CLASS XII CHEMISTRY.........SOLID

General Characteristics of Solid State:
Solids have definite mass, volume and shape.
Intermolecular forces are strong.
The constituent particles (atoms, molecules or ions) have fixed positions and can only oscillate about their mean positions.
They are incompressible and rigid. Solids are classified as crystalline and amorphous on the basis of order of arrangement of constituent particles.

Crystalline solid

1.Solids which have regular orderly arrangements of constituent particles (Long range order) 2. They have sharp melting point. 3. They give regular structure on cleavage. 4. They are anisotropic, i.e. ; they have different optical and electrical properties in different directions due to different arrangements of particles in different directions. 5. They have high and fixed heat of fusion. 6. Diamond, Graphite, NaCl, Metal (Fe, Cu, Ag etc) ice.

Amorphous solid

1.Solids which have irregular arrangement of constituent particles Short range order 2. They melt over a range of temperature. 3. They give irregular structure on cleavage. 4. They are isotropic, i.e. ; the value of physical properties is same in all directions due to irregular arrangement in all directions. 5. They do not have fixed heat of fusion. 6.Glass,rubber,plastics, Quartz glass


Crystal lattice: A regular arrangement of atoms, molecules or ions in the three dimensional space. Unit Cell: The smallest repeating portion of a crystal lattice which, when repeated in different direction generates the entire lattice. Lattice sites (points): The positions which are occupied by the constituent particles in the crystal lattice.

Crystalline solids are classified on the basis of nature of intermolecular forces (bonding) as

(SEE THE TABLE FROM YOUR TEXT)

Types of Unit cells Primitive Unit Cell: The unit cell which contains constituent particles at its corner positions.
Centred unit cells: The unit cells which contains constituent particles at other positions in addition to the corner positions.
Centred Unit cells are of three types:
Body centered unit cells: it contains constituent particles at its body centre in addition to the corner positions.
Face centered unit cell: it contains constituent particles in all the corner positions and also at the centre of each face.
 End centered unit cell: it contains constituent particles at the centre of any two opposite faces (end faces) in addition to the corner positions.


The seven crystal systems are: cubic tetragonal orthorhombic, hexagonal, trigonal, monoclinic, triclinic

 Bravais Lattices: there are 14 types of crystal lattices (space lattices) corresponding to seven crystals systems.
UNIT CELL          CALCULATION OF No. ATOMS                   No. OF ATOMS PER UNIT CELL

Primitive              8 (corner atoms)× 1/8(atom per unit cell)               1


Body centred    8 (corner atoms)× 1/8 (atom per unit cell) + 1           2

Face centred   8 (corner atoms) × 1/8 (atom per unit cell)                                                                                                                      
                       + 6 (face centred atoms) × 1/2 (atom per unit cell)       4


Close packed structures: various types of close packing of  constituent particles. Close packing in three dimensions Simple Cubic Lattice or Structure:  AAA….type arrangement generates simple cubic lattice in three dimension  Its unit cell is primitive cubic unit cell.  Packing efficiency = 52.4%

Hexagonal Close Packing Structure (HCP):  ABAB….Type arrangement  Example: Mg, Zn.  Packing efficiency = 74% o Coordination number = 12  It has N no of octahedral voids and 2N no. of tetrahedral voids if it has N no. of spheres.

Cubic Close Packing Structure (CCP):  ABCABC….Type arrangement  CCP structure is also called FCC structure. Example: Ag, Cu.  Packing efficiency = 74%  Coordination number = 12.  If there is N no. of close packed spheres then it has 2N no. of tetrahedral voids and N no. of octahedral voids.

 Voids (Interstitial Voids or Sites or Hole): Free space or vacant between close packed constituent particles. Types of voids: 
Tetrahedral void
Octahedral void

 Coordination number: the no of nearest neighbors of a particle or the no of spheres which are touching a given sphere.

Packing efficiency: the percentage of total space filled by the particles in a crystal.  Example: Packing efficiency of BCC structure = 68%

Density of the crystal: Density = mass of unit cell /Volume of unit cell