Tamilnadu Samacheer Kalvi 12th Chemistry Notes Chapter 6 Solid State Notes
Solids – The substances having definite shape mass and volume are called solids.
General characteristics of solids –
- Solids have definite volume and shape.
- Solids are rigid and incompressible
- Solids have strong cohesive forces.
- Solids have short inter atomic, ionic or molecular distances.
- Their constituents ( atoms , ions or molecules) have fixed positions and can only oscillate about their mean positions
Types of solids – They are two types.
- Crystalline solids
- Amorphous solids
Crystalline solids – In a crystalline solid, the particles are arranged in a regular and repetitive three dimensional arrangement.
Amorphous solids – In a amorphous solid, the particles are arranged in an irregular and non- repetitive three dimensional arrangement.
Isotropy – Isotropy means uniformity in all directions. In solid state isotropy means having identical values of physical properties such as refractive index electrical conductance etc., in all directions.
Anisotropy – Anisotropy is the property which depends on the direction of measurement.
Ionic solids – The structural units of an ionic crystal are cations and anions. They are bound together by strong electrostatic attractive forces. To maximize the attractive force, cations are surrounded by as many anions as possible and vice versa. Ex., NaCl crystal
Characteristics of ionic crystals
- Ionic solids have high melting points.
- These solids do not conduct electricity, because the ions are fixed in their lattice positions.
- They do conduct electricity in molten state (or) when dissolved in water because, the ions are free to move in the molten state or solution.
- They are hard as only strong external force can change the relative positions of ions.
Covalent solids – In covalent solids the constituents are bound together in a three dimensional network entirely by covalent bonds. Ex., Diamond
Molecular solids – In molecular solids, the constituents are neutral molecules. They are held together by weak vander waals forces. Ex., Naphthalene, solid CO2, urea
Metallic solids – In metallic solids, the lattice points are occupied by positive metal ions and a cloud of electrons pervades the space. Ex., all kind of metals (Cu, Ni, Fe).
Crystal lattice – Crystalline solid is characterised by a definite orientation of atoms, ions or molecules, relative to one another in a three dimensional pattern. The regular arrangement of these species throughout the crystal is called a crystal lattice.
Unit Cell – A basic repeating structural unit of a crystalline solid is called a unit cell.
Primitive and non-primitive unit cell:
- A unit cell that contains only one lattice points is called a primitive unit cell, which is made up from the lattice points at each of the comers.
- In case of non-primitive unit cells there are additional lattice points, either on a face of the unit cell or with in the unit cell.
Seven classes of primitive crystal systems :
|System||Crystallographic axes||Crystallographic angles|
|(i) Cubic||a = b = c||α = β = γ = 90°|
|(ii) Rhombohedral||a = b = c||α = β = γ ≠ 90°|
|(iii) Hexagonal||a = b ≠ c||α – β = 90°, γ = 120°|
|(iv) Tetragonal||a ≠ b ≠ c||α = β = γ = 90°|
|(v) Orthorhombic||A ≠ b ≠ c||α = β = γ = 90°|
|(vi) Monoclinic||a ≠ b ≠ c||α = β = 90°, β ≠ 90°|
|(vii) Triclinic||a ≠ b ≠ c||α ≠ β ≠ γ|
Packing efficiency – There is some free space between the spheres of a single layer and the spheres of successive layers. The percentage of total volume occupied by these constituent spheres gives the packing efficiency of an arrangement.
Schottky defect: – Schottky defect arises due to the missing of equal number of cations and anions from the crystal lattice. This effect does not change the stoichiometry of the crystal. Example :NaCl.
Frenkel defect: – Frenkel defect arises due to the dislocation of ions from its crystal lattice. The ion which is missing from the lattice point occupies an interstitial position.
Metal excess defect: – Metal excess defect arises due to the presence of more number of metal ions as compared to anions.Ex., Alkali halides (NaCl, KCl).
Metal deficiency defect: – Metal deficiency defect arises due to the presence of less number of cations than the anions. This defect is observed in a crystal in which, the cations have variable oxidation states. Example: FeO
Impurity defect: – This defect arises due to cation vacancies. In some ionic compounds presence of impurity produce some defects called impurity defects.
Ex, (i) presence of Cd++ ion as impurity in AgCl.
(ii) presence of Sr++ ion as impurity in AgCl.