About "Energy bands" (Atomic Structure)

Energy bands and classification.

What is energy band ?

In case of a single isolated atom,the electron in an Orbit process definite energy. However an atom in a solid is greatly influenced by the closely packed neighbouring atoms. The reason is that the electron in an Orbit of such an atom can have a range of energies rather than a single energy this is known as in energy band.
The range of energies possessed by an electron in a solid is known as energy band.

There are a number of energy bands in solid, the following are particular importance.

Valance band

The range of energy possessed by valence electron is known as valence band. The electrons in the outermost orbit of an atom are known as valence electrons. In a normal atom valence band has the electron of highest energy. This brand may be completely or partially filled. For intense in case of inert gases, the valence band is full whereas for other materials, it is only partially filled. The partially filled then can accomodate more electrons.

Conduction band 

In certain materials (e.g. metals), the valence electrons are loosely attached to the nucleus. Even at ordinary temperature some of the valence electrons may get detached to become free electrons. In fact, it is these free electrons which are responsible for the conduction of current in a conductor. For this reason they are called conduction electron. The range of energy possessed by conduction band electrons is known as conduction band. All electrons in the conduction band are free electrons. If substance has empty conduction band, it means current conduction is not possible in that substance. Generally insulator have empty conduction band on the other hand, It is partially filled for conductors. 

Forbidden gap 

separation between conduction band and valence band on the energy level diagram is knowyn as forbidden energy gap. No electron of a solid can stay in a forbidden energy gap as there is no allowed in energy state in this region. The width of the forbidden energy gap is a measure of the bondage of valence electrons to the atom. The greater the energy gap, more tightly the valence electrons are bound to the nucleus. In order to push electron from valence band to conduction band external energy equal to the forbidden energy gap must be supplied.

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