In case of such insulating material, there exists a large forbidden gap in between conduction band and the valence band. Practically it is impossible for an electron to jump from the valence band to the conduction band. Hence such materials cannot conduct and called insulators.
The forbidden gap is very wide, approximately of about 7 eV is present in insulators. For a diamond, which is an insulator, the forbidden gap is about 6 eV. Such materials may conduct only at very high temperatures or if they are subjected to high voltage. Such a conduction is rare and is called breakdown of an insulator.
Gases that serves as dielectrics are air, nitrogen, hydrogen, sulphur hexafluoride ete. Liquid dielectrics that are commonly used are mineral oils, synthetic hydrocarbons, etc. and they are used not only as insulating media but also to improve heat conduction property of apparatus.
Solid insulators are extremely diverse in origin and properties, they may be natural organic substances like paper, cloth, rubber etc. or inorganic materials like mica, glass and ceramic or synthetic materials like plastics.
Dielectric materials provide electrical insulation between conductors and also acts as stores of electrical charges.
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