Depending upon the electrical conductivities the solids are classified in given three types:-
- The solids which have the conductivity in range 104 to 107 ohm-1 are called the conductors metals are called the best conductors of electricity.
- The solids which have extremely low conductivities ranging between 10-20 to 10-10 ohm are called insulators.
- Example: Plastics, Wood, Rubber etc.
- The solids which have conductivity in between those of conductors and insulators are called the semiconductors.
- Example: Silicon, Germanium etc.
BAND THEORY OF METALS:-
- In metals orbitals of atoms are so closed in energy that they overlap to form a large no. of molecular orbitals very close in energy. Thus set of molecular orbitals is called a band.
- For example in case of lithium the lower half of molecular orbitals is completely filled with electron while upper half is empty. On applying the electric current electrons can move into vacant levels. Thus electric current flows.
- Thus the band formed by lower energy valance orbitals is called valance bond it generally contains electrons.
- And band formed by slightly higher energy orbitals is called as the conduction band.
Explanation of behavior of conductors, insulators and semiconductors on the basis of bond theory:-
1. In case of metals [conductor]:- In conductor valance bond is partially filled with electrons and it overlaps with higher energy or unoccupied conduction band thus electrons can easily flow on application of electrical field thus metals have high conductivity.
2. Insulators:- In case of insulators the gap between the conduction band and valance band is too much that electron cannot jump from valance band to conduction band. Thus they not exhibit conductivity.
3. Semiconductors:- If the gap between valance band and conduction band is small (as in semiconductors) some electrons may jump from valance band to conduction band. Thus substance show a moderate conductivity.
Effect of temperature on conductivity:-
- The electrical conductivity of metals is decreases with increase in the temperature or heating.
- The electrical conductivity of semiconductors is increases on heating, due to the fact that more electrons can jump from valance band to conduction band.
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