Energy Band

Energy Band:

 In case of a single isolated atom, the electrons in any orbit possess definite energy. However, an atom in a solid is greatly influenced by the closely-packed neighbouring atoms. The result is that the electron in any orbit of such an atom can have a range of energies rather than a single energy. This is known as energy band. 

The range of energies possessed by an electron in a solid is known as energy band.

(i) Valence band:

The range of energies (i.e. band) possessed by valence electrons 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 electrons of highest energy. This band may be completely or partially filled. For instance, in case of inert gases, the valence band is full whereas for other materials, it is only partially filled. The partially filled band can accommodate more electrons. 

(ii) 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 electrons. 

The range of energies (i.e. band) possessed by conduction band electrons is known as conduction band. 

All electrons in the conduction band are free electrons. If a substance has empty conduction band, it means current conduction is not possible in that substance. Generally, insulators have empty conduction band. On the other hand, it is partially filled for conductors. 

(iii) Forbidden energy gap:

 The separation between conduction band and valence band on the energy level diagram is known as forbidden energy gap. 

No electron of a solid can stay in a forbidden energy gap as there is no allowed 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 an electron from valence band to the conduction band (i.e. to make the valence electron free), external energy equal to the forbidden energy gap must be supplied.