NEET Chemistry Notes Solid State – Concept of Solids
Concept of Solids
Concept of Solids
A solid is defined as that form of matter which has rigidity and due to which possesses a definite volume and a definite shape.
Classification of Solids
On the basis of the arrangement of their atoms or ions or molecules, solids are broadly classified into two groups:
- Crystalline Solids
They have systematic and regular arrangement of particles and sharp melting point. They have flat faces and sharp edges.
On the basis of types of bond present, they are further divided into four groups:
- Molecular crystal or van der Waals’crystals, e.g. I2(s),H20 (s), etc.
- Atomic crystals or covalent crystals, e.g. graphite, diamond, etc.
- Metallic crystals, e.g. all metallic solids, etc.
- Ionic crystals, e.g. NaCl, CsCl, ZnS, etc.
- Amorphous Solids
They have unsystematic and irregular arrangement of particles.
Bragg’s Equation
The fundamental equation which gives a simple relation among the wavelength of the X-rays, the inter planar distance in the crystal and the angle of reflection, is known as the Bragg’s equation.
where,
λ= wavelength of X-ray used.
θ= angle between incident X-rays and the plane of the crystal.
d = distance between planes of the constituent particles in a crystal.
n = an integer (1, 2, 3, 4, …) which represents the serial order of diffracted beams.
Applications of Bragg’s Equation
- Structure of solids are determined by X-ray diffraction methods.
- Structures of complex substances such as proteins and nucleic acids are also determined by this method.
Space Lattice or Crystal Lattice
An array of lattice points showing arrangement of constituent particles in different position in three dimensional space, is known as space lattice or crystal lattice.
There are seven crystal lattice which can be tabulated with edge length and interfacial angles as:
Unit Cell
It is the smallest group of lattice points which when repeated in all directions will develop the entire lattice. It can be
- Simple unit cell or primitive unit cell, in which particles are present only at comers.
Number of atoms per unit cell
- Face centred unit cell, in which the particles are present at the corners as well as at the centre of each face.
Number of atoms per unit cell
- Body centred unit cell, in which the particles are present at the comers of the cube as well as one particle is present at the centre within the body.
Number of atoms per unit cell
Packing in Crystals
The packing of spheres of equal size takes place as follows:
One Dimensional Packing
When the spheres are placed in horizontal row, touching each other, an edge of the crystal is formed.
Two Dimensional Packing
It is of two types:
- Square Close Packing (SCP) The particles, when placed in the adjacent rows, show a horizontal as well as vertical alignment and form squares.
- Hexagonal Close Packing (HCP) The particles in every next row are placed in the depression between the particles of the first row. The particles in the third row will be vertically aligned with those in the first row.
Three Dimensional Packing
It is of two types:
- Hexagonal Closed Packing (HCP) When the first layer is placed as layer A and second layer as layer B, then the arrangement is called AB AB… pattern or Hexagonal Closed Packing (HCP).
- Cubic Closed Packing (CCP) Whe.n the third layer is placed over the second layer in such a way that spheres cover the octahedral voids, a layer different from layers A and B is produced. Let us call it as layer C. This pattern of stacking spheres is called ABC ABC… pattern or Cubic Closed Packing (CCP). It is similar to Face Centred Cubic (FCC) packing.