Energy bands and band gaps

Insulators have a full valence band separated from the next energy band, which is empty, by a large, forbidden gap. Diamond is an excellent insulator. It has a band gap of approximately 6eV which is very large. This means very few electrons have sufficient energy to be promoted and the conductivity is negligibly small. (fig. 5) When conductivity of insulators is able to be measured it is found to increase with temperature like the non-metallic solids.

IONIC SOLIDS – bonding due to transfer of charge from one atom to another. Energy bands formed from the atomic orbitals of anions and cations.

COVALENT SOLIDS – bonding due to overlap and sharing of electrons. Bands formed from bonding molecular orbitals (filled bands) and antibonding orbitals (empty bands).

METALLIC SOLIDS – bonding due to orbital overlap forming a delocalised cloud of electrons. Overlap of atomic orbitals can be so strong that bands are formed which are much broader than the original energy separation of the orbitals. Orbitals lose their individuality and you can look at it as the electrons moving freely.

The band gap is important as the size of it determines whether you are looking at a semiconductor or an insulator. Band gap determines the minimum photon energy required to excite an electron up to the conduction band from the valence band.
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