The Solid State

The coordination number of Na in Na₂O is 4.Na⁺ ion is surrounded by four oxide ion and O^2- ion is surrounded by eight Na⁺ ions.
 Co-ordination number of Na⁺ = 4.
Co-ordination number of O^2– = 8.

ionic bonds are the electrostatic force of attraction between oppositely charged ion. The oppositely charged ion are arranged in a regular way to form giant ionic lattices. Ionic crystal of NaCl is a non-conductor of electricity because it has its electron in fully filled lower energy state. There are no free electrons in crystal lattice.

 The conduction of electricity in semi- conductors is because of the  Electrons and holes.

Phosphorus doped semiconductors produced n- type semiconductor while when silicon doped with gallium, it produces p- type semiconductor.