Solid State - Notes : Class 12 Chemistry Chapter 1 : Maharashtra State Board New Syllabus

Notes, Imp Points, Definitions, Formulae, etc.

 

 Class 12 Chemistry Chapter 1 Solid State

INTRODUCTION :

Particles : smallest constituent particles of solids (in this chapter)

1.2 Types of Solids

1) Crystalline Solids  --------------- ---------------- 2) Amorphous Solids

Crystalline Solids : Properties 

1. There is a regularity and periodicity in the arrangement of constituent particles in crystalline solids. The ordered arrangement of particles extends over a long range.
2. Crystalline solids have sharp melting points, that is, they melt at a definite temperature.
3. All crystalline substances except those having cubic structure are anisotropic. In other words their properties like refractive index, thermal and electrical conductivity, etc, are different in different directions.

Examples : Ice, salts such as NaCl, metals such as sodium, gold, copper and materials such as diamond, graphite, ceramics

Amorphous Solids : Properties 

1. The constituent particles in amorphous solids are randomly arranged. The particles do not have long range ordered structure, but they do have a short range order.
2. Amorphous solids do not have sharp melting points. They melt gradually over  a temperature interval. On heating, amorphous solids gradually and continuously soften and start to flow.
3. solids are isotropic. In other words, their properties such as refractive index, conductivity are all independent of direction of measurement. They exhibit the same magnitude for any property in every direction.

Examples : Glass, plastic, rubber, tar, and metallic glass (metal-metalloid alloy)

Isomorphism and Polymorphism 

Similarity or dissimilarity in crystal structure of different solids is described as isomorphism and polymorphism.

Isomorphism : Two or more substances having the same crystal structure are said to be isomorphous. 

For Example : (i) NaF and MgO (ii) NaNO3 CaCO3 are isomorphous pairs, and have the and same atomic ratios, 1:1 and 1:1:3, respectively, of the constituent atoms.

Polymorphism : A single substance that exists in two or more forms or crystalline structures is said to be polymorphous.

For Example : Calcite and aragonite are two forms of calcium carbonate; α-quartz, b-quartz and cristobalite are three of the several forms of silica. 

Allotropy : Polymorphism occuring in elements

Example : three polymorphic (allotropic) forms of carbon are diamond, graphite and fullerene.

1.3 Classification of Crystalline Solids 

1) ionic solids, 

2) covalent network solids, 

3) molecular solids

4) metallic solids.

Properties of different types of solids

Lattice - Lattice is a geometrical arrangement of points in a three dimensional periodic array. 

UNIT CELL : The smallest repeating structural unit of a crystalline solid is called unit cell.

Types of unit cell : There are four types of unit cells.

i. Primitive or simple unit cell : In primitive unit cell, the constituent particles are present at its corners only. 

ii. Body-centred unit cell :   In this type of unit cell, one constituent particle is present at the centre of its body in addition to the corner particles.

iii. Face-centred unit cell :   This unit cell consists of particles at the centre of each of the faces in addition to the corner particles. 

iv. Base-centred unit cell : This unit cell consists of particles at the centre of any two of its opposite faces in addition to the corner particles.

14 lattices, which describe the crystal structure, are called Bravais lattices.

seven crystal systems are associated with 14 Bravais lattices also called 14 unit cells

The seven crystal monoclinic, systems are

named as cubic, tetragonal, orthorhombic, rhombohedral,

hexagonal system.

Cubic system : There are three kinds of unit cells in cubic system :

 primitive or simple cubic (sc), body-centred cubic (bcc) and facecentred cubic (fcc)

i. Simple cubic unit cell (sc) has a particle at each of the eight corners of a cube.

ii. Body-centred cubic unit cell (bcc) has particles at its eight corners and an additional particle in the center of the cube. 

iii. Face-centred cubic unit cell (fcc) has particle at the centre of each of six faces in addition to the particles at eight corners of the cube.

Number of particles in cubic unit cell

 i. Primitive or simple cubic unit cell (sc) : 

A simple cubic cell has particles at its eight corners. When these unit cells are stacked together, particle at each corner of a given unit cell is shared with seven other neighbouring cubes that come together at that corner. 

As a result the corner particle contributes its 1/8th part to the given unit cell. Thus, a simple cubic cell has 1/8 × 8 = 1 particle per unit cell. 

ii. Body-centred cubic unit cell (bcc) : 

A bcc unit cell has eight corner particles and an additional particle at the centre of the cube. One eighth of each particle from eight corners belongs to the given unit cell as mentioned in simple cubic unit cell.  

The particle at the centre of a given

cube is not shared by any other cube. Hence, it belongs entirely to the given unit cell. Thus bcc unit cell has one particle from eight corners plus one particle in the centre of the cube, making total of 2 particles per bcc unit cell. 

iii. Face-centred cubic unit cell (fcc) :

 A fcc unit cell has particles at the eight corners plus particles at the centre of its six faces. As described in simple cubic unit cell, one particle from eight corners belongs to the given unit cell.

Each particle at the centre of the six faces is shared with one neighbouring cube. Thus,1/2 of each face particle belongs to the given unit cell. From six faces, 1/2 × 6 = 3 particles belong to the given unit cell.

Therefore, fcc unit cell has one corner particle plus 3 face particles, total 4 particles per unit cell.

Remember...

Each corner particle of a cube is shared by 8 cubes, each face particle is shared by 2 cubes and each edge particle is shared by 4 cubes.

1.6 Packing of particles in crystal lattice

Coordination Number : The number of neighbouring spheres that touch any given sphere is its coordination number.

 The larger the coordination number, the closer are the spheres to each other.

1.6.1 Close packed structures 

a. Close packing in one dimension : A close packed one dimensional structure results by arranging the spheres to touch each other in a row. 

b. Close packing in two dimensions : A close packed two dimensional (planar)

structure results by stacking the rows together such that they are in contact with each other. There are two ways to obtain close packing in two dimensions.

i. Square close packing : One dimensional rows of close packed spheres are stacked over each other such that the spheres align vertically and horizontally 

 this arrangement is called A, A, A, A..... type two dimensional arrangement.

two dimensional coordination number,  here, is 4.

ii. Hexagonal close packing : If the second row is arranged in such a way that its spheres fit in the depressions of the first row, a staggered arrangement results

The resulting two dimensional arrangement is 'ABAB...' type

the two dimensional coordination number in this packing is 6. 

the free space in this arrangement is less than in square packing, making it more effecient packing than square packing.