Band theory of metals is based on valence band and conduction band. It is also known as band theory of solids or zone theory of solids. It defines conductors, semiconductors and insulators very clearly and distinctly. Before understanding the band theory, you need to have knowledge of following terms –
Valence Band – It is made up of those valence shell orbitals which have electrons in them. For example, a sodium valence band is made up of 3s1 orbital. As electronic configuration of sodium is 1s2, 2s2 2p6, 3s1.
Conduction Band – It is made up of those orbitals which are unoccupied by electrons either in valence shell or higher unoccupied shell. Thus, orbitals of the conduction band are empty. Again, let’s take the example of sodium as after orbital 3s next orbital 3p is empty so it forms a conduction band.
Thus, in other words we can say the highest energy band that is filled is known as the valence band. While the next available band in the energy structure which is empty is called conduction band.
Band structure of sodium can be shown as follows –
Forbidden Gap – The gap or energy difference between valence band and conduction band is called forbidden gap.
Conductors – Those materials which allow electricity to pass through them are called conductors. Examples – copper, iron, zinc etc.
Semiconductors – Those materials which show conductivity between conductors and insulators are called semiconductors. Examples – Si, Ge, As etc.
Insulators – Those materials which do not allow electricity to pass through them are called insulators. Examples – wood, glass, stones etc.
After understanding all these terms now, you are ready to understand band theory.
In chemistry, according to band theory of solids electrons jump from valence band to conduction band even at ordinary temperature and if this happens then the solid conducts electricity. Conductivity depends on the gap between valence band and conduction band. If the gap or energy difference between valence band and conduction band is more than or equal to 5ev then material will behave as insulator.
If the energy difference between valence band and conduction band is equal to or less than 3ev then the material or solid is called semiconductor. If valence band and conduction band overlap each other then the solid is called conductors. The reasoning behind it is that electrons can jump from valence band to conduction band and thus conducts electricity. While if the gap is too much between valence band and conduction band (more than or equal to 5ev) then electrons can’t jump from valence band to conduction band, hence these materials or solids can’t conduct electricity. If valence band and conduction band are neither overlapping nor at too much distance then few electrons may jump from valence band to conduction band and these materials are called semiconductors.
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