Solid states are the fundamental states of matter. They are used more often than liquids or gases in the surrounding. Solid states have been widely used in the industry, research, productions and by manufacturers. We need solid in different applications with different properties. Hence there is a need to understand the structure of solids. This is a very important chapter in chemistry for 12th standard students of CBSE Board. Solutions to all the questions in this chapter can be easily found in any good solution key that provides NCERT Solutions for Class 12 Chemistry . The correlation between the structure and properties can discover other solid states with desired properties. The solid state should have properties like high temperature, magnetic materials, biodegradable polymers etc.
The general characteristics of solid state are as follows:
- Definite mass, shape and volume
- Short intermolecular distances
- Strong intermolecular force
- Constituent particles have fixed position and can oscillate about their mean positions
- Rigid
- Incompressible
Solids can be classified as amorphous and crystalline solids.
Crystalline solids: The solids have more number of small crystals each having a definite geometric shape. For example, Sodium chloride and quartz
Amorphous solids: The solids consist of particles with irregular shapes. For example, quartz glass. The difference between Crystalline and Amorphous solids are:
| Property | Crystalline solids | Amorphous solids |
| Shape | Definite geometric shape | Irregular shape |
| Melting point | Melt at sharp and characteristic temperature | Softens gradually over range of temperature |
| Cleavage property | When cut with a sharp edge tool, split into two equal parts with plain and smooth surfaces | When cut with a sharp edge tool, split into two with irregular surfaces |
| Heat of fusion | Have | Does not have |
| Anisotropy | Anisotropy in nature | Isotropic in nature |
| Order of arrangement of particles | Long range order | Short range order |
Classification of crystalline solids
Crystalline solids are classified based on the nature of intermolecular forces. They are as follows:
- Molecular solids: molecules are the particles of molecular solids. They are further divided into
- Non polar molecular solids, for example, Ar, CCl4,
- Polar molecular solids, for example, HCl, SO2
- Hydrogen bonded molecular solids, for example, H2O (ice)
- Ionic solids: ions are the particles of ionic solids. Solids are formed by cation and anion bound by electrostatic forces. For example, NaCl, MgO
- Metallic solids: metals are the particles of metallic solids. They are positive ions surrounded by free electrons. These are mobile and widely spread throughout the solids. For example, Fe, Cu
- Covalent or network solids: covalent bonds are present in the solids that are strong and directional in nature. Hence they are held strongly in their position making them hard and brittle. For example, quartz and diamond.
Crystal lattice and unit cells
When three dimensional arrangements of particles in a crystal in which each particle is depicted as a point is known as crystal lattice. Whereas unit cells are the smallest portion of crystal lattice that gets repeated in different directions to form entire lattice. Unit cells are broadly classified into primitive and centered unit cells.
Number of atoms in unit cell
Crystal lattice is made of a large number of unit cells and every lattice is occupied by one constituent particle. They are further classified into primitive cubic unit cells, body centered cubic unit cell and face centered cubic unit cell.
Close packed structures
The solids consist of particles that are closely packed. There are three steps to build a three dimensional structure.
- Close packing in one direction: the particles are arranged in row and touching each other.
- Close packing in two dimension: the particles are arranged in stacking the rows of close packed spheres
- Close packing in three dimensions: the particles are arranged by stacking two dimensional layers one above the other.
Magnetic properties
All the substances have magnetic properties. Each electron acts as a magnet. On the basis of their magnetic properties, they are classified as paramagnetic, diamagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic.
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