CLASS 12 CHAPTER 1- Solid State

Amongst the three states of matter, liquid and gases are called fluids because of their ability to flow. But solid cannot flow. The reason for this is that the particles in a solid are not free to move in available space.
There is a strong intermolecular force of attraction in between the particles in a solid. The constituent particles in a solid have a fixed position and can only oscillate about their mean position.
Due to their specific arrangements, it shows a huge range of properties and numerous applications like magnetic materials, superconductors, polymers etc. The following are the characteristic properties of the solid-state:
They have a definite shape, volume and mass.
Intermolecular distances are short.
Intermolecular forces are strong.
Their constituent particles (atoms, molecules or ions) have fixed positions and can only oscillate about their mean positions.
They are rigid and incompressible.

Solids can be classified as crystalline or amorphous on the basis of the nature of order present in the arrangement of their constituent particles.
The term amorphous comes from the Greek word amorphous, meaning no form. The arrangement of constituent particles (atoms, molecules or ions) are irregular in these solids and a regular and periodically repeating pattern is observed in small areas only. Examples are quartz (crystalline) and quartz glass.
Amorphous solids are isotropic in nature.
Amorphous solids soften, melt and start flowing over a range of temperatures and can be moulded into various shapes.
Their properties such as mechanical strength, refractive index and electrical conductivity, etc., are the same in all directions. It is because there is no long-range order in them. Thus, the overall arrangement becomes equivalent in all directions. Therefore, the extent of any physical property would be the same in any direction.
They may become crystalline at some temperature when heated.

Crystalline solids possess a regular pattern of arrangement of particles that repeats itself periodically over the entire crystal. Examples of crystalline solids are Sodium chloride and quartz.
Like liquids, amorphous solids have a tendency to flow, though very slowly. Hence, these are also called pseudo solids or supercooled liquids.
Crystalline solids have a sharp melting point which means that they melt abruptly and become liquid.
Crystalline solids are anisotropic in nature, this means some of their physical properties like electrical resistance or refractive index show different values when measured along with different directions in the same crystals.

Crystalline solids can be classified on the basis of the nature of intermolecular forces or bonds present between the constituents. These forces are Van der Waals forces, ionic bonds, Covalent bonds, metallic bonds etc.
On this basis crystalline solids are classified into the following 4 types:
Molecular
Ionic
Metallic
Covalent

Types of solid	Constituent particles	Attractive force	Examples	Physical nature	Electrical conductivity	Melting point
Molecular solids 1) Non -polar	Molecules	Dispersion/ London forces	H_2, I_2,Cl₂	soft	Do not conduct electricity	very low
2) polar	molecules	strong dipole-dipole interactions	HCl, SO₂	soft	Do not conduct electricity	higher
3) Hydrogen bonded	molecules	Strong hydrogen bonds	H₂O		Do not conduct electricity	low
Ionic solids	ions	Strong electrostatic forces	NaCl	Hard and brittle solid	Non- conductors but conductors in molten state/ solutions	high
Metallic solids	Kernels in the sea of electrons	Metallic bonding	Fe, Cu	hard	Conduct electricity	high
Covalent or network solids	atoms	Covalent bonding	SiO₂	Hard/ soft	Do not conduct electricity	Very high

The chief characteristic feature of all crystalline solids is the regular and repeating arrangement of constituent units in space. The regular three-dimensional arrangement of constituent units in a crystal is called the crystal lattice.
Each point in a crystal lattice is called a lattice point or lattice site.
Each point in a crystal lattice signifies one constituent particle which can be an atom, a molecule or an ion.
This structural unit is known as the base or motif.
The smallest part of the crystal lattice is known as the unit cell. Look at the role of a unit cell in the crystal lattice.

Crystal lattice

The smallest part of a crystal lattice is known as unit cell.
In the three-dimensional crystal structure, unit cell is characterised by:
its dimensions along the three edges a, b and c. These edges may or may not be mutually perpendicular.
angles between the edges, α (between b and c), β (between a and c) and γ (between a and b). Thus, a unit cell is characterised by six parameters a, b, c, α, β and γ.

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