In insulator, the atoms are electrically much
stable. The other most shell of the atoms are completed. So there is no chance
of gaining or loosing electron by these atoms. In other words, the forbidden
band gap between valance band and conduction band is sufficient large in
insulator.
The energy required by the electrons to cross
over to the conduction band is impractically large. That is why insulators cannot
conduct electric current easily. Wood, Glass, Plastic, Mica are well known
example of insulator.
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