Important Points in Solid State
Important Points in Solid State
- Physical state in which a substance exists is determined by the resultant of the disordering effect of thermal energy and ordering effect of the cohesive forces.
- A substance exists in the solid-state if the cohesive forces between the molecules are much greater than the thermal energy.
- The constituent particles in a solid, have fixed positions and can only oscillate about their mean positions.
- Solids have definite shape and volume due to the fixed positions of their constituent particles.
- Solids are hard and rigid because of the strong attractive forces between the constituent units due to which, they have fixed positions.
- Solids have higher density because the constituent units in solids are closely packed leading to decrease in volume.
- Solids are nearly incompressible because of lack of space between the constituent particles.
- The diffusion in solids is extremely slow because molecules in solids have fixed positions.
- A solid is said to be isotropic if its physical properties such as conductance, mechanical strength, refractive index, etc. are the same in all directions.
- Glass is a supercooled liquid.
- Amorphous solids cannot be obtained in the form of crystals under ordinary conditions.
- The interfacial angle is the angle formed by the two intersecting faces.
- Ideal crystals are the crystal which has the same unit cell containing the same lattice points in the whole crystal.
- The size of the crystal depends upon the rate at which it is formed, the slower the rate, the bigger is the crystal.
- The interfacial angle is measured with the help of Goniometer.
- Alkali metals are body centred cubic; nickel, copper and aluminium are face centred cubic;
magnesium, beryllium and zinc have a hexagonal close-packed arrangement.
- In hcp and ccp, each sphere is touching six others in its own layer, three spheres in the layer above, three spheres in the layers below. Thus, in close packing, a sphere is touching 12 other spheres. This number of nearest neighbours in packing is called coordination number.
Stacking Pattern of Solids
|Structure||Coordination Number||Stacking Pattern|
|Hexagonal Close Packing||12||AB AB AB….|
|Cubic Close Packing||12||ABC ABC ABC….|
- In close packing, the number of tetrahedral voids is double the number of spheres;
rvoid/rsphere = 0.225
- In close packing, the number of octahedral voids is double the number of spheres;
rvoid/rsphere = 0.414
The octahedral void is larger than the tetrahedral void.
Characters of Unit Cell
Number of particles per unit cell:
- Simple cubic= 1
- Body centred cubic= 2
- Face centred cubic= 4
Packing fraction (space occupied):
- Simple cubic= 52.4%
- Body Centred cubic= 68%
- Face Centred cubic= 74%
- Simple cubic= a/2
- Body Centred cubic= √3/4
- Face Centred cubic= a/ (2√2)
Distance Between Nearest Neighbours
- Simple cubic= a
- Body Centred cubic= a√3/2
- Face Centred cubic= a/√2
Density of Unit cell
= ZM/(N*a3*10-30) g/cm3
Where a is expressed in pm.
- In NaCl, Cl ions have ccp arrangement and Na ions occupy all the octahedral voids. Coordination Number of Na and Cl is 6:6.
- In CsCl, Cl ions are in the cubic arrangement and Cs ions occupy cubic voids. The coordination Number is 8:8.
- The defects caused by missing or misplaced atoms or ions in the crystal are called Point Defects.
- Equal Number of Cations and anions are missing from the lattice sites of crystals having Schottky Defect. Density decreases due to the Schottky defect.
- The number of point defects depends on the temperature, they are sometimes referred to as thermodynamic defects.
- Photovoltaic is a solid used for converting sunlight into electricity and effect is called Photovoltaic effect, e.g., Amorphous Silica.
- Curie Temperature is the temperature above which a ferromagnetic substance becomes paramagnetic.
- Piezoelectricity is produced by applying mechanical stress on polar crystals resulting in the displacement of ions. Piezoelectric crystals are used as pickups in record players.
- Pyroelectricity is the current produced by heating the polar crystals.
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