UNIT-1: THE SOLID STATE
GENERAL CHARACTERISTICS OF SOLIDS
- Have definite mass, volume and shape.
- Intermolecular distances are short.
- Intermolecular forces are strong.
- Constituent particles have fixed positions.
- They can only oscillate about their mean positions.
- Incompressible and rigid.
CLASSIFICATION OF SOLIDS
Solids can be classified into two types. They are
- Crystalline Solids
- Amorphous Solids
DIFFERENCE BETWEEN CRYSTALLINE AND AMORPHOUS SOLIDS
|Regular geometrical arrangement of large number of constituent particles||Irregular geometrical arrangement of constituent particles.|
|Long Range Order||Short Range Order|
|Regarded as true solids||Regarded as supercooled liquids or pseudo solids|
|sharp melting point||Not sharp melting point|
|Anisotropic in nature||Isotropic in nature|
|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|
- The physical properties like electrical resistance or refractive index show different values when measured along different directions in the same crystal.
- The arrangement of particles is different in different directions.
- The value of any physical property would be same along any directions.
- No long-range order and the arrangement is irregular along all the directions.
PSUEDO SOLIDS OR SUPER COOLED LIQUIDS
- Amorphous solids have a tendency to flow very slowly.
- Therefore, they are called pseudo solids or super cooled liquids.
Do You Know?
- Glass objects from ancient civilizations are milky in appearance due to crystallization.
- Glass is an amorphous solid.
- On heating, glass become crystalline at some temperature.
- Glass panes fixed to windows or doors of old buildings are found to be slightly thicker at the bottom than at the top.
- Glass flows down very slowly and makes the bottom portion slightly thicker.
CLASSIFICATION OF CRYSTALLINE SOLIDS
Classification is based on the intermolecular forces. Crystalline solids are classified into four. They are
- Molecular Solids
- Ionic Solids
- Metallic Solids
- Covalent solids
In molecular solids, molecules are the constituent particles.Molecular solids are classified into three types. They are
Non-Polar Molecular Solids
- Polar Molecular Solids
- Hydrogen Bonded Molecular Solids
- 1. NON-POLAR MOLECULAR SOLIDS
- The atoms or molecules are held by weak dispersion forces or London forces.
- They are Soft and non-conductors of electricity.
- They have low melting points.
- Exist in liquid or gaseous state at room temperature and pressure.
- Eg: H2, Cl2, I2 etc.
- 2. POLAR MOLECULAR SOLIDS
- The molecules are held together by dipole-dipole interactions.
- They are soft and non-conductors of electricity.
- Their melting points are higher than those of non-polar molecular solids.
- Eg: Solid SO2 and Solid NH3.
HYDROGEN BONDED MOLECULAR SOLIDS
- The molecules are held together by strong hydrogen bonding.
- They are non-conductors of electricity.
- They are generally volatile liquids or soft solids under room temperature and pressure.
- Eg: H2O.
- Ions are the constituent particles.
- Formed by the 3-dimensional arrangements of cations and anions bound by strong electrostatic forces.
- These solids are hard and brittle in nature.
- They have high melting and boiling points.
- They are insulators in solid state.
- In aqueous solutions, they conduct electricity.
. METALLIC SOLIDS
Consist of +ve ions in a sea of mobile electrons.
Held together by strong electrostatic force of attraction.
They are malleable and ductile.
Good conductors of heat and electricity.
COVALENT OR NETWORK SOLIDS
- The atoms are held together by strong covalent bonds.
- They are very hard and brittle.
- They are insulators.
- Eg: Diamond, Silicon Carbide etc.