Valence band
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To understand valence band, study should be started from the formation of a metal. For example Li (Lithium) atoms with electronic configuration 1s22s1can form only one covalent bond. But when forming a continuous metal, Li atoms come to a resonance structure by taking 1 electron from its neighbouring Li atom and the resultant electronic configuration becomes 1s22s12p1 (e-). As a result of this electron sharing its neighbouring Li atom looses an electron and comes to an electronic configuration of 1s2 (e+). The Li (e-) atoms now gain the capability to form two covalent bonds thus can form a continuous metal. Inside the metal Li (e+) remain alone (not bonded) but nullify the negative charges of the neighbouring Li atoms thus forming a Lithium metal matrix.
In a 3D metal structure of Li, the molecular orbital formation starts from the lower energy level orbitals, i.e. first 1s, then 2s, then 2p. The molecular bond formation is a rapid process and as a result it is seen that 2s orbitals come to totally fulfilled condition whereas the 2p orbitals only partially filled during that time span, and the remaining part of the 2p orbitals remain empty (no electron). In between there is a overlapped zone of totally fulfilled 2s and totally fulfilled 2p orbitals, called Overlapped Zone.
Now, for any metal this is the rule of forming a metal from its atoms. The totally fulfilled orbitals with highest range of electron is called Valence Band (the fulfilled 2s orbitals in the example of Li), the empty orbitals with no electron is called the Conduction band (the empty region of 2p orbitals in the example of Li).
The overlapping depends on the interatomic distance (rd) and also on the energy level of the orbitals. If (rd) is large or the orbitals are of large energy level then there may be small overlapping or no overlapping leaving a band gap called Forbidden Zone (Eg). The electrical conductivity of a metal depends on its capability to flow electrons from valence band to conduction band. Hence in case of a metal with large overlapped region the electrical conductivity is high along with good metallic property. If there is a small forbidden zone i.e. Eg=120 kJmol-1 or 4eV, then the flow of electron from valence to conduction band is only possible if an external energy (thermal etc.) is supplied and these groups with small Eg are called Semiconductors. If the Eg is sufficiently high then flow of electron from valence to conduction band become negligible under normal conditions, these groups are called Insulators. But it is not possible to achieve a perfect insulator (0 electron flow).
In solids, the valence band is the highest range of electron energies where electrons are normally present at absolute zero.
In semiconductors and insulators, there is a band gap above the valence band, followed by a conduction band above that. In metals, the conduction band has no energy gap separating it from the valence band (basically, this is correct only for semimetals. All solids have forbidden energy levels between the energy bands). The rest of this article refers to the valence band in semiconductors and insulators.
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Semiconductor band structure
See electrical conduction and semiconductor for a more detailed description of band structure.
Semiconductors and insulators owe their low conductivity to the properties of the valence band in those materials. It just so happens that the number of electrons is precisely equal to the number of states available up to the top of the valence band. There are no available states in the band gap. This means that when an electric field is applied, the electrons cannot increase their energy (i.e., accelerate) because there are no states available to the electrons where they would be moving faster than they are already going.
There is some conductivity in insulators, however. This is due to thermal excitation—some of the electrons get enough energy to jump the band gap in one go. Once they are in the conduction band, they can conduct electricity, as can the hole they left behind in the valence band. The hole is an empty state that allows electrons in the valence band some degree of freedom.
It is a common misconception to refer to electrons in insulators as "bound"—as if they were somehow attached to the nucleus and could not move. Electrons in insulators are free to move. They are also delocalized, having no well-defined position within the sample.
[edit] See also
- Semiconductor for a full explanation of the band structure of materials.
- Electrical conduction for more information about conduction in solids, and another description of band structure.
- Conduction band
- Band Theory
- Fermi sea
