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Solid State
Introduction
Intermolecular forces and thermal energy are the two factors on which physical states of matter
depend. While the intermolecular forces of attraction tend to keep the particles closer; the thermal
energy tends to keep the particles apart from each other by making them move faster.
When the net resultant of these two opposing forces, i.e. intermolecular forces and thermal energy,
makes the particles cling together and forces them to occupy fixed positions, matters exist in solid
state.
Characteristic properties of solid state
a.
b.
c.
d.
e.
f.
Solids have definite mass, volume and shape
Solids are incompressible and rigid
In solids, intermolecular distances are very short
In solids, intermolecular forces are very strong
The constituent particles of solids have fixed positions.
The constituent particles of solids can only oscillate about their mean positions.
Classification of solids – Solids can be classified into two types on the basis of the arrangements of
their constituent particles (atoms, molecules or ions). These two types are Crystalline Solid and
Amorphous Solid.
Crystalline Solid
Solids having large number of crystals; each with definite characteristic geometrical shape; are called
crystalline solids.
The constituent particles of crystalline solid are arranged in regular pattern which is repeated
periodically over the entire crystal. Such type of arrangement is called long range order. Crystalline
solids are anisotropic in nature, i.e. many physical properties, such as electrical resistance, refractive
index, etc. are different along different axes. Crystal of NaCl, Quartz, Ice, HCl, Iron, etc. are some
examples of crystalline solid.
Characteristics of crystalline solid –
a. Crystalline solids have definite characteristic
geometrical shape.
b. Crystalline solids have sharp characteristic melting
point.
c. Crystalline solids have definite and characteristic
heat of fusion.
d. Crystalline solids produce pieces with plain and
smooth surface when cut with a tool of sharp
edge.
e. Crystalline solids are anisotropic in nature.
f. Crystalline solids are true solid.
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g. Constituent particles of crystalline solids are arranged in long range order.
Amorphous Solid
Solids having irregular shapes of particles are known as Amorphous Solids. The word ‘Amorphous’
came from Greek ‘Amorphos’ which means no shape.
The constituent particles of amorphous solids have only short range order of arrangement, i.e. regular
and periodical arrangement of particles is seen to a short distance only. The structures of amorphous
solids are similar to that of liquids. Glass, rubber, plastics, etc. are some of the examples of amorphous
solids. Amorphous solids are isotropic in nature, i.e. physical properties of amorphous solids are same
in all directions.
In old buildings, it is often seen that glasses of windows get slightly thickened at bottom, this happens
because glass which is an amorphous solid; flows down very slowly. Some very old glasses get milky
appearance because of some crystallization. This happens because on heating, glasses get
crystallized at some temperature. This is the cause; amorphous solids are also known as Pseudo
Solids or Super Cooled Liquids.
Characteristic of amorphous solid –
a.
b.
c.
d.
e.
Particles of amorphous solids are irregular in shape.
Amorphous solids soften gradually over a range of temperature.
Amorphous solids produce pieces of irregular shapes when they are cut into two pieces.
Amorphous solids do not have definite heat of fusion.
Amorphous solids are isotropic in nature, i.e. they have same physical properties in all
directions.
f. Amorphous solids are not true solids and hence these are also known as Pseudo Solid or Super
Cooled Liquid.
g. The arrangement of constituent particles is in short range order.
Crystalline Solids:
Amorphous solids are very useful but most of the solids are crystalline in nature. Crystalline solids are
classified into four types; based on the intermolecular forces operating in them.
1.
2.
3.
4.
Molecular Solids
Ionic Solids
Metallic Solids
Covalent solids
1 - Molecular Solids – Solids having molecules as their constituent particles are called Molecular solids.
For, example, Hydrogen, Chlorine, Water, HCl, solid carbon dioxide, sucrose, etc.
Molecular solids are classified into three types on the basis of their bond:
a. Non-Polar Molecular solids
b. Polar Molecular Solids
c. Hydrogen Bonded Molecular Solids
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(a) Non Polar Molecular Solids – Solids which are comprised of atoms only, such as helium and argon
or molecules; formed because of the non polar covalent bonds are known as Non-Polar Molecular
Solids. For example – H2, Cl2, I2, etc.
Characteristic of Non-Polar Molecular Solids –





The molecules of non-polar molecular solids are held together by weak dispersion forces or
London forces.
Non-Polar Molecular Solids are soft.
Non-polar molecular solids are non-conductor of electricity.
Non-polar molecular solids have low melting points.
Non-polar molecular solids are usually in liquid or gaseous state at the room temperature and
pressure.
(b) Polar Molecular Solids – The solids which are formed by polar covalent bonds are known as Polar
Molecular solids. For example – HCl, SO2, NH3, etc.
Characteristic of Polar Molecular Solids –






The molecules in polar molecular solids are held together with dipole-dipole interactions.
Polar molecular solids are generally soft in nature.
Polar molecular solids are non-conductor of electricity.
Polar molecular solids have higher melting points in comparison to non-polar molecular solids.
Most of the polar molecular solids are gases or liquids at room temperature and pressure.
Solid SO2 and solid NH3 are some examples of polar molecular solids.
(c) Hydrogen bonded Molecular Solids – The molecules of hydrogen bonded molecular solids contain
polar covalent bond between H and O, F or N. In solids such as H2O (ice) molecules are bound
together strongly with hydrogen bond. HF, H2O (ice), etc are the examples of hydrogen bound
molecular solids.
Characteristics –


Hydrogen bound molecular solids are generally volatile liquid or soft solids at room temperature
and pressure.
Hydrogen bound molecular solids are non-conductor of electricity.
2 - Ionic Solids – Solids, in which ions are the constituent particles, are called ionic solids. These solids
are formed because of three dimensional arrangements of cations and anions bound together with
strong electrostatic forces (coulombic forces). For example NaCl.
Characteristics of Ionic Solids –




High melting and boiling points.
Non-conductor of electricity in solid state.
Conductor of electricity in molten state.
Conducted electricity when dissolved in water.
3 - Metallic Solids – All metals are referred as Metallic solids. Their constituent particles are positive
ions. These positive ions are surrounded by free moving electrons. For example – iron, aluminium, etc.
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Characteristics –




High melting points.
Good conductors of electricity and heat.
Lustrous, and are of specific colors.
Hard but malleable and ductile in nature
4 - Covalent Solids – Crystalline solids are formed by non metals because of formation of covalent
bonds between the adjacent molecules throughout the crystal. These are also known as Network
Solids. These are also called giant molecules. For example – diamond, graphite, silicon carbide, etc.
Characteristic of Covalent Solids –




They are very hard and brittle except graphite which is soft.
Very high melting points.
Do not conduct electricity except graphite.
Also called giant molecules.
Crystal Lattices and Unit Cells
Crystal lattice is the depiction of three dimensional arrangements of constituent particles (atoms,
molecules, ions) of crystalline solids as points. Or the geometric arrangement of constituent particles of
crystalline solids as point in space is called crystal lattice.
Characteristics of crystal lattice:




Each constituent particle is represented by one point in a crystal lattice.
These points are known as lattice point or lattice site.
Lattice points in a crystal lattice are joined together by straight lines.
By joining the lattice points with straight lines the geometry of the crystal
lattice is formed.
Unit Cell – The smallest portion of a crystal lattice is called Unit Cell. By repeating in different directions
unit cell generates the entire lattice.
Parameters of a unit cell:



 A unit cell is characterized by six parameters. These parameters are
three edges (a, b and c) and angles between them (α, β and γ).
Dimensions along the edges of a unit cell is represented by a, b and c.
Edges of unit cell may or may not be mutually perpendicular.
The angle between b and c is represented by α, between a and c by β and between a and b by
γ.
Types of Unit Cell - There are two types of unit cells – Primitive and Centred Unit Cells.
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Primitive Unit Cells – When particles in unit cell are present only at the corners, it is called the primitive
unit cell.
Centred Unit Cells – When particles are present at other positions in addition to those at corners in a
unit cell, it is called a Centred Unit Cell.
There are three types of Centred Unit Cell.
(a) Body Centred Unit Cells – If one constituent particle lies at the centre of the body of a unit cell in
addition to the particles lying at the corners, it is called Body-Centred Unit Cell.
(b) Face-Centred Unit Cells – If one constituent particle lies at the centre of each face besides the
particles lying at the corner, it is known as Face-Centred Unit Cells.
(c) End-Centred Unit Cell – If one constituent particle lies at the centre of any two opposite faces
besides the particles lying at the corners, it is known as End-Centred Unit Cell. It is also known as
base-centred unit cell.
There are seven types of unit cell formed. These are Cubic, Tetragonal, Orthorhombic, Monoclinic,
Hexagonal, Rhombohedral or Trigonal and Triclinic.
Bravais Lattices
There are only 14 possible crystal lattices, which are called Bravais Lattices.
Cubic Lattice – There are three types of lattice possible for cubic lattice.
Primitive or Simple, Body centred, Face centred lattices. In these types of lattices all sides are of equal
length. The angles between their faces are 900 in a cubic lattice.
Tetragonal Lattice – There are two possible types of
tetragonal lattices. Primitive and Body centred unit
cells. In these lattices one side is different in length and angles between faces are equal to 900.
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Orthorhombic Lattice – Four types of orthorhombic lattice are possible. They are Primitive, Endcentred, Body centred and Face centred. They have unequal sides. The Angles between their faces
are equal to 900.
Monoclinic Lattice – There are two possible types of monoclinic lattice. They are Primitive and End
centred. They have unequal sides and two faces have angles other than 900.
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Hexagonal lattice – Hexagonal lattice is of one type only. It has one side is different in length to the
other two and the angles on two faces are 600.
Rhombohedral Lattice – Only one type of lattice is possible for
Rhombohedral lattice. It has all sides equal and angles on two faces
are less than 900.
Triclinic Lattice – Triclinic lattice has only one type of lattice. It has unequal sides and none of the
angles between faces are equal to 900.
Number of Atoms in a Unit Cell
A crystal lattice is made of very large number of unit cells and lattice points are the representation of
constituent particles. Therefore, the number of atoms in a unit cell of a crystal lattice can be calculated.
Number of atoms in Primitive Cubic Unit Cell –
In primitive unit cell, atoms are present at corners only. In a crystal lattice every corner is shared by
eight adjacent unit cells. Therefore, only 1/8 of an atom, or other constituent particles, belong to a
particular unit cell.
Therefore,
Since, there are 8 atoms present in a unit cell on every corner,
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Thus, 1 atom is present in a Primitive Cubic Unit Cell.
Body Centred Cubic (bcc) Unit Cell – There are eight atoms at each corner and one atom present at
the centre of body in a body centred cubic (bcc) unit cell.
Therefore, the number of atoms present in a Body Centred Cubic (bcc) Unit Cell
Face – Centred Cubic (fcc) Unit Cell
–
In a face centred cubic unit cell, there are eight atoms present at each corner. A cube has six faces,
therefore total six atoms are present at the centre of each of the face.
Each atom present at corners is shared by adjacent eight atoms and each atom present at the centre of
face is shared between adjacent two atoms.
Therefore, number of atoms in an fcc unit cell -
Close Packed Structure
Matters exist in solid state because of close packing of their constituent particles. There are two types
of close packing found in solids. These are Cubic Close Packed (ccp) and Hexagonal Close Packed
(hcp) lattice.
Cubic Close packed (ccp)
In this type of packing, the spheres of molecules are adjacent to each other that each row of spheres
in a particular dimension is a repetition of the pervious row. The spheres of a particular row don’t fit in
the depressions between two adjacent spheres of the previous row. This types of arrangement is called
AAAA type arrangement. This is also known as face centered cubic (fcc). This type of close packing of
constituent particles is found in metals like copper, silver, etc.
Lattice of this cubic close packed is simple cubic and its unit cell is primitive cubic unit cell.
Hexagonal Close packed (hcp)
In this type of packing, the spheres of molecules of a particular row in a particular dimension are in a
position that they fit into depressions between adjacent spheres of the previous row. This type of
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arrangement is called ABAB type arrangement. This type of packed lattice is found in many metals
such as magnesium, zinc, etc.
Coordination number: The number of adjacent particles of atoms is called coordination number.
In both ccp and hcp, each sphere is surrounded by 12 adjacent atoms, thus coordination number is
equal to 12 in each case.
Formation of voids in close packing:
Empty space left after the packing is called void. Two types of voids are formed in ccp and hcp
structures. These are tetrahedral voids and octahedral voids.
Tetrahedral voids are formed because of formation of tetrahedron between the layers of atoms. Thus,
voids in the shape of tetrahedron are called tetrahedral voids.
Octahedral voids are formed because of
formation of octahedron between the
layers of atoms. Thus, voids in the
shape of octahedron are called
octahedral voids.
Number of voids:
The number of formation of voids
depends upon the number of close
packed spheres. The number of
tetrahedral voids is formed twice as the
number of octahedral voids while close
packing of atoms in ccp and hcp
structures.
Thus, if number of close packed
spheres is equal to ‘N’.
Therefore, number of octahedral voids formed = N
And the number of tetrahedral voids formed = 2N
Formula of a compound and number of voids filled:
Bigger ions, usually anions, form close packed structure and smaller ions, usually cations occupy the
voids in ionic solids. If cations are bigger in size, they occupy octahedral voids and if are smaller
enough then they occupy tetrahedral voids.
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The occupation of number of voids depends upon the chemical formula of compound. It may be
possible to occupy all the voids or fraction of voids.
Example –
(a) If cation of an ionic solid occupies all the octahedral voids, then the formula of the compound can be
obtained as follows:
Let ‘A’ are cations and ‘B’ are anions in the compound.
Since the number of close packed sphere is equal to the number of octahedral voids formed, thus the
cations and anions must be in the ratio of 1:1.
Therefore, A and B will be combined in the ratio of A:B.
Thus the formula of the compound will be AB.
(b) If there are two ions A and B in an ionic compound and cations occupy all the tetrahedral voids
formed because of close packing, then the formula of the compound can be obtained as follows:
Let A is the cation and B is the anion in given compound.
Since, number of tetrahedral voids formed = 2 X number of close packed spheres.
This means A and B will combined in the ratio of 1:2
Therefore, formula of the compound will be AB2
Packing Efficiency of Close Packed Structure - 1
Both ccp and hcp are highly efficient lattice; in terms of packing. The packing efficiency of both types of
close packed structure is 74%, i.e. 74% of the space in hcp and ccp is filled. The hcp and ccp structure
are equally efficient; in terms of packing.
The packing efficiency of simple cubic lattice is 52.4% and that of body centered cubic lattice (bcc) is
68%.
Calculation of pacing efficiency in hcp and ccp structure:
The packing efficiency can be calculated by the percent of space occupied by spheres present in a unit
cell.
Let the side of an unit cell = a
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And diagonal AC = b
Now, in ∆ ABC,
AB is perpendicular, DC is base and AC is diagonal
Thus,packing efficiency of hcp or ccp structure=74%
Packing efficiency of body centered cubic (bcc) structure:
In body centered cubic unit cell, one atom is present in body
center apart from 4 atoms at its corners. Therefore, total
number of atoms present in bcc unit cell is equal to 2.
Let a unit cell of bcc structure with side a.
Let FD (diagonal) = b and diagonal AF = c
Let the radius of atom present in unit cell = r
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Now, in ∆EFD
After subtituting the value of a from equation (vi) we get
Thus,packing efficiency of bcc structure=68%
Packing Efficiency of Close Packed Structure - 2
Packing efficiency in Simple Cubic Lattice:
A unit cell of simple cubic lattice contains one atom.
Let the side of a simple cubic lattice is ‘a’ and radius of atom present in
it is ‘r’.
Since, edges of atoms touch each other, therefore, a = 2r
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Thus,packing efficiency of bcc structure=52.4%
Calculation of dimensions of a unit cell:
Let
The edge of a unit cell is ‘a’.
The density of unit cell is ‘d’
Molar mass of unit cell is ‘M’.
Number of atoms present in unit cell is ‘z’.
Mass of each atoms present in unit cell is ‘m’.
Where, d is density, z is number of atoms present in unit cell, a is length of edge, and NA is Avogadro
constant.
Above expression has five parameters, d, z, a, m and NA . By knowing any four of them fifth can be
calculated.
Imperfections in Solids or Crystal defects:
Irregularity in the arrangement of constituent particles in solids is called crystal defect or imperfection in
solids. There are two types of crystal defects - Point Defects and Line Defects.
Point Defects: Irregularities or deviation from ideal arrangement of constituent particles around the
point or atom in a crystalline solid is known as point defects.
Line Defects: Irregularities or deviation from ideal arrangement of constituent particles in entire row of
lattice is known as line defects.
Point Defects: Point Defects are divided into three types:
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(i) Stoichiometric Defects
(ii) Impurities Defects
(iii) Non-stoichiometric Defects
(i) Stoichiometric Defects – It is a type of point defects which does not disturb the stoichiometry of solid.
This is also known as Intrinsic or Thermodynamic Defects.
Types of stoichiometric defects: Vacancy Defects, Interstitial defects, Frenkel Defects, Schottky
Defects.
Vacancy defects and Interstitial defects are found in non-ionic compounds while similar defects found in
ionic compounds are known as Frenkel Defects and Schottky Defects.
(a) Vacancy Defects: When some lattice sites left vacant while the formation of crystal, the defect is
called Vacancy Defects.
In vacancy defects, an atom is missing from its regular atomic site. Because of missing of atom the
density of substance decreases, i.e. because of vacancy defects.
The vacancy defect develops on heating of substance.
(b) Interstitial Defects: Sometime in the formation of lattice structure some of the atoms occupy
interstitial site, the defect arising because of this is called Interstitial Defects.
In interstitial defect, some atoms occupy sites at which; generally there is no atom in the crystal
structure. Because of the interstitial defects, the number of atoms becomes larger than the number of
lattice sites.
Increase in number of atoms increases the density of substance, i.e. interstitial defects increase the
density of substance.
The vacancy defects and interstitial defects are found only in non-ionic compounds. Such defects found
in ionic compounds are known as Frenkel Defects and Schottky Defects.
(c) Frenkel Defects: It is a type of vacancy defect. In ionic compounds, some of the ions (usually
smaller in size) get dislocated from their original site and create defect. This defect is known as Frenkel
Defects. Since this defect arises because of dislocation of ions, thus it is also known as Dislocation
Defects. As there are a number of cations and anions (which remain equal even because of defect); the
density of the substance does not increase or decrease.
Ionic compounds; having large difference in the size between their cations and anions; show Frenkel
Defects, such as ZnS, AgCl, AgBr, AgI, etc. These compounds have smaller size of cations compared
to anions.
(d) Schottky Defects: Schottky Defect is type of simple vacancy defect and shown by ionic solids
having cations and anions; almost similar in size, such as NaCl, KCl, CsCl, etc. AgBr shows both types
of defects, i.e. Schottky and Frenkel Defects.
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When cations and anions both are missing from regular sites, the defect is called Schottky Defect. In
Schottky Defects, the number of missing cations is equal to the number of missing anions in order to
maintain the electrical neutrality of the ionic compound.
Since, Schottky Defects arises because of mission of constituent particles, thus it decreases the
density of ionic compound.
(ii) Impurities Defects: Defects in ionic compounds because of replacement of ions by the ions of other
compound is called impurities defects.
In NaCl; during crystallization; a little amount of SrCl2 is also
crystallized. In this process, Sr++ ions get the place of Na+
ions and create impurities defects in the crystal of NaCl. In
this defect, each of the Sr++ ion replaces two Na+ ions. Sr++
ion occupies one site of Na+ ion; leaving other site vacant.
Hence it creates cationic vacancies equal number of Sr++
ions. CaCl2, AgCl, etc. also shows impurities defects.
(iii) Non-stoichiometric Defects: There are large numbers of
inorganic solids found which contain the constituent particles
in non-stoichiometric ratio because of defects in their crystal structure. Thus, defects because of
presence of constituent particles in non-stoichiometric ratio in the crystal structure are called Nonstoichiometric Defects.
Non-stoichiometric Defects is mainly of two types – Metal Excess Defects and Metal Deficiency
Defects.
Metal Excess Defects: Metal excess defects are of two types:
(a) Metal excess defects due to anionic vacancies:
These type of defects seen because of missing of anions from regular site leaving a hole which is
occupied by electron to maintain the neutrality of the compound. Hole occupied by electron is called Fcentre and responsible for showing colour by the compound.
This defect is common in NaCl, KCl, LiCl, etc. Sodium atoms get deposited on the surface of crystal
when sodium chloride is heated in an atmosphere of sodium vapour. In this process, the chloride ions
get diffused with sodium ion to form sodium chloride. In this process, sodium atom releases electron to
form sodium ion. This released electron gets diffused and occupies the anionic sites in the crystal of
sodium chloride; creating anionic vacancies and resulting in the excess of sodium metal.
The anionic site occupied by unpaired electron is called F-centre. When visible light falls over the
crystal of NaCl, the unpaired electron present gets excited because of absorption of energy and impart
yellow colour.
Because of similar defect if present, crystal of LiCl imparts pink colour and KCl imparts violet.
(b) Metal excess defect due to presence of extra cations at interstitial sites:
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Zinc oxide loses oxygen on heating resulting the number of cations (zinc ion) become more than anions
present in zinc oxide.
The excess cations (Zn++ions) move to interstitial site and electrons move to neighbouring interstitial
sites. Because of this zinc oxide imparts yellow colour when heated. Such defects are called metal
excess defects.
Metal Deficiency Defects:
Many solids show metal deficiency defects as they have less metals compare to ideal stoichiometric
proportion. The less proportion of metal is compensated by same metals having higher valency. Such
defects are shown generally by transition elements. Thus, when metal present less than ideal
stoichiometric proportion in a solid, it is called metal deficiency defect.
Example – FeO is generally found in composion of Fe0.95O. In the crystal of FeO, missing Fe++ ions
are compensated with Fe+++ ions in order to maintain neutrality.
Electrical Properties:
Solids show amazing range of electrical conductivities. Electrical conductivity is the reciprocal of
resistivity. Whereas resistivity is the property of solids to resist flow of electricity, conductivity is the
property to conduct electricity.
The SI unit of resistivity is ohm meter. Since, conductivity is the reciprocal of resistivity, thus its unit is
reciprocal of ohm meter, i.e. ohm -1 m -1. Conductivity is generally represented by Greek letter σ
(sigma). The SI unit of conductivity is Siemens per meter, i.e. S/m.
On the basis of magnitude of range of conductivities, i.e. from 10 -20 to 107 ohm-1 m-1, solids can be
classified into three types:
(a) Conductor: Solids having magnitude of range of conductivities from 10 4 to 107 ohm-1 m-1 are
classified as conductors. Metals are good conductor of electricity. Silver has conductivity in the order of
107 ohm-1 m-1 is considered as very good conductor.
(b) Insulator: Solids having range of conductivity from 10-20 to 10-10 ohm-1 m-1 are considered as
insulators.
(c) Semiconductor: Solids having intermediate range of conductivity, i.e. from 10-6 to 104 ohm-1 m-1
are called semiconductors.
Conduction of Electricity in Metals:
Metals show electrical conductivity because of movement of electrons. Electrolytes show electrical
conductivity because of movement of ions. Metals show electrical conductivity in solid and molten
states both while electrolytes show electrical conductivity in molten state and aqueous solution.
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Conductivity in metals depends upon presence of unpaired electrons in their valence shell per atom.
Electrons present in valence shell of metals are free to move and allow conducting electricity in metals.
Energy level (atomic orbital) with electrons and vacant energy levels present in metals have if minute
difference in energy they together are called energy band or simply band. The empty energy levels or
unoccupied energy levels are known as conduction band also since they helps in conduction of
electricity.
When partially filled energy levels (atomic orbital) are too close or overlapped with unoccupied energy
level or conduction band; electrons can easily flow between them under the electrical field. Because of
flows of unpaired electrons from occupied energy level to conduction band metals conduct electricity.
Conduction of electricity in Insulators:
In insulators the difference in energy between occupied energy level and unoccupied energy level
(conduction band) is higher because of which electrons do not flow from occupied energy band to the
next higher unoccupied energy band resulting insulators do not conduct electricity as electrons do not
flow.
Conduction of electricity in Semiconductor:
In semiconductors like silicon and germanium, the energy gap between valence shell and conduction
band is so smaller that electrons may jump from filled orbital to conduction band when put under
electrical field. Because of this behavior, i.e. lower gap between valence band and conduction band
semiconductor show the conduction of electricity.
The conduction of electricity in semiconductors increases with increase in temperature. Elements such
as silicon and germanium show such behavior and are called intrinsic semiconductors.
Doping:
Intrinsic semiconductors show very low conductivity and thus cannot be used practically. Thus, the
conductivity of intrinsic semiconductors is increased by adding suitable impurities. Addition of
appropriate amount of suitable impurities to elements, such as intrinsic semiconductors is called
doping.
Doping is done with electron rich or electron deficient element (impurities) to the intrinsic
semiconductors. Doping with electron rich or electron deficient elements creates electronic defects in
semiconductors.
(a) Doping with electron rich impurities: n-type of semiconductor:
Silicon and/or germanium are doped with electron rich impurities to
increase their electrical conductivity. Semiconductors so formed after
are called n-type semiconductors.
Silicon and germanium, each has four valence electrons as they
belong to 14th group of periodic table. Arsenic and phosphorous
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belong to 15th group of periodic table and they have valence electrons equal to 5. When silicon or
germanium is doped with phosphorous or arsenic, four electrons of phosphorous or arsenic out of five;
make covalent bonds with four electrons of silicon or germanium leaving one electron free; which
increases the electrical conductivity of silicon or germanium.
Since the electrical conductivity of silicon or phosphorous is increased because of negatively charged
particle (electron), thus this is known as n-type of semiconductor.
(b) Doping with electron deficient impurities – p-type semiconductor:
Electrical conductivity of silicon or germanium is doped with elements, such as Boron, Aluminium or
Gallium which belong to group 13th in periodic table also. Elements belong to group 13th have valence
electrons equal to 3. Three valence electrons present in these elements make covalent bonds with
three electrons present in valence shell out of four of silicon or germanium leaving one electron
delocalized. The place from where one electron is missing is called electron hole or electron vacancy.
When the silicon or germanium is placed under electrical field, electron from neighbouring atom fill the
electron hole, but in doing so another electron hole is created at the place of movement of electron. In
the influence of electrical filed electron moves toward positively charge plate through electron hole as
appearing the electron hole as positively charged and are moving towards negatively charged plate.
Semiconductor formed by the doping with electron deficient impurities; are called p-type
semiconductors.
Applications of n-type and p-type semiconductors





Both n-type and p-type semiconductors are used in making electronic components.
As diode which is the combination of n-type and p-type semiconductors.
As integrated circuit (ICs).
In photoelectric cell
As transistors, to amplify radio and audio signal
Magnetic Properties:
Substance shows magnetic properties because of presence of electrons in them. Each electron in an
atom behaves like a magnet because of its two types of motions - one is around their axis and other
around the nucleus. Electrons in an atom because of charge over then and in motion continuously;
possess small loop of current which shows the magnetic moment.
Substances are classified into five types on the basis of magnetic properties:
a.
b.
c.
d.
e.
Paramagnetic
Diamagnetic
Ferromagnetic
Antiferromagnetic
Ferrimagnetic
(a) Paramagnetism: Substances which are attracted slightly by magnetic field and do not retain the
magnetic property after removal of magnetic field are called paramagnetic substances. For example
O2, Cu2+, Fe3+, Cr3+, Magnesium, molybdenum, lithium, etc.
Smallworld Institute Of Technology / APEX Institute Er. Harish Srivastava 011-65920613/ 9268966434
Substances show paramagnetism because of presence of unpaired electrons. These unpaired
electrons are attracted by magnetic field.
(b) Diamagnetism: Diamagnetic substances are just opposite to that of paramagnetic. Substances
which are repelled slightly by magnetic field are called diamagnetic substances. For example; H2O,
NaCl, C6H6, etc. Diamangetic substances are magnetized slightly when put under magnetic field but in
opposite direction.
Substances show diamagnetic property because of presence of paired electrons and no unpaired
electron. Thus, pairing of electrons cancel the magnetic property.
(c) Ferromagnetism: Substances that are attracted strongly with magnetic field are called ferromagnetic
substances, such as cobalt, nickel, iron, gadolinium, chromium oxide, etc. Ferromagnetic substances
can be permanently magnetized also.
Metal ions of ferromagnetic substances are randomly oriented in normal condition and substances do
not act as a magnet. But when metal ions are grouped together in small regions, called domains, each
domains act like a tiny magnet and produce strong magnetic field, in such condition ferromagnetic
substance act like a magnet. When the ordering of domains in group persists even after removal of
magnetic field a ferromagnetic substance becomes a permanent magnet.
(d) Antiferromagnetism: Substances in which domain structure are similar to ferromagnetic substances
but are oriented oppositely, which cancel the magnetic property are called antiferromagnetic
substances and this property is called antiferromagnetism. For example; MnO.
(e) Ferrimagnetism: Substances which are slightly attracted in magnetic field and in which domains are
grouped in parallel and anti-parallel direction but in unequal number, are called ferromagnetic
substances and this property is called ferrimagnetism. For example, magnetite (Fe3O4), ferrite
(MgFe2O4), ZnFe2O4, etc.
Ferrimagnetic
ferrimagnetism
paramagnetic.
substances
on heating and
lose
become
NCERT Solution
In Text Questions and Answer - 1
Question: 1.1 - Why are solids rigid?
Answer: The particles of solids are close packed and can only oscillate about their fixed positions.
These properties make solids rigid.
Question: 1.2 - Why do solids have a definite volume?
Answer: The intermolecular force of attraction make the particles of solid closely packed and force
them to only oscillate at their fixed positions. These give solids a definite volume.
Smallworld Institute Of Technology / APEX Institute Er. Harish Srivastava 011-65920613/ 9268966434
Question: 1.3 - Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene,
benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper.
Answer:
Polyurethane, Teflon, cellophane, polyvinyl chloride, fibre glass – Amorphous solids
Naphthalene, benzoic acid, potassium nitrate, copper – Crystalline solids.
Question: 1.4 - Why is glass considered a super cooled liquid?
Answer: Glass is an amorphous solids, it has tendency to flow but very slowly. This is the cause that
glass is considered as super cooled liquid.
Question: 1.5 - Refractive index of a solid is observed to have the same value along all directions.
Comment on the nature of this solid. Would it show cleavage property?
Answer: Amorphous solids are isotropic in nature, i.e. they have short range order of arrangement of
particles. Because of this amorphous solids have same value of refractive index along all directions.
Amorphous solids do not show cleavage property, i.e. when cut into two pieces with a sharp knife, they
give pieces with irregular surface.
Question: 1.6 - Classify the following solids in different categories based on the nature of intermolecular
forces operating in them:
Potassium sulphate, tin, benzene, urea, ammonia, water, zinc sulphide, graphite, rubidium, argon,
silicon carbide.
Answer:
Potassium sulphate, Zinc sulphate – Ionic solid
Benzene, urea, water, argon, ammonia – Molecular solid
Tin, rubidium – Metallic solid
Graphite, silicon carbide – Covalent solids or network solids
Question: 1.7 - Solid A is a very hard electrical insulator in solid as well as in molten state and melts at
extremely high temperature. What type of solid is it?
Answer: Given solid ‘A’ is a covalent solids, such as diamond.
Question: 1.8 - Ionic solids conduct electricity in molten state but not in solid state. Explain.
Answer: Ionic solids conduct electricity because of movement of their ions. In solid state ions present in
ionic solids do not move hence do not conduct electricity while in molten state ions can move and thus
conduct electricity.
Smallworld Institute Of Technology / APEX Institute Er. Harish Srivastava 011-65920613/ 9268966434
Question: 1.9 - What type of solids are electrical conductors, malleable and ductile?
Answer:
Metallic solids are conductor of electricity, malleable and ductile.
Question: 1.10 - Give the significance of a ‘lattice point’.
Answer:
Lattice point denotea the position of constituent particles (molecule, atom or ion) in space. When lattice
points are joined together by straight line they give the geometry of lattice.
Question: 1.11 - Name the parameters that characterise a unit cell.
Answer:
Unit cells are characterize on six parameters – dimensions along three edges and three angles
between their edges, i.e. a, b, c which are edges and α, β and γ which are angles between the edges.
Question: 1.12 - Distinguish between
(i) Hexagonal and monoclinic unit cells
Answer:
(ii) Face-centred and end-centred unit cells.
Answer:
There are four atoms present in face centered unit cell while there are only 2 atoms present in end
centered unit cell.
In face centered unit cell one constituent
particles are present at the center of each
of the faces besides one at each corner.
In end centered unit cell two constituent
particles are present at the center of any of the two faces besides one at each corner of the unit cell.