Band Theory

Atomic Structure

In the National 5 Course, materials could be classed in one of two ways :-

1. Conductors - Any material that allows Current to flow easily.

2. Insulators - Any material that does not allow Current to flow easily.

The reason between the two different groups is based upon the atomic structure of the material.

The diagram below shows the electron configuration of the element Potassium:-

As can be seen above, all Electrons within an atom exist within discrete energy levels (sometimes called "shells") which can hold a set number of Electrons.

For light atoms (up to 20Ca) the innermost shell can hold 2 Electrons, whereas the other shells can hold 8 electrons.

Whether or not an atom is a conductor or an insulator depends on the number of electrons in the outermost shell:-

1. Outermost shell full - Electrons are tightly bound to the nucleus and are not free to move, making the material an insulator.

2. Outermost shell incomplete - Electrons are loosely bound to the nucleus and are free to move, making the material a conductor.

Band Theory

So far only isolated atoms have been considered above, but in real materials billions of atoms interact together to form the material structure. This has an effect on the shape of the shells surrounding the nucleus.

Due to the Pauli exclusion principle (see AH Physics for more information), electrons in adjacent atoms cannot occupy the same Energy level as each other, so more energy levels spontaneously come into existence. This creates "Bands" of allowed energies that an electron can possess. These Bands can be completely full of Electrons or only partially full, just like the isolated atoms energy levels.

There are two main Bands within an atom, the Valence Band and the Conduction Band, as can be seen in the diagram below:-

The two Bands are defined as :-

1. Valence Band - Outermost band that contains electrons. Electrons within the Valence Band are bound to the atom.

2. Conduction Band - Electrons with Energies in the Conduction Band are free to move through the material, giving an electric Current.

The diagram below shows how the Valence and Conduction Bands are structured in a conductor and an insulator:-

The following explains why materials are conductors or insulators:-

Insulator - there is a large Band Gap between the full Valence Band and the empty Conduction Band. As there is no Electrons in the Conduction Band, and it takes a large amount of Energy to move a Valence Electron up to it, no charges can move through the material, and no Current flows.

Conductor - The Valence Band and Conduction Bands overlap, meaning there are Electrons within the Conduction Band, therefore Electrons can move through it, allowing a Current to flow.