 1 bar pressure is continuously bubbled through the solution at 298 K. The oxidation or reduction takes place at the Platinum foil. The standard hydrogen electrode can act as both anode and cathode.

• If the standard hydrogen electrode acts as an anode:

If the standard hydrogen electrode acts as a cathode:

In the electrochemical series, various elements are arranged as per their standard reduction potential values.

A substance with higher reduction potential value means that it has a higher tendency to get reduced. So, it acts as a good oxidising agent.

A substance with lower reduction potential value means that it has a higher tendency to get oxidised. So, it acts as a good reducing agent.

The electrode with higher reduction potential acts as a cathode while the electrode with a lower reduction potential acts as an anode.

The potential difference between the 2 electrodes of a galvanic cell is called cell potential and is measured in Volts.

The cell potential is the difference between the reduction potential of cathode and anode.

E cell = E cathode – E anode

Cell potential is called the electromotive force of the cell (EMF) when no current is drawn through the cell.

Nernst studied the variation of electrode potential of an electrode with temperature and concentration of electrolyte.

Nernst formulated a relationship between standard electrode potential E0 and electrode potential E. ^[1]

At equilibrium, cell potential Ecteell becomes zero.

Relationship between equilibrium constant Kc and standard cell potential E0cell:

Work done by an electrochemical cell is equal to the decrease in Gibbs energy

The substances which allow the passage of electricity through them are known as conductors.

Every conducting material offers some obstruction to the flow of electricity which is called resistance. It is denoted by R and is measured in ohm.

The resistance of any object is directly proportional to its length l and inversely proportional to its area of cross section A.

Where p is called specific resistance or resistivity.

The SI unit of specific resistivity is ohm metre.

The inverse of resistance is known as conductance, G

Unit of conductance is ohm-1 or mho. It is also expressed in Siemens denoted by S. The inverse of resistivity is known as conductivity. It is represented by the symbol The SI unit of conductivity is Sm-1. But it is also expressed in Scm-1.

Conductivity = Conductance * Cell constant

For measuring the resistance of an ionic solution, there are 2 problems:

1. Firstly, passing direct current changes the composition of the solution
2. Secondly, a solution cannot be connected to the bridge like a metallic wire or a solid conductor.

Conductivity cell: The problem of measuring the resistance of an ionic solution can be resolved by using a source of alternating current and the second problem is resolved by using a specially designed vessel called conductivity cell.

A conductivity cell consists of 2 Pt electrodes coated with Pt black. They have area of cross section A and are separated by a distance T. Resistance of such a column of solution is given by the equation:

Whereis called cell constant and is denoted by the symbol

Molar conductivity of a solution: It is defined as the conducting power of all the ions

Material Downloaded From ImperialStudy

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produced by dissolving 1 mole of an electrolyte in solution.

Molar conductivity

WhereConductivity and M is the molarity Unit of Molar conductivity is Scm2 mol- 1

• Equivalent conductivity: It is the conductivity of all the ions produced by dissolving one gram equivalent of an electrolyte in solution. Unit of equivalent conductivity is S cm2 (g equiv) -1

Equivalent conductivity:

• Kohlrausch’s Law of independent migration of ions: According to this law, molar conductivity of an electrolyte, at infinite dilution, can be expressed as the sum of individual contributions from its individual ions.
• If the limiting molar conductivity of the cations is denoted byand that of the anions bythen the limiting molar conductivity of electrolyte is:

Molar conductivity,

Where v+ and v- are the number of cations and anions per formula of electrolyte

• Degree of dissociation: It is ratio of molar conductivity at a specific concentration ‘c’ to the molar conductivity at infinite dilution. It is denoted by.

• Dissociation constant:WhereKa is acid dissociation constant, ‘c’ is

concentration of electrolyte, a is degree of ionization.

• Faraday constant: It is equal to charge on 1 mol of electrons. It is equal to 96487 C mol-1 or approximately equal to 96500 C mol-1.
• Faraday’s first law of electrolysis: The amount of substance deposited during electrolysis is directly proportional to quantity of electricity passed.
• Faraday’s second law of electrolysis: If same charge is passed through different electrolytes, the mass of substance deposited will be proportional to their equivalent weights.
• Products of electrolysis: The products of electrolysis depend upon
• The nature of electrolyte being electrolyzed and the nature of electrodes. If electrode is inert like platinum or gold, they do not take part in chemical reaction i.e. they neither lose nor gain electrons. If the electrodes are reactive then they will take part in chemical reaction and products will be different as compared to inert electrodes.
• The electrode potentials of oxidizing and reducing species. Some of the

electrochemical processes although feasible but slow in their rates at lower voltage, these require extra voltage, i.e. over voltage at which these processes will take place. The products of electrolysis also differ in molten state and aqueous solution of electrolyte.

• Primary cells: A primary cell is a cell in which electrical energy is produced by the reaction occurring in the cell, e.g. Daniel cell, dry cell, mercury cell. It cannot be recharged.
• Dry Cell:

At anode At cathode

The net reaction:

• Mercury Cell: The electrolyte is a paste of KOH and ZnO.

At Anode:

At cathode:

The net reaction:

• Secondary cells: Those cells which are used for storing electricity, e.g., lead storage battery, nickel – cadmium cell. They can be recharged.
• Lead storage battery:

Anode:

Cathode:

The overall cell reaction consisting of cathode and anode reactions is:

On recharging the battery, the reaction is reversed.

• Nickel cadmium cell: It is another type of secondary cell which has longer life than lead storage cell but more expensive to manufacture.

The overall reaction during discharge is

• Fuel cells:

At Anode:

At cathode:

• Overall reaction:

• Corrosion:

Oxidation:

Reduction:

• Galvanization: It is a process of coating zinc over iron so as to protect it from rusting.
• Cathodic protection: Instead of coating more reactive metal on iron, the use of such metal is made as sacrificial anode.

1. Electrode potential increases with increase in the concentration of the electrolyte and decrease in temperature.

   Nernst equation when applied to a cell, it helps in calculating the cell potential. ↑

 

CBSE Class 12 Chemistry
Quick Revision Notes
Chapter 2
Solutions

The difference in boiling points of solution Tb and pure solvent T® is called elevation in boiling point

• Solutions: Solutions are the homogeneous mixtures of two or more than two components.
• Binary solution: A solution having two components is called a binary solution.
• Components of a binary solution.

It includes solute and solvent.

1. When the solvent is in solid state, solution is called solid solution.
2. When the solvent is in liquid state, solution is called liquid solution.
3. When the solvent is in gaseous state, solution is called gaseous solution.

• Concentration: It is the amount of solute in given amount of solution.
• Mass by volume percentage (w/v): Mass of the solute dissolved in 100 mL of solution.
• Molality (m) is the number of moles of solute present in 1kg of solvent.
• Molarity (M) is the number of moles of solute present in 1L of solution.
• Normality is the number of gram equivalent of solute dissolved per litre of solution.
• Solubility: It is the maximum amount that can be dissolved in a specified amount of solvent at a specified temperature.
• Saturated solution: It is a solution in which no more solute can be dissolved at the same temperature and pressure.
• In a nearly saturated solution if dissolution process is an endothermic process, solubility increases with increase in temperature.
• In a nearly saturated solution if dissolution process is an exothermic process, solubility decreases with increase in temperature.
• Henry’s Law: It states “at a constant temperature the solubility of gas in a liquid is directly proportional to the pressure of gas”. In other words, “the partial pressure of gas in vapour phase is proportional to the mole fraction of the gas in the solution”.
• When a non-volatile solute is dissolved in a volatile solvent, the vapour pressure of solution is less than that of pure solvent.
• Raoult’s law: It states that “for a solution of volatile liquids the partial vapour pressure of each component in the solution is directly proportional to its mole fraction”.
• Using Dalton’s law of partial pressure the total pressure of solution is calculated.

• Comparison of Raoult’ law and Henry’s law: It is observed that the partial pressure of volatile component or gas is directly proportional to its mole fraction in solution. In case of Henry’s Law the proportionality constant is KH and it is different from p10 which is partial pressure of pure component. Raoult’s Law becomes a special case of Henry’s Law when KH becomes equal to p10 in Henry’s law.
• Classification of liquid-liquid solutions: It can be classified into ideal and non-ideal solutions on basis of Raoult’s Law.

• Ideal solutions:

1. The solutions that obey Raoult’s Law over the entire range of concentrations are known as ideal solutions.

3. The intermolecular attractive forces between solute molecules and solvent

molecules are nearly equal to those present between solute and solvent molecules i.e. A-A and B-B interactions are nearly equal to those between A-B.

Non-ideal solutions:

1. When a solution does not obey Raoult’s Law over the entire range of concentration, then it is called non-ideal solution.

3. The intermolecular attractive forces between solute molecules and solvent

molecules are not equal to those present between solute and solvent molecules i.e. A-A and B-B interactions are not equal to those between A-B

• Types of non- ideal solutions:

1. Non ideal solution showing positive deviation
2. Non ideal solution showing negative deviation

• Non ideal solution showing positive deviation

1. The vapour pressure of a solution is higher than that predicted by Raoult’s Law.
2. The intermolecular attractive forces between solute-solvent molecules are weaker than those between solute-solute and solvent-solvent molecules i.e., A-B < A-A and B-B interactions.

• Non ideal solution showing negative deviation

1. The vapour pressure of a solution is lower than that predicted by Raoult’s Law.
2. The intermolecular attractive forces between solute-solvent molecules are stronger than those between solute-solute and solvent-solvent molecules i.e. A-B > A-A and B-B interactions.

• Azeotopes: These are binary mixtures having same composition in liquid and vapour

phase and boil at constant temperature. Liquids forming azeotrope cannot be

separated by fractional distillation.

• Types of azeotropes: There are two types of azeotropes namely,

1. Minimum boiling azeotrope
2. Maximum boiling azeotrope

• The solutions which show a large positive deviation from Raoult’s law form minimum

boiling azeotrope at a specific composition.

• The solutions that show large negative deviation from Raoult’s law form maximum

boiling azeotrope at a specific composition.

• Colligative properties: The properties of solution which depends on only the number of solute particles but not on the nature of solute are called colligative properties.
• Types of colligative properties: There are four colligative properties namely,

1. Relative lowering of vapour pressure
2. Elevation of boiling point
3. Depression of freezing point
4. Osmotic pressure

• Relative lowering of vapour pressure: The difference in the vapour pressure of pure solvent p\j^[1] and solution pi represents lowering in vapour pressure(p® —pi).
• Relative lowering of vapour pressure: Dividing lowering in vapour pressure by vapour pressure of pure solvent is called relative lowering of vapour pressure

• Relative lowering of vapour pressure is directly proportional to mole fraction of solute. Hence it is a colligative property.

• For a dilute solution elevation of boiling point is directly proportional to molal concentration of the solute in solution. Hence it is a colligative property.

• Depression of freezing point: The lowering of vapour pressure ofsolution causes a lowering of freezing point compared to that of pure solvent.The difference in freezing point of the pure solvent T® and solution Tf is called the depression in freezing point.

passage of solvent into solution through a semipermeable membrane is called osmotic pressure.

• Osmotic pressure is a colligative property as it depends on the number of solute particles and not on their identity.
• For a dilute solution, osmotic pressure (7r) is directly proportional to the molarity (C) of the solution i.e. 7r= CRT
• Osmotic pressurecan also be used to determine the molar mass of solute using the equatioi

• Isotonic solution: Two solutions having same osmotic pressure at a given temperature are called isotonic solution.
• Hypertonic solution: If a solution has more osmotic pressure than other solution it is called hypertonic solution.
• Hypotonic solution: If a solution has less osmotic pressure than other solution it is called hypotonic solution.
• Reverse osmosis: The process of movement of solvent through a semipermeable membrane from the solution to the pure solvent by applyingexcess pressure on the solution side is called reverse osmosis.
• Colligative properties help in calculation of molar mass of solutes.
• Abnormal molar mass: Molar mass that is either lower or higher than expected or normalmolar mass is called as abnormal molar mass.
• Van’t Hoff factor: Van’t Hoff factor (i)accounts for the extent of dissociation or association.

• For a dilute solution depression in freezing point is a colligative property because it is

directly proportional to molal concentration of solute.

• Osmosis: The phenomenon of flow of solvent molecules through a semi permeable membrane from pure solvent to solution is called osmosis.
• Osmotic pressure: The excess pressure that must be applied to solution to prevent the

1. • Value of i is less than unity in case solute undergo association and the value of i is greater than unity in case solute undergo dissociation.

2. Inclusion of van’t Hoff factor modifies the equations for colligative properties as: ↑

CBSE Class 12 Chemistry
Quick Revision Notes
Chapter 1
The Solid State

Solid: Solid is a state of matter in which the constituting particles are arranged very closely.The constituent particles can be atoms, molecules or ions.

Properties of solids:

1. They have definite mass, volume and shape.
2. They are compressible and rigid.
3. Intermolecular distances are very short and hence the intermolecular forces are strong.
4. Their constituent particles have fixed position. sand can only oscillate about their mean positions.

Classification of on the basis of the arrangement of constituent particles:

• Properties of crystalline solids:
• They have a definite geometrical shape.
• They have a long range order.
• They have a sharp melting point.
• They are anisotropic in nature i.e. their physical properties show different values when measured along different directions in the same crystal.
• They have a definite and characteristic heat of fusion.
• They are called true solids.
• When cut with a sharp edged tool , they split into two pieces and the newly generated surfaces are plain and smooth.

• Polymorphic forms or polymorphs:

The different crystalline forms of a substance are known as polymorphic forms or polymorphs .For example: graphite and diamond.

• Characteristics of amorphous solids:

1. They have an irregular shape.
2. They have a short range order.
3. They gradually soften over arrange of temperature.
4. They are isotropic in nature i.e. their physical properties are the same in all directions.
5. When cut with a sharp edged tool, they cut into two pieces with irregular surfaces.
6. They do not have definite heat of fusion.
7. They are called pseudo solids or super cooled liquids. This is because they have a tendency to flow,though very slowly.

• Types of crystalline solids:

A. Molecular Solids

Constituent Particles: Molecules

Type of solid	Constituent Particles	Bonding/ Attractive Forces	Electrical conductivity	physical nature	Melting point	Examples
Non polar solids	Molecules	Dispersion or London forces	Insulator	Soft	Very low	Ar,CCl4,H2,I2,C02
Polar solids	Molecules	Dipole- dipole interactions	Insulator	Soft	low	HCl, solid SO2, solid NH3
Hydrogen bonded	Molecules	Hydrogen bonding	Insulator	Hard	low	H20 (ice)

B. Ionic Solids

Constituent Particles: Ions

Bonding/Attractive Forces: Coulombic or Electrostatic

Electrical Conductivity: Insulators in solid state but conducts in molten state and in

aqueous solutions

Physical Nature: Hard but brittle

Melting Point: High

Examples: CaF2, ZnS, MgO, NaCl

C. Metallic Solids

Constituent Particles: Positive ions in a sea of delocalized electrons Bonding/Attractive Forces: Metallic bonding

Electrical Conductivity: Conductors in solid state as well as in molten state Physical Nature: Hard but malleable and ductile Melting Point: Fairly high

Examples: Fe ,Cu, Ag, Mg

D. Covalent or NetworkSolids Constituent Particles: Atoms Bonding/Attractive Forces: Covalent bonding

Electrical Conductivity: Conductors in solid state as well as in molten state Physical Nature: Hard but malleable and ductile Melting Point: Fairly high

Examples: Si02, (quartz), SiC, C (diamond), C(graphite)

Network structure of graphite

• Crystal lattice: A regular ordered arrangement of constituent particles in three dimensions is called crystal lattice.

• Lattice points or lattice sites:the fixed positions on which the constituent particles are presentare called lattice points or lattice sites. A group of lattice points which when repeated over and over againin3dimensions give the complete crystal lattice.
• Unit cell: It is defined as the smallest repeating unit in space lattice which when repeated over and over again generates the complete crystal lattice. The crystal can consist of an infinite number of unit cells.

1. Dimensions of the unit cell along the three edges ,a, b and c:these edges may or may not be mutually perpendicular.
2. Inclination of the edges to each other:this is denoted by the angle between the edges a,fi , andrespectively.cdsthe angle between the edges b and c,/3isthe angle between the edges a and c ,and7is the angle between a and b.

i — 1 c! i
J0L-A–

Parameters which characterise a unit cell

• Seven crystal systems:

1. Cubic: o:=/3=7=90^o ,a=b=c
2. Tetragonal: a=/3=7=90° ; a=by^c
3. Orthorhombic: cl={3=j=90°; a^by^c
4. Monoclinic: o:=7=90°,^:90°; ay^by^c
5. Hexagonal: o:=/3=90°,7=120°; a=by^c
6. Rhombohedral or trigonal: a= /3=7y^90°;a=b=c
7. Triclinic: o:y^/3y^7y^90O;ay^by^c

• Types of unit cells:

1. Primitive or simple unit cells have constituent particles only at its corners.
2. Centered unit cells are those unit cells in which one or more constituent particles are present at positions in addition to those present at the corners.

• Types of centered unit cells:

1. Face centered unit cell: It consists of one constituent particle present at the centre of each face in addition to those present at the corners.
2. Body centered unit cell: It consists of a one constituent particle is present at its body centre in addition to those present at the corners.
3. End centered unit cell: It consists of one constituent particle present at the centre of any

two opposite faces in addition to those present at the corners.

• End centre: f an atom is present at the edge centre, it is shared by four unit cells. So, only one fourth of an atom belongs to the unit cell.
• Number of atoms in different unit cells:

1. Primitive unit cell have latom

1. Face centered unit cell have 3 atoms
2. Body centered unit cell have 2atoms

• Coordination number: Coordination number is the number of nearest neighbours of a particle.
• Close packed structures:

ccccccce

One dimensional close packing of spheres

• Close packing in two dimensions: It is generated by stacking the rows of close packed spheres in two ways:

i) Square close packing and ii) Hexagonal close packing.

• Close packing in three dimensions: They can be obtained by stacking the two dimensional layers one above the other. It can be obtained in two ways:

i) Square close packed layers and ii) Hexagonal close packed layers.

• Square close packing: Here, the spheres of the second row ware placed exactly above those of the first row. This way the spheres are aligned horizontally as well as vertically. The arrangement is AAA type. The coordination number is 4.

• Hexagonal close packing: Here, these spheres of these bond row are placed above the first one in as taggered manner in such a way that its spheres fit in the depression of the first row. The arrangement is ABAB type. The coordination number is 6.

Hexagonal dose packing of spheres in two dimensions

• Three dimensional close packing from two dimensional square close packed

Covering the octahedral voids: Here, octahedral voids of these bond layer may be covered by the spheres of the third layer. It gives rise to ABCABCABC type pattern. The three dimensional structure is called cubic close packed structure or face centered cubic structure. The coordination number is 12.Example: Cu, Ag.

• In hexagonal close packing or cubic close packing arrangement, the octa hedral 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.

For example:

If the number of close packed particles = n

Number of particles present in octahedral voids = n

Then, the number of particles present in tetrahedral voids = 2n

• Packing efficiency: It is the percentage of total space occupied by constituent particles (atoms, molecules orions).

x 100%

Packing Efficiency =

Volume occupied by spheres
Total volume of unit cell

• Packing efficiency for face centered unit cell =74%
• Packing efficiency for body centered cubic unit cell =68%
• Packing efficiency for simple cubic unit cell =52.4%
• 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.

i =o-⁴¹⁴

• 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.

i = °-²²⁵

• Density of a unit cell is same as the density of the substance.
• Relationship between radius of constituent particle(r) and edge length(a):

Simple cubic unit cell: a=2r Face centered unit cell: a=2y/2r Body centered unit cell: a=

1.

2.

3.

1.

2.

v³

• Volume of a unit cell=(edge length)3=a3

Simple cubic unit cell: Volume= (2r)3

2

Face centered unit cell: Volume= (2\/2r) Body centered unit cell: Volume=

• Number of atoms in a unit cell(z):

1. Simple cubic unit cell: z=1
2. Face centered unit cell: z=4
3. Body centered unit cell: z=2

• Density of unit cell=
• Crystal defects are basically irregularities in the arrangement of constituent particles.
• Types of defects:

1. Point defects- Point defects are the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance.
2. Line defects- Line defects are the irregularities or deviations from ideal arrange ment in entire rows of lattice points.

Impurity defect!

Different types of point defects:

Different types of stoichiometric defects for non- ionic solids:

Vacancy defect

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

Interstitial defect

• Different types of stoichiometric defects for ionic solids:

Schottky defects

• 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 a tits original site and an interstitial defect a tits new location. It does not change the density of the solid. Frenkel defect is shown by ionic substance in which there is a larged difference in the size of ions. It includes ZnS,AgCl,AgBrand Agl.

Frenkei defects

• Different types of non-stoichiometric defects:

• Metal deficiency: This defect arises because of absence of metal ions from its lattice sites. The electrical neutrality is maintained by an adjacention having a higher positive charge.
• Reasons for the cause of metal excess defect:

1. Anionic vacancies: A compound may have an extra metal ion if the negative ion is absent from its lattice site.This empty lattice site is called a hole.To maintain electrical neutrality this site is occupied by an electron. The hole occupied by an electron is called f-centre or Farbenz enter centre. The F- centre is responsible for the colour of the compound.
2. Presence of extracations: A compound is said to have extracations if a cation is present in the interstitial site. An electron is present in the interstitial site to maintain the electrical neutrality.

• Classification of solids based on their electrical conductivities:

1. Conductors: The solids with conductivities ranging between 10⁴ to 10 ⁷o/im^_1m^_1 are called conductors.

nrj

1. Insulators: These are the solids with very low conductivities ranging between to

.

1. Semi- conductors: These are the solids with conductivities in the intermediate range from

tol0⁴ofom^_1m^_1.

1. Intrinsic semiconductors: 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.
2. Extrinsic semiconductors: When an appropriate impurity is added to an intrinsic semiconductor, it is called extrinsic semi conductors. Their electrical conductivity is high.

• Doping: The process of adding an appropriate amount of suitable impurity to increase the conductivity of semiconductors is known as doping.

1. The 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.
2. The 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.

• Types of extrinsic semiconductors:
• Diode: It is a combination of n-type and p-type semiconductors and is used as a rectifier.
• Transistors: They are made by sandwiching a layer of one type of semiconductor between two layers of the other type of semi conductor. The npn and pnp type of transistors are used to detector amplify radio or audio signals.
• The 12- 16 compounds: These compounds are formed by the combination of group 12 and group 16 compounds.They possess an average valency of 4.Examples – ZnS,CdS,CdSe and HgTe.
• The 13- 15 compounds: These compounds are formed by the combination of group 13 and group 15 compounds.They possess an average valency of 4.Examples – InSb,AlP and GaAs.
• 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 at in y 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.

• Classification of substances based on their magnetic properties:

1. Paramagnetic substances: These are those substances which are weakly attracted by the magnetic field. It is due to presence of one or more unpaired electrons.
2. Diamagnetic substances: 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.
3. Ferromagnetic substances: These are those substances which are attracted every strongly by a magnetic field.
4. Anti ferromagnetic substances: They have equal number of parallel and anti parallel magnetic dipoles resulting in a zero net dipolemoment.
5. Ferrimagnetic substances: They have unequal number of parallel and anti parallel magnetic dipoles resulting in an at dipole moment.