The coordination number is defined as the number of nearest neighbours around any lattice point( or atom) in the crystal lattice.
( a) For sc, coordination number is 6.
( b) For bee, coordination number is 8.( c) For fcc, coordination number is 12.( d) For sc, atomic radius is a / 2.( e) For bcc, atomic is a √3 / 4.( f) For fcc. atomic radius is a / 2√2.
Classification of solids on the basis of conductivity
( i) Conductor Conductors are those substances through which electricity can pass easily, e. g., all metals are conductors.
( ii) Insulator Insulators are those substances through which electricity cannot pass, e. g., wood. rubber, mica etc.
( iii) Semiconductor Semiconductors are those substances whose conductivity lies between conductors and insulators. e. g., germanium, silicon, carbon etc.
Energy Bands of Solids
1. Energy Band
In a crystal due to interatomic interaction valence electrons of one atom are shared by more than one atom in the crystal. Now splitting of energy levels takes place. The collection of these closely spaced energy levels is called an energy band.
2. Valence Band
This energy band contains valence electrons. This band may be PartIally or completely filled with electrons but never be empty. The electrons in this band are not capable of gaining energy from external electric field to take part in conduction of current.
3. Conduction Band
This band contains conduction electrons. This band is either empty or Partially filled with electrons.
Electrons present in this band take part in the conduction of current.