ARBIND SINGH ACADEMY
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.
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Characteristics of crystalline solid –
a.
b.
c.
d.
Crystalline solids have definite characteristic geometrical shape.
Crystalline solids have sharp characteristic melting point.
Crystalline solids have definite and characteristic heat of fusion.
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.
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. Particles of amorphous solids are irregular in shape.
b. Amorphous solids soften gradually over a range of temperature.
c. Amorphous solids produce pieces of irregular shapes when they are cut into two
pieces.
d. Amorphous solids do not have definite heat of fusion.
e. 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.
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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
(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.
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ARBIND SINGH ACADEMY


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.
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.
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


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:
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



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.
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.
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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.
Orthorhombic Lattice – Four types of orthorhombic lattice are possible. They are
Primitive, End-centred, Body centred and Face centred. They have unequal sides. The
Angles
between
their
faces
are
equal
to
90 0.
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.
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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,
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
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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 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.
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ARBIND SINGH ACADEMY
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.
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.
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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 the packing efficiency 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
And diagonal AC = b
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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:
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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
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.
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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
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’.
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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:
(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.
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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.
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
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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 nonstoichiometric ratio in the crystal structure are called Non-stoichiometric 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 F-centre and responsible for showing colour by the
compound.
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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:
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
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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 ohm1 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.
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:
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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 belong to 15th group of periodic table and they
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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
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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. Paramagnetic
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ARBIND SINGH ACADEMY
b.
c.
d.
e.
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.
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.
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Ferrimagnetic substances lose ferrimagnetism on heating and become paramagnetic.
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.
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?
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ARBIND SINGH ACADEMY
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.
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:
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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.
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ARBIND SINGH ACADEMY
NCERT Solution
In Text Questions and Answer - 2
Question: 1.13 - Explain how much portion of an atom located at (i) corner and (ii)
bodycentre of a cubic unit cell is part of its neighbouring unit cell.
Answer: (i) Atom located at the corner is shared among eight adjacent unit cell, thus only 1/8 th
portion of the atom is located at corner.
(ii) Atom located at body center does not share any part of its neighbouring unit cell,
thus whole portion of atom is located at body center of cubic unit cell.
Question: 1.14 - What is the two dimensional coordination number of a molecule in
square close-packed layer?
Answer: -The coordination number of a molecule in two dimensions in square close
packed layer is 4.
Question: 1.15 - A compound forms hexagonal close-packed structure. What is the total
number of voids in 0.5 mol of it? How many of these are tetrahedral voids?
Answer:
Question: 1.16 - A compound is formed by two elements M and N. The element N forms
ccp and atoms of M occupy 1/3rd of tetrahedral voids. What is the formula of the
compound?
Answer:
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ARBIND SINGH ACADEMY
Let the number of octahedral voids occupied by element N = a
Therefore total number of tetrahedral voids = 2a
Question: 1.17 - Which of the following lattices has the highest packing efficiency (i)
simple cubic (ii) body-centred cubic and (iii) hexagonal close-packed lattice?
Answer:
The packing efficiency of simple cubic lattice is 52.4%, body centered is 68% and that of
hexagonal close packed lattice is 74%.
Therefore, (iii) hexagonal close packed lattice has highest packing efficiency, i.e. 74%.
Question: 1.18 - An element with molar mass 2.7×10-2 kg mol-1 forms a cubic unit cell
with edge length 405 pm. If its density is 2.7×103 kg m-3, what is the nature of the cubic
unit cell?
Answer:
By knowing the number of atom in the cubic unit cell of given lattice, its nature can be
determined.
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ARBIND SINGH ACADEMY
NCERT Solution
In Text Questions and Answer - 3
Question: 1.19 - What type of defect can arise when a solid is heated? Which physical
property is affected by it and in what way?
Answer:
When a solid is heated a vacancy defect may arise. Because of vacancy defect the
density of the solid decreases because of leaving of some of the constituent particles.
Question: 1.20 - What type of stoichiometric defect is shown by:
(i) ZnS (ii) AgBr
Answer:
(i) ZnS – Since there is large difference in the size of ions, thus it shows Frenkel defect.
(ii) AgBr – AgBr shows Frenkel defects and Schottky defects both.
Question: 1.21 - Explain how vacancies are introduced in an ionic solid when a cation of
higher valence is added as an impurity in it.
Answer: In ionic solids, such as NaCl; during crystallization; a little amount of SrCl2 is also
crystallized. In this process, Sr++ ions get the place of Na+ ions and create 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 and is called vacancy defects.
This is also known as impurities defects.
Question: 1.22 - Ionic solids, which have anionic vacancies due to metal excess defect,
develop colour. Explain with the help of a suitable example.
Answer: Metal excess defects are 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 F-centre 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.
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In this process, the chloride ions get diffused with sodium ion to form sodium chloride
and 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.
Question: 1.23 - A group 14 element is to be converted into n-type semiconductor by
doping it with a suitable impurity. To which group should this impurity belong?
Answer:
Silicon and germanium, each has four valence electrons and they belong to 14th group
of periodic table. Arsenic and phosphorous 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.
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ARBIND SINGH ACADEMY
The impurities belong to group 15 which is introduced with elements of group 14 to
produce n-type of semiconductor.
Question: 1.24 - What type of substances would make better permanent magnets,
ferromagnetic or ferrimagnetic. Justify your answer.
Answer: 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. While domains are
grouped in parallel and anti-parallel direction but in unequal number in ferromagnetic
compounds.
Thus, Ferromagnetic substances, such as Ni, Co, Fe would make better permanent
magnets rather than ferromagnetic substances.
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