Class 12 Chemistry Book Chapter SOLID STATE (chapter 1) Notes based on CBSE book.

SOLID STATE (chapter 1)

The liquids and Gases are called fluids because of their ability to flow. The fluidity in both of these states is due to the fact that the molecules are free to move about.

The constituent particles in solids have fixed positions and oscillate about their mean positions. These properties depend upon the nature of constituent particles and the binding forces operating between them.

General Characteristics of Solid State

The following are the characteristic properties of the solid state:

(i) They have definite mass, volume and shape.

(ii) Intermolecular distances are short.

(iii) Intermolecular forces are strong.

(iv) Their constituent particles (atoms, molecules or ions) have fixed positions and can only oscillate about their mean positions.

(v) They are incompressible and rigid.

Amorphous and Crystalline Solids

Solids can be classified as crystalline or amorphous on the basis of the nature of order present in the arrangement of their constituent particles.

A crystalline solid

A crystalline solid usually consists of a large number of small crystals, each of them having a definite characteristic geometrical shape.
The arrangement of constituent particles (atoms, molecules or ions) in a crystal is ordered and repetitive in three dimensions. Crystal has a long range order which means that there is a regular pattern of arrangement of particles which repeats itself periodically over the entire crystal.
Sodium chloride and quartz are examples of crystalline solids.

Crystalline solids have a sharp melting point. At a characteristic temperature they melt abruptly and become liquid.

Crystalline solids are anisotropic in nature, that is, some of their physical properties like electrical resistance or refractive index show different values when measured along different directions in the same crystals. This arises from different arrangement of particles in different directions.

Mechanical property such as resistance to shearing stress might be quite different in two directions indicated.

A amorphous solid

Glass, rubber and many plastics do not form crystals when their liquids solidify on cooling. These are called amorphous solids. The term amorphous comes from the Greek word amorphos, meaning no form.

The arrangement of constituent particles (atoms, molecules or ions) in such a solid has only short range order. In such an arrangement, a regular and periodically repeating pattern is observed over short distances only. Regular patterns are scattered and in between the arrangement is disordered.

The structure of amorphous solids is similar to that of liquids.

Due to the differences in the arrangement of the constituent particles, the two types of solids differ in their properties.

Amorphous solids soften, melt and start flowing over a range of temperature and can be moulded and blown into various shapes. Amorphous solids have the same structural features as liquids and Amorphous solids are conveniently regarded as extremely viscous liquids.

Amorphous solids may become crystalline at some temperature.

Like liquids, amorphous solids have a tendency to flow, though very slowly. Therefore, sometimes Amorphous solids are called pseudo solids or super cooled liquids.

Amorphous solids are isotropic in nature. Their properties such as mechanical strength, refractive index and electrical conductivity, etc., are same in all directions.

Amorphous solids have no long range order in them and arrangement of particles is not definite along all the directions. In Amorphous solids overall arrangement are equivalent in all directions.

The value of any physical property of Amorphous solids would be same along any direction.

Amorphous solids are useful materials. Glass, rubber and plastics find many applications in our daily lives.

Amorphous silicon is one of the best photovoltaic material available for conversion of sunlight into electricity.

A polycrystalline solids

Besides crystalline and amorphous solids, there are some solids which apparently appear amorphous but have microcrystalline structures. These are called polycrystalline solids. Metals often occur in polycrystalline condition. Individual crystals are randomly oriented so a metallic sample may appear to be isotropic even though a single crystal is anisotropic.

Distinction between Crystalline and Amorphous Solids

Property	Crystalline solids	Amorphous solids
Shape	Definite characteristic geometrical shape	Irregular shape
Melting point	Melt at a sharp and characteristic temperature	Gradually soften over a range of temperature
Cleavage property	When cut with a sharp edged tool, they split into two pieces and the newly generated surfaces are plain and smooth	When cut with a sharp edged tool, they cut into two pieces with irregular surfaces
Heat of fusion	They have a definite and characteristic enthalpy of fusion	They do not have definite enthalpy of fusion
Anisotropy	Anisotropic in nature	Isotropic in nature
Nature	True solids	Pseudo solids or super cooled liquids
Order in arrangement of constituent particles	Long range order	Only short range order

Classification of Crystalline Solids

All the metallic elements like iron, copper and silver; non-metallic elements like sulphur, phosphorus and iodine and compounds like sodium chloride, zinc sulphide and naphthalene are crystalline in nature.

Crystalline solids can be classified in various ways.

On the basis of nature of intermolecular forces or bonds and Van der waals forces, Ionic bonds, Covalent bonds, and, Metallic bonds, Crystalline solids are classified into four categories viz.,

Molecular solids.
Ionic solids.
Metallic solids.
Covalent solids.

Molecular solids

Molecules are the constituent particles of molecular solids.

Molecular solids are divided into the Three categories:

(i) Non polar Molecular Solids:

The molecules formed by non polar covalent bonds, for example, H2 , Cl2 and I2 .

In these solids, the atoms or molecules are held by weak dispersion forces or London forces.

These solids are soft and non-conductors of electricity.

They have low melting points and are usually in liquid or gaseous state at room temperature and pressure.

(ii) Polar Molecular Solids:

The molecules of substances like HCl, SO2, etc. are formed by polar covalent bonds.

The molecules in polar molecules solids are held together by relatively stronger dipole-dipole interactions.

These solids are soft and non-conductors of electricity.

Their melting points are higher than those of non polar molecular solids yet most of these are gases or liquids under room temperature and pressure.

Solid SO2 and solid NH3 are some examples of such solids.

(iii) Hydrogen Bonded Molecular Solids:

The molecules of such solids contain polar covalent bonds between H and F, O or N atoms.

Strong hydrogen bonding binds molecules of such solids like H2O (ice).

They are non-conductors of electricity.

Hydrogen Bonded Molecular Solids are volatile liquids or soft solids under room temperature and pressure.

Ionic solids

Ions are the constituent particles of ionic solids.

Ionic solids are formed by the three dimensional arrangements of cations and anions bound by strong coulombic (electrostatic) forces.

Ionic solids are hard and brittle in nature.

Ionic have high melting and boiling points. Since the ions are not free to move about,

Ionic are electrical insulators in the solid state.

In the molten state or when dissolved in water, the ions become free to move about and they conduct electricity.

Metallic solids

Metals are orderly collection of positive ions surrounded by and held together by a sea of free electrons. These electrons are mobile and are evenly spread out throughout the crystal. Each metal atom contributes one or more electrons towards this sea of mobile electrons. These free and mobile electrons are responsible for high electrical and thermal conductivity of metals. When an electric field is applied, these electrons flow through the network of positive ions.

When heat is supplied to one portion of a metal, the thermal energy is uniformly spread throughout by free electrons.

Lustre and colour in Solid is also due to the presence of free electrons in them.

Metals are highly malleable and ductile.

Covalent or Network Solids

A wide variety of crystalline solids of non-metals result from the formation of covalent bonds between adjacent atoms throughout the crystal. They are also called giant molecules.

Covalent bonds are strong and directional in nature, therefore atoms are held very strongly at their positions.

Covalent solids are very hard and brittle.

Covalent solids have extremely high melting points and may even decompose before melting.

Covalent solids are insulators and do not conduct electricity. Diamond, Graphite and silicon carbide are examples of Covalent solids.

Graphite is soft and is a conductor of electricity. Its exceptional properties are due to its typical structure. Carbon atoms are arranged in different layers and each atom is covalently bonded to three of its neighbouring atoms in the same layer. The fourth valence electron of each atom is present between different layers and is free to move about. These free electrons make graphite a good conductor of electricity. Different layers can slide one over the other. This makes graphite a soft solid and a good solid lubricant.

Type of Solid	Constituent Particles	Bonding/Attractive Forces	Examples	Physical Nature	Electrical Conductivity	Melting Point
(1) Molecular solids (i) Non polar	Molecules	Dispersion or London forces	Ar, CCl4 , H2 , I2 , CO2	Soft	Insulator	Very Low
(ii) Polar		Dipole-Dipole interactions	HCl, SO2	Soft	Insulator	Low
(iii) Hydrogen bonded		Hydrogen bonding	H2O (ice)	Hard	Insulator	Low
(2) Ionic solids	Ions	electrostatic or electrostatic	NaCl, MgO, ZnS, CaF2	Hard but brittle	Insulator in solid state but Conductor in molten state and in aqueous solution	Fairly High
(3) Metallic solids	Positive ions of in a sea of delocalised electrons	electrostatic bonding	Fe, Cu, Ag, Mg	Hard but malleable and ductile	Conductor in solid state as well as molten state	High
(4) Covalent solids	Atoms	Covalent bonding	SiO2(quartz), SiC, C (diamond), AlN, C(graphite)	Hard soft	Insulator	Very High

CLASS 12TH Chemistry Book SOLID STATE Chapter Notes