March 20, 2012

Solid state - II Three Marks

1. Determine the number of CsCl units per unit cell. CsCl has BCC arrangement.
CsCl is body centered cubic system. The Cl- ions are at the corners of a cube where as Cs+ ion is at the centre of the cube or vice versa. Each Cs+ ion is connected with eight Cl- ion and Cl- is connected with eight Cs+ ions.
 Number of chloride ions per unit Nc /  8 = 8 / 8 = 1
Number of cesium ion per unit = Nb / 1 = 1 / 1 = 1
Thus, number of CsCl units per unit cell is one.

2. How are glasses formed?
When certain liquids are cooled rapidly there is no formation of crystals at a definite temperature, such as occurring on slow cooling. The viscosity of the liquid increases steadily and finally a glassy substance is formed.
3. Sketch the following lattices: a) Simple cubic b) Face - centred cubic c) Body centred cubic.
4. State Bragg's law.
Mathematically, Bragg’s equation / law is defined as = 2d sinθ
n = Order of reflection
λ = Wavelength of X-rays
d = Interplanar distance in the crystal
θ = Angle of reflection
5. What are superconductors? Give its applications / uses.
Substances having the property of virtually zero electrical resistance are called super conductors.
Application / uses of superconductors
1. It is a basis of new generation of energy saving power systems. Super conducting generators are smaller in size and weight when we compare with conventional generators. These generators consume very low energy and so we can save more energy.
2. High efficiency ore separating machines may be built using superconducting
3. Superconducting solenoids are used in Nuclear Magnetic Resonance Imaging equipment which is whole body scan equipment.
6. What is a vitreous state?
Vitreous state is a condition in which certain substance can exist, lying between the solid and liquid states.
7. What is meant by super conducting transition temperature?
Super conducting transition temperature ‘Tc’ of a material is defined as a critical temperature at which the resistivity of the material is suddenly changed to zero.
8. What is molecular crystal? Give an example / Write a note on molecular crystals.
1. The lattice points in molecular crystals consist of molecules which do not carry any charge.
2. The forces binding the molecules together are of two types
1. Dipole-dipole interaction and 2. Vanderwaal’s forces.
3. Dipole-dipole forces occur in solids which consists of polar molecules e.g., Ice.
4. The Vanderwaal’s forces are more general and occur in all kinds of molecular solids.
9. Write a note on Frenkel defect.
1. Frenkel defect arise when an ion occupies an interstitial position between the lattice points.
2. This defect occurs generally in ionic crystals in which the anion is much larger in size than the cation.
3. Example - AgBr
4. One of the Ag+ ion occupies a position in the interstitial space rather than its own appropriate site in the lattice is shown in Fig
5. The crystal remains neutral since the number of positive ions is the same as the number of negative ions.
10. Write a note on the assignment of atoms per unit cell in fcc.
A face atom is shared equally between two unit cells and therefore a face atom contributes only (Nf / 2) to the unit cell.

The number of atoms per unit cell in fcc     = Nc / 8 + Nf  / 2
     = 8 / 8 + 6 / 2
      = 1 + 3
      = 4
11. Write Bragg’s equation and explain the terms.
Bragg’s equation is nλ = 2d sinθ
n is the order of reflection
λ is the wavelength of X - rays
d is the interplanar distance in the crystal
θ is the angle of reflection
12. Write about metal excess defect
If a crystal of NaCl is heated in sodium vapour, it acquires a yellow colour. This yellow colour is due to the formation of a non-stoichiometric compound of NaCl in which there is a slight excess of sodium ions. This defect is called the metal excess defect.
13. What is metal deficiency defect? Give an example
In certain cases, one of the positive ions is missing from its lattice site and the extra negative charge is balanced by some nearby metal ion acquiring additional charges instead of original charge. This type of defect is generally found in compounds of transition metals which can exhibit variable valency. FeO and FeS show this type of defects.

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