Band theory

Band theory is the theory that metals have bands riddled with electrons, which explains why materials are conductors or insulators. This begs the question however, how is a material determined to be an insulator or conductor?

Band theory originated from Felix Bloch who suggested in 1928 who took the quantum theory and applied it to metals. The quantum theory in the most basic and oversimplified term in the study of the nature and behaviour of mass and energy. Lucking for Felix Bloch the year prior Walter Hietlar (good luck explaining who he is without mentioning a 1940s dictator) and Fritz London discovered that metals have bands, small orbitals very closely spaced to each other. 

With this knowledge Felix discovered that certain metals had different band structure. Conductors have their bands overlap ,which explained why they conducted electricity better than other materials. Insulators on the other hand had far too big of a gap to let electrons pass through them (I'll come back to this). Semi conductors however could conduct electrons if presented with light or energy (typically heat energy). So in conclusion there are three types of materials in electricity, Conductors, Insulators and Semi - Conductors.

Let us take a deeper look into Band theory. There are two bands, the conduction band and the valance band. The conduction band is the band that lets the electrons flow, the valance band is the band with electrons that can not flow and the energy gap is the gap between the conduction and valance band.

"Conductors", we have all come across this word in GCSE physics and it is the word that refers to materials that can conduct electricity. In the case of Band theory conductors have no energy gaps and the bands overlap, hence why they don't need any external form of energy like heat or light to make the material conduct electricity.

"Insulators", this word is used to describe materials that can not conduct electricity. Or can they? When you did GCSE physics insulators where materials such as plastic, glass and water. However with the use of band theory you can explain why these insulators can conduct electricity.

"Semi - Conductors", this term becomes important at A - level as you begin to study topics like the photoelectric effect or in this case band theory. Semi - conductors are usually metals like silicon as they are good semi conductors. the valance band and conduction band are not too far apart and usually requires only a small amount of energy or light to let the electrons in the valance band to move to the conduction band allowing an electric current to flow.

There is still one little piece of knowledge left in order to grasp the basic concept of band theory. When an electron leaves the valance band and goes into the conduction band a gap is left ethic is filled by the next electron in line to jump from the valance band to the conduction band. 

That's it. You now have the initial basic knowledge of band theory. I know that these posts are more academic, much like my first post on thermodynamics. On the other hand however these are some of my more favourite posts to write.