# Solid State-Bsc I And NEB

## Solid State-Bsc I And NEB

Solid is that state of matter in which molecules are closely packed with high inter-molecular force.

Based on the arrangement of consituent particles solid is divided into two types:

• Crystalline Solid
• Amorphous Solid

Crystalline Solid
In crystalline solid, the consituent particles such as atoms or ions or molecules are regularly arranged in a definite pattern for long range. They have sharp melting point. They are anisotropic i.e the magnitude of physical properties like refractive index, thermal conductivity etc are different in all direction . They have definite heat of fusion.

Types of Crystalline Solid

1. Molecular Solids
In molecular solids , the consituent particles are molecules. They are held together by dipole-dipole interaction or Vander Waal’s force. Eg: H_2, 0_2H2,02 etc.
Characteristics
1. Generally soft in nature.
2. Low melting and boiling point and volatile.
4. Low heat of vaporization.
2. Metallic Solids
In metallic solids , the consituent particles are metal ions or free electrons. They are held together by metallic bonds. Eg: Cu ,Fe etc.
Characteristics
• Soft to hard
1. Possess metallic lustre
2. Malleable and Ductile
3. Good conductor of electricity
1. Covalent Solids
In covalent solids, the constituent particles are atoms of same or different elements having similar electro negativities and held together by covalent bonds. Eg: Graphite, Diamond etc.
Characteristics
• Very hard and Soluble in organic solvents
1. High M.P
2. Poor conductor of electricity.
3. High heat of fusion.
1. Ionic Solids
In metallic solids , the constituent particles are ions of opposite charges and are held together by ionic bonds. Ions of opposite charges are closely packed in a definite geometric pattern.
Eg: NaCl , KCl etc.
Characteristics
• Hard , brittle and soluble in polar solvent.
1. High M.P and density.
2. Non conductor in solid state but conductor in aqueous form.
3. High heat of vaporization.

Amorphous Solid
In amorphous solid, the consituent particles such as atoms or ions or molecules are irregularly arranged. They do not have a sharp melting point. They are isotropic i.e the magnitude of physical properties like refractive index, thermal conductivity etc are same in all direction .

Crystal Lattice and Unit cells
We know that, the main characteristic of crystalline solids is its regular and repeating pattern of constituent particles. When we replace these particles with representive points , a crystal lattice is found. Thus, crystal lattice can be defined as three dimensional arrangement of constituent particles represented digramatically such that each particles is denoted as point.

Unit cell is the small portion of crystal lattice when repeated in different direction forms entire lattice. A unit cell is represented by 3 edges (a,b,c) represented along x y and z axes. They may or may not be mutually perpendicular to each other. The angle between them are denoted by \alphaα(between b and c) , \betaβ(between a and c) and \gammaγ(between a and b)

Depending upon the values of a,b and c and \alpha , \beta and \gamma , crystal lattice is divided into following types:

 S.N Crystal System Edges Angles Example 1 Cubic a=b=c \alpha = \beta =\gamma= 90α=β=γ=90 NaCl 2 Tetragonal a=b \neq ca=b​=c \alpha = \beta =\gamma=90α=β=γ=90 SnO_{2}SnO2​ 3 Orthorhombic a \neq b \neq ca​=b​=c \alpha = \beta =\gamma=90α=β=γ=90 KNO_{3}KNO3​ 4 Monoclinic a \neq b \neq ca​=b​=c \alpha = \beta=90 and \gamma \neq 90α=β=90andγ​=90 CaSO_{4}. 2H_{2}OCaSO4​.2H2​O 5 Hexagonal a=b \neq ca=b​=c \alpha = \beta=90 and \gamma = 120α=β=90andγ=120 ZnO 6 Trigonal a=b=c \alpha \neq \beta \neq \gamma \neq 90α​=β​=γ​=90 CaCO_{3}CaCO3​ 7 Triclinic a \neq b \neq ca​=b​=c \alpha = \beta =\gamma \neq 90α=β=γ​=90 CuSO_{4}. 5H_{2}OCuSO4​.5H2​O

Types of Unit Cells in Cubic System

1. Simple Cubic Unit Cell
In this cell , one particle is situated at each corners. Hence, eight corner contains eight particles.
2. Body Centered Cubic Unit Cell
In this cell , one particles are at each corner. Also, one particle is situated at the center of the cube.
3. Face Centered Cubic Unit Cell
In this cell , one particles are at each corner , also at the center of each six faces of the cube.

Miller Indices
Miller indicies are se of integers(h,k,l) which are used to describe a given plane of crystal. The miller indices of a face of a crystal are inversely proportional to the intercepts of that face with axes.

Give reason:

1. Why ionic crystals have high MP that molecular crystal?
In ionic crystals, the electro static force of attraction between the oppositively charged ions are very strong , therefore high amount of energy is required to seperate ions from one another.
On other hand, in molecular crystals the molecules are held together by weak Vander Waal’s force. Thus less energy can seperate the molecules. Hence, ionic crystals have high MP that molecular crystal.
2. Graphite can conduct electricity , but not diamond although both are covalent solids . Why ?
In diamond every carbon atom exists in SP_3 hybrid state. Each atom has four electrons available for sharing and is covalently linked to four other carbon atoms tetrahedrally resulting three dimensional network of carbons. Thus it has no free electron and cannot conduct.
But , in graphite the carbon atoms are in SP_2 hybrid state. Each carbon atom is linked to three other carbon atoms through single bonds in the same plane to form hexagonal rings. Hence, fourth valence electrons of each carbon atom is free and hence conduct electricity.