1) Crystalline solids have a definite geometric structure due to orderly arrangement of particles, while amorphous solids do not. 2) Crystalline solids have sharp melting points and properties that depend on direction, while amorphous solids melt over a range of temperatures and are isotropic. 3) Elements and compounds can exist in multiple crystalline forms called polymorphs or allotropes, with a transition temperature between forms. Examples are the allotropes of oxygen, sulfur, tin, and carbon.

1. SOLIDS
CHAPTER 6
XI FDC CHEMISTRY
SIDRA JAVED

2. KINETIC MOLECULAR
THEORY OF SOLIDS
● Attractive forces: The attractive forces
among solids are maximum due to closest
packing of solid molecules.
● Rigidity: Due to close packing of molecules,
their movement is restricted. As a result they
are rigid in nature. Solid molecules vibrate
about their mean positions.

3. ● High Density: Molecules of solids occupy
less volume due to close packing. As
density is inversely proportional to
volume, therefore solids have high
density.
● Collisions: As there is no translational
motion of particles in solids, therefore
there are no collisions among molecules.
● Kinetic Energy: Molecules of solid
possess only Vibrational Kinetic Energy.

4. ● Geometric Shape: The crystalline solids have definite distinctive
geometrical shape. It is due to definite orderly arrangement of atoms,
ions or molecules in 3D shape.

5. PROPERTIES
OF SOLIDS
Some simple properties of solids are:
● Diffusion
● Compression
● Expansion
● Motion of molecules
● Spaces between molecules
● Intermolecular forces
● Kinetic Energy

6. Diffusion in solids
Diffusion depends upon
velocity of molecules. As the
movement of molecules is
very slow, therefore, the
diffusion will be minimum.
Had there been diffusion in
solids?
The centuries old buildings
would have collapsed in no
time.

7. Compression
There is practically no effect of
pressure on solids as the molecules
are closely packed.
The decrease in volume per unit
volume when the pressure is
increased by 1 atm is defined as
Compressibility (𝛽).

8. Expansion
The solids expand when heated. Increase
in temperature decreases the
intermolecular attractive forces. As a
result the volume increases.
The increase in volume per unit volume
when the temperature is increased by 1oC
is called Coefficient of expansion (𝛂)

9. Melting Point
The temperature at which a
solid changes into the liquid
form is called melting point
of the solid.

10. Motion of molecules
There is no translational or rotational motion in
solids due to strong intermolecular forces of
attraction. However the molecules vibrate about
their mean positions.

11. Intermolecular
Forces
In solids, the intermolecular forces are
maximum in between the particles.
The molecules are held together in fixed
positions by strong attractive forces.

12. Kinetic Energy of
solids
According to KMT, the attractive forces between solids particles are maximum.
This is due to minimum distance between them and, therefore the molecules of
solids do not possess translational and rotational kinetic energies.
However they can vibrate about theri mean positions so they possess Vibrational
Kinetic Energy.

13. Types of Solids
There are two types of solids:
1. Amorphous solids: These solids
have no definite geometric
shapes e.g. glass, rubber, dust etc.
2. Crystalline solids: These solids
have definite regular and 3D
geometric shapes e.g NaCl,
CuSO4.5H2O etc

14. Differences between
Amorphous and
Crystalline solids

15. 1.Geometrical Shapes
Crystalline solids possess definite geometrical structure e.g. NaCl is cubic in
nature. Amorphous solids do not have definite geometric structure such as
glass.

16. 2. Regular arrangement of particles
There is complete regularity of arrangement of atoms, ions or molecules in a
crystalline solid whereas the atoms, ions or molecules are not arranged in a
regular manner in amorphous solids.

17. 3. Water of crystallization
In a crystalline solid, water molecules are a part of crystal e.g. FeSO4.7H2O,
CuSO4.5H2O etc while the amorphous substances have no such water of
crystallization. Sometimes the color of crystal is due to water of crystallization.

18. 4. Sharp melting
points and effect
of heat
The crystalline substances have a sharp
melting point while the amorphous
substances do not have sharp melting
point. They may be called as super cooled
liquids such as glass, plastics etc.
The amorphous solids softens on heating
and ultimately they become liquid over a
wide range of temperature while the
crystalline solid have no such property.

19. Properties of
Crystalline solids

20. Geometrical Shape
Crystalline solids have a definite
distinctive geometrical shape
because the molecules have fixed
position. Therefore the cannot
move appreciably. Moreover the
solids have orderly arrangement of
atoms ions of molecules in three
dimensional space. E.g. NaCl is cubic
Melting Point
Pure crystalline solids have sharp MP.
When a solid is heated, the atoms, ions
or molecules present in a solid start
vibrating at higher frequency and
transfer their K.E throughout the solid.
At the MP, their vibrational energies
becomes so much that they leave their
fixed positions simultaneously and
become a liquid.

21. Cleavage Plane
When some external pressure is applied to a
crystalline solid, it changes into small crystals of the
same size and shape as that of the original one.
The breaking up of a larger crystal into smaller one
with identical size and shape is called Cleavage. The
plane which contains the direction of cleavage is called
Cleavage Plane.

22. Habit of a Crystal
The shape of crystal in which it usually
grows is called habit of crystal. E.g. cubic
crystals of NaCl are obtained from its
aqueous solution.
If the conditions are changed, the shape of
crystal also changes. If 10% urea is present
in the aqueous solution of NaCl, octahedral
crystals are obtained.

23. Crystal Growth
The crystal growth takes place when
the heated solution of a substance is
allowed to cool in a slow manner. The
outer appearance or shape of crystals
depends on how it is prepared and
under what conditions.
The size of a crystal is controlled by its
rate of growth. A slowly going crystal
has larger size.

24. Anisotropy
A crystalline substance may possess different
properties in different directions. It is because crystal
has different arrangements in different directions.
A substance which shows different intensity of
properties in different directions is called anisotropic
and this property is called anisotropy e.g. refractive
index, co-efficient of thermal expansion, electrical
and thermal conductivities etc.

25. Symmetry
The rotations which brings the crystal into
its original shape is called symmetry of
element. A regular cube is rotated about its
axis at an angle of 90o, the identical face is
obtained, rotating to 180o, 270o and 360o,
identical faces are obtained. An axis
containing 4 identical faces is called 4 fold
axis of rotation.

26. Symmetry
The process through which the crystal was
brought back to its identical position is called
symmetry operation.
An imaginary plane passing through crystal
that divides the crystal into two identical
halves is called plane of symmetry.
The symmetry elements that occur in a crystal
are plane of symmetry, centre of symmetry,
axis of symmetry and angle of symmetry.

27. Isomorphism
Different crystalline substance
having same crystalline shapes are
called isomorphs and this
phenomenon is called isomorphism.
Isomorphic solids have same ratios of
atoms.

28. Polymorphism
The substance existing in more than one
crystalline form is called polymorphous and
the phenomena as polymorphism.
Calcium Carbonate - Aroganite
(Orthorhombic)
Calcium Carbonate - Calcite (Trigonal)

29. Allotropy
An element may exist in different crystalline forms. These forms are called
allotropes and this phenomenon is called allotropy.

30. Transition temperature
The temperature at which more than one form of a given substance can
exist in equilibrium is called transition temperature. Above and below this
temperature only one polymorph or allotrope can exist.
Substance Crystalline Form Transition Temperature
Tin (Grey)
Tin (White)
Orthorhombic
Tetragonal
18oC
Sulphur Monoclinic
Orthorhombic
95.6oC

31. Allotropes of
Oxygen and
Sulphur

32. Allotropes of Oxygen
Dioxygen or Molecular Oxygen (O2) and
Trioxygen or Ozone (O3) are two allotropic
forms of Oxygen. These are obtained by
absorption of heat from atmosphere.
3O2 ⇋ 2O3 ΔH = 142 KJmol-1

33. Formation of
allotropes of Oxygen
The UV radiation bring about the
photo-chemical reaction.
The conversion is spontaneous and
one directional therefore it is called
Monotropic.
Maximum concentration of O3 is
about 10 ppm, occurs 24-30 Km from
the surface of the earth i.e.
Stratosphere

35. Structures
Oxygen Molecules O=O has a sigma
and a pi bond between two atoms.
In O3 molecule, there is an angle of
117o
O3 has a characteristic smell. In
concentration above 1000 ppm, it
causes health hazards.

36. Allotropes of
Sulphur
Sulphur exist in 4 allotropic forms:
1. Rhombic Sulphur or α sulphur
2. Monoclinic Sulphur or 𝛽 sulphur
3. Plastic Sulphur or 𝛄 sulphur
4. Amorphous sulphur

37. Rhombic Sulphur
It is bright yellow in color and stable below 96oC
It is crystalline in nature and made up of S8 molecules

38. Monoclinic sulphur
It is a crstalline solid and stable between 96oC and 119oC.
It is converted to Rhombic sulphur at room temperature.

39. Plastic Sulphur
It is a super cooled liquid.If yellow
sulphur is heated to boiling and
poured into water, it will roll up and
produce plastic like ribbons. It is not
considered to be a true allotrope of
sulphur because it is soft and elastic in
nature and insoluble in H2S.

40. Amorphous Sulphur
It has irregular crystalline shape which
may be called as amorphous. It is not
found in free state. It may be prepared
by passing H2S gas through water for a
long time. The saturated solution of
H2S so obtained is exposed to air.
Amorphous sulphur so produce has
almost white color.
2H2S + O2 → 2H2O + 2S

41. End of Lesson
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