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 .