NCERT Textbook: The Solid State (Old NCERT) | Chemistry Class 12 - NEET PDF Download

 Page 1


From our earlier studies, we know that liquids and gases
are called fluids because of their ability to flow. The
fluidity in both of these states is due to the fact that the
molecules are free to move about. On the contrary, the
constituent particles in solids have fixed positions and
can only oscillate about their mean positions. This
explains the rigidity in solids. These properties depend
upon the nature of constituent particles and the binding
forces operating between them. The correlation between
structure and properties helps in the discovery of new
solid materials with desired properties. For example,
carbon nanotubes are new materials that have potential
to provide material that are tougher than steel, lighter
than aluminium and have more conductive property than
copper. Such materials may play an expanding role in
future development of science and society. Some other
materials which are expected to play an important role
in future are high temperature superconductors,
magnetic materials, biodegradable polymers for
packaging, biocompliant solids for surgical implants, etc.
Thus, the study of this state becomes more important in
the present scenario.
In this Unit, we shall discuss different possible
arrangements of particles resulting in several types of
structures and explore why different arrangements of
structural units lend different properties to solids. We
will also learn how these properties get modified due to
the structural imperfections or by the presence of
impurities in minute amounts.
After studying this Unit, you will be
able to
• describe general characteristics of
solid state;
• distinguish between amorphous
and crystalline solids;
• classify crystalline solids on the
basis of the nature of binding
forces;
• define crystal lattice and unit cell;
• explain close packing of particles;
• describe different types of voids
and close packed structures;
• calculate the packing efficiency of
different types of cubic unit cells;
• correlate the density of a
substance with its unit cell
properties;
• describe the imperfections in
solids and their effect on
properties;
• correlate the electrical and
magnetic properties of solids and
their structure.
Objectives
The vast majority of solid substances like high temperature
superconductors, biocompatible plastics, silicon chips, etc. are destined
to play an ever expanding role in future development of science.
The Solid State
1
Unit Unit Unit Unit Unit
1
The Solid State
2022-23
Page 2


From our earlier studies, we know that liquids and gases
are called fluids because of their ability to flow. The
fluidity in both of these states is due to the fact that the
molecules are free to move about. On the contrary, the
constituent particles in solids have fixed positions and
can only oscillate about their mean positions. This
explains the rigidity in solids. These properties depend
upon the nature of constituent particles and the binding
forces operating between them. The correlation between
structure and properties helps in the discovery of new
solid materials with desired properties. For example,
carbon nanotubes are new materials that have potential
to provide material that are tougher than steel, lighter
than aluminium and have more conductive property than
copper. Such materials may play an expanding role in
future development of science and society. Some other
materials which are expected to play an important role
in future are high temperature superconductors,
magnetic materials, biodegradable polymers for
packaging, biocompliant solids for surgical implants, etc.
Thus, the study of this state becomes more important in
the present scenario.
In this Unit, we shall discuss different possible
arrangements of particles resulting in several types of
structures and explore why different arrangements of
structural units lend different properties to solids. We
will also learn how these properties get modified due to
the structural imperfections or by the presence of
impurities in minute amounts.
After studying this Unit, you will be
able to
• describe general characteristics of
solid state;
• distinguish between amorphous
and crystalline solids;
• classify crystalline solids on the
basis of the nature of binding
forces;
• define crystal lattice and unit cell;
• explain close packing of particles;
• describe different types of voids
and close packed structures;
• calculate the packing efficiency of
different types of cubic unit cells;
• correlate the density of a
substance with its unit cell
properties;
• describe the imperfections in
solids and their effect on
properties;
• correlate the electrical and
magnetic properties of solids and
their structure.
Objectives
The vast majority of solid substances like high temperature
superconductors, biocompatible plastics, silicon chips, etc. are destined
to play an ever expanding role in future development of science.
The Solid State
1
Unit Unit Unit Unit Unit
1
The Solid State
2022-23
2 Chemistry
In Class XI you have learnt that matter can exist in three states namely,
solid, liquid and gas.  Under a given set of conditions of temperature and
pressure, which of these would be the most stable state of a given
substance depends upon the net effect of two opposing factors. These
are intermolecular forces which tend to keep the molecules (or atoms
or ions) closer, and the thermal energy, which tends to keep them apart
by making them move faster.  At sufficiently low temperature, the thermal
energy is low and intermolecular forces bring them so close that they
cling to one another and occupy fixed positions. These can still oscillate
about their mean positions and the substance exists in solid state.  The
following are the characteristic properties of the solid state:
(i) They have definite mass, volume and shape.
(ii) Intermolecular distances are short.
(iii) Intermolecular forces are strong.
(iv) Their constituent particles (atoms, molecules or ions) have fixed
positions and can only oscillate about their mean positions.
(v) They are incompressible and rigid.
Solids can be classified as crystalline or amorphous on the basis of the
nature of order present in the arrangement of their constituent particles.
A crystalline solid usually consists of a large number of small crystals,
each of them having a definite characteristic geometrical shape. The
arrangement of constituent particles (atoms, molecules or ions) in a crystal
is ordered and repetitive in three dimensions. If we observe the pattern in
one region of the crystal, we can predict accurately the position of particles
in any other region of the crystal however far they may be from the place
of observation. Thus, crystal has a long range order which means that
there is a regular pattern of arrangement of particles which repeats itself
periodically over the entire crystal.  Sodium chloride and quartz are typical
examples of crystalline solids. Glass, rubber and many plastics do not
form crystals when their liquids solidify on cooling. These are called
amorphous solids. The term amorphous comes from the Greek word
amorphos, meaning no form.The arrangement of constituent particles
(atoms, molecules or ions) in such a solid has only short range order. In
such an arrangement, a regular and
periodically repeating pattern is observed
over short distances only. Regular patterns
are scattered and in between the
arrangement is disordered. The structures
of quartz (crystalline) and quartz glass
(amorphous) are shown in Fig. 1.1 (a) and
(b) respectively.  While the two structures
are almost identical, yet in the case of
amorphous quartz glass there is no long
range order. The structure of amorphous
solids is similar to that of liquids. Due to
the differences in the arrangement of the
constituent particles, the two types of solids
differ in their properties.
1.1 1.1 1.1 1.1 1.1 General General General General General
Characteristics Characteristics Characteristics Characteristics Characteristics
of Solid State of Solid State of Solid State of Solid State of Solid State
1.2 1.2 1.2 1.2 1.2 Amorphous Amorphous Amorphous Amorphous Amorphous
and Crystalline and Crystalline and Crystalline and Crystalline and Crystalline
Solids Solids Solids Solids Solids
Fig. 1.1: Two dimensional structure of
(a) quartz and (b) quartz glass
2022-23
Page 3


From our earlier studies, we know that liquids and gases
are called fluids because of their ability to flow. The
fluidity in both of these states is due to the fact that the
molecules are free to move about. On the contrary, the
constituent particles in solids have fixed positions and
can only oscillate about their mean positions. This
explains the rigidity in solids. These properties depend
upon the nature of constituent particles and the binding
forces operating between them. The correlation between
structure and properties helps in the discovery of new
solid materials with desired properties. For example,
carbon nanotubes are new materials that have potential
to provide material that are tougher than steel, lighter
than aluminium and have more conductive property than
copper. Such materials may play an expanding role in
future development of science and society. Some other
materials which are expected to play an important role
in future are high temperature superconductors,
magnetic materials, biodegradable polymers for
packaging, biocompliant solids for surgical implants, etc.
Thus, the study of this state becomes more important in
the present scenario.
In this Unit, we shall discuss different possible
arrangements of particles resulting in several types of
structures and explore why different arrangements of
structural units lend different properties to solids. We
will also learn how these properties get modified due to
the structural imperfections or by the presence of
impurities in minute amounts.
After studying this Unit, you will be
able to
• describe general characteristics of
solid state;
• distinguish between amorphous
and crystalline solids;
• classify crystalline solids on the
basis of the nature of binding
forces;
• define crystal lattice and unit cell;
• explain close packing of particles;
• describe different types of voids
and close packed structures;
• calculate the packing efficiency of
different types of cubic unit cells;
• correlate the density of a
substance with its unit cell
properties;
• describe the imperfections in
solids and their effect on
properties;
• correlate the electrical and
magnetic properties of solids and
their structure.
Objectives
The vast majority of solid substances like high temperature
superconductors, biocompatible plastics, silicon chips, etc. are destined
to play an ever expanding role in future development of science.
The Solid State
1
Unit Unit Unit Unit Unit
1
The Solid State
2022-23
2 Chemistry
In Class XI you have learnt that matter can exist in three states namely,
solid, liquid and gas.  Under a given set of conditions of temperature and
pressure, which of these would be the most stable state of a given
substance depends upon the net effect of two opposing factors. These
are intermolecular forces which tend to keep the molecules (or atoms
or ions) closer, and the thermal energy, which tends to keep them apart
by making them move faster.  At sufficiently low temperature, the thermal
energy is low and intermolecular forces bring them so close that they
cling to one another and occupy fixed positions. These can still oscillate
about their mean positions and the substance exists in solid state.  The
following are the characteristic properties of the solid state:
(i) They have definite mass, volume and shape.
(ii) Intermolecular distances are short.
(iii) Intermolecular forces are strong.
(iv) Their constituent particles (atoms, molecules or ions) have fixed
positions and can only oscillate about their mean positions.
(v) They are incompressible and rigid.
Solids can be classified as crystalline or amorphous on the basis of the
nature of order present in the arrangement of their constituent particles.
A crystalline solid usually consists of a large number of small crystals,
each of them having a definite characteristic geometrical shape. The
arrangement of constituent particles (atoms, molecules or ions) in a crystal
is ordered and repetitive in three dimensions. If we observe the pattern in
one region of the crystal, we can predict accurately the position of particles
in any other region of the crystal however far they may be from the place
of observation. Thus, crystal has a long range order which means that
there is a regular pattern of arrangement of particles which repeats itself
periodically over the entire crystal.  Sodium chloride and quartz are typical
examples of crystalline solids. Glass, rubber and many plastics do not
form crystals when their liquids solidify on cooling. These are called
amorphous solids. The term amorphous comes from the Greek word
amorphos, meaning no form.The arrangement of constituent particles
(atoms, molecules or ions) in such a solid has only short range order. In
such an arrangement, a regular and
periodically repeating pattern is observed
over short distances only. Regular patterns
are scattered and in between the
arrangement is disordered. The structures
of quartz (crystalline) and quartz glass
(amorphous) are shown in Fig. 1.1 (a) and
(b) respectively.  While the two structures
are almost identical, yet in the case of
amorphous quartz glass there is no long
range order. The structure of amorphous
solids is similar to that of liquids. Due to
the differences in the arrangement of the
constituent particles, the two types of solids
differ in their properties.
1.1 1.1 1.1 1.1 1.1 General General General General General
Characteristics Characteristics Characteristics Characteristics Characteristics
of Solid State of Solid State of Solid State of Solid State of Solid State
1.2 1.2 1.2 1.2 1.2 Amorphous Amorphous Amorphous Amorphous Amorphous
and Crystalline and Crystalline and Crystalline and Crystalline and Crystalline
Solids Solids Solids Solids Solids
Fig. 1.1: Two dimensional structure of
(a) quartz and (b) quartz glass
2022-23
3 The Solid State
Crystalline solids have a sharp melting point. At a characteristic
temperature they melt abruptly and become liquid. On the other hand,
amorphous solids soften, melt and start flowing over a range of
temperature and can be moulded and blown into various shapes.
Amorphous solids have the same structural features as liquids and are
conveniently regarded as extremely viscous liquids. They may become
crystalline at some temperature. Some glass objects from ancient
civilisations are found to become milky in appearance because of some
crystallisation. Like liquids, amorphous solids have a tendency to flow,
though very slowly. Therefore, sometimes these are called pseudo
solids or super cooled liquids.
Amorphous solids are isotropic in nature. Their properties such as
mechanical strength, refractive index and electrical conductivity, etc.,
are same in all directions. It is because there is no long range order in
them and arrangement of particles is not definite along all the directions.
Hence, the overall arrangement becomes equivalent in all directions.
Therefore, value of any physical property would be same along
any direction.
Crystalline solids are anisotropic in nature, that
is, some of their physical properties like electrical
resistance or refractive index show different values
when measured along different directions in the
same crystals. This arises from different arrangement
of particles in different directions. This is illustrated
in Fig. 1.2. This figure shows a simple two-
dimensional pattern of arrangement of two kinds of
atoms. Mechanical property such as resistance to
shearing stress might be quite different in two
directions indicated in the figure. Deformation in CD
direction displaces row which has two different types
of atoms while in AB direction rows made of one type
of atoms are displaced. The differences between the
crystalline solids and amorphous solids are
summarised in Table 1.1.
D
C
B
A
Fig. 1.2: Anisotropy in crystals is due
to different arrangement of
particles along different
directions.
Table 1.1: Distinction between Crystalline and Amorphous Solids
Shape
Melting point
Cleavage
property
Heat of fusion
Definite characteristic geometrical shape
Melt at a sharp and characteristic
temperature
When cut with a sharp edged tool, they
split into two pieces and the newly
generated surfaces are plain and
smooth
They have a definite and characteristic
enthalpy of fusion
Irregular shape
Gradually soften over a range of
temperature
When cut with a sharp edged tool, they
cut into two pieces with irregular
surfaces
They do not have definite enthalpy of
fusion
Property Crystalline solids Amorphous solids
2022-23
Page 4


From our earlier studies, we know that liquids and gases
are called fluids because of their ability to flow. The
fluidity in both of these states is due to the fact that the
molecules are free to move about. On the contrary, the
constituent particles in solids have fixed positions and
can only oscillate about their mean positions. This
explains the rigidity in solids. These properties depend
upon the nature of constituent particles and the binding
forces operating between them. The correlation between
structure and properties helps in the discovery of new
solid materials with desired properties. For example,
carbon nanotubes are new materials that have potential
to provide material that are tougher than steel, lighter
than aluminium and have more conductive property than
copper. Such materials may play an expanding role in
future development of science and society. Some other
materials which are expected to play an important role
in future are high temperature superconductors,
magnetic materials, biodegradable polymers for
packaging, biocompliant solids for surgical implants, etc.
Thus, the study of this state becomes more important in
the present scenario.
In this Unit, we shall discuss different possible
arrangements of particles resulting in several types of
structures and explore why different arrangements of
structural units lend different properties to solids. We
will also learn how these properties get modified due to
the structural imperfections or by the presence of
impurities in minute amounts.
After studying this Unit, you will be
able to
• describe general characteristics of
solid state;
• distinguish between amorphous
and crystalline solids;
• classify crystalline solids on the
basis of the nature of binding
forces;
• define crystal lattice and unit cell;
• explain close packing of particles;
• describe different types of voids
and close packed structures;
• calculate the packing efficiency of
different types of cubic unit cells;
• correlate the density of a
substance with its unit cell
properties;
• describe the imperfections in
solids and their effect on
properties;
• correlate the electrical and
magnetic properties of solids and
their structure.
Objectives
The vast majority of solid substances like high temperature
superconductors, biocompatible plastics, silicon chips, etc. are destined
to play an ever expanding role in future development of science.
The Solid State
1
Unit Unit Unit Unit Unit
1
The Solid State
2022-23
2 Chemistry
In Class XI you have learnt that matter can exist in three states namely,
solid, liquid and gas.  Under a given set of conditions of temperature and
pressure, which of these would be the most stable state of a given
substance depends upon the net effect of two opposing factors. These
are intermolecular forces which tend to keep the molecules (or atoms
or ions) closer, and the thermal energy, which tends to keep them apart
by making them move faster.  At sufficiently low temperature, the thermal
energy is low and intermolecular forces bring them so close that they
cling to one another and occupy fixed positions. These can still oscillate
about their mean positions and the substance exists in solid state.  The
following are the characteristic properties of the solid state:
(i) They have definite mass, volume and shape.
(ii) Intermolecular distances are short.
(iii) Intermolecular forces are strong.
(iv) Their constituent particles (atoms, molecules or ions) have fixed
positions and can only oscillate about their mean positions.
(v) They are incompressible and rigid.
Solids can be classified as crystalline or amorphous on the basis of the
nature of order present in the arrangement of their constituent particles.
A crystalline solid usually consists of a large number of small crystals,
each of them having a definite characteristic geometrical shape. The
arrangement of constituent particles (atoms, molecules or ions) in a crystal
is ordered and repetitive in three dimensions. If we observe the pattern in
one region of the crystal, we can predict accurately the position of particles
in any other region of the crystal however far they may be from the place
of observation. Thus, crystal has a long range order which means that
there is a regular pattern of arrangement of particles which repeats itself
periodically over the entire crystal.  Sodium chloride and quartz are typical
examples of crystalline solids. Glass, rubber and many plastics do not
form crystals when their liquids solidify on cooling. These are called
amorphous solids. The term amorphous comes from the Greek word
amorphos, meaning no form.The arrangement of constituent particles
(atoms, molecules or ions) in such a solid has only short range order. In
such an arrangement, a regular and
periodically repeating pattern is observed
over short distances only. Regular patterns
are scattered and in between the
arrangement is disordered. The structures
of quartz (crystalline) and quartz glass
(amorphous) are shown in Fig. 1.1 (a) and
(b) respectively.  While the two structures
are almost identical, yet in the case of
amorphous quartz glass there is no long
range order. The structure of amorphous
solids is similar to that of liquids. Due to
the differences in the arrangement of the
constituent particles, the two types of solids
differ in their properties.
1.1 1.1 1.1 1.1 1.1 General General General General General
Characteristics Characteristics Characteristics Characteristics Characteristics
of Solid State of Solid State of Solid State of Solid State of Solid State
1.2 1.2 1.2 1.2 1.2 Amorphous Amorphous Amorphous Amorphous Amorphous
and Crystalline and Crystalline and Crystalline and Crystalline and Crystalline
Solids Solids Solids Solids Solids
Fig. 1.1: Two dimensional structure of
(a) quartz and (b) quartz glass
2022-23
3 The Solid State
Crystalline solids have a sharp melting point. At a characteristic
temperature they melt abruptly and become liquid. On the other hand,
amorphous solids soften, melt and start flowing over a range of
temperature and can be moulded and blown into various shapes.
Amorphous solids have the same structural features as liquids and are
conveniently regarded as extremely viscous liquids. They may become
crystalline at some temperature. Some glass objects from ancient
civilisations are found to become milky in appearance because of some
crystallisation. Like liquids, amorphous solids have a tendency to flow,
though very slowly. Therefore, sometimes these are called pseudo
solids or super cooled liquids.
Amorphous solids are isotropic in nature. Their properties such as
mechanical strength, refractive index and electrical conductivity, etc.,
are same in all directions. It is because there is no long range order in
them and arrangement of particles is not definite along all the directions.
Hence, the overall arrangement becomes equivalent in all directions.
Therefore, value of any physical property would be same along
any direction.
Crystalline solids are anisotropic in nature, that
is, some of their physical properties like electrical
resistance or refractive index show different values
when measured along different directions in the
same crystals. This arises from different arrangement
of particles in different directions. This is illustrated
in Fig. 1.2. This figure shows a simple two-
dimensional pattern of arrangement of two kinds of
atoms. Mechanical property such as resistance to
shearing stress might be quite different in two
directions indicated in the figure. Deformation in CD
direction displaces row which has two different types
of atoms while in AB direction rows made of one type
of atoms are displaced. The differences between the
crystalline solids and amorphous solids are
summarised in Table 1.1.
D
C
B
A
Fig. 1.2: Anisotropy in crystals is due
to different arrangement of
particles along different
directions.
Table 1.1: Distinction between Crystalline and Amorphous Solids
Shape
Melting point
Cleavage
property
Heat of fusion
Definite characteristic geometrical shape
Melt at a sharp and characteristic
temperature
When cut with a sharp edged tool, they
split into two pieces and the newly
generated surfaces are plain and
smooth
They have a definite and characteristic
enthalpy of fusion
Irregular shape
Gradually soften over a range of
temperature
When cut with a sharp edged tool, they
cut into two pieces with irregular
surfaces
They do not have definite enthalpy of
fusion
Property Crystalline solids Amorphous solids
2022-23
4 Chemistry
Besides crystalline and amorphous solids, there are some solids
which apparently appear amorphous but have microcrystalline
structures. These are called polycrystalline solids. Metals often occur
in polycrystalline condition. Individual crystals are randomly oriented
so a metallic sample may appear to be isotropic even though a single
crystal is anisotropic.
Amorphous solids are useful materials.  Glass, rubber and plastics
find many applications in our daily lives.  Amorphous silicon is one of  the
best photovoltaic material available for conversion of sunlight into electricity.
In Section 1.2, we have learnt about amorphous substances and that
they have only short range order.  However, most of the solid substances
are crystalline in nature. For example, all the metallic elements like iron,
copper and silver; non-metallic elements like sulphur, phosphorus and
iodine and compounds like sodium chloride, zinc sulphide and
naphthalene form crystalline solids.
Crystalline solids can be classified in various ways. The method
depends on the purpose in hand. Here, we will classify crystalline solids
on the basis of nature of intermolecular forces or bonds that hold the
constituent particles together. These are — (i) Van der waals forces;
(ii) Ionic bonds; (iii) Covalent bonds; and (iv) Metallic bonds. On this basis,
crystalline solids are classified into four categories viz., molecular, ionic,
metallic and covalent solids.  Let us now learn about these categories.
Molecules are the constituent particles of molecular solids. These are
further sub divided into the following categories:
(i) Non polar Molecular Solids: They comprise either atoms, for example,
argon and helium or the molecules formed by non polar covalent
1.3 1.3 1.3 1.3 1.3 Classification Classification Classification Classification Classification
of Crystalline of Crystalline of Crystalline of Crystalline of Crystalline
Solids Solids Solids Solids Solids
1.3.1 Molecular
Solids
Intext Questions Intext Questions Intext Questions Intext Questions Intext Questions
1.1 Why are solids rigid?
1.2 Why do solids have a definite volume?
1.3 Classify the following as amorphous or crystalline solids: Polyurethane,
naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl
chloride, fibre glass, copper.
1.4 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?
Anisotropy
Nature
Order in
arrangement
of constituent
particles
Anisotropic in nature
True solids
Long range order
Isotropic in nature
Pseudo solids or super cooled liquids
Only short range order.
2022-23
Page 5


From our earlier studies, we know that liquids and gases
are called fluids because of their ability to flow. The
fluidity in both of these states is due to the fact that the
molecules are free to move about. On the contrary, the
constituent particles in solids have fixed positions and
can only oscillate about their mean positions. This
explains the rigidity in solids. These properties depend
upon the nature of constituent particles and the binding
forces operating between them. The correlation between
structure and properties helps in the discovery of new
solid materials with desired properties. For example,
carbon nanotubes are new materials that have potential
to provide material that are tougher than steel, lighter
than aluminium and have more conductive property than
copper. Such materials may play an expanding role in
future development of science and society. Some other
materials which are expected to play an important role
in future are high temperature superconductors,
magnetic materials, biodegradable polymers for
packaging, biocompliant solids for surgical implants, etc.
Thus, the study of this state becomes more important in
the present scenario.
In this Unit, we shall discuss different possible
arrangements of particles resulting in several types of
structures and explore why different arrangements of
structural units lend different properties to solids. We
will also learn how these properties get modified due to
the structural imperfections or by the presence of
impurities in minute amounts.
After studying this Unit, you will be
able to
• describe general characteristics of
solid state;
• distinguish between amorphous
and crystalline solids;
• classify crystalline solids on the
basis of the nature of binding
forces;
• define crystal lattice and unit cell;
• explain close packing of particles;
• describe different types of voids
and close packed structures;
• calculate the packing efficiency of
different types of cubic unit cells;
• correlate the density of a
substance with its unit cell
properties;
• describe the imperfections in
solids and their effect on
properties;
• correlate the electrical and
magnetic properties of solids and
their structure.
Objectives
The vast majority of solid substances like high temperature
superconductors, biocompatible plastics, silicon chips, etc. are destined
to play an ever expanding role in future development of science.
The Solid State
1
Unit Unit Unit Unit Unit
1
The Solid State
2022-23
2 Chemistry
In Class XI you have learnt that matter can exist in three states namely,
solid, liquid and gas.  Under a given set of conditions of temperature and
pressure, which of these would be the most stable state of a given
substance depends upon the net effect of two opposing factors. These
are intermolecular forces which tend to keep the molecules (or atoms
or ions) closer, and the thermal energy, which tends to keep them apart
by making them move faster.  At sufficiently low temperature, the thermal
energy is low and intermolecular forces bring them so close that they
cling to one another and occupy fixed positions. These can still oscillate
about their mean positions and the substance exists in solid state.  The
following are the characteristic properties of the solid state:
(i) They have definite mass, volume and shape.
(ii) Intermolecular distances are short.
(iii) Intermolecular forces are strong.
(iv) Their constituent particles (atoms, molecules or ions) have fixed
positions and can only oscillate about their mean positions.
(v) They are incompressible and rigid.
Solids can be classified as crystalline or amorphous on the basis of the
nature of order present in the arrangement of their constituent particles.
A crystalline solid usually consists of a large number of small crystals,
each of them having a definite characteristic geometrical shape. The
arrangement of constituent particles (atoms, molecules or ions) in a crystal
is ordered and repetitive in three dimensions. If we observe the pattern in
one region of the crystal, we can predict accurately the position of particles
in any other region of the crystal however far they may be from the place
of observation. Thus, crystal has a long range order which means that
there is a regular pattern of arrangement of particles which repeats itself
periodically over the entire crystal.  Sodium chloride and quartz are typical
examples of crystalline solids. Glass, rubber and many plastics do not
form crystals when their liquids solidify on cooling. These are called
amorphous solids. The term amorphous comes from the Greek word
amorphos, meaning no form.The arrangement of constituent particles
(atoms, molecules or ions) in such a solid has only short range order. In
such an arrangement, a regular and
periodically repeating pattern is observed
over short distances only. Regular patterns
are scattered and in between the
arrangement is disordered. The structures
of quartz (crystalline) and quartz glass
(amorphous) are shown in Fig. 1.1 (a) and
(b) respectively.  While the two structures
are almost identical, yet in the case of
amorphous quartz glass there is no long
range order. The structure of amorphous
solids is similar to that of liquids. Due to
the differences in the arrangement of the
constituent particles, the two types of solids
differ in their properties.
1.1 1.1 1.1 1.1 1.1 General General General General General
Characteristics Characteristics Characteristics Characteristics Characteristics
of Solid State of Solid State of Solid State of Solid State of Solid State
1.2 1.2 1.2 1.2 1.2 Amorphous Amorphous Amorphous Amorphous Amorphous
and Crystalline and Crystalline and Crystalline and Crystalline and Crystalline
Solids Solids Solids Solids Solids
Fig. 1.1: Two dimensional structure of
(a) quartz and (b) quartz glass
2022-23
3 The Solid State
Crystalline solids have a sharp melting point. At a characteristic
temperature they melt abruptly and become liquid. On the other hand,
amorphous solids soften, melt and start flowing over a range of
temperature and can be moulded and blown into various shapes.
Amorphous solids have the same structural features as liquids and are
conveniently regarded as extremely viscous liquids. They may become
crystalline at some temperature. Some glass objects from ancient
civilisations are found to become milky in appearance because of some
crystallisation. Like liquids, amorphous solids have a tendency to flow,
though very slowly. Therefore, sometimes these are called pseudo
solids or super cooled liquids.
Amorphous solids are isotropic in nature. Their properties such as
mechanical strength, refractive index and electrical conductivity, etc.,
are same in all directions. It is because there is no long range order in
them and arrangement of particles is not definite along all the directions.
Hence, the overall arrangement becomes equivalent in all directions.
Therefore, value of any physical property would be same along
any direction.
Crystalline solids are anisotropic in nature, that
is, some of their physical properties like electrical
resistance or refractive index show different values
when measured along different directions in the
same crystals. This arises from different arrangement
of particles in different directions. This is illustrated
in Fig. 1.2. This figure shows a simple two-
dimensional pattern of arrangement of two kinds of
atoms. Mechanical property such as resistance to
shearing stress might be quite different in two
directions indicated in the figure. Deformation in CD
direction displaces row which has two different types
of atoms while in AB direction rows made of one type
of atoms are displaced. The differences between the
crystalline solids and amorphous solids are
summarised in Table 1.1.
D
C
B
A
Fig. 1.2: Anisotropy in crystals is due
to different arrangement of
particles along different
directions.
Table 1.1: Distinction between Crystalline and Amorphous Solids
Shape
Melting point
Cleavage
property
Heat of fusion
Definite characteristic geometrical shape
Melt at a sharp and characteristic
temperature
When cut with a sharp edged tool, they
split into two pieces and the newly
generated surfaces are plain and
smooth
They have a definite and characteristic
enthalpy of fusion
Irregular shape
Gradually soften over a range of
temperature
When cut with a sharp edged tool, they
cut into two pieces with irregular
surfaces
They do not have definite enthalpy of
fusion
Property Crystalline solids Amorphous solids
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4 Chemistry
Besides crystalline and amorphous solids, there are some solids
which apparently appear amorphous but have microcrystalline
structures. These are called polycrystalline solids. Metals often occur
in polycrystalline condition. Individual crystals are randomly oriented
so a metallic sample may appear to be isotropic even though a single
crystal is anisotropic.
Amorphous solids are useful materials.  Glass, rubber and plastics
find many applications in our daily lives.  Amorphous silicon is one of  the
best photovoltaic material available for conversion of sunlight into electricity.
In Section 1.2, we have learnt about amorphous substances and that
they have only short range order.  However, most of the solid substances
are crystalline in nature. For example, all the metallic elements like iron,
copper and silver; non-metallic elements like sulphur, phosphorus and
iodine and compounds like sodium chloride, zinc sulphide and
naphthalene form crystalline solids.
Crystalline solids can be classified in various ways. The method
depends on the purpose in hand. Here, we will classify crystalline solids
on the basis of nature of intermolecular forces or bonds that hold the
constituent particles together. These are — (i) Van der waals forces;
(ii) Ionic bonds; (iii) Covalent bonds; and (iv) Metallic bonds. On this basis,
crystalline solids are classified into four categories viz., molecular, ionic,
metallic and covalent solids.  Let us now learn about these categories.
Molecules are the constituent particles of molecular solids. These are
further sub divided into the following categories:
(i) Non polar Molecular Solids: They comprise either atoms, for example,
argon and helium or the molecules formed by non polar covalent
1.3 1.3 1.3 1.3 1.3 Classification Classification Classification Classification Classification
of Crystalline of Crystalline of Crystalline of Crystalline of Crystalline
Solids Solids Solids Solids Solids
1.3.1 Molecular
Solids
Intext Questions Intext Questions Intext Questions Intext Questions Intext Questions
1.1 Why are solids rigid?
1.2 Why do solids have a definite volume?
1.3 Classify the following as amorphous or crystalline solids: Polyurethane,
naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl
chloride, fibre glass, copper.
1.4 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?
Anisotropy
Nature
Order in
arrangement
of constituent
particles
Anisotropic in nature
True solids
Long range order
Isotropic in nature
Pseudo solids or super cooled liquids
Only short range order.
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5 The Solid State
1.3.3 Metallic
Solids
1.3.4 Covalent or
Network
Solids
1.3.2 Ionic Solids
bonds, for example, H
2
, Cl
2
 and I
2
. In these solids, the atoms or
molecules are held by weak dispersion forces or London forces
about which you have learnt in Class XI. These solids are soft and
non-conductors of electricity. They have low melting points and are
usually in liquid or gaseous state at room temperature and pressure.
(ii) Polar Molecular Solids: The molecules of substances like HCl, SO
2,
etc. are formed by polar covalent bonds. The molecules in such
solids are held together by relatively stronger dipole-dipole
interactions. These solids are soft and non-conductors of electricity.
Their melting points are higher than those of non polar molecular
solids yet most of these are gases or liquids under room
temperature and pressure. Solid SO
2
 and solid NH
3
 are some
examples of such solids.
(iii) Hydrogen Bonded Molecular Solids: The molecules of such solids
contain polar covalent bonds between H and F, O or N atoms.
Strong hydrogen bonding binds molecules of such solids like H
2
O
(ice).  They are non-conductors of electricity. Generally they are
volatile liquids or soft solids under room temperature and pressure.
Ions are the constituent particles of ionic solids. Such solids are formed
by the three dimensional arrangements of cations and anions bound
by strong coulombic (electrostatic) forces. These solids are hard and
brittle in nature. They have high melting and boiling points. Since the
ions are not free to move about, they are electrical insulators in the
solid state. However,  in the molten state or when dissolved in water,
the ions become free to move about and they conduct electricity.
Metals are orderly collection of positive ions surrounded by and held
together by a sea of free electrons. These electrons are mobile and are
evenly spread out throughout the crystal. Each metal atom contributes
one or more electrons towards this sea of mobile electrons. These free
and mobile electrons are responsible for high electrical and thermal
conductivity of metals. When an electric field is applied, these electrons
flow through the network of positive ions. Similarly, when heat is
supplied to one portion of a metal, the thermal energy is uniformly
spread throughout by free electrons. Another important characteristic
of metals is their lustre and colour in certain cases. This is also due
to the presence of free electrons in them. Metals are highly malleable
and ductile.
A wide variety of crystalline solids of non-metals result from the
formation of covalent bonds between adjacent atoms throughout the
crystal. They are also called giant molecules. Covalent bonds are
strong and directional in nature, therefore atoms are held very strongly
at their positions. Such solids are very hard and brittle. They have
extremely high melting points and may even decompose before melting.
They are insulators and do not conduct electricity. Diamond (Fig. 1.3)
and silicon carbide are typical examples of such solids. Although
Graphite (Fig. 1.4) also belongs to this class of crystals, but it is soft
and is a conductor of electricity. Its exceptional properties are due to
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