You will read in this tutorial about band theory of solids. Different types of bands present in solids like Valance band, Conduction band and Forbidden gap which is also known as energy band gap.
Also you will know about different roles of electrons, like valance electron, free electron etc.
First of all, what is a band? It is a collection of energy levels.
BAND THEORY OF SOLIDS
VALANCE BAND AND VALANCE ELECTRON
Electrons in outer shell are called valence electrons. In an element there are many atoms present and the valance electrons of these atoms have different energies associated with them because
Electrons of the atoms also get affected from the nucleus or electrons (e–) of neighbouring atoms (attractive force from neighbouring nucleus), so electrons have different energy levels, for example – suppose there are two electrons present in valance shell,
- One electron of valance band means one valance electron has energy of 4 but due to some forces from the neighbour atom valance electron loses its energy from 4 to 3.6
- While second electron has energy 4 and gains energy from the neighbouring atom from 4 to 4.4
So the range of these energy levels is 3.6 to 4.4 this is called a band and this band is called as valance band.
However, the energy levels of inner orbit electrons are not much affected by the presence of neighbouring atoms because they are tightly attracted to their own nucleus.
Valance electrons that get enough energy to jump from valance band to conduction band are called as free electrons. In other words valance electrons that get enough energy to leave the outer shell of an atom are called as free electrons.
Valance electrons when becomes free electrons are responsible for conduction. In simple words student near the exit gate can easily go out from the class, now as they are free they can roam in the school or college premises.
These free electrons are responsible for conduction in a substance.
Free electrons have different energies (energy levels) and the Range of energy levels of these free electrons forms a band, this band is called conduction band. Electrons in conduction band have higher energy than electrons in valance band or in forbidden band gap.
When a crystalline structure (ordered arrangement of atoms) (any object like lunch box, water bottle etc. are crystalline structure) forms, the free electrons cause current to flow through the surface of it (conductors or semiconductors) by jumping from valance band to conduction band.
When an electron of less energy gets some more energy it jumps to upper energy level i.e. from nth shell to n+1th shell (n+1th shell have more energy than nth shell).
For conduction, a valance electron must need the energy that should be equal or greater than the forbidden energy gap to jump from valance band to conduction band.
FORBIDDEN OR ENERGY BAND GAP
The energy levels difference between valance band and conduction band forms an energy band, this band is called as forbidden band gap. In other words the energy required by electrons to excite or to jump or to move from valance band to conduction band is known as Energy band gap or forbidden band gap.
ATOM, ORBITS AND ENERGY LEVELS