[Chemistry Class Notes] on Band Theory Pdf for Exam

The band theory of metals is based on the valence band and conduction band. It is also known as the 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 the 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. The 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, the 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 the conduction band. 

 

The band structure of sodium can be shown as follows –

Forbidden Gap – The gap or energy difference between the valence band and conduction band is called the 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. 

 

What is Band Theory? 

In chemistry, according to the 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 the valence band and conduction band. If the gap or energy difference between the valence band and conduction band is more than or equal to 5ev then the material will behave as an insulator. 

 

If the energy difference between the valence band and conduction band is equal to or less than 3ev then the material or solid is called a semiconductor. If the 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 conduct electricity. While if the gap is too much between the valence band and conduction band (more than or equal to 5ev) then electrons can’t jump from the valence band to the conduction band, hence these materials or solids can’t conduct electricity. If the valence band and conduction band are neither overlapping nor at too much distance then a few electrons may jump from the valence band to the conduction band and these materials are called semiconductors.

 

This was a brief of Band Theory, if you want to know more about the topic then register yourself on or download the learning app for classes 6-10, IITJEE and NEET.