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.
Important Points in Solid State

Stacking Pattern of Solids

StructureCoordination NumberStacking Pattern
Hexagonal Close Packing12AB AB AB….
Cubic Close Packing12ABC 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:

  1. Simple cubic= 1
  2. Body centred cubic= 2
  3. Face centred cubic= 4

Packing fraction (space occupied):

  1. Simple cubic= 52.4%
  2. Body Centred cubic= 68%
  3. Face Centred cubic= 74%

Radius (r):

  1. Simple cubic= a/2
  2. Body Centred cubic= √3/4
  3. 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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