Solid State Physics computerized notes free download

COURSE CONTENTS:

✓ To introduce solid as a state of matter

✓ To introduce classification of solids

✓ To discuss terms involved in solid-state

INTRODUCTION

Matter exists mainly in three states, viz. solids, liquids, and gases. The existence of matter in any of these three forms depends upon two factors

1. Intermolecular forces of attraction (keeps the particle closer)

2. Thermal energy (keeps particles apart)

Some of the common properties of solids, which distinguish they form the other two states of matter are:

• Solids are rigid and have definite shapes.

• Solids have a definite volume irrespective of the size or shape of the container in which they are placed.

• Solids are almost incompressible.

• Solids diffuse very slowly as compared to liquids and gases. Constituent particles are

very closely packed in solids permitting very little space for their movement.

• Solids have a much higher density (mass to volume ratio) than that of gases and liquids.

• Most solids become liquids when heated. Some undergo sublimation on heating. The temperature at which a solid changes into liquid is called the melting point and the process is called melting. Due to the varying natures of solids, their melting temperatures vary considerably.

CLASSIFICATION OF SOLIDS

Solids are divided into two classes, namely crystalline and amorphous solids. A solid is said

to be crystalline if the constituents regularly arrange themselves throughout the

three-dimensional network. The ordered arrangement of building constituents extends over

a large distance (long-range order). On the other hand, in amorphous solids, the

arrangement of building constituents is not regular (short-range order).

i) Crystalline solids: A crystalline solid is a homogeneous solid in which the constituent particles

(atoms, ions, or molecule) are arranged in a definite repeating pattern.

Example: Diamond, Quartz, NaCl, K2SO4 etc.

General characteristics of Crystalline solids:

i) A crystalline solid is a homogeneous solid in which the constituent particles (i.e., atoms, ions, or molecules) are arranged in a definite repeating pattern in all dimensions.

ii) The total intermolecular force of attraction in crystalline solid is maximum thus imparting maximum stability, the forces responsible for the stability involves ionic bonds,

covalent bonds, hydrogen bonds, and Van der Waal’s forces.

iii) A crystalline solid usually consist of a large number of small tiny crystals called a unit cell, each having a definite characteristic geometrical shape.

iv) Crystalline solid has regular arrangement of particles which repeats periodically over the entire crystal, thus exhibiting short- and long-range order.

v) Crystalline solids have a sharp melting point, thus have definite heat of fusion.

vi) Crystalline solids are true solids.

vii) Crystalline solid on cutting gives a clean cleavage.

viii) Crystalline solid shows different physical properties in different directions, this type of behavior is called anisotropy and the substances exhibiting this type of behavior are

called anisotropic. The properties like electrical conductivity, refractive index, thermal expansion, etc. have a different value in a different direction.

ix) Two or more crystalline substances having the same crystal structure are said to be isomorphous. The isomorphous substance contains the constituent atom of the same atomic ratio.

Example: a) NaF and MgO (Ratio is 1:1)

b) NaNO3 and CaCO3 (Ratio is 1:1:3)

c) K2SO4 and K2SeO4 (Ratio is 2:1:4)

d) Cr2O3 and Fe2O3 (Ratio is 2 : 3)

Exceptional: NaCl and KCl have all properties identical [same atomic ratio, similar molecular formula or similar chemical properties] but are not isomorphous.

x) A single substance that crystallizes in two or more forms under different conditions is called polymorphous. (allotropic forms)

. Example: a) Carbon has two allotropes graphite and diamond.

b) Sulfur has two polymorphic forms monoclinic and rhombic.

c) CaCO3 and SiO2 have two allotropic forms.

ii) Amorphous solids: Substances that appear like solids but do not have perfectly ordered

crystalline structure and no regular arrangement of constituent particles in structure is

called amorphous solids.

Example: Tar, glass, plastic, rubber, butter, etc.

General characteristics of Amorphous solids:

i) Amorphous substances appear like solids but they do not have a perfectly ordered crystalline structure, hence they are not real solids.

ii) An amorphous solid does not have a regular arrangement of constituent particles.

iii) The arrangement of constituent particles like atoms or molecules have only short-range order hence periodically repeating regular the pattern is only over a short distance.

iv) Regular patterns are scattered and hence the arrangement is disordered.

v) Amorphous solids are called supercooled liquids of very high viscosity or pseudo solids.

vi) Physical properties do not change with change in directions hence amorphous solids are isotropic in nature.

vii) Amorphous solids behave like fluids and very slowly float under gravity.

viii) Amorphous solids do not have sharp melting points.

ix) When cut, they split into pieces with irregular and rough surfaces.

Uses of amorphous solids:

i) The most widely used amorphous solid are the inorganic glasses viz. construction, houseware,

laboratory ware, etc.

ii) Used as rubber in making tires, shoe soles, etc.

iii) Used in plastics.

iv) Amorphous silica is used for converting sunlight into electricity (in the photovoltaic cell).

Anisotropy: The ability of crystalline solids to change their

physical properties when measured indifferent directions

is called anisotropy.

Explanation: This property is due to the different arrangement of constituents in different directions.

Different types of particles fall on the way of measurements

in different directions. Hence, the composition of crystalline solids changes with directions changing their physical properties.

Isotropy: The ability of amorphous solids to have the same physical properties when measured in different directions is called isotropy.

Explanation: This property is due to no regular arrangement of particles in any direction.

Hence the properties like electrical conductivity, thermal expansion is identical in all directions.

Types of crystalline solids: Crystalline solids are classified into four main types as follows:

i) Molecular solids: They are further classified into three types:

a. Polar molecular solids.

b. Non-polar molecular solids.

c. Hydrogen bonded molecular solids.

ii) Ionic solids.

iii) Metallic solids.

iv) Covalent solids.

Molecular solids:

i) In the crystalline molecular solids, the constituent particles are molecules of the same compound.

ii) Depending upon the type of molecules involved in crystal formation and the nature of the intermolecular force of attraction between the neighboring molecules, they are further sub-divided as:

a) Non-polar molecular solids:

1. These are those crystalline solid in which the constituent particles are either atoms [Noble gases] or non-polar molecules [H2, Cl2, I2, CH4, etc.] or weakly polar molecules like CO or other hydrocarbons.